Patent Publication Number: US-9844335-B2

Title: Measurement device for the muscular-skeletal system having load distribution plates

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation-In-Part of U.S. application Ser. Nos. 13/406,484, 13/406,488 13/406,494, 13/406,500, 13/406,510, 13/406,512 13/406,515 all filed on Feb. 27, 2012, the entire contents of each application are hereby incorporated by reference. 
    
    
     FIELD 
     The present invention pertains generally to measurement of physical parameters, and particularly to, but not exclusively, medical electronic devices for high precision sensing. 
     BACKGROUND 
     The skeletal system of a mammal is subject to variations among species. Further changes can occur due to environmental factors, degradation through use, and aging. An orthopedic joint of the skeletal system typically comprises two or more bones that move in relation to one another. Movement is enabled by muscle tissue and tendons attached to the skeletal system of the joint. Ligaments hold and stabilize the one or more joint bones positionally. Cartilage is a wear surface that prevents bone-to-bone contact, distributes load, and lowers friction. 
     There has been substantial growth in the repair of the human skeletal system. In general, orthopedic joints have evolved using information from simulations, mechanical prototypes, and patient data that is collected and used to initiate improved designs. Similarly, the tools being used for orthopedic surgery have been refined over the years but have not changed substantially. Thus, the basic procedure for replacement of an orthopedic joint has been standardized to meet the general needs of a wide distribution of the population. Although the tools, procedure, and artificial joint meet a general need, each replacement procedure is subject to significant variation from patient to patient. The correction of these individual variations relies on the skill of the surgeon to adapt and fit the replacement joint using the available tools to the specific circumstance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features of the system are set forth with particularity in the appended claims. The embodiments herein, can be understood by reference to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a sensor placed in contact between a femur and a tibia for measuring a parameter in accordance with an example embodiment; 
         FIG. 2  illustrates a block diagram of an zero-crossing receiver in accordance with an example embodiment; 
         FIG. 3  illustrates a block diagram of the integrated zero-crossing receiver coupled to a sensing assembly in accordance with an example embodiment; 
         FIG. 4  illustrates a propagation tuned oscillator (PTO) incorporating a zero-crossing receiver or an edge detect receiver to maintain positive closed-loop feedback in accordance with an example embodiment; 
         FIG. 5  illustrates a sensor interface incorporating the zero-crossing receiver in a continuous wave multiplexing arrangement for maintaining positive closed-loop feedback in accordance with an example embodiment; 
         FIG. 6  illustrates a block diagram of a propagation tuned oscillator (PTO) incorporating the integrated zero-crossing receiver for operation in continuous wave mode; 
         FIG. 7  illustrates a sensor interface diagram incorporating the integrated zero-crossing receiver in a pulse multiplexing arrangement for maintaining positive closed-loop feedback in accordance with an example embodiment; 
         FIG. 8  illustrates a block diagram of a propagation tuned oscillator (PTO) incorporating the integrated zero-crossing receiver for operation in pulse mode in accordance with an example embodiment; 
         FIG. 9  illustrates a block diagram of an edge-detect receiver circuit in accordance with an example embodiment; 
         FIG. 10  illustrates a block diagram of the edge-detect receiver circuit coupled to a sensing assembly; 
         FIG. 11  illustrates a sensor interface diagram incorporating the edge-detect receiver circuit in a pulse-echo multiplexing arrangement for maintaining positive closed-loop feedback in accordance with an example embodiment; 
         FIG. 12  illustrates a block diagram of a propagation tuned oscillator (PTO) incorporating the edge-detect receiver circuit for operation in pulse echo mode; 
         FIG. 13  illustrates a simplified cross-sectional view of a sensing module in accordance with an example embodiment; 
         FIG. 14  illustrates an assemblage for illustrating reflectance and unidirectional modes of operation in accordance with an example embodiment; 
         FIG. 15  illustrates an assemblage that illustrates propagation of ultrasound waves within a waveguide in the bi-directional mode of operation of this assemblage; 
         FIG. 16  illustrates a cross-sectional view of a sensor element to illustrate changes in the propagation of ultrasound waves with changes in the length of a waveguide; 
         FIG. 17  illustrates a simplified flow chart of method steps for high precision processing and measurement data in accordance with an example embodiment; 
         FIG. 18  illustrates a block diagram of a medical sensing system in accordance with an example embodiment; 
         FIG. 19  illustrates an oscillator configured to generate a measurement cycle corresponding to a capacitor in accordance with an example embodiment; 
         FIG. 20  illustrates a method of force, pressure, or load sensing in accordance with an example embodiment; 
         FIG. 21  illustrates a cross-sectional view of a capacitor in accordance with an example embodiment; 
         FIG. 22  illustrates the capacitor of  FIG. 21  comprising more than one capacitor coupled mechanically in series in accordance with an example embodiment; 
         FIG. 23  illustrates the capacitor of  FIG. 21  comprising more than one capacitor coupled electrically in parallel in accordance with an example embodiment; 
         FIG. 24  illustrates a top view of a conductive region of the capacitor of  FIG. 21  and interconnect thereto in accordance with an example embodiment; 
         FIG. 25  illustrates a cross-sectional view of the interconnect coupled to the capacitor of  FIG. 21  in accordance with an example embodiment; 
         FIG. 26  illustrates a diagram of a method of measuring a force, pressure, or load in accordance with an example embodiment; 
         FIG. 27  illustrates a medical device having a plurality of sensors in accordance with an example embodiment; 
         FIG. 28  illustrates one or more prosthetic components having sensors coupled to and conforming with non-planar surfaces in accordance with an example embodiment; 
         FIG. 29  illustrates a tool having one or more shielded sensors coupled to a non-planar surface in accordance with an example embodiment; 
         FIG. 30  illustrates a diagram of a method of using a capacitor as a sensor to measure a parameter of the muscular-skeletal system in accordance with an example embodiment; 
         FIG. 31  illustrates a prosthetic component having a plurality of sensors in accordance with an example embodiment; 
         FIG. 32  illustrates a cross-sectional view of a structure of the prosthetic component in accordance with an example embodiment; 
         FIG. 33  illustrates the prosthetic component and an insert in accordance with an example embodiment; 
         FIG. 34  illustrates electronic circuitry coupled to interconnect in accordance with an example embodiment; 
         FIG. 35  illustrates an assembled the prosthetic component in accordance with an example embodiment; 
         FIG. 36  illustrates a partial cross-sectional view of the prosthetic component in accordance with an example embodiment; 
         FIG. 37  illustrates the structure and electronic circuitry in accordance with an example embodiment; 
         FIG. 38  illustrates the prosthetic component and a remote system in accordance with an example embodiment; 
         FIG. 39  is an illustration of the electronic circuitry and the structure in accordance with an example embodiment; 
         FIG. 40  is an illustration of the electronic circuitry and the structure in accordance with an example embodiment; 
         FIG. 41  depicts an exemplary diagrammatic representation of a machine in the form of a system within which a set of instructions are executed in accordance with an example embodiment; 
         FIG. 42  is an illustration of a communication network for measurement and reporting in accordance with an example embodiment; 
         FIG. 43  is an illustration of a measurement device for measuring a force, pressure, or load of the muscular-skeletal system in accordance with an example embodiment; 
         FIG. 44  is an illustration of a support structure and load plates in accordance with an example embodiment; 
         FIG. 45  is an illustration of the support structure and load plates in accordance with an example embodiment; 
         FIG. 46  is an illustration of the measurement system prior to coupling the support structures together in accordance with an example embodiment; 
         FIG. 47  is an illustration of a cross-section of the measurement system in accordance with an example embodiment; 
         FIG. 48  is an illustration of the assembled measurement system in accordance with an example embodiment; 
         FIG. 49  is an illustration of the measurement system coupled to a prosthetic component in accordance with an example embodiment; 
         FIG. 50  is an illustration of the measurement system in the muscular-skeletal system in accordance with an example embodiment; and 
         FIG. 51  is a method of assembling a device for measuring a force, pressure, or load measurement device that couples to the muscular-skeletal system in accordance with an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention are broadly directed to measurement of physical parameters, and more particularly, to fast-response circuitry that supports accurate measurement of small sensor changes. 
     The following description of exemplary embodiment(s) is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. 
     Processes, techniques, apparatus, and materials as known by one of ordinary skill in the art may not be discussed in detail but are intended to be part of the enabling description where appropriate. For example specific computer code may not be listed for achieving each of the steps discussed, however one of ordinary skill would be able, without undo experimentation, to write such code given the enabling disclosure herein. Such code is intended to fall within the scope of at least one exemplary embodiment. 
     In all of the examples illustrated and discussed herein, any specific materials, such as temperatures, times, energies, and material properties for process steps or specific structure implementations should be interpreted to be illustrative only and non-limiting. Processes, techniques, apparatus, and materials as known by one of ordinary skill in the art may not be discussed in detail but are intended to be part of an enabling description where appropriate. It should also be noted that the word “coupled” used herein implies that elements may be directly coupled together or may be coupled through one or more intervening elements. 
     Additionally, the sizes of structures used in exemplary embodiments are not limited by any discussion herein (e.g., the sizes of structures can be macro (centimeter, meter, and larger sizes), micro (micrometer), and nanometer size and smaller). 
     Notice that similar reference numerals and letters refer to similar items in the following figures, and thus once an item is defined in one figure, it may not be discussed or further defined in the following figures. 
     In a first embodiment, an ultrasonic measurement system comprises one or more ultrasonic transducers, an ultrasonic waveguide, and a propagation tuned oscillator (PTO) or Phase Locked Loop (PLL). The ultrasonic measurement system in this embodiment employs a continuous mode (CM) of operation to evaluate propagation characteristics of continuous ultrasonic waves in the waveguide by way of closed-loop feedback to determine levels of applied forces on the waveguide. 
     In a second embodiment, an ultrasonic measurement system comprises one or more ultrasonic transducers, an ultrasonic waveguide, and a propagation tuned oscillator (PTO) or Phase Locked Loop (PLL). The ultrasonic measurement system in this embodiment employs a pulse mode (PM) of operation to evaluate propagation characteristics of pulsed ultrasonic waves in the waveguide by way of closed-loop feedback to determine levels of applied forces on the waveguide. 
     In a third embodiment, an ultrasonic measurement system comprises one or more ultrasonic transducers, an ultrasonic waveguide, and a propagation tuned oscillator (PTO) or Phase Locked Loop (PLL). The ultrasonic measurement system in this embodiment employs a pulse echo mode (PE) of operation to evaluate propagation characteristics of ultrasonic echo reflections in the waveguide by way of closed-loop feedback to determine levels of applied forces on the waveguide. 
       FIG. 1  is an illustration of a sensor  100  placed in contact between a femur  102  and a tibia  108  for measuring a parameter in accordance with an exemplary embodiment. In general, a sensor  100  is placed in contact with or in proximity to the muscular-skeletal system to measure a parameter. In a non-limiting example, sensor  100  is used to measure a parameter of a muscular-skeletal system during a procedure such as an installation of an artificial joint. Embodiments of sensor  100  are broadly directed to measurement of physical parameters, and more particularly, to evaluating changes in the transit time of a pulsed energy wave propagating through a medium. In-situ measurements during orthopedic joint implant surgery would be of substantial benefit to verify an implant is in balance and under appropriate loading or tension. In one embodiment, the instrument is similar to and operates familiarly with other instruments currently used by surgeons. This will increase acceptance and reduce the adoption cycle for a new technology. The measurements will allow the surgeon to ensure that the implanted components are installed within predetermined ranges that maximize the working life of the joint prosthesis and reduce costly revisions. Providing quantitative measurement and assessment of the procedure using real-time data will produce results that are more consistent. A further issue is that there is little or no implant data generated from the implant surgery, post-operatively, and long term. Sensor  100  can provide implant status data to the orthopedic manufacturers and surgeons. Moreover, data generated by direct measurement of the implanted joint itself would greatly improve the knowledge of implanted joint operation and joint wear thereby leading to improved design and materials. 
     In at least one exemplary embodiment, an energy pulse is directed within one or more waveguides in sensor  100  by way of pulse mode operations and pulse shaping. The waveguide is a conduit that directs the energy pulse in a predetermined direction. The energy pulse is typically confined within the waveguide. In one embodiment, the waveguide comprises a polymer material. For example, urethane or polyethylene are polymers suitable for forming a waveguide. The polymer waveguide can be compressed and has little or no hysteresis in the system. Alternatively, the energy pulse can be directed through the muscular-skeletal system. In one embodiment, the energy pulse is directed through bone of the muscular-skeletal system to measure bone density. A transit time of an energy pulse is related to the material properties of a medium through which it traverses. This relationship is used to generate accurate measurements of parameters such as distance, weight, strain, pressure, wear, vibration, viscosity, and density to name but a few. 
     Sensor  100  can be size constrained by form factor requirements of fitting within a region the muscular-skeletal system or a component such as a tool, equipment, or artificial joint. In a non-limiting example, sensor  100  is used to measure load and balance of an installed artificial knee joint. A knee prosthesis comprises a femoral prosthetic component  104 , an insert, and a tibial prosthetic component  106 . A distal end of femur  102  is prepared and receives femoral prosthetic component  104 . Femoral prosthetic component  104  typically has two condyle surfaces that mimic a natural femur. As shown, femoral prosthetic component  104  has single condyle surface being coupled to femur  102 . Femoral prosthetic component  104  is typically made of a metal or metal alloy. 
     A proximal end of tibia  108  is prepared to receive tibial prosthetic component  106 . Tibial prosthetic component  106  is a support structure that is fastened to the proximal end of the tibia and is usually made of a metal or metal alloy. The tibial prosthetic component  106  also retains the insert in a fixed position with respect to tibia  108 . The insert is fitted between femoral prosthetic component  104  and tibial prosthetic component  106 . The insert has at least one bearing surface that is in contact with at least condyle surface of femoral prosthetic component  104 . The condyle surface can move in relation to the bearing surface of the insert such that the lower leg can rotate under load. The insert is typically made of a high wear plastic material that minimizes friction. 
     In a knee joint replacement process, the surgeon affixes femoral prosthetic component  104  to the femur  102  and tibial prosthetic component  106  to tibia  108 . The tibial prosthetic component  106  can include a tray or plate affixed to the planarized proximal end of the tibia  108 . Sensor  100  is placed between a condyle surface of femoral prosthetic component  104  and a major surface of tibial prosthetic component  106 . The condyle surface contacts a major surface of sensor  100 . The major surface of sensor  100  approximates a surface of the insert. Tibial prosthetic component  106  can include a cavity or tray on the major surface that receives and retains sensor  100  during a measurement process. Tibial prosthetic component  106  and sensor  100  has a combined thickness that represents a combined thickness of tibial prosthetic component  106  and a final (or chronic) insert of the knee joint. 
     In one embodiment, two sensors  100  are fitted into two separate cavities, the cavities are within a trial insert (that may also be referred to as the tibial insert, rather than the tibial component itself) that is held in position by tibial component  106 . One or two sensors  100  may be inserted between femoral prosthetic component  104  and tibial prosthetic component  106 . Each sensor is independent and each measures a respective condyle of femur  102 . Separate sensors also accommodate a situation where a single condyle is repaired and only a single sensor is used. Alternatively, the electronics can be shared between two sensors to lower cost and complexity of the system. The shared electronics can multiplex between each sensor module to take measurements when appropriate. Measurements taken by sensor  100  aid the surgeon in modifying the absolute loading on each condyle and the balance between condyles. Although shown for a knee implant, sensor  100  can be used to measure other orthopedic joints such as the spine, hip, shoulder, elbow, ankle, wrist, interphalangeal joint, metatarsophalangeal joint, metacarpophalangeal joints, and others. Alternatively, sensor  100  can also be adapted to orthopedic tools to provide measurements. 
     The prosthesis incorporating sensor  100  emulates the function of a natural knee joint. Sensor  100  can measure loads or other parameters at various points throughout the range of motion. Data from sensor  100  is transmitted to a receiving station  110  via wired or wireless communications. In a first embodiment, sensor  100  is a disposable system. Sensor  100  can be disposed of after using sensor  100  to optimally fit the joint implant. Sensor  100  is a low cost disposable system that reduces capital costs, operating costs, facilitates rapid adoption of quantitative measurement, and initiates evidentiary based orthopedic medicine. In a second embodiment, a methodology can be put in place to clean and sterilize sensor  100  for reuse. In a third embodiment, sensor  100  can be incorporated in a tool instead of being a component of the replacement joint. The tool can be disposable or be cleaned and sterilized for reuse. In a fourth embodiment, sensor  100  can be a permanent component of the replacement joint. Sensor  100  can be used to provide both short term and long term post-operative data on the implanted joint. In a fifth embodiment, sensor  100  can be coupled to the muscular-skeletal system. In all of the embodiments, receiving station  110  can include data processing, storage, or display, or combination thereof and provide real time graphical representation of the level and distribution of the load. Receiving station  110  can record and provide accounting information of sensor  100  to an appropriate authority. 
     In an intra-operative example, sensor  100  can measure forces (Fx, Fy, Fz) with corresponding locations and torques (e.g. Tx, Ty, and Tz) on the femoral prosthetic component  104  and the tibial prosthetic component  106 . The measured force and torque data is transmitted to receiving station  110  to provide real-time visualization for assisting the surgeon in identifying any adjustments needed to achieve optimal joint pressure and balancing. The data has substantial value in determining ranges of load and alignment tolerances required to minimize rework and maximize patient function and longevity of the joint. 
     As mentioned previously, sensor  100  can be used for other joint surgeries; it is not limited to knee replacement implant or implants. Moreover, sensor  100  is not limited to trial measurements. Sensor  100  can be incorporated into the final joint system to provide data post-operatively to determine if the implanted joint is functioning correctly. Early determination of a problem using sensor  100  can reduce catastrophic failure of the joint by bringing awareness to a problem that the patient cannot detect. The problem can often be rectified with a minimal invasive procedure at lower cost and stress to the patient. Similarly, longer term monitoring of the joint can determine wear or misalignment that if detected early can be adjusted for optimal life or replacement of a wear surface with minimal surgery thereby extending the life of the implant. In general, sensor  100  can be shaped such that it can be placed or engaged or affixed to or within load bearing surfaces used in many orthopedic applications (or used in any orthopedic application) related to the musculoskeletal system, joints, and tools associated therewith. Sensor  100  can provide information on a combination of one or more performance parameters of interest such as wear, stress, kinematics, kinetics, fixation strength, ligament balance, anatomical fit and balance. 
       FIG. 2  is a block diagram of a zero-crossing receiver  200  in accordance with one embodiment. In a first embodiment, the zero-crossing receiver  200  is provided to detect transition states of energy waves, such as the transition of each energy wave through a mid-point of a symmetrical or cyclical waveform. This enables capturing of parameters including, but not limited to, transit time, phase, or frequency of the energy waves. The receiver rapidly responds to a signal transition and outputs a digital pulse that is consistent with the energy wave transition characteristics and with minimal delay. The zero-crossing receiver  200  further discriminates between noise and the energy waves of interest, including very low level waves by way of adjustable levels of noise reduction. A noise reduction section  218  comprises a filtering stage and an offset adjustment stage to perform noise suppression accurately over a wide range of amplitudes including low level waves. 
     In a second embodiment, a zero-crossing receiver is provided to convert an incoming symmetrical, cyclical, or sine wave to a square or rectangular digital pulse sequence with superior performance for very low level input signals. The digital pulse sequence represents pulse timing intervals that are consistent with the energy wave transition times. The zero-crossing receiver is coupled with a sensing assembly to generate the digital pulse sequence responsive to evaluating transitions of the incoming sine wave. This digital pulse sequence conveys timing information related to parameters of interest, such as applied forces, associated with the physical changes in the sensing assembly. 
     In a third embodiment, the integrated zero-crossing receiver is incorporated within a propagation tuned oscillator (PTO) to maintain positive closed-loop feedback when operating in a continuous wave mode or pulse-loop mode. The integrated edge zero-crossing receiver is electrically integrated with the PTO by multiplexing input and output circuitry to achieve ultra low-power and small compact size. Electrical components of the PTO are integrated with components of the zero-crossing receiver to assure adequate sensitivity to low-level signals. 
     In one embodiment, low power zero-crossing receiver  200  can be integrated with other circuitry of the propagation tuned oscillator to further improve performance at low signal levels. The zero-crossing receiver  200  comprises a preamplifier  206 , a filter  208 , an offset adjustment circuitry  210 , a comparator  212 , and a digital pulse circuit  214 . The filter  208  and offset adjustment circuitry  210  constitute a noise reduction section  218  as will be explained ahead. The zero-crossing receiver  200  can be implemented in discrete analog components, digital components or combination thereof. The integrated zero-crossing receiver  200  practices measurement methods that detect the midpoint of energy waves at specified locations, and under specified conditions, to enable capturing parameters including, but not limited to, transit time, phase, or frequency of energy waves. A brief description of the method of operation is as follows. 
     An incoming energy wave  202  is coupled from an electrical connection, antenna, or transducer to an input  204  of zero-crossing receiver  200 . Input  204  of zero-crossing receiver  200  is coupled to pre-amplifier  206  to amplify the incoming energy wave  202 . The amplified signal is filtered by filter  208 . Filter  208  is coupled to an output of pre-amplifier  206  and an input of offset adjustment circuitry  210 . In one configuration, filter  208  is a low-pass filter to remove high frequency components above the incoming energy wave  202  bandwidth. In another arrangement, the filter is a band-pass filter with a pass-band corresponding to the bandwidth of the incoming energy wave  202 . It is not however limited to either arrangement. The offset of the filtered amplified wave is adjusted by offset adjustment circuitry  210 . An input of comparator  212  is coupled to an output of offset adjustment circuitry  210 . Comparator  212  monitors the amplified waveforms and triggers digital pulse circuitry  214  whenever the preset trigger level is detected. Digital pulse circuit  214  has an input coupled to the output of comparator  212  and an output for providing digital pulse  216 . The digital pulse  216  can be further coupled to signal processing circuitry, as will be explained ahead. 
     In a preferred embodiment, the electronic components are operatively coupled together as blocks of integrated circuits. As will be shown ahead, this integrated arrangement performs its specific functions efficiently with a minimum number of components. This is because the circuit components are partitioned between structures within an integrated circuit and discrete components, as well as innovative partitioning of analog and digital functions, to achieve the required performance with a minimum number of components and minimum power consumption. 
       FIG. 3  illustrates a block diagram of the integrated zero-crossing receiver  200  coupled to a sensing assembly  300  in accordance with an exemplary embodiment. The pre-amplifier  206  and the digital pulse circuit  214  are shown for reference and discussion. In one embodiment, sensing assembly  300  comprises a transmitter transducer  302 , an energy propagating structure (or medium)  304 , and a receiver transducer  306 . As will be explained further hereinbelow, the sensing assembly  300  in one embodiment is part of a sensory device that measures a parameter such as force, pressure, or load. In a non-limiting example, an external parameter such as an applied force  308  affects the sensing assembly  200 . As shown, applied force  308  modifies propagating structure  304  dimensionally. In general, the sensing assembly  300  conveys one or more parameters of interest such as distance, force, weight, strain, pressure, wear, vibration, viscosity, density, direction, and displacement related to a change in energy propagating structure  304 . An example is measuring loading applied by a joint of the muscular-skeletal system as disclosed above using sensing assembly  300  between the bones of the joint. 
     A transducer driver circuit (not shown) drives the transmitter transducer  302  of the sensing assembly  300  to produce energy waves  310  that are directed into the energy propagating structure  304 . Changes in the energy propagating medium  304  due to an applied parameter such as applied forces  308  change the frequency, phase, and transit time of energy waves  310  (or pulses). In one embodiment, applied forces  308  affect the length of propagating structure  304  in a direction of a path of propagation of energy waves  310 . The zero-crossing receiver  200  is coupled to the receiver transducer  306  to detect zero-crossings of the reproduced energy wave  202 . Upon detecting a zero-crossing digital pulse circuit  214  is triggered to output a pulse  216 . The timing of the digital pulse  216  conveys the parameters of interest (e.g., distance, force weight, strain, pressure, wear, vibration, viscosity, density, direction, displacement, etc.). 
     Measurement methods that rely on such propagation of energy waves  310  or pulses of energy waves are required to achieve highly accurate and controlled detection of energy waves or pulses. Moreover, pulses of energy waves may contain multiple energy waves with complex waveforms therein leading to potential ambiguity of detection. In particular, directing energy waves  310  into the energy propagating structure  304  can generate interference patterns caused by nulls and resonances of the waveguide, as well as characteristics of the generated energy waves  310 . These interference patterns can multiply excited waveforms that result in distortion of the edges of the original energy wave. 
     Briefly referring back to  FIG. 2 , to reliably detect the arrival of a pulse of energy waves, the zero-crossing receiver  200  leverages noise reduction section  218  that incorporates two forms of noise reduction. Frequencies above the operating frequencies for physical measurements of the parameters of interest are attenuated with the filter  208 . In addition, the offset level of the incoming waveform is adjusted by the offset adjustment  210  to optimize the voltage level at which the comparator  212  triggers an output pulse. This is more reliable than amplifying the incoming waveform because it does not add additional amplification of noise present on the input. The combination of rapid response to the arrival of incoming symmetrical, cyclical, or sine waves with adjustable levels of noise reduction achieves reliable zero-crossing detection by way of the ultra low power zero-crossing receiver  200  with superior performance for very low level signals. 
     There are a wide range of applications for compact measurement modules or devices having ultra low power circuitry that enables the design and construction of highly performing measurement modules or devices that can be tailored to fit a wide range of nonmedical and medical applications. Applications for highly compact measurement modules or devices may include, but are not limited to, disposable modules or devices as well as reusable modules or devices and modules or devices for long term use. In addition to nonmedical applications, examples of a wide range of potential medical applications may include, but are not limited to, implantable devices, modules within implantable devices, intra-operative implants or modules within intra-operative implants or trial inserts, modules within inserted or ingested devices, modules within wearable devices, modules within handheld devices, modules within instruments, appliances, equipment, or accessories of all of these, or disposables within implants, trial inserts, inserted or ingested devices, wearable devices, handheld devices, instruments, appliances, equipment, or accessories to these devices, instruments, appliances, or equipment. 
       FIG. 4  is an exemplary block diagram  400  of a propagation tuned oscillator (PTO)  404  to maintain positive closed-loop feedback in accordance with an exemplary embodiment. The measurement system includes a sensing assemblage  401  and propagation tuned oscillator (PTO)  404  that detects energy waves  402  in one or more waveguides  403  of the sensing assemblage  401 . In one embodiment, energy waves  402  are ultrasound waves. A pulse  411  is generated in response to the detection of energy waves  402  to initiate a propagation of a new energy wave in waveguide  403 . It should be noted that ultrasound energy pulses or waves, the emission of ultrasound pulses or waves by ultrasound resonators or transducers, transmitted through ultrasound waveguides, and detected by ultrasound resonators or transducers are used merely as examples of energy pulses, waves, and propagation structures and media. Other embodiments herein contemplated can utilize other waveforms, such as, light. 
     The sensing assemblage  401  comprises transducer  405 , transducer  406 , and a waveguide  403  (or energy propagating structure). In a non-limiting example, sensing assemblage  401  is affixed to load bearing or contacting surfaces  408 . External forces applied to the contacting surfaces  408  compress the waveguide  403  and change the length of the waveguide  403 . Under compression, transducers  405  and  406  will also be move closer together. The change in distance affects the transit time  407  of energy waves  402  transmitted and received between transducers  405  and  406 . The propagation tuned oscillator  404  in response to these physical changes will detect each energy wave sooner (e.g. shorter transit time) and initiate the propagation of new energy waves associated with the shorter transit time. As will be explained below, this is accomplished by way of PTO  404  in conjunction with the pulse generator  410 , the mode control  412 , and the phase detector  414 . 
     Notably, changes in the waveguide  403  (energy propagating structure or structures) alter the propagation properties of the medium of propagation (e.g. transit time  407 ). The energy wave can be a continuous wave or a pulsed energy wave. A pulsed energy wave approach reduces power dissipation allowing for a temporary power source such as a battery or capacitor to power the system during the course of operation. In at least one exemplary embodiment, a continuous wave energy wave or a pulsed energy wave is provided by transducer  405  to a first surface of waveguide  403 . Transducer  405  generates energy waves  402  that are coupled into waveguide  403 . In a non-limiting example, transducer  405  is a piezo-electric device capable of transmitting and receiving acoustic signals in the ultrasonic frequency range. 
     Transducer  406  is coupled to a second surface of waveguide  403  to receive the propagated pulsed signal and generates a corresponding electrical signal. The electrical signal output by transducer  406  is coupled to phase detector  414 . In general, phase detector  414  is a detection circuit that compares the timing of a selected point on the waveform of the detected energy wave with respect to the timing of the same point on the waveform of other propagated energy waves. In a first embodiment, phase detector  414  can be a zero-crossing receiver. In a second embodiment, phase detector  414  can be an edge-detect receiver. In a third embodiment, phase detector  414  can be a phase locked loop. In the example where sensing assemblage  401  is compressed, the detection of the propagated energy waves  402  occurs earlier (due to the length/distance reduction of waveguide  403 ) than a signal prior to external forces being applied to contacting surfaces. Pulse generator  410  generates a new pulse in response to detection of the propagated energy waves  402  by phase detector  414 . The new pulse is provided to transducer  405  to initiate a new energy wave sequence. Thus, each energy wave sequence is an individual event of energy wave propagation, energy wave detection, and energy wave emission that maintains energy waves  402  propagating in waveguide  403 . 
     The transit time  407  of a propagated energy wave is the time it takes an energy wave to propagate from the first surface of waveguide  403  to the second surface. There is delay associated with each circuit described above. Typically, the total delay of the circuitry is significantly less than the propagation time of an energy wave through waveguide  403 . In addition, under equilibrium conditions variations in circuit delay are minimal. Multiple pulse to pulse timings can be used to generate an average time period when change in external forces occur relatively slowly in relation to the pulsed signal propagation time such as in a physiologic or mechanical system. The digital counter  420  in conjunction with electronic components counts the number of propagated energy waves to determine a corresponding change in the length of the waveguide  403 . These changes in length change in direct proportion to the external force thus enabling the conversion of changes in parameter or parameters of interest into electrical signals. 
     The block diagram  400  further includes counting and timing circuitry. More specifically, the timing, counting, and clock circuitry comprises a digital timer  420 , a digital timer  422 , a digital clock  426 , and a data register  424 . The digital clock  426  provides a clock signal to digital counter  420  and digital timer  422  during a measurement sequence. The digital counter  420  is coupled to the propagation tuned oscillator  404 . Digital timer  422  is coupled to data register  424 . Digital timer  420 , digital timer,  422 , digital clock  426  and data register  424  capture transit time  407  of energy waves  402  emitted by ultrasound resonator or transducer  405 , propagated through waveguide  403 , and detected by or ultrasound resonator or transducer  405  or  406  depending on the mode of the measurement of the physical parameters of interest applied to surfaces  408 . The operation of the timing and counting circuitry is disclosed in more detail hereinbelow. 
     The measurement data can be analyzed to achieve accurate, repeatable, high precision and high resolution measurements. This method enables the setting of the level of precision or resolution of captured data to optimize trade-offs between measurement resolution versus frequency, including the bandwidth of the sensing and data processing operations, thus enabling a sensing module or device to operate at its optimal operating point without compromising resolution of the measurements. This is achieved by the accumulation of multiple cycles of excitation and transit time instead of averaging transit time of multiple individual excitation and transit cycles. The result is accurate, repeatable, high precision and high resolution measurements of parameters of interest in physical systems. 
     In at least one exemplary embodiment, propagation tuned oscillator  404  in conjunction with one or more sensing assemblages  401  are used to take measurements on a muscular-skeletal system. In a non-limiting example, sensing assemblage  401  is placed between a femoral prosthetic component and tibial prosthetic component to provide measured load information that aids in the installation of an artificial knee joint. Sensing assemblage  401  can also be a permanent component or a muscular-skeletal joint or artificial muscular-skeletal joint to monitor joint function. The measurements can be made in extension and in flexion. In the example, assemblage  401  is used to measure the condyle loading to determine if it falls within a predetermined range and location. Based on the measurement, the surgeon can select the thickness of the insert such that the measured loading and incidence with the final insert in place will fall within the predetermined range. Soft tissue tensioning can be used by a surgeon to further optimize the force or pressure. Similarly, two assemblages  401  can be used to measure both condyles simultaneously or multiplexed. The difference in loading (e.g. balance) between condyles can be measured. Soft tissue tensioning can be used to reduce the force on the condyle having the higher measured loading to reduce the measured pressure difference between condyles. 
     One method of operation holds the number of energy waves propagating through waveguide  403  as a constant integer number. A time period of an energy wave corresponds to energy wave periodicity. A stable time period is one in which the time period changes very little over a number of energy waves. This occurs when conditions that affect sensing assemblage  401  stay consistent or constant. Holding the number of energy waves propagating through waveguide  403  to an integer number is a constraint that forces a change in the time between pulses when the length of waveguide  403  changes. The resulting change in time period of each energy wave corresponds to a change in aggregate energy wave time period that is captured using digital counter  420  as a measurement of changes in external forces or conditions applied to contacting surfaces  408 . 
     A further method of operation according to one embodiment is described hereinbelow for energy waves  402  propagating from transducer  405  and received by transducer  406 . In at least one exemplary embodiment, energy waves  402  are an ultrasonic energy wave. Transducers  405  and  406  are piezo-electric resonator transducers. Although not described, wave propagation can occur in the opposite direction being initiated by transducer  406  and received by transducer  405 . Furthermore, detecting ultrasound resonator transducer  406  can be a separate ultrasound resonator as shown or transducer  405  can be used solely depending on the selected mode of propagation (e.g. reflective sensing). Changes in external forces or conditions applied to contacting surfaces  408  affect the propagation characteristics of waveguide  403  and alter transit time  407 . As mentioned previously, propagation tuned oscillator  404  holds constant an integer number of energy waves  402  propagating through waveguide  403  (e.g. an integer number of pulsed energy wave time periods) thereby controlling the repetition rate. As noted above, once PTO  404  stabilizes, the digital counter  420  digitizes the repetition rate of pulsed energy waves, for example, by way of edge-detection, as will be explained hereinbelow in more detail. 
     In an alternate embodiment, the repetition rate of pulsed energy waves  402  emitted by transducer  405  can be controlled by pulse generator  410 . The operation remains similar where the parameter to be measured corresponds to the measurement of the transit time  407  of pulsed energy waves  402  within waveguide  403 . It should be noted that an individual ultrasonic pulse can comprise one or more energy waves with a damping wave shape. The energy wave shape is determined by the electrical and mechanical parameters of pulse generator  410 , interface material or materials, where required, and ultrasound resonator or transducer  405 . The frequency of the energy waves within individual pulses is determined by the response of the emitting ultrasound resonator  404  to excitation by an electrical pulse  411 . The mode of the propagation of the pulsed energy waves  402  through waveguide  403  is controlled by mode control circuitry  412  (e.g., reflectance or uni-directional). The detecting ultrasound resonator or transducer may either be a separate ultrasound resonator or transducer  406  or the emitting resonator or transducer  405  depending on the selected mode of propagation (reflectance or unidirectional). 
     In general, accurate measurement of physical parameters is achieved at an equilibrium point having the property that an integer number of pulses are propagating through the energy propagating structure at any point in time. Measurement of changes in the “time-of-flight” or transit time of ultrasound energy waves within a waveguide of known length can be achieved by modulating the repetition rate of the ultrasound energy waves as a function of changes in distance or velocity through the medium of propagation, or a combination of changes in distance and velocity, caused by changes in the parameter or parameters of interest. 
     Measurement methods that rely on the propagation of energy waves, or energy waves within energy pulses, may require the detection of a specific point of energy waves at specified locations, or under specified conditions, to enable capturing parameters including, but not limited to, transit time, phase, or frequency of the energy waves. Measurement of the changes in the physical length of individual ultrasound waveguides may be made in several modes. Each assemblage of one or two ultrasound resonators or transducers combined with an ultrasound waveguide may be controlled to operate in six different modes. This includes two wave shape modes: continuous wave or pulsed waves, and three propagation modes: reflectance, unidirectional, and bi-directional propagation of the ultrasound wave. The resolution of these measurements can be further enhanced by advanced processing of the measurement data to enable optimization of the trade-offs between measurement resolution versus length of the waveguide, frequency of the ultrasound waves, and the bandwidth of the sensing and data capture operations, thus achieving an optimal operating point for a sensing module or device. 
     Measurement by propagation tuned oscillator  404  and sensing assemblage  401  enables high sensitivity and high signal-to-noise ratio. The time-based measurements are largely insensitive to most sources of error that may influence voltage or current driven sensing methods and devices. The resulting changes in the transit time of operation correspond to frequency, which can be measured rapidly, and with high resolution. This achieves the required measurement accuracy and precision thus capturing changes in the physical parameters of interest and enabling analysis of their dynamic and static behavior. 
     These measurements may be implemented with an integrated wireless sensing module or device having an encapsulating structure that supports sensors and load bearing or contacting surfaces and an electronic assemblage that integrates a power supply, sensing elements, energy transducer or transducers and elastic energy propagating structure or structures, biasing spring or springs or other form of elastic members, an accelerometer, antennas and electronic circuitry that processes measurement data as well as controls all operations of ultrasound generation, propagation, and detection and wireless communications. The electronics assemblage also supports testability and calibration features that assure the quality, accuracy, and reliability of the completed wireless sensing module or device. 
     The level of accuracy and resolution achieved by the integration of energy transducers and an energy propagating structure or structures coupled with the electronic components of the propagation tuned oscillator enables the construction of, but is not limited to, compact ultra low power modules or devices for monitoring or measuring the parameters of interest. The flexibility to construct sensing modules or devices over a wide range of sizes enables sensing modules to be tailored to fit a wide range of applications such that the sensing module or device may be engaged with, or placed, attached, or affixed to, on, or within a body, instrument, appliance, vehicle, equipment, or other physical system and monitor or collect data on physical parameters of interest without disturbing the operation of the body, instrument, appliance, vehicle, equipment, or physical system. 
     Referring to  FIG. 17 , a simplified flow chart  1700  of method steps for high precision processing and measurement data is shown in accordance with an exemplary embodiment. The method  1700  can be practiced with more or less than the steps shown, and is not limited to the order of steps shown. The method steps correspond to  FIG. 4  to be practiced with the aforementioned components or any other components suitable for such processing, for example, electrical circuitry to control the emission of energy pulses or waves and to capture the repetition rate of the energy pulses or frequency of the energy waves propagating through the elastic energy propagating structure or medium. 
     In a step  1702 , the process initiates a measurement operation. In a step  1704 , a known state is established by resetting digital timer  422  and data register  424 . In a step  1706 , digital counter  420  is preset to the number of measurement cycles over which measurements will be taken and collected. In a step  1708 , the measurement cycle is initiated and a clock output of digital clock  426  is enabled. A clock signal from digital clock  426  is provided to both digital counter  420  and digital timer  422 . An elapsed time is counted by digital timer  420  based on the frequency of the clock signal output by digital clock  426 . In a step  1710 , digital timer  422  begins tracking the elapsed time. Simultaneously, digital counter  420  starts decrementing a count after each measurement sequence. In one embodiment, digital counter  420  is decremented as each energy wave propagates through waveguide  403  and is detected by transducer  406 . Digital counter  420  counts down until the preset number of measurement cycles has been completed. In a step  1712 , energy wave propagation is sustained by propagation tuned oscillator  404 , as digital counter  420  is decremented by the detection of a propagated energy wave. In a step  1714 , energy wave detection, emission, and propagation continue while the count in digital counter  420  is greater than zero. In a step  1716 , the clock input of digital timer  422  is disabled upon reaching a zero count on digital counter  420  thus preventing digital counter  420  and digital timer  422  from being clocked. In one embodiment, the preset number of measurement cycles provided to digital counter  420  is divided by the elapsed time measured by digital timer  422  to calculate a frequency of propagated energy waves. Conversely, the number can be calculated as a transit time by dividing the elapsed time from digital timer  422  by the preset number of measurement cycles. Finally, in a step  1718 , the resulting value is transferred to register  424 . The number in data register  424  can be wirelessly transmitted to a display and database. The data from data register  424  can be correlated to a parameter being measured. The parameter such as a force or load is applied to the propagation medium (e.g. waveguide  403 ) such that parameter changes also change the frequency or transit time calculation of the measurement. A relationship between the material characteristics of the propagation medium and the parameter is used with the measurement value (e.g. frequency, transit time, phase) to calculate a parameter value. 
     The method  1700  practiced by the example assemblage of  FIG. 4 , and by way of the digital counter  420 , digital timer  422 , digital clock  426  and associated electronic circuitry analyzes the digitized measurement data according to operating point conditions. In particular, these components accumulate multiple digitized data values to improve the level of resolution of measurement of changes in length or other aspect of an elastic energy propagating structure or medium that can alter the transit time of energy pulses or waves propagating within the elastic energy propagating structure or medium. The digitized data is summed by controlling the digital counter  420  to run through multiple measurement cycles, each cycle having excitation and transit phases such that there is not lag between successive measurement cycles, and capturing the total elapsed time. The counter is sized to count the total elapsed time of as many measurement cycles as required to achieve the required resolution without overflowing its accumulation capacity and without compromising the resolution of the least significant bit of the counter. The digitized measurement of the total elapsed transit time is subsequently divided by the number of measurement cycles to estimate the time of the individual measurement cycles and thus the transit time of individual cycles of excitation, propagation through the elastic energy propagating structure or medium, and detection of energy pulses or waves. Accurate estimates of changes in the transit time of the energy pulses or waves through the elastic energy propagating structure or medium are captured as elapsed times for excitation and detection of the energy pulses or waves are fixed. 
     Summing individual measurements before dividing to estimate the average measurement value data values produces superior results to averaging the same number of samples. The resolution of count data collected from a digital counter is limited by the resolution of the least-significant-bit in the counter. Capturing a series of counts and averaging them does not produce greater precision than this least-significant-bit, that is the precision of a single count. Averaging does reduce the randomness of the final estimate if there is random variation between individual measurements. Summing the counts of a large number of measurement cycles to obtain a cumulative count then calculating the average over the entire measurement period improves the precision of the measurement by interpolating the component of the measurement that is less than the least significant bit of the counter. The precision gained by this procedure is on the order of the resolution of the least-significant-bit of the counter divided by the number of measurement cycles summed. 
     The size of the digital counter and the number of measurement cycles accumulated may be greater than the required level of resolution. This not only assures performance that achieves the level of resolution required, but also averages random component within individual counts producing highly repeatable measurements that reliably meet the required level of resolution. 
     The number of measurement cycles is greater than the required level of resolution. This not only assures performance that achieves the level of resolution required, but also averages any random component within individual counts producing highly repeatable measurements that reliably meet the required level of resolution. 
       FIG. 5  is a sensor interface diagram incorporating the zero-crossing receiver  200  in a continuous wave multiplexing arrangement for maintaining positive closed-loop feedback in accordance with one embodiment. The positive closed-loop feedback is illustrated by the bold line path. Initially, multiplexer (mux)  502  receives as input a clock signal  504 , which is passed to the transducer driver  506  to produce the drive line signal  508 . Analog multiplexer (mux)  510  receives drive line signal  508 , which is passed to the transmitter transducer  512  to generate energy waves  514 . Transducer  512  is located at a first location of an energy propagating medium. The emitted energy waves  514  propagate through the energy propagating medium. Receiver transducer  516  is located at a second location of the energy propagating medium. Receiver transducer  516  captures the energy waves  514 , which are fed to analog mux  520  and passed to the zero-crossing receiver  200 . The captured energy waves by transducer  516  are indicated by electrical waves  518  provided to mux  520 . Zero-crossing receiver  200  outputs a pulse corresponding to each zero crossing detected from captured electrical waves  518 . The zero crossings are counted and used to determine changes in the phase and frequency of the energy waves propagating through the energy propagating medium. In a non-limiting example, a parameter such as applied force is measured by relating the measured phase and frequency to a known relationship between the parameter (e.g. force) and the material properties of the energy propagating medium. In general, pulse sequence  522  corresponds to the detected signal frequency. The zero-crossing receiver  200  is in a feedback path of the propagation tuned oscillator. The pulse sequence  522  is coupled through mux  502  in a positive closed-loop feedback path. The pulse sequence  522  disables the clock signal  504  such that the path providing pulse sequence  522  is coupled to driver  506  to continue emission of energy waves into the energy propagating medium and the path of clock signal  504  to driver  506  is disabled. 
       FIG. 6  is an exemplary block diagram of a propagation tuned oscillator (PTO) incorporating the zero-crossing receiver  640  for operation in continuous wave mode. In particular, with respect to  FIG. 4 , it illustrates closed loop measurement of the transit time  412  of ultrasound waves  414  within the waveguide  408  by the operation of the propagation tuned oscillator  416 . This example is for operation in continuous wave mode. The system can also be operated in pulse mode and a pulse-echo mode. Pulse mode and pulsed echo-mode use a pulsed energy wave. Pulse-echo mode uses reflection to direct an energy wave within the energy propagation medium. Briefly, the digital logic circuit  646  digitizes the frequency of operation of the propagation tuned oscillator. 
     In continuous wave mode of operation a sensor comprising transducer  604 , propagating structure  602 , and transducer  606  is used to measure the parameter. In general, the parameter to be measured affects the properties of the propagating medium. For example, an external force or condition  612  is applied to propagating structure  602  that changes the length of the waveguide in a path of a propagating energy wave. A change in length corresponds to a change in transit time  608  of the propagating wave. Similarly, the length of propagating structure  602  corresponds to the applied force  612 . A length reduction corresponds to a higher force being applied to the propagating structure  602 . Conversely, a length increase corresponds to a lowering of the applied force  612  to the propagating structure  602 . The length of propagating structure  602  is measured and is converted to force by way of a known length to force relationship. 
     Transducer  604  is an emitting device in continuous wave mode. The sensor for measuring a parameter comprises transducer  604  coupled to propagating structure  602  at a first location. A transducer  606  is coupled to propagating structure  602  at a second location. Transducer  606  is a receiving transducer for capturing propagating energy waves. In one embodiment, the captured propagated energy waves are electrical sine waves  634  that are output by transducer  606 . 
     A measurement sequence is initiated when control circuitry  618  closes switch  620  coupling oscillator output  624  of oscillator  622  to the input of amplifier  626 . One or more pulses provided to amplifier  626  initiates an action to propagate energy waves  610  having simple or complex waveforms through energy propagating structure or medium  602 . Amplifier  626  comprises a digital driver  628  and matching network  630 . In one embodiment, amplifier  626  transforms the oscillator output of oscillator  622  into sine waves of electrical waves  632  having the same repetition rate as oscillator output  624  and sufficient amplitude to excite transducer  604 . 
     Emitting transducer  604  converts the sine waves  632  into energy waves  610  of the same frequency and emits them at the first location into energy propagating structure or medium  602 . The energy waves  610  propagate through energy propagating structure or medium  602 . Upon reaching transducer  606  at the second location, energy waves  610  are captured, sensed, or detected. The captured energy waves are converted by transducer  606  into sine waves  634  that are electrical waves having the same frequency. 
     Amplifier  636  comprises a pre-amplifier  634  and zero-cross receiver  640 . Amplifier  636  converts the sine waves  634  into digital pulses  642  of sufficient duration to sustain the behavior of the closed loop circuit. Control circuitry  618  responds to digital pulses  642  from amplifier  636  by opening switch  620  and closing switch  644 . Opening switch  620  decouples oscillator output  624  from the input of amplifier  626 . Closing switch  644  creates a closed loop circuit coupling the output of amplifier  636  to the input of amplifier  626  and sustaining the emission, propagation, and detection of energy waves through energy propagating structure or medium  602 . 
     An equilibrium state is attained by maintaining unity gain around this closed loop circuit wherein sine waves  632  input into transducer  604  and sine waves  634  output by transducer  606  are in phase with a small but constant offset. Transducer  606  as disclosed above, outputs the sine waves  634  upon detecting energy waves propagating to the second location. In the equilibrium state, an integer number of energy waves  610  propagate through energy propagating structure or medium  602 . 
     Movement or changes in the physical properties of energy propagating structure or medium  602  change a transit time  608  of energy waves  610 . The transit time  608  comprises the time for an energy wave to propagate from the first location to the second location of propagating structure  602 . Thus, the change in the physical property of propagating structure  602  results in a corresponding time period change of the energy waves  610  within energy propagating structure or medium  602 . These changes in the time period of the energy waves  610  alter the equilibrium point of the closed loop circuit and frequency of operation of the closed loop circuit. The closed loop circuit adjusts such that sine waves  632  and  634  correspond to the new equilibrium point. The frequency of energy waves  610  and changes to the frequency correlate to changes in the physical attributes of energy propagating structure or medium  602 . 
     The physical changes may be imposed on energy propagating structure  602  by external forces or conditions  612  thus translating the levels and changes of the parameter or parameters of interest into signals that may be digitized for subsequent processing, storage, and display. Translation of the operating frequency into digital binary numbers facilitates communication, additional processing, storage, and display of information about the level and changes in physical parameters of interest. Similarly, the frequency of energy waves  610  during the operation of the closed loop circuit, and changes in this frequency, may be used to measure movement or changes in physical attributes of energy propagating structure or medium  602 . 
     Prior to measurement of the frequency or operation of the propagation tuned oscillator, control logic  618  loads the loop count into digital counter  650  that is stored in count register  648 . The first digital pulses  642  initiates closed loop operation within the propagation tuned oscillator and signals control circuit  618  to start measurement operations. At the start of closed loop operation, control logic  618  enables digital counter  650  and digital timer  652 . In one embodiment, digital counter  650  decrements its value on the rising edge of each digital pulse output by zero-crossing receiver  640 . Digital timer  652  increments its value on each rising edge of clock pulses  656 . When the number of digital pulses  642  has decremented, the value within digital counter  650  to zero a stop signal is output from digital counter  650 . The stop signal disables digital timer  652  and triggers control circuit  618  to output a load command to data register  654 . Data register  654  loads a binary number from digital timer  652  that is equal to the period of the energy waves or pulses times the value in counter  648  divided by clock period  656 . With a constant clock period  656 , the value in data register  654  is directly proportional to the aggregate period of the energy waves or pulses accumulated during the measurement operation. Duration of the measurement operation and the resolution of measurements may be adjusted by increasing or decreasing the value preset in the count register  648 . 
       FIG. 7  is a sensor interface diagram incorporating the integrated zero-crossing receiver  200  in a pulse multiplexing arrangement for maintaining positive closed-loop feedback in accordance with one embodiment. In one embodiment, the circuitry other than the sensor is integrated on an application specific integrated circuit (ASIC). The positive closed-loop feedback is illustrated by the bold line path. Initially, mux  702  is enabled to couple one or more digital pulses  704  to the transducer driver  706 . Transducer driver  706  generates a pulse sequence  708  corresponding to digital pulses  704 . Analog mux  710  is enabled to couple pulse sequence  708  to the transmitter transducer  712 . Transducer  712  is coupled to a medium at a first location. Transducer  712  responds to pulse sequence  708  and generates corresponding energy pulses  714  that are emitted into the medium at the first location. The energy pulses  714  propagate through the medium. A receiver transducer  716  is located at a second location on the medium. Receiver transducer  716  captures the energy pulses  714  and generates a corresponding signal of electrical pulses  718 . Transducer  716  is coupled to a mux  720 . Mux  720  is enabled to couple to zero-cross receiver  200 . Electrical pulses  718  from transducer  716  are coupled to zero-cross receiver  200 . Zero-cross receiver  200  counts zero crossings of electrical pulses  718  to determine changes in phase and frequency of the energy pulses responsive to an applied force, as previously explained. Zero-cross receiver  200  outputs a pulse sequence  722  corresponding to the detected signal frequency. Pulse sequence  722  is coupled to mux  702 . Mux  702  is decoupled from coupling digital pulses  704  to driver  706  upon detection of pulses  722 . Conversely, mux  702  is enabled to couple pulses  722  to driver  706  upon detection of pulses  722  thereby creating a positive closed-loop feedback path. Thus, in pulse mode, zero-cross receiver  200  is part of the closed-loop feedback path that continues emission of energy pulses into the medium at the first location and detection at the second location to measure a transit time and changes in transit time of pulses through the medium. 
       FIG. 8  is an exemplary block diagram of a propagation tuned oscillator (PTO) incorporating the zero-crossing receiver  640  for operation in pulse mode. In particular, with respect to  FIG. 4 , it illustrates closed loop measurement of the transit time  412  of ultrasound waves  414  within the waveguide  408  by the operation of the propagation tuned oscillator  416 . This example is for operation in pulse mode. The system can also be operated in continuous wave mode and a pulse-echo mode. Continuous wave mode uses a continuous wave signal. Pulse-echo mode uses reflection to direct an energy wave within the energy propagation medium. Briefly, the digital logic circuit  646  digitizes the frequency of operation of the propagation tuned oscillator. 
     In pulse mode of operation, a sensor comprising transducer  604 , propagating structure  602 , and transducer  606  is used to measure the parameter. In general, the parameter to be measured affects the properties of the propagating medium. For example, an external force or condition  612  is applied to propagating structure  602  that changes the length of the waveguide in a path of a propagating energy wave. A change in length corresponds to a change in transit time  608  of the propagating wave. The length of propagating structure  602  is measured and is converted to force by way of a known length to force relationship. One benefit of pulse mode operation is the use of a high magnitude pulsed energy wave. In one embodiment, the magnitude of the energy wave decays as it propagates through the medium. The use of a high magnitude pulse is a power efficient method to produce a detectable signal if the energy wave has to traverse a substantial distance or is subject to a reduction in magnitude as it propagated due to the medium. 
     A measurement sequence is initiated when control circuitry  618  closes switch  620  coupling oscillator output  624  of oscillator  622  to the input of amplifier  626 . One or more pulses provided to amplifier  626  initiates an action to propagate energy waves  610  having simple or complex waveforms through energy propagating structure or medium  602 . Amplifier  626  comprises a digital driver  628  and matching network  630 . In one embodiment, amplifier  626  transforms the oscillator output of oscillator  622  into analog pulses of electrical waves  832  having the same repetition rate as oscillator output  624  and sufficient amplitude to excite transducer  604 . 
     Emitting transducer  604  converts the analog pulses  832  into energy waves  610  of the same frequency and emits them at a first location into energy propagating structure or medium  602 . The energy waves  610  propagate through energy propagating structure or medium  602 . Upon reaching transducer  606  at the second location, energy waves  610  are captured, sensed, or detected. The captured energy waves are converted by transducer  606  into analog pulses  834  that are electrical waves having the same frequency. 
     Amplifier  636  comprises a pre-amplifier  638  and zero-cross receiver  640 . Amplifier  636  converts the analog pulses  834  into digital pulses  642  of sufficient duration to sustain the behavior of the closed loop circuit. Control circuitry  618  responds to digital pulses  642  from amplifier  636  by opening switch  620  and closing switch  644 . Opening switch  620  decouples oscillator output  624  from the input of amplifier  626 . Closing switch  644  creates a closed loop circuit coupling the output of amplifier  636  to the input of amplifier  626  and sustaining the emission, propagation, and detection of energy waves through energy propagating structure or medium  602 . 
     An equilibrium state is attained by maintaining unity gain around this closed loop circuit wherein pulses  832  input into transducer  604  and pulses  834  output by transducer  606  are in phase with a small but constant offset. Transducer  606  as disclosed above, outputs the pulses  834  upon detecting energy waves propagating to the second location. In the equilibrium state, an integer number of energy waves  610  propagate through energy propagating structure or medium  602 . 
     Movement or changes in the physical properties of energy propagating structure or medium  602  change a transit time  608  of energy waves  610 . The transit time  608  comprises the time for an energy wave to propagate from the first location to the second location of propagating structure  602 . Thus, the change in the physical property of propagating structure  602  results in a corresponding time period change of the energy waves  610  within energy propagating structure or medium  602 . These changes in the time period of the energy waves  610  alter the equilibrium point of the closed loop circuit and frequency of operation of the closed loop circuit. The closed loop circuit adjusts such that pulses  832  and  834  correspond to the new equilibrium point. The frequency of energy waves  610  and changes to the frequency correlate to changes in the physical attributes of energy propagating structure or medium  602 . 
     The physical changes may be imposed on energy propagating structure  602  by external forces or conditions  612  thus translating the levels and changes of the parameter or parameters of interest into signals that may be digitized for subsequent processing, storage, and display. Translation of the operating frequency into digital binary numbers facilitates communication, additional processing, storage, and display of information about the level and changes in physical parameters of interest as disclosed in more detail hereinabove. Similarly, the frequency of energy waves  610  during the operation of the closed loop circuit, and changes in this frequency, may be used to measure movement or changes in physical attributes of energy propagating structure or medium  602 . 
       FIG. 9  illustrates a block diagram of an edge-detect receiver circuit  900  in accordance with an exemplary embodiment. In a first embodiment, edge-detect receiver  900  is provided to detect wave fronts of pulses of energy waves. This enables capturing of parameters including, but not limited to, transit time, phase, or frequency of the energy waves. Circuitry of the integrated edge-detect receiver  900  provides rapid on-set detection and quickly responds to the arrival of an energy pulse. It reliably triggers thereafter a digital output pulse at a same point on the initial wave front of each captured energy pulse or pulsed energy wave. The digital pulse can be optimally configured to output with minimal and constant delay. The edge-detect receiver  900  can isolate and precisely detect the specified point on the initial energy wave or the wave front in the presence of interference and distortion signals thereby overcoming problems commonly associated with detecting one of multiple generated complex signals in energy propagating mediums. The edge-detect receiver  900  performs these functions accurately over a wide range of amplitudes including very low-level energy pulses. 
     In a second embodiment, the edge-detect receiver  900  is incorporated within a propagation tuned oscillator (PTO) to maintain positive closed-loop feedback when operating in a pulse or pulse-echo mode. The edge-detect receiver  900  can be integrated with other circuitry of the PTO by multiplexing input and output circuitry to achieve ultra low-power and small compact size. Integration of the circuitry of the PTO with the edge-detect receiver provides the benefit of increasing sensitivity to low-level signals. 
     The block diagram illustrates one embodiment of a low power edge-detect receiver circuit  900  with superior performance at low signal levels. The edge-detect receiver  900  comprises a preamplifier  912 , a differentiator  914 , a digital pulse circuit  916  and a deblank circuit  918 . The edge-detect receiver circuit  900  can be implemented in discrete analog components, digital components or combination thereof. In one embodiment, edge-detect receiver  900  is integrated into an ASIC as part of a sensor system described hereinbelow. The edge-detect receiver circuit  900  practices measurement methods that detect energy pulses or pulsed energy waves at specified locations and under specified conditions to enable capturing parameters including, but not limited to, transit time, phase, frequency, or amplitude of energy pulses. A brief description of the method of operation is as follows. In a non-limiting example, a pre-amplifier triggers a comparator circuit responsive to small changes in the slope of an input signal. The comparator and other edge-detect circuitry responds rapidly with minimum delay. Detection of small changes in the input signal assures rapid detection of the arrival of a pulse of energy waves. The minimum phase design reduces extraneous delay thereby introducing less variation into the measurement of the transit time, phase, frequency, or amplitude of the incoming energy pulses. 
     An input  920  of edge-detect receiver  900  is coupled to pre-amplifier  912 . As an example, the incoming wave  910  to the edge-detect receiver circuit  900  can be received from an electrical connection, antenna, or transducer. The incoming wave  910  is amplified by pre-amplifier  912 , which assures adequate sensitivity to small signals. Differentiator circuitry  914  monitors the output of pre-amplifier  912  and triggers digital pulse circuitry  916  whenever a signal change corresponding to a pulsed energy wave is detected. For example, a signal change that identifies the pulsed energy wave is the initial wave front or the leading edge of the pulsed energy wave. In one arrangement, differentiator  914  detects current flow, and more specifically changes in the slope of the energy wave  910  by detecting small changes in current flow instead of measuring changes in voltage level to achieve rapid detection of slope. Alternatively, differentiator  914  can be implemented to trigger on changes in voltage. Together, preamplifier  912  and differentiator  916  monitor the quiescent input currents for the arrival of wave front of energy wave(s)  910 . Preamplifier  912  and differentiator  916  detect the arrival of low level pulses of energy waves as well as larger pulses of energy waves. This detection methodology achieves superior performance for very low level signals. Differentiator circuitry  912  triggers digital pulse circuitry  916  whenever current flow driven by the initial signal ramp of the incoming wave  910  is detected. The digital pulse is coupled to deblank circuit  918  that desensitizes pre-amplifier  912 . For example, the desensitization of pre-amplifier  912  can comprise a reduction in gain, decoupling of input  920  from energy wave  910 , or changing the frequency response. The deblank circuit  918  also disregards voltage or current levels for a specified or predetermined duration of time to effectively skip over the interference sections or distorted portions of the energy wave  910 . In general, energy wave  910  can comprise more than one change in slope and is typically a damped wave form. Additional signals or waves of the pulsed energy wave on the input  920  of pre-amplifier  912  are not processed during the preset blanking period. In this example, the digital output pulse  928  can then be coupled to signal processing circuitry as explained hereinbelow. In one embodiment, the electronic components are operatively coupled as blocks within an integrated circuit. As will be shown ahead, this integration arrangement performs its specific functions efficiently with a minimum number of components. This is because the circuit components are partitioned between structures within an integrated circuit and discrete components, as well as innovative partitioning of analog and digital functions, to achieve the required performance with a minimum number of components and minimum power consumption. 
       FIG. 10  illustrates a block diagram of the edge-detect receiver circuit  900  coupled to a sensing assembly  1000 . The pre-amplifier  912  and the digital pulse circuit  916  are shown for reference and discussion. The sensing assembly  1000  comprises a transmitter transducer  1002 , an energy propagating medium  1004 , and a receiver transducer  1006 . The transmitter transducer  1002  is coupled to propagating medium  1004  at a first location. The receiver transducer  1006  is coupled to energy propagating medium  1004  at a second location. Alternatively, a reflecting surface can replace receiver transducer  1006 . The reflecting surface reflects an energy wave back towards the first location. Transducer  1006  can be enabled to be a transmitting transducer and a receiving transducer thereby saving the cost of a transducer. As will be explained ahead in further detail, the sensing assembly  1000  in one embodiment is part of a sensory device that assess loading, in particular, the externally applied forces  1008  on the sensing assembly  1000 . A transducer driver circuit (not shown) drives the transmitter transducer  1002  of the sensing assembly  1000  to produce energy waves  1010  that are directed into the energy propagating medium  1004 . In the non-limiting example, changes in the energy propagating medium  1004  due to the externally applied forces  1008  change the frequency, phase, and transit time  1012  of energy waves  1010  propagating from the first location to the second location of energy propagating medium  1004 . The integrated edge-detect receiver circuit  900  is coupled to the receiver transducer  1006  to detect edges of the reproduced energy wave  910  and trigger the digital pulse  928 . In general, the timing of the digital pulse  928  conveys the parameters of interest (e.g., distance, force weight, strain, pressure, wear, vibration, viscosity, density, direction, displacement, etc.) related to the change in energy propagating structure  1004  due to an external parameter. For example, sensing assembly  1000  placed in a knee joint as described hereinabove. 
     Measurement methods that rely on the propagation of energy pulses require the detection of energy pulses at specified locations or under specified conditions to enable capturing parameters including, but not limited to, transit time, phase, frequency, or amplitude of the energy pulses. Measurement methods that rely on such propagation of energy waves  1010  or pulses of energy waves are required to achieve highly accurate and controlled detection of energy waves or pulses. Moreover, pulses of energy waves may contain multiple energy waves with complex waveforms therein leading to potential ambiguity of detection. In particular, directing energy waves  1010  into the energy propagating structure  1004  can generate interference patterns caused by nulls and resonances of the waveguide, as well as characteristics of the generated energy wave  1010 . These interference patterns can generate multiply excited waveforms that result in distortion of the edges of the original energy wave. To reliably detect the arrival of a pulse of energy waves, the edge-detect receiver  900  only responds to the leading edge of the first energy wave within each pulse. This is achieved in part by blanking the edge-detect circuitry  900  for the duration of each energy pulse. As an example, the deblank circuit  918  disregards voltage or current levels for a specified duration of time to effectively skip over the interference sections or distorted portions of the waveform. 
       FIG. 11  is a sensor interface diagram incorporating the edge-detect receiver circuit  900  in a pulse-echo multiplexing arrangement for maintaining positive closed-loop feedback in accordance with one embodiment. The positive closed-loop feedback is illustrated by the bold line path. Initially, multiplexer (mux)  1102  receives as input a digital pulse  1104 , which is passed to the transducer driver  1106  to produce the pulse sequence  1108 . Analog multiplexer (mux)  1110  receives pulse sequence  1108 , which is passed to the transducer  1112  to generate energy pulses  1114 . Energy pulses  1114  are emitted into a first location of a medium and propagate through the medium. In the pulse-echo example, energy pulses  1114  are reflected off a surface  1116  at a second location of the medium, for example, the end of a waveguide or reflector, and echoed back to the transducer  1112 . The transducer  1112  proceeds to then capture the reflected pulse echo. In pulsed echo mode, the transducer  1112  performs as both a transmitter and a receiver. As disclosed above, transducer  1112  toggles back and forth between emitting and receiving energy waves. Transducer  1112  captures the reflected echo pulses, which are coupled to analog mux  1110  and directed to the edge-detect receiver  900 . The captured reflected echo pulses is indicated by electrical waves  1120 . Edge-detect receiver  900  locks on pulse edges corresponding to the wave front of a propagated energy wave to determine changes in phase and frequency of the energy pulses  1114  responsive to an applied force, as previously explained. Among other parameters, it generates a pulse sequence  1118  corresponding to the detected signal frequency. The pulse sequence  1118  is coupled to mux  1102  and directed to driver  1106  to initiate one or more energy waves being emitted into the medium by transducer  1112 . Pulse  1104  is decoupled from being provided to driver  1106 . Thus, a positive closed loop feedback is formed that repeatably emits energy waves into the medium until mux  1102  prevents a signal from being provided to driver  1106 . The edge-detect receiver  900  is coupled to a second location of the medium and is in the feedback path. The edge-detect receiver  900  initiates a pulsed energy wave being provided at the first location of the medium upon detecting a wave front at the second location when the feedback path is closed. 
       FIG. 12  is an exemplary block diagram of a propagation tuned oscillator (PTO) incorporating the edge-detect receiver circuit  900  for operation in pulse echo mode. In particular, with respect to  FIG. 4 , it illustrates closed loop measurement of the transit time  412  of ultrasound waves  414  within the waveguide  408  by the operation of the propagation tuned oscillator  416 . This example is for operation in a pulse echo mode. The system can also be operated in pulse mode and a continuous wave mode. Pulse mode does not use a reflected signal. Continuous wave mode uses a continuous signal. Briefly, the digital logic circuit  1246  digitizes the frequency of operation of the propagation tuned oscillator. 
     In pulse-echo mode of operation a sensor comprising transducer  1204 , propagating structure  1202 , and reflecting surface  1206  is used to measure the parameter. In general, the parameter to be measured affects the properties of the propagating medium. For example, an external force or condition  1212  is applied to propagating structure  1202  that changes the length of the waveguide in a path of a propagating energy wave. A change in length corresponds to a change in transit time of the propagating wave. Similarly, the length of propagating structure  1202  corresponds to the applied force  1212 . A length reduction corresponds to a higher force being applied to the propagating structure  1202 . Conversely, a length increase corresponds to a lowering of the applied force  1212  to the propagating structure  1202 . The length of propagating structure  1202  is measured and is converted to force by way of a known length to force relationship. 
     Transducer  1204  is both an emitting device and a receiving device in pulse-echo mode. The sensor for measuring a parameter comprises transducer  1204  coupled to propagating structure  1202  at a first location. A reflecting surface is coupled to propagating structure  1202  at a second location. Transducer  1204  has two modes of operation comprising an emitting mode and receiving mode. Transducer  1204  emits an energy wave into the propagating structure  1202  at the first location in the emitting mode. The energy wave propagates to a second location and is reflected by reflecting surface  1206 . The reflected energy wave is reflected towards the first location and transducer  1204  subsequently generates a signal in the receiving mode corresponding to the reflected energy wave. 
     A measurement sequence in pulse echo mode is initiated when control circuitry  1218  closes switch  1220  coupling digital output  1224  of oscillator  1222  to the input of amplifier  1226 . One or more pulses provided to amplifier  1226  starts a process to emit one or more energy waves  1210  having simple or complex waveforms into energy propagating structure or medium  1202 . Amplifier  1226  comprises a digital driver  1228  and matching network  1230 . In one embodiment, amplifier  1226  transforms the digital output of oscillator  1222  into pulses of electrical waves  1232  having the same repetition rate as digital output  1224  and sufficient amplitude to excite transducer  1204 . 
     Transducer  1204  converts the pulses of electrical waves  1232  into pulses of energy waves  1210  of the same repetition rate and emits them into energy propagating structure or medium  1202 . The pulses of energy waves  1210  propagate through energy propagating structure or medium  1202  as shown by arrow  1214  towards reflecting surface  1206 . Upon reaching reflecting surface  1206 , energy waves  1210  are reflected by reflecting surface  1206 . Reflected energy waves propagate towards transducer  1204  as shown by arrow  1216 . The reflected energy waves are detected by transducer  1204  and converted into pulses of electrical waves  1234  having the same repetition rate. 
     Amplifier  1236  comprises a pre-amplifier  1234  and edge-detect receiver  1240 . Amplifier  1236  converts the pulses of electrical waves  1234  into digital pulses  1242  of sufficient duration to sustain the pulse behavior of the closed loop circuit. Control circuitry  1218  responds to digital output pulses  1242  from amplifier  1236  by opening switch  1220  and closing switch  1244 . Opening switch  1220  decouples oscillator output  1224  from the input of amplifier  1226 . Closing switch  1244  creates a closed loop circuit coupling the output of amplifier  1236  to the input of amplifier  1226  and sustaining the emission, propagation, and detection of energy pulses through energy propagating structure or medium  1202 . 
     An equilibrium state is attained by maintaining unity gain around this closed loop circuit wherein electrical waves  1232  input into transducer  1204  and electrical waves  1234  output by transducer  1204  are in phase with a small but constant offset. Transducer  1204  as disclosed above, outputs the electrical waves  1234  upon detecting reflected energy waves reflected from reflecting surface  1206 . In the equilibrium state, an integer number of pulses of energy waves  1210  propagate through energy propagating structure or medium  1202 . 
     Movement or changes in the physical properties of energy propagating structure or medium  1202  change a transit time  1208  of energy waves  1210 . The transit time  1208  comprises the time for an energy wave to propagate from the first location to the second location of propagating structure  1202  and the time for the reflected energy wave to propagate from the second location to the first location of propagating structure  1202 . Thus, the change in the physical property of propagating structure  1202  results in a corresponding time period change of the energy waves  1210  within energy propagating structure or medium  1202 . These changes in the time period of the repetition rate of the energy pulses  1210  alter the equilibrium point of the closed loop circuit and repetition rate of operation of the closed loop circuit. The closed loop circuit adjusts such that electrical waves  1232  and  1234  correspond to the new equilibrium point. The repetition rate of energy waves  1210  and changes to the repetition rate correlate to changes in the physical attributes of energy propagating structure or medium  1202 . 
     The physical changes may be imposed on energy propagating structure  1202  by external forces or conditions  1212  thus translating the levels and changes of the parameter or parameters of interest into signals that may be digitized for subsequent processing, storage, and display. Translation of the operating frequency into digital binary numbers facilitates communication, additional processing, storage, and display of information about the level and changes in physical parameters of interest. Similarly, the frequency of energy waves  1210  during the operation of the closed loop circuit, and changes in this frequency, may be used to measure movement or changes in physical attributes of energy propagating structure or medium  1202 . 
     Prior to measurement of the frequency or operation of the propagation tuned oscillator, control logic  1218  loads the loop count into digital counter  1250  that is stored in count register  1248 . The first digital pulses  1242  initiates closed loop operation within the propagation tuned oscillator and signals control circuit  1218  to start measurement operations. At the start of closed loop operation, control logic  1218  enables digital counter  1250  and digital timer  1252 . In one embodiment, digital counter  1250  decrements its value on the rising edge of each digital pulse output by edge-detect receiver  1240 . Digital timer  1252  increments its value on each rising edge of clock pulses  1256 . When the number of digital pulses  1242  has decremented, the value within digital counter  1250  to zero a stop signal is output from digital counter  1250 . The stop signal disables digital timer  1252  and triggers control circuit  1218  to output a load command to data register  1254 . Data register  1254  loads a binary number from digital timer  1252  that is equal to the period of the energy waves or pulses times the value in counter  1248  divided by clock period  1256 . With a constant clock period  1256 , the value in data register  1254  is directly proportional to the aggregate period of the energy waves or pulses accumulated during the measurement operation. Duration of the measurement operation and the resolution of measurements may be adjusted by increasing or decreasing the value preset in the count register  1248 . 
       FIG. 13  is a simplified cross-sectional view of a sensing module  1301  in accordance with an exemplary embodiment. The sensing module (or assemblage) is an electro-mechanical assembly comprising electrical components and mechanical components that when configured and operated in accordance with a sensing mode performs as a positive feedback closed-loop measurement system. The measurement system can precisely measure applied forces, such as loading, on the electro-mechanical assembly. The sensing mode can be a continuous mode, a pulse mode, or a pulse echo-mode. 
     In one embodiment, the electrical components can include ultrasound resonators or transducers  405  and  406 , ultrasound waveguides  403 , and signal processing electronics  1310 , but are not limited to these. The mechanical components can include biasing springs  1332 , spring retainers and posts, and load platforms  1306 , but are not limited to these. The electrical components and mechanical components can be inter-assembled (or integrated) onto a printed circuit board  1336  to operate as a coherent ultrasonic measurement system within sensing module  1301  and according to the sensing mode. As will be explained ahead in more detail, the signal processing electronics incorporate a propagation tuned oscillator (PTO) or a phase locked loop (PLL) to control the operating frequency of the ultrasound resonators or transducers for providing high precision sensing. Furthermore, the signal processing electronics incorporate detect circuitry that consistently detects an energy wave after it has propagated through a medium. The detection initiates the generation of a new energy wave by an ultrasound resonator or transducer that is coupled to the medium for propagation therethrough. A change in transit time of an energy wave through the medium is measured and correlates to a change in material property of the medium due to one or more parameters applied thereto. 
     Sensing module  1301  comprises one or more assemblages  401  each comprised one or more ultrasound resonators  405  and  406 . As illustrated, waveguide  403  is coupled between transducers ( 405 ,  406 ) and affixed to load bearing or contacting surfaces  408 . In one exemplary embodiment, an ultrasound signal is coupled for propagation through waveguide  403 . The sensing module  1301  is placed, attached to, or affixed to, or within a body, instrument, or other physical system  1318  having a member or members  1316  in contact with the load bearing or contacting surfaces  408  of the sensing module  401 . This arrangement facilitates translating the parameters of interest into changes in the length or compression or extension of the waveguide or waveguides  403  within the sensing module  1301  and converting these changes in length into electrical signals. This facilitates capturing data, measuring parameters of interest and digitizing that data, and then subsequently communicating that data through antenna  1334  to external equipment with minimal disturbance to the operation of the body, instrument, appliance, vehicle, equipment, or physical system  1318  for a wide range of applications. 
     The sensing module  401  supports three modes of operation of energy wave propagation and measurement: reflectance, unidirectional, and bi-directional. These modes can be used as appropriate for each individual application. In unidirectional and bi-directional modes, a chosen ultrasound resonator or transducer is controlled to emit pulses of ultrasound waves into the ultrasound waveguide and one or more other ultrasound resonators or transducers are controlled to detect the propagation of the pulses of ultrasound waves at a specified location or locations within the ultrasound waveguide. In reflectance or pulse-echo mode, a single ultrasound or transducer emits pulses of ultrasound waves into waveguide  403  and subsequently detects pulses of echo waves after reflection from a selected feature or termination of the waveguide. In pulse-echo mode, echoes of the pulses can be detected by controlling the actions of the emitting ultrasound resonator or transducer to alternate between emitting and detecting modes of operation. Pulse and pulse-echo modes of operation may require operation with more than one pulsed energy wave propagating within the waveguide at equilibrium. 
     Many parameters of interest within physical systems or bodies can be measured by evaluating changes in the transit time of energy pulses. The frequency, as defined by the reciprocal of the average period of a continuous or discontinuous signal, and type of the energy pulse is determined by factors such as distance of measurement, medium in which the signal travels, accuracy required by the measurement, precision required by the measurement, form factor of that will function with the system, power constraints, and cost. The physical parameter or parameters of interest can include, but are not limited to, measurement of load, force, pressure, displacement, density, viscosity, localized temperature. These parameters can be evaluated by measuring changes in the propagation time of energy pulses or waves relative to orientation, alignment, direction, or position as well as movement, rotation, or acceleration along an axis or combination of axes by wireless sensing modules or devices positioned on or within a body, instrument, appliance, vehicle, equipment, or other physical system. 
     In the non-limiting example, pulses of ultrasound energy provide accurate markers for measuring transit time of the pulses within waveguide  403 . In general, an ultrasonic signal is an acoustic signal having a frequency above the human hearing range (e.g. &gt;20 KHz) including frequencies well into the megahertz range. In one embodiment, a change in transit time of an ultrasonic energy pulse corresponds to a difference in the physical dimension of the waveguide from a previous state. For example, a force or pressure applied across the knee joint compresses waveguide  403  to a new length and changes the transit time of the energy pulse. When integrated as a sensing module and inserted or coupled to a physical system or body, these changes are directly correlated to the physical changes on the system or body and can be readily measured as a pressure or a force. 
       FIG. 14  is an exemplary assemblage  1400  for illustrating reflectance and unidirectional modes of operation in accordance with an exemplary embodiment. It comprises one or more transducers  1402 ,  1404 , and  1406 , one or more waveguides  1414 , and one or more optional reflecting surfaces  1416 . The assemblage  1400  illustrates propagation of ultrasound waves  1418  within the waveguide  1414  in the reflectance and unidirectional modes of operation. Either ultrasound resonator or transducer  1402  and  1404  in combination with interfacing material or materials  1408  and  1410 , if required, can be selected to emit ultrasound waves  1418  into the waveguide  1414 . 
     In unidirectional mode, either of the ultrasound resonators or transducers for example  1402  can be enabled to emit ultrasound waves  1418  into the waveguide  1414 . The non-emitting ultrasound resonator or transducer  1404  is enabled to detect the ultrasound waves  1418  emitted by the ultrasound resonator or transducer  1402 . 
     In reflectance mode, the ultrasound waves  1418  are detected by the emitting ultrasound resonator or transducer  1402  after reflecting from a surface, interface, or body at the opposite end of the waveguide  1414 . In this mode, either of the ultrasound resonators or transducers  1402  or  1404  can be selected to emit and detect ultrasound waves. Additional reflection features  1416  can be added within the waveguide structure to reflect ultrasound waves. This can support operation in a combination of unidirectional and reflectance modes. In this mode of operation, one of the ultrasound resonators, for example resonator  1402  is controlled to emit ultrasound waves  1418  into the waveguide  1414 . Another ultrasound resonator or transducer  1406  is controlled to detect the ultrasound waves  1418  emitted by the emitting ultrasound resonator  1402  (or transducer) subsequent to their reflection by reflecting feature  1416 . 
       FIG. 15  is an exemplary assemblage  1500  that illustrates propagation of ultrasound waves  1510  within the waveguide  1506  in the bi-directional mode of operation of this assemblage. In this mode, the selection of the roles of the two individual ultrasound resonators ( 1502 ,  1504 ) or transducers affixed to interfacing material  1520  and  1522 , if required, are periodically reversed. In the bi-directional mode the transit time of ultrasound waves propagating in either direction within the waveguide  1506  can be measured. This can enable adjustment for Doppler effects in applications where the sensing module  1508  is operating while in motion  1516 . Furthermore, this mode of operation helps assure accurate measurement of the applied load, force, pressure, or displacement by capturing data for computing adjustments to offset this external motion  1516 . An advantage is provided in situations wherein the body, instrument, appliance, vehicle, equipment, or other physical system  1514 , is itself operating or moving during sensing of load, pressure, or displacement. Similarly, the capability can also correct in situation where the body, instrument, appliance, vehicle, equipment, or other physical system, is causing the portion  1512  of the body, instrument, appliance, vehicle, equipment, or other physical system being measured to be in motion  1516  during sensing of load, force, pressure, or displacement. Other adjustments to the measurement for physical changes to system  1514  are contemplated and can be compensated for in a similar fashion. For example, temperature of system  1514  can be measured and a lookup table or equation having a relationship of temperature versus transit time can be used to normalize measurements. Differential measurement techniques can also be used to cancel many types of common factors as is known in the art. 
     The use of waveguide  1506  enables the construction of low cost sensing modules and devices over a wide range of sizes, including highly compact sensing modules, disposable modules for bio-medical applications, and devices, using standard components and manufacturing processes. The flexibility to construct sensing modules and devices with very high levels of measurement accuracy, repeatability, and resolution that can scale over a wide range of sizes enables sensing modules and devices to the tailored to fit and collect data on the physical parameter or parameters of interest for a wide range of medical and non-medical applications. 
     For example, sensing modules or devices may be placed on or within, or attached or affixed to or within, a wide range of physical systems including, but not limited to instruments, appliances, vehicles, equipments, or other physical systems as well as animal and human bodies, for sensing the parameter or parameters of interest in real time without disturbing the operation of the body, instrument, appliance, vehicle, equipment, or physical system. 
     In addition to non-medical applications, examples of a wide range of potential medical applications may include, but are not limited to, implantable devices, modules within implantable devices, modules or devices within intra-operative implants or trial inserts, modules within inserted or ingested devices, modules within wearable devices, modules within handheld devices, modules within instruments, appliances, equipment, or accessories of all of these, or disposables within implants, trial inserts, inserted or ingested devices, wearable devices, handheld devices, instruments, appliances, equipment, or accessories to these devices, instruments, appliances, or equipment. Many physiological parameters within animal or human bodies may be measured including, but not limited to, loading within individual joints, bone density, movement, various parameters of interstitial fluids including, but not limited to, viscosity, pressure, and localized temperature with applications throughout the vascular, lymph, respiratory, and digestive systems, as well as within or affecting muscles, bones, joints, and soft tissue areas. For example, orthopedic applications may include, but are not limited to, load bearing prosthetic components, or provisional or trial prosthetic components for, but not limited to, surgical procedures for knees, hips, shoulders, elbows, wrists, ankles, and spines; any other orthopedic or musculoskeletal implant, or any combination of these. 
       FIG. 16  is an exemplary cross-sectional view of a sensor element  1600  to illustrate changes in the propagation of ultrasound waves  1614  with changes in the length of a waveguide  1606 . In general, the measurement of a parameter is achieved by relating displacement to the parameter. In one embodiment, the displacement required over the entire measurement range is measured in microns. For example, an external force  1608  compresses waveguide  1606  thereby changing the length of waveguide  1606 . Sensing circuitry (not shown) measures propagation characteristics of ultrasonic signals in the waveguide  1606  to determine the change in the length of the waveguide  1606 . These changes in length change in direct proportion to the parameters of interest thus enabling the conversion of changes in the parameter or parameters of interest into electrical signals. 
     As illustrated, external force  1608  compresses waveguide  1606  and moves the transducers  1602  and  1604  closer to one another by a distance  1610 . This changes the length of waveguide  1606  by distance  1612  of the waveguide propagation path between transducers  1602  and  1604 . Depending on the operating mode, the sensing circuitry measures the change in length of the waveguide  1606  by analyzing characteristics of the propagation of ultrasound waves within the waveguide. 
     One interpretation of  FIG. 16  illustrates waves emitting from transducer  1602  at one end of waveguide  1606  and propagating to transducer  1604  at the other end of the waveguide  1606 . The interpretation includes the effect of movement of waveguide  1606  and thus the velocity of waves propagating within waveguide  1606  (without changing shape or width of individual waves) and therefore the transit time between transducers  1602  and  1604  at each end of the waveguide. The interpretation further includes the opposite effect on waves propagating in the opposite direction and is evaluated to estimate the velocity of the waveguide and remove it by averaging the transit time of waves propagating in both directions. 
     Changes in the parameter or parameters of interest are measured by measuring changes in the transit time of energy pulses or waves within the propagating medium. Closed loop measurement of changes in the parameter or parameters of interest is achieved by modulating the repetition rate of energy pulses or the frequency of energy waves as a function of the propagation characteristics of the elastic energy propagating structure. 
     In a continuous wave mode of operation, a phase detector (not shown) evaluates the frequency and changes in the frequency of resonant ultrasonic waves in the waveguide  1606 . As will be described below, positive feedback closed-loop circuit operation in continuous wave (CW) mode adjusts the frequency of ultrasonic waves  1614  in the waveguide  1606  to maintain a same number or integer number of periods of ultrasonic waves in the waveguide  1606 . The CW operation persists as long as the rate of change of the length of the waveguide is not so rapid that changes of more than a quarter wavelength occur before the frequency of the Propagation Tuned Oscillator (PTO) can respond. This restriction exemplifies one advantageous difference between the performance of a PTO and a Phase Locked Loop (PLL). Assuming the transducers are producing ultrasonic waves, for example, at 2.4 MHz, the wavelength in air, assuming a velocity of 343 microns per microsecond, is about 143μ, although the wavelength within a waveguide may be longer than in unrestricted air. 
     In a pulse mode of operation, the phase detector measures a time of flight (TOF) between when an ultrasonic pulse is transmitted by transducer  1602  and received at transducer  1604 . The time of flight determines the length of the waveguide propagating path, and accordingly reveals the change in length of the waveguide  1606 . In another arrangement, differential time of flight measurements (or phase differences) can be used to determine the change in length of the waveguide  1606 . A pulse consists of a pulse of one or more waves. The waves may have equal amplitude and frequency (square wave pulse) or they may have different amplitudes, for example, decaying amplitude (trapezoidal pulse) or some other complex waveform. The PTO is holding the phase of the leading edge of the pulses propagating through the waveguide constant. In pulse mode operation the PTO detects the leading edge of the first wave of each pulse with an edge-detect receiver rather than a zero-crossing receiver circuitry as used in CW mode. 
       FIG. 18  illustrates a block diagram of a medical sensing system  1800  in accordance with an example embodiment. The medical sensing system operates similar to the systems described in  FIG. 4 ,  FIG. 6 ,  FIG. 8 , and  FIG. 12  to measure a medical parameter. The sensor of system  1800  is capacitor  1802 . Capacitor  1802  is a variable capacitor that varies with the medical parameter being measured. A capacitance value of capacitor  1802  correlates to a value of the parameter. In a first example, the parameter being measured is temperature. The capacitance of capacitor  1802  is coupled to the temperature to be measured. The capacitance of capacitor  1802  at “temperature” can be accurately measured by system  1800  and correlated back to a temperature value. Another example of a parameter is a force, pressure, or load. In one embodiment, the force, pressure, or load can be applied to capacitor  1802 . The capacitance of capacitor  1802  at the “force, pressure, or load” is measured by system  1800  and correlated back to a force, pressure, or load value. In either example, the capacitance will change by a known manner over the parameter measurement range. In general, the change in capacitance over the parameter measurement range occurs in a regular manner. Irregularities in capacitance change within the parameter System  1800  can be calibrated over the parameter measurement range to account for any irregularities in capacitance change or to further refine measurement accuracy. 
     System  1800  comprises a capacitor  1802 , a signal generator  1804 , a digital clock  1806 , a digital counter  1808 , a digital timer  1810 , a counter register  1812 , and a data register  1814 . Signal generator  1804  is coupled to capacitor  1802  and has an output for providing a signal. Signal generator  1804  generates a signal  1816  or waveform that corresponds to the capacitance of capacitor  1802 . The signal  1816  changes as the capacitance of capacitor  1802  changes. For example, a time period of a measurement cycle of signal  1816  can relate to the capacitance of capacitor  1802 . 
     In one embodiment, signal generator  1804  is an oscillator. A digital clock  1806  is coupled to digital counter  1808  and digital timer  1810 . Digital clock  1806  provides a clock signal to digital counter  1808  and digital timer  1810  during a measurement sequence. Digital counter  1808  couples to counter register  1812  and couples to the output of signal generator  1804 . Counter register  1812  provides a predetermined count corresponding to the measurement sequence. In general, measurement accuracy can be increased by raising the predetermined count. Digital counter  1808  receives the predetermined count from counter register  1812 . After initiating the measurement sequence the digital counter compares the number of measurement cycles at the output of signal generator  1804  to the predetermined count. The measurement sequence ends when the count of measurement cycles equals the predetermined count. In one embodiment, each measurement cycle output by signal generator  1804  decrements digital counter  1808  until a zero count is reached which signifies an end of the measurement sequence. Digital timer  1810  measures a time period of the measurement sequence. In other words, digital timer  1810  measures an elapsed time required for signal generator  1804  to output the predetermined count of measurement cycles. Data register  1814  couples to digital timer  1810  and stores a value corresponding to the time period or elapsed time of the measurement sequence. The elapsed time of the measurement sequence corresponds to a statistically large number of measurements of capacitor  1802 . The elapsed time corresponds to an aggregate of the predetermined count of measurement cycles or capacitance measurements. The value stored in data register  1814  can be a translation of the elapsed time to a force, pressure, or load value. The parameter being measured should produce a stable capacitance value during the time period of the measurement sequence. 
       FIG. 19  illustrates an oscillator  1900  generating a signal corresponding to a capacitor  1802  in accordance with an example embodiment. Oscillator  1900  corresponds to signal generator  1804  of  FIG. 18 . Oscillator  1900  is an example of a circuit used to generate signal  1816  of  FIG. 18 . Oscillator  1900  comprises a current source  1902 , a current source  1904 , a comparator  1906 , a switch  1908 , a switch  1910 , and a switch control  1912 . Capacitor  1802  is coupled to current sources  1902  and  1904 . Current sources  1902  and  1904  respectively source and sink current from capacitor  1802 . Current source  1902  sources a current I. Current source  1904  sinks a current  21  or twice the current provided by current source  1902 . Switch  1910  enables current source  1904  to sink current when coupled to ground. Comparator  1906  includes a positive input coupled to capacitor  1802 , a negative input coupled to switch  1908 , and an output. The output of comparator  1906  couples to switch control  1912 . Switch control  1912  couples to switches  1908  and  1910  to control switch position. The output of comparator  1906  is a control signal to switch control  1912 . 
     In general, current sources  1902  and  1904  respectively charge and discharge capacitor  1802 . Capacitor  1802  is charged by current source  1902  when the output of comparator  1906  is in a low state. Switch control  1912  opens switch  1910  and a reference voltage Vref is coupled to the negative input of comparator  1906  by switch  1908  when the output of comparator  1906  transitions to the low state. The voltage on capacitor  1802  rises as the current I from current source  1902  charges the capacitance. The slew rate of the change in voltage on the capacitor is related to the capacitance of capacitor  1802  and the current I. The output of comparator  1906  transitions from a low state to a high state when the voltage on capacitor  1802  is greater than or equal to the reference voltage Vref. Switch control  1912  closes switch  1910  and a reference voltage Vref/2 is coupled to the negative input of comparator  1906  by switch  1908  when the output of comparator  1906  transitions to the high state. The sink current of current source  1904  is 2I or twice as large as the current sourced by current source  1902 . Current source  1904  sinks a current I from capacitor  1802  and an equal current from current source  1902 . The voltage on capacitor  1802  falls as charge is removed. The output of comparator changes from the high state to a low state when the voltage on the capacitor is less than or equal to the reference voltage Vref/2. In the example, voltage on capacitor  1802  will transition between the reference voltages Vref and Vref/2. The slew rate of the rising edge and falling edge of the capacitor voltage is symmetrical. A repeating saw tooth pattern is generated by oscillator  1900  until the sequence is stopped. A measurement cycle corresponds to the time to generate a single triangle shaped waveform. The triangle shaped waveform constitutes the time to transition the voltage on capacitor  1802  from Vref/2 to Vref and from Vref to Vref/2. It should be noted that the measurement cycle relates to the capacitance of capacitor  1802 . Increasing the capacitance of capacitor  1802  correspondingly increases the measurement cycle. Conversely, decreasing the capacitance of capacitor  1802  correspondingly decreases the measurement cycle. The signal at the output of the comparator  1906  also corresponds to signal  1816 . Thus, a relation is established by the signal output by oscillator  1900  to the capacitance of capacitor  1802 . 
     Referring briefly to  FIG. 1 , a sensor  100  is coupled to the muscular-skeletal system. In the example, a prosthetic knee joint is illustrated and the sensor  100  is coupled to the knee region. Sensor  100  can be capacitor  1802  coupled to the muscular-skeletal system. Capacitor  1802  can be coupled to an articular surface of the prosthetic knee joint to measure a force, pressure, or load. In one embodiment, the force, pressure, or load applied to the articular surface is coupled to capacitor  1802  whereby the capacitance varies with the force, pressure, or load applied thereto. Although a knee joint is shown, capacitor  1802  and system  1800  of  FIG. 18  can be used in medical devices, tools, equipment, and prosthetic components to measure parameters that affect capacitance of capacitor  1802 . Similarly, although a knee joint is described as an example, capacitor  1802  can be integrated into muscular-skeletal medical devices, tools, equipment, and prosthetic components to measure an applied force, pressure, or load. Moreover, capacitor  1802  and system  1800  of  FIG. 18  is not limited to the knee but can be integrated into prosthetic components for parameter measurement such as bone, tissue, shoulder, ankle, hip, knee, spine, elbow, hand, and foot. 
     Referring back to  FIGS. 18 and 19 , signal generator  1804  outputs a repeating waveform that corresponds to the capacitance of capacitor  1802 . Oscillator  1900  is an implementation of signal generator  1804  that oscillates or generates a repeating waveform. In the example, oscillator  1900  outputs a repeating sawtooth waveform that has symmetrical rising and falling edges. The measurement cycle of the waveform is the time required to transition from Vref/2 to Vref and transition back to Vref/2. The time of the measurement cycle corresponds to the capacitance of the capacitor. The time of each measurement cycle will be substantially equal if the capacitance of capacitor  1802  remains constant during the measurement sequence. In one embodiment, counter register  1812  is loaded with a predetermined count. The measurement sequence can be initiated at a predetermined point of the waveform. For example, a voltage Vref/2 can be detected to start on the waveform to start the measurement sequence. Each subsequent time the voltage Vref/2 is detected the digital counter  1808  is decremented. The measurement sequence ends when digital counter decrements to zero. Digital timer  1810  measures the elapsed time of the measurement sequence corresponding to the predetermined count of measurement cycles of the sawtooth waveform. Alternatively, the output of comparator  1906  can be used as the oscillating or repeating waveform. A rising or falling edge of the output of comparator  1906  can be used to initiate and decrement digital counter  1808 . The measurement sequence is configured to be initiated during a period when the parameter to be measured and by relation the capacitance of capacitor  1802  is substantially constant. The process measures the capacitance  1802  a number of times equal to the predetermined count. Variations in the measurement can be averaged out by having a large predetermined count. The process also allows for very small changes in capacitance to be measured very accurately. The accuracy of the measurement can be increased by raising the predetermined count of the measurement cycles. In one embodiment, the measured capacitance is an average determined by the measured elapsed time and the predetermined count of measurement cycles. The measured capacitance can be translated to the parameter being measured such as a force, pressure, or load. Data register  1814  can be configured to store the parameter measurement or a number corresponding to the parameter measurement. 
       FIG. 20  discloses a method  2000  for measuring a force, pressure, or load. The method description relates to and can reference  FIGS. 1, 4, 6, 8, 12, 13, and 19 . The example disclosed herein uses a prosthetic component implementation but method  2000  can be practiced in any other suitable system or device. The steps of method  2000  are not limited to the order disclosed. Moreover, method  2000  can also have a greater number of steps or a fewer number of steps than shown. 
     At a step  2002 , a force, pressure, or load is applied to a capacitor. Changes in the force, pressure, or load produce a corresponding change in a capacitance of the capacitor. At a step  2004 , a repeating signal is generated. A time period of a single waveform of the repeating signal is a measurement cycle. The time period of the measurement cycle corresponds to the capacitance of the capacitor. At a step  2006 , the waveform or signal is repeated a predetermined number of times. A measurement sequence comprises the repeated waveform for the predetermined number of times. At a step  2008 , an elapsed time of the measurement sequence is measured. The elapsed time is the time required to generate the predetermined number of waveforms. At a step  2010 , the force, pressure, or load is maintained during the measurement sequence. In general, the force, pressure, or load coupled to the capacitor should be constant during the measurement sequence. At a step  2012 , the measured elapsed time is correlated to the force, pressure, or load measurement. Typically, a measurement range is known for the force, pressure, or load being applied to the capacitor. The capacitor or capacitor type being used can be characterized using known force, pressure, and loads throughout the measurement range prior to use. Thus, a correlation between capacitance and force, pressure, or load is known. For example, the relationship between capacitance and force, pressure, or load can be stored in a look up table or by a mathematical expression. In one embodiment, the capacitor responds approximately linear throughout the measurement range. The average capacitance of the capacitor can be calculated using the measured elapsed time to generate the predetermined number of waveforms during the measurement sequence. The force, pressure, or load can then be determined from the previous characterization. Further refinement can be achieved by using calibration techniques during final testing of the capacitor. The calibration data on the capacitor can be used in the calculation of the force, pressure, or load to further reduce measurement error. At a step  2014 , the predetermined number of waveforms can be increased to raise measurement accuracy. The measurement resolution can be increased by this technique if the force, pressure, or load is substantially constant over the increased number of predetermined number waveforms. Moreover, the resolution supports measurement where the capacitance changes are relatively small over the force, pressure, or load measurement range. 
       FIG. 21  illustrates a capacitor  2100  in accordance with an example embodiment. In general, a sensor for use in a medical environment is accurate, reliable, low cost, and have a form factor suitable for the application. Sensors that produce an electrical signal require a wired or wireless interconnect to electronic circuitry to receive, analyze, and provide the measurement data. Capacitor  2100  meets the above listed requirements. Capacitor  2100  can be used in medical devices, tools, and equipment for measurement of different medical parameters. In the example, capacitor  2100  can be integrated into devices, tools, equipment, and prosthetic components for measuring parameters of the muscular-skeletal system. Capacitor  2100  is suitable for intra-operative and implantable prosthetic components that support installation and long-term measurement of the installed structures. 
     Capacitor  2100  comprises a dielectric layer  2102 , a dielectric layer  2104 , and a dielectric layer  2106 . Capacitor  2100  comprises more than two capacitors in series mechanically. In one embodiment, capacitor  2100  comprises 3 capacitors in mechanical series. Referring briefly to  FIG. 22 , capacitor  2100  of  FIG. 21  comprises capacitors  2206 ,  2204 , and  2208 . Capacitors  2206 ,  2204 , and  2208  are coupled mechanically in series. A compressive force, pressure, or load  2202  is applied to the series coupled capacitors  2206 ,  2204 , and  2208 . Referring back to  FIG. 21 , a first capacitor comprises a conductive region  2108 , dielectric layer  2102 , and conductive region  2110 . The first capacitor corresponds to capacitor  2204  of  FIG. 22 . Conductive regions  2108  and  2110  have a predetermined area such that the predetermined area, dielectric constant of dielectric layer  2102 , and the thickness of dielectric layer  2102  determine the capacitance of capacitor  2204 . In one embodiment, conductive layer  2108  overlies, has substantially equal area, and is aligned to conductive layer  2110 . 
     A second capacitor comprises conductive region  2108 , dielectric layer  2104 , and a conductive region  2112 . The second capacitor corresponds to capacitor  2206  of  FIG. 22 . In one embodiment, conductive region  2112  overlies, has approximately equal area, and is aligned to conductive region  2108 . A load pad  2114  is formed overlying conductive region  2112 . Load pad  2114  protects and prevents damage to conductive layer  2112  due to a force, pressure or load applied to capacitor  2100 . 
     A third capacitor comprises conductive region  2110 , dielectric layer  2106 , and a conductive layer  2116 . The third capacitor corresponds to capacitor  2208  of  FIG. 22 . In one embodiment, conductive region  2116  overlies, has approximately equal area, and is aligned to conductive region  2110 . A load pad  2118  is formed overlying conductive region  2116 . Load pad  2118  protects and prevents damage to conductive layer  2116  due to a force, pressure or load applied to capacitor  2100 . In general, load pads  2114  and  2118  comprise a non-compressible material. Load pads  2114  and  2218  can comprise metal, composite material, or a polymer. 
     Capacitor  2100  couples to electronic circuitry as disclosed in  FIG. 18 . Capacitor  2100  can comprise more than one capacitor in parallel. In one embodiment, conductive regions  2108  and  2110  can be coupled in common. In the example, conductive regions  2108  and  2110  are coupled in common by conductive via  2120 . Conductive regions  2112  and  2116  are also coupled in common or to a common voltage potential. In one embodiment, conductive regions  2112  and  2116  are coupled to ground forming a shield. Referring briefly to  FIG. 23 , capacitor  2100  comprises capacitors  2206  and  2208 . Capacitors  2206  and  2208  are coupled electrically in parallel having a terminal coupled to ground and a terminal comprising conductive regions  2108  and  2110  coupled in common. Capacitor  2204  is not shown in the electrical equivalent circuit of capacitor  2100  because the conductive regions of capacitor  2204  are shorted together. Referring back to  FIG. 21 , capacitor  2206  and capacitor  2208  can be formed having substantially equal capacitance. Thus, capacitor  2100  comprises more than one capacitor that are mechanically in series and comprises more than one capacitor that are coupled electrically in parallel. 
     In the example, capacitor  2100  can be used as a force, pressure, or load sensor for the muscular-skeletal system. Capacitor  2100  can be integrated into a prosthetic component to measure the force, pressure, or load applied by the muscular-skeletal system. The measurement has supports the installation of prosthetic components and can be used for long-term data collection on the implanted system. The size and shape of capacitor  2100  is beneficial to biological sensing applications. The form factor of capacitor  2100  can be made very small. Moreover, capacitor  2100  can be made very thin which supports integration and placement in regions of the body that could not be achieved with conventional sensors. A thickness of less 2.5 millimeters and typically less than 1 millimeter for capacitor  2100  can be manufactured. 
     In one embodiment, a multi-layered interconnect can be used to form capacitor  2100 . Multi-layer interconnect comprises alternating conductive layers and dielectric layers. The conductive layers can be patterned to form conductive regions and interconnect. Applying a force, pressure, or load to multi-layer interconnect can deform the dielectric layers. It has been found that for small deformations the dielectric layers of interconnect will rebound elastically when the stimulus is removed. Deformation of the dielectric layer changes the dielectric thickness of capacitor  2100  and the capacitance value thereof. System  1800  of  FIG. 18  supports high resolution of small changes in capacitance that makes the use of capacitor  2100  viable. 
     In general, the dielectric material for the interconnect can comprise a polymer, polyester, an aramid, an adhesive, silicon, glass, or composite material. Capacitor  2100  includes at least one dielectric layer comprising polyimide. In one example, dielectric layers  2102 ,  2104 , and  2106  comprise polyimide. Alternatively, layer  2102  can be an adhesive layer that couples capacitors  2206  and  2208  together. Under testing, polyimide has been shown to compress elastically under load values typical for prosthetic component load measurement. In general, capacitor  2100  compresses less than 20% of thickness of each capacitor to maintain operation in an elastic region of the dielectric. In one embodiment, the dielectric of capacitor  2100  is compressed less than 10% of the dielectric thickness over the operating range. For example, the polyimide layer can be approximately 0.0254 millimeters thick. Compression of the polyimide can be less than 0.0022 millimeters over the entire load measurement range for a prosthetic knee application. The interconnect can be flexible allowing placement on non-planar regions. Moreover, capacitor  2100  can be conformal to different surface shapes if required. Alternatively, capacitor  2100  can be formed as a compressible structure that does not flex or conform. 
     As mentioned previously, capacitor  2100  is coupled to electronic circuitry such as that disclosed in  FIG. 18 . Using interconnect to form capacitor  2100  provides the further benefit of being able to integrate capacitor  2100  with the interconnect that couples to the electronic circuitry. This eliminates a connection between the sensor and the interconnect as they are formed as a single structure. The integrated capacitor and interconnect also increases sensor reliability, lowers cost, and simplifies assembly. 
     Referring briefly to  FIG. 24 , a top view illustrates conductive region  2112  formed overlying dielectric layer  2104 . In general, the force, pressure, or load is applied uniformly on the conductive regions of the sensor capacitor. The load pad can support the distribution of the force, pressure, or load across the entire conductive region. The area of the conductive region is of sufficient size to maintain elastic compression of the dielectric material over the entire force, pressure, or load range of the application. The area of the conductive regions can be increased to reduce the force, pressure, or load per unit area thereby lowering dielectric compression over the measurement range for improved reliability. In the knee prosthetic component example, conductive region  2112  can have a circular shape. The area of conductive region  2112  is a function of the force, pressure, or load range being measured. The diameter of conductive region  2112  is approximately 2.0 millimeters for a sensor for a knee application. The dashed line indicates a periphery of conductive region  2108  that underlies conductive region  2112 . In the example, conductive region  2108  has a diameter of approximately 2.2 millimeters. More than one of the sensors can fit within a prosthetic component of the knee. An interconnect  2124  is coupled to conductive region  2112 . Interconnect  2124  can be formed on the same layer as conductive region  2112 . Referring back to  FIG. 21 , conductive region  2116  can have a similar circular shape as conductive region  2112 . The diameter of conductive region  2116  is approximately 2.0 millimeters for a sensor for a knee application. The conductive region  2110  that overlies conductive region  2112  is approximately 2.2 millimeters in diameter. An interconnect  2126  can be formed overlying the polyimide layer  2106  and couple to conductive region  2116 . 
     In the example, a force, pressure, or load is applied by the muscular-skeletal system to load pads  2114  and  2118 . The force, pressure, or load compresses capacitors  2206 ,  2204 , and  2208  that are mechanically in series that comprise capacitor  2100 . Dielectric layers  2202 ,  2204 , and  2206  compress under the force, pressure, or load. The plates of capacitor  2204  are coupled in common and do not contribute to a capacitance of capacitor  2100 . The structure of capacitor  2100  minimizes the effect of parasitic capacitance. Conductive regions  2112  and  2116  are coupled to ground. Conductive regions  2112  and  2116  respectively overlie and underlie conductive regions  2108  and  2110  thereby acting as a ground shield. The shield minimizes or blocks external capacitive interaction that could occur with conductive regions  2112  and  2116  that can effect measurement accuracy. 
     Referring briefly to  FIG. 25 , a cross-sectional view of interconnect  2122 ,  2124 , and  2126  in an example embodiment is provided. As described hereinabove, conductive regions  2108  and  2110  are coupled in common by via  2120 . An interconnect  2122  couples to conductive regions  2108  and  2110 . Interconnect  2122 ,  2124 , and  2126  can couple capacitor  2100  to system  1800  of  FIG. 18 . Interconnect  2124  and  2126  are coupled to ground. Interconnect  2124  and  2126  overlie and underlie interconnect  2122  thereby acting as a shield. In one embodiment, interconnect  2122  has a width less than interconnects  2124  and  2126 . Interconnects  2124  and  2126  shield and block potential capacitive interaction with interconnect  2122  as it is routed and coupled to system  1800  of  FIG. 18 . 
     Referring back to  FIG. 21 , parasitic capacitance related to capacitor  2100  remains substantially constant throughout the parameter measurement range. A first parasitic capacitance comprises interconnect  2124 , dielectric layer  2104 , and interconnect  2122 . A second parasitic capacitance comprises interconnect  2126 , dielectric layer  2106 , and interconnect  2122 . The first and second parasitic capacitances add together to increase the capacitance of capacitor  2100 . The force, pressure, or load is not applied to first and second parasitic capacitances thereby remaining constant during measurement. Thus, the change in capacitance of capacitor  2100  can be measured by system  1800  over the force, pressure, or load range using the method disclosed herein with secondary affects due to changes in parasitic capacitance being minimized. 
       FIG. 26  discloses a method  2600  for measuring a force, pressure, or load. The method description relates to and can reference  FIGS. 1, 4, 6, 8, 12, 13, 19, and 21-25 . The steps of method  2600  are not limited to the order disclosed. Moreover, method  2600  can also have a greater number of steps or a fewer number of steps than shown. At a step  2602 , more than one capacitor in series is compressed. A sensor capacitor can comprise more than one capacitor coupled in series. The force, pressure, or load is applied across the series coupled capacitors. At a step  2604 , a capacitance of more than one capacitor in parallel is measured. The sensor capacitor can comprise more than one capacitor electrically coupled in parallel. 
     At a step  2606 , a repeating signal is generated having a measurement cycle corresponding to capacitance of the more than one capacitor in parallel. In one embodiment, the more than one capacitor in parallel is coupled to a signal generator circuit. The signal generator circuit coupled to the more than one capacitor in parallel is configured to oscillate. The repeating signal comprises a repeating measurement cycle. A time period of each measurement cycle generated by the signal generator corresponds to the capacitance of the more than one capacitor in parallel. 
     At a step  2608 , an elapsed time is measured of the repeating signal. In one embodiment, the repeating signal is repeated a predetermined number of times. In other words, the measurement cycle is repeated the predetermined number of times and the elapsed time of the predetermined number of measurement cycles is measured. At a step  2610 , the elapsed time is correlated to the capacitance of the more than one capacitor in parallel. As disclosed herein, the capacitance of the more than one capacitor in parallel corresponds to the applied force, pressure, or load. Measuring a large number of measurement cycles while the applied force, pressure, or load is substantially constant supports an accurate correlation between capacitance and the force, pressure, or load. 
       FIG. 27  illustrates a medical device having a plurality of sensors in accordance with an example embodiment. In general, embodiments of the invention are broadly directed to the measurement of physical parameters. The medical device includes an electro-mechanical system that is configured to measure medical parameters and in the example related to the measurement of the muscular-skeletal system. Many physical parameters of interest within physical systems or bodies are currently not measured due to size, cost, time, or measurement precision. For example, joint implants such as knee, hip, spine, shoulder, and ankle implants would benefit substantially from in-situ measurements taken during surgery to aid the surgeon in the installation and fine-tuning of a prosthetic system. Measurements can supplement the subjective feedback of the surgeon to ensure optimal installation. Permanent sensors in the final prosthetic components can provide periodic data related to the status of the implant in use. Data collected intra-operatively and long term can be used to determine parameter ranges for surgical installation and to improve future prosthetic components. 
     The physical parameter or parameters of interest can include, but are not limited to, measurement of load, force, pressure, position, displacement, density, viscosity, pH, spurious accelerations, humidity, and localized temperature. Often, a measured parameter is used in conjunction with another measured parameter to make a qualitative assessment. In joint reconstruction, portions of the muscular-skeletal system are prepared to receive prosthetic components. Preparation includes bone cuts or bone shaping to mate with one or more prosthesis. Parameters can be evaluated relative to orientation, alignment, direction, displacement, or position as well as movement, rotation, or acceleration along an axis or combination of axes by wireless sensing modules or devices positioned on or within a body, instrument, appliance, vehicle, equipment, or other physical system. 
     In the present invention parameters are measured with an integrated wireless sensing module or device comprising an i) encapsulating structure that supports sensors and contacting surfaces and ii) an electronic assemblage that integrates a power supply, sensing elements, an accelerometer, antennas, electronic circuitry that controls and processes a measurement sequence, and wireless communication circuitry. The wireless sensing module or device can be positioned on or within, or engaged with, or attached or affixed to or within, a wide range of physical systems including, but not limited to instruments, equipment, devices, appliances, vehicles, equipment, or other physical systems as well as animal and human bodies, for sensing and communicating parameters of interest in real time. 
     Sensors are disclosed that can indirectly measure the parameter such as a capacitor having a capacitance that varies with the parameter. The capacitance or related factor (e.g. time) is measured and then converted to the parameter. The measurement system has a form factor, power usage, and material that is compatible with human body dynamics. The physical parameter or parameters of interest can include, but are not limited to, measurement of load, force, pressure, displacement, density, viscosity, pH, humidity, distance, volume, pain, infection, spurious acceleration, and localized temperature to name a few. These parameters can be evaluated by sensor measurement, alignment, direction, or position as well as movement, rotation, or acceleration along an axis or combination of axes by wireless sensing modules or devices positioned on or within a body, instrument, appliance, vehicle, equipment, or other physical system. 
     In the example, an insert  2700  illustrates a device having a medical sensor for measuring a parameter of the muscular-skeletal system. Prosthetic insert  2700  is a component of a joint replacement system that allows articulation of the muscular-skeletal system. The prosthetic insert  2700  is a wear component of the joint replacement system. The prosthetic insert  2700  has one or more articular surfaces that allow joint articulation. In a joint replacement, a prosthetic component has a surface that couples to the articular surface of the insert  2700 . The articular surface is low friction and can absorb loading that occurs naturally based on situation or position. The contact area between surfaces of the articulating joint can vary over the range of motion. The articular surface of insert  2700  will wear over time due to friction produced by the prosthetic component surface contacting the articular surface during movement of the joint. Ligaments, muscle, and tendons hold the joint together and motivate the joint throughout the range of motion. 
     Insert  2700  is an active device having a power source  2702 , electronic circuitry  2704 , load pads  2722 , transmit capability, and sensors within the body of the prosthetic component. Electronic circuitry  2704  includes the circuitry of  FIG. 18  and  FIG. 19 . In the example, sensors underlie load pads  2722 . The sensors are capacitors formed in an interconnect  2718  that couples to electronic circuitry  2704 . Interconnect  2718  can be flexible and conformal to non-planar shapes. In one embodiment, insert  2700  is used intra-operatively to measure parameters of the muscular-skeletal system to aid in the installation of one or more prosthetic components. As will be disclosed hereinbelow, operation of insert  2700  is shown as a knee insert to illustrate operation and measurement of a parameter such as load and balance. Referring briefly to  FIG. 1 , a typical knee joint replacement system comprises an insert, femoral prosthetic component  104 , and tibial prosthetic component  106 . Although housed in the insert, sensor capacitors can also be housed within or coupled to femoral prosthetic component  104  or tibial prosthetic component  106 . Referring back to  FIG. 27 , insert  2700  can be adapted for use in other prosthetic joints having articular surfaces such as the hip, spine, shoulder, ankle, and others. Alternatively, insert  2700  can be a permanent active device that can be used to take parameter measurements over the life of the implant. The sensing system is not limited to the prosthetic component example. The system can also be implemented in medical tools, devices, and equipment. 
     Insert  2700  is substantially equal in dimensions to a passive final prosthetic insert. The substantially equal dimensions correspond to a size and shape that allow insert  2700  to fit substantially equal to the passive final prosthetic insert in a tibial prosthetic component. In the intra-operative example, the measured load and balance using insert  2700  as a trial insert would be substantially equal to the loading and balance seen by a final passive insert under equal conditions. It should be noted that insert  2700  for intra-operative measurement could be dissimilar in shape or have missing features that do not benefit the trial during operation. Insert  2700  should be positionally stable throughout the range of motion equal to that of the final insert. 
     The exterior structure of insert  2700  comprises two components. In the embodiment shown, insert  2700  comprises a support structure  2706  and a support structure  2708 . Support structures  2706  and  2708  have major support surfaces that are loaded by the muscular-skeletal system. As previously mentioned, insert  2700  is shown as a knee insert to illustrate general concepts and is not limited to this configuration. Support structure  2706  has an articular surface  2710  and an articular surface  2712 . Condyles of a femoral prosthetic component articulate with surfaces  2710  and  2712 . Loading on the prosthetic knee joint is distributed over a contact area of the articular surfaces  2710  and  2712 . Support structure  2708  has a load-bearing surface  2724 . The load-bearing surface  2724  couples to the tibial prosthetic component. The loading on load-bearing surface  2724  is much lower than that applied to the articular surfaces due to the larger surface area for distributing a force, pressure, or load. 
     A region  2714  of the support structure  2706  is unloaded or is lightly loaded over the range of motion. Region  2714  is located between the articular surfaces  2710  and  2712 . It should be noted that there is a minimum area of contact on articular surfaces  2710  and  2712  to minimize wear while maintaining joint performance. The contact location and contact area size can vary depending on the position of the muscular-skeletal system. Anomalies may occur if the contact area falls outside a predetermined area range within articular surfaces  2710  and  2712  over the range of motion. In one embodiment, the location where the load is applied on articular surfaces  2710  and  2712  can be determined by the sensing system. This is beneficial because the surgeon now has quantitative information where the loading is applied. The surgeon can then make adjustments that move the location of the applied load within the predetermined area using real-time feedback from the sensing system to track the result of each correction. 
     The support structure  2708  can be formed to support the sensors and electronic circuitry  2704  that measure loading on each articular surface of insert  2700 . A load plate  2716  underlies articular surface  2710 . Similarly, a load plate  2720  underlies articular surface  2712 . Interconnect  2718  underlies load plate  2720 . Capacitor sensors underlie load pads  2722  in the vertices of the triangular shaped interconnect  2718  in support structure  2708 . In one embodiment, the capacitor sensors are formed in the interconnect  2718 . Interconnect  2718  couples the sensors to electronic circuitry  2704 . A shield is formed in interconnect  2718  that minimizes parasitic capacitance and coupling to ensure accuracy over the measurement range. Load plate  2720  couples to the capacitor sensors through load pads  2722 . Load plate  2720  distributes the load applied to articular surface  2712  to the capacitor sensors at predetermined locations within insert  2700 . The measurements from the three sensors underlying articular surface  2712  can be used to determine the location of the applied load to insert  2700 . Load plate  2716  operates similarly underlying articular surface  2710 . Although the surface of load plates  2716  and  2720  as illustrated are planar they can be non-planar with the sensors conforming to the non-planar surface. Similarly, the capacitor sensors can be formed having a non-planar shape. 
     A force, pressure, or load applied by the muscular-skeletal system is coupled to the articular surfaces  2710  and  2712  of prosthetic component insert  2700 , which respectively couples to plates  2716  and  2720 . In one embodiment, each capacitor elastically compresses due to the force, pressure, or load. Electronic circuitry  2704  is operatively coupled to the capacitor sensors underlying load plates  2716  and  2720 . A signal is generated that corresponds to the capacitance of the capacitor being measured. The signal is repeated a predetermined number of times or for a predetermined count. The elapsed time of the predetermined count is measured. The elapsed time corresponds to the capacitance of the capacitor. The relationship between capacitance and force, pressure, or load is known and used to determine the measurement value. Furthermore, the measurement data can be processed and transmitted to a receiver external to insert  2700  for display and analysis. 
     In one embodiment, the physical location of the sensors and electronic circuitry  2704  is housed in insert  2700  thereby protecting the active components from an external environment. Electronic circuitry  2704  can be located between articular surfaces  2710  and  2712  underlying region  2714  of support structure  2700 . A cavity for housing the electronic circuitry  2704  can underlie region  2714 . Support structure  2708  has a surface within the cavity having retaining features extending therefrom to locate and retain electronic circuitry  2704  within the cavity. Region  2714  is an unloaded or a lightly loaded region of insert  2700  thereby reducing the potential of damaging the electronic circuitry  2704  due to a high compressive force during surgery or as the joint is used by the patient. In one embodiment, a temporary power source such as a battery, capacitor, inductor, or other storage medium is located within insert  2700  to power the sensors and electronic circuitry  2704 . 
     Support structure  2706  attaches to support structure  2708  to form an insert casing or housing. In one embodiment, internal surfaces of support structures  2706  and  2708  mate together. Moreover, the internal surfaces of support structures  2706  and  2708  can have cavities or extrusions to house and retain components of the sensing system. Externally, support structures  2706  and  2708  provide load bearing and articular surfaces that interface to the other prosthetic components of the joint. The load-bearing surface  2724  of support structure  2708  couples to the tibial prosthetic component. Load-bearing surface  2724  can have one or more features or a shape that supports coupling to the tibial prosthetic component. 
     The support structures  2706  and  2708  can be temporarily or permanently coupled, attached, or fastened together. As shown, insert  2700  can be taken apart to separate support structures  2706  and  2708 . A seal can be located peripherally on an interior surface of support structure  2708 . In one embodiment, the seal can be an O-ring that comprises a compliant and compressible material. The O-ring compresses and forms a seal against the interior surface of support structures  2706  and  2708  when attached together. Support structures  2706  and  2708  form a housing whereby the cavities or recesses within a boundary of the seal are isolated from an external environment. In one embodiment support structures  2706  and  2708  are coupled together when the O-ring is compressed sufficiently to interlock fastening elements. Support structures  2706  and  2708  are held together by the fastening elements under force or pressure provided by the O-ring or other means such as a spring. 
     In one embodiment, support structure  2700  comprises material commonly used for passive inserts. For example, ultra high molecular weight polyethylene can be used. The material can be molded, formed, or machined to provide the appropriate support and articular surface thickness for a final insert. Alternatively, support structures  2706  and  2708  can be made of metal, plastic, or polymer material of sufficient strength for a trial application. In an intra-operative example, support structures  2706  and  2708  can be formed of polycarbonate. It should be noted that the long-term wear of the articular surfaces is a lesser issue for the short duration of the joint installation. The joint moves similarly to a final insert when moved throughout the range of motion with a polycarbonate articular surface. Support structures  2706  and  2708  can be a formed as a composite where a bearing material such as ultra high molecular weight polyethylene is part of the composite material that allows the sensing system to be used both intra-operatively and as a final insert. 
       FIG. 28  illustrates one or more prosthetic components having sensors coupled to and conforming with non-planar surfaces in accordance with an example embodiment. Hip joint prosthetic components are used as an example to illustrate non-planar sensors. The hip joint prosthesis comprises an acetabular cup  2806 , an insert  2808 , and a femoral prosthetic component  2810 . The acetabular cup  2806  couples to a pelvis. Cup  2806  can be cemented to pelvis  2802  thereby fastening the prosthetic component in a permanent spatial orientation for receiving femoral prosthetic component  2810 . Insert  2808  is inserted into acetabular cup  2806  having an exposed articular surface. A femoral head of femoral prosthetic component  2810  can be placed into insert  2808 . Insert  2808  retains the femoral head. The articular surface of insert  2808  couples to the femoral head of femoral prosthetic component  2810  allowing rotation of the joint. The loading is distributed over an area of the articular surface of insert  2808  that varies depending on the leg position. A shaft of femoral prosthetic component  2810  is coupled to a femur  2804 . Cement can be used to fasten the shaft of femoral prosthetic component  2810  to femur  2804 . Tissue such as tendons, ligaments, and muscle couple to pelvis  2802  and femur  2804  to retain and support movement of the hip joint. The sensors and electronic circuitry disclosed herein are not limited to prosthetic hip components and can be applied similarly to other parts of the anatomy including but not limited to the muscular-skeletal system, bone, organs, skull, knee, shoulder, spine, ankle, elbow, hands, and feet. 
     In one embodiment, femoral prosthetic component  2810  can house electronic circuitry  2812  thereby protecting the active components from an external environment. The electronic circuitry  2812  can include the circuitry disclosed in  FIG. 18  and  FIG. 19  to measure capacitance of a capacitor sensor. The electronic circuitry  2812  can further include a power source, power management circuitry, conversion circuitry, digital logic, processors, multiple input/output circuitry, and communication circuitry. The electronic circuitry  2812  can be a module having a form factor that can fit within a prosthetic component. Similarly, electronic circuitry  2812  can be integrated into a tool, device, or equipment. Alternatively, electronic circuitry  2812  can be a separate component that couples through a wired or wireless connection to sensors. 
     The femoral head of the prosthetic component  2810  is spherical in shape. Capacitors  2814  are sensors that conform and couple to the curved surface of the femoral head. In first embodiment, capacitors  2814  can underlie an external surface of the femoral head. A force, pressure, or load applied to the femoral head couples to and can elastically compress capacitors  2814 . Capacitors  2814  and electronic circuitry  2812  are protected from an external environment such that the prosthetic component is suitable for long term monitoring of the joint. In a second embodiment, capacitors  2814  can be exposed on portions of the surface conforming to a spherical shape of the femoral head. In a third embodiment, capacitors  2814  can be formed having the non-planar shape. Capacitors  2814  can be in a trial prosthetic component that is disposed of after a single use. As disclosed herein, capacitors  2814  can be formed in interconnect as disclosed in  FIGS. 21-25 . The interconnect can be flexible and can conform to non-planar surfaces. In the example, capacitors  2814  are formed in interconnect that couples to electronic circuitry  2812  to receive and process measurement data. The interconnect and more specifically capacitors  2814  are positioned within and coupled to the spherical femoral head surface whereby force, pressure, or loads can be measured at predetermined locations. Thus, the sensor system can be housed entirely within a prosthetic component. Similarly, the sensors can be placed on, within or between acetabular cup  2806  and insert  2808 . As an example, capacitors  2816  are shown placed between acetabular cup  2806  and insert  2808 . Capacitors  2816  can also underlie or comprise a portion of the articular surface of insert  2808 . Similarly, capacitors  2816  can underlie or comprise a portion of the curved surface of acetabular cup  2806 . Capacitors  2816  can be configured to measure force, pressure, or load applied to different regions of the articular surface of insert  2808 . Electronic circuitry coupled to capacitors  2816  can be in proximity to or housed in acetabular cup  2806 , insert  2808 . Force, pressure, or load measurements on bone can be supported by the system. Capacitors  2822  can be embedded in bone such as pelvis  2802  to measure forces applied thereto. 
     In the example, capacitors  2814  are located at predetermined locations of the femoral head of femoral prosthetic component  2810 . The capacitance of capacitors  2814  relate to the force, pressure, or load applied to the femoral head by the muscular-skeletal system thereby providing measurement data at the different locations of the femoral head. In one embodiment, measurement data from capacitors  2814  can be wirelessly transmitted to a remote system  2818  in real-time. Remote system  2818  includes a display  2820  configured to display the measurement data. Remote system  2818  can be a computer that further processes the measurement data. The measurement data can be provided in an audible, visual, or haptic format that allows the user to rapidly assess the information. Rotating and moving the leg over the range of motion can provide quantitative data on how the loading varies over the range of motion of the hip joint for the installation. The leg movement couples capacitors  2814  to different areas of the articular surface of insert  2808 . Capacitors  2814  move in an arc when the leg is moved in a constant plane. The measurements data can indicate variations in loading that can require modification to the joint installation. The installation can be done in workflow steps that are supported by remote system  2818 . Moreover, clinical evidence from quantitative measurements over a statistically significant number of patients as target values or ranges for an optimal fit. The surgeon can further fine-tune the installation based on the actual measured quantitative data and subjective feedback from the patient installation. 
       FIG. 29  illustrates a tool having one or more shielded sensors coupled to a non-planar surface in accordance with an example embodiment. A reamer  2902  is used as an example of a medical device, tool, equipment, or component having one or more sensors. Reamer  2902  can be used in a hip prosthetic joint replacement surgery for removing bone in a pelvis  2908  to accept a prosthetic component such as an acetabular cup. Reamer  2902  has spherical shaped surface  2904  having cutting blades or abrasives for removing bone in an acetabular region  2910  to form a spherical shaped bone region. The cutting head of reamer  2902  is sized to cut acetabular region  2910  region substantial equal in dimensions to the acetabular cup to be fitted therein. 
     In one embodiment, more than one sensor can be coupled to the cutting head of reamer  2902 . In a non-limiting example, the sensors can be used to measure a force, pressure, or load. More specifically, the sensors can be positioned corresponding to locations on surface  2904  of the cutting head. The sensors are coupled to surface  2904  but are internal to the cutting head of reamer  2902 . The force, pressure, or load is coupled from surface  2904  to the sensors. The sensors provide quantitative data on the force, pressure, or load applied to the different locations of surface  2904 . The quantitative data can be used as feedback to the material removal process for optimal fit of the acetabular cup. For example, placing too much force in one direction can result in too much material being removed in a location thereby affecting the shape of the bone cut. 
     Capacitors  2906  are an example of sensors for measuring a force, pressure, or load. Capacitors  2906  are elastically compressible over the measurable range of reamer  2902 . More specifically, the dielectric material comprising capacitors  2906  compresses under an applied force, pressure, or load. The capacitance of a capacitor increases as the dielectric material decreases in thickness due to the force, pressure, or load. Conversely, the dielectric material increases in thickness as the force, pressure, or load applied to the capacitor is reduced thereby decreasing a capacitance value. Capacitors  2906  are coupled to different locations of surface  2904  of the cutting head of reamer  2902 . The capacitors  2906  are distributed across surface  2904  to provide force, pressure, or load magnitudes and differential force, pressure, or load magnitudes for different surface regions during a material removal process. The surface regions being measured by capacitors  2906  will change with the trajectory of reamer  2902 . The measurement data can be used to support a bone reaming process for optimal prosthetic component fit. 
     In one embodiment, capacitors  2906  are formed within an interconnect as disclosed in  FIGS. 21-25 . The interconnect can include one or more dielectric layers or substrates comprising polyimide. The polyimide layers are flexible, can conform to a non-planar surface, or be formed having a predetermined shape. Capacitors  2906  include one or more shields to reduce capacitive coupling to the device. A shield can be coupled to ground and be physically between a conductive region of capacitors  2906  and an external environment of the interconnect. The shield can be a conductive region of the capacitor. In one embodiment, a first shield is formed overlying a conductive region of a capacitor and a second shield is formed underlying the conductive region of the capacitor. The shield minimizes parasitic capacitances that can change a capacitance value of capacitors  2906 . 
     Interconnect can be formed on the one or more polyimide layers that couples to the conductive regions of capacitors  2906 . The interconnect can couple capacitors  2906  to electronic circuitry (not shown) for generating a signal corresponding to a capacitance of each capacitor. Capacitors  2906  couple to surface  2904  of the cutting head of reamer  2902 . In the example, capacitors  2906  conform to a curved or non-planar surface corresponding to a shape of surface  2904 . In one embodiment, the interconnect and capacitors  2906  are internal to the cutting head thereby isolated from an external environment. The interconnect couples to electronic circuitry for measuring capacitance of capacitors  2906 . The electronic circuitry can be housed in the cutting head or the handle of reamer  2902 . The electronic circuitry can include a power source such as a battery, inductive power source, super capacitor, or other storage medium. As mentioned previously, the capacitance of capacitors  2906  can be related to a force, pressure, or load applied thereto. In the example, the electronic circuitry generates a signal for each capacitor of capacitors  2906  that relates to a capacitance value. The electronic circuitry can include transmit and receive circuitry for sending measurement data from capacitors  2906 . In one embodiment, the measured data is transmitted to a remote system  2818 . Remote system  2818  can include a display  2820  for presenting the measurement data. Data processing can be performed by remote system  2818  to convert the measurement data to a force, pressure, or load. Trajectory data and force, pressure, or load measurements can be provided in a visual format that allows rapid assessment of the information. Audible feedback can be provided to supplement display  2820  when the user requires direct viewing of an operational area. Remote system  2818  can analyze the quantitative measurement data and transmit information to reamer  2902  that provides haptic or other types of feedback to the device that affects trajectory or force, pressure, or load as directed by the user. Quantitative data provided by reamer  2902  is provided in real-time allowing the user to see how the changes affect bone removal on pelvis  2908  on display  2820 . 
       FIG. 30  discloses a method  3000  for measuring a force, pressure, or load. The method description relates to and can reference  FIGS. 1, 4, 6, 8, 12, 13, 19, 21-25 , and  27 - 29 . The steps of method  3000  are not limited to the order disclosed. Moreover, method  3000  can also have a greater number of steps or a fewer number of steps than shown. At a step  3002 , a force, pressure, or load is applied to a capacitor. Changes in the force, pressure, or load produce a corresponding change in a capacitance of the capacitor. In one embodiment, the capacitor is formed on or in an interconnect. The dielectric material of the capacitor can be elastically compressible. In a step  3004 , at least one conductive region of the capacitor is shielded to reduce capacitive coupling. In one embodiment, the shield can comprise a conductive region of the capacitor that is a plate of the capacitor. Alternatively, the shield can be a separate structure. The shield can be grounded to minimize parasitic capacitance or coupling to the capacitor. The shield can be between an external environment of the capacitor and the active conductive region or plate of the capacitor being shielded. Furthermore, the shield reduces variable parasitic capacitance that can affect measurement accuracy. The grounded conductive region can be between the active conductive region and the external environment. In a step  3006 , interconnect coupling the capacitor to electronic circuitry is shielded to further reduce capacitive coupling. The shield can be an interconnect of the capacitor. For example, a grounded interconnect can be placed between the interconnect carrying a signal and an external environment to prevent capacitive coupling from circuitry in the external environment. Alternatively, the shield can be a separate structure. Shielding for the capacitor and the interconnect supports the measurement of very small capacitive values. The change in measured capacitance can be small in comparison to the total capacitance. Shielding prevents the total capacitance from changing thereby allowing a capacitance change of less than 10 picofarads to be measured. 
     Thus, a system is provided herein for measuring small capacitive values and small changes in capacitance. The system further supports a small form factor, high reliability, measurement accuracy, and low cost. Capacitors for force, pressure, and load measurement can be formed in interconnect used to couple the capacitors to electronic circuitry. The capacitors are operated within a substantially elastically compressible region of the dielectric material. Forming the capacitors in the interconnect reduces system complexity, improves reliability, product consistency, and reduces assembly steps. 
     A signal is generated corresponding to a capacitance of the capacitor under a force, pressure, or load. The signal is repeated for a predetermined count. Measuring an elapsed time of a large number of measurement cycles can be used to generate an average time period of a measurement cycle when change in the parameter being measured occurs slowly in relation to physiological changes such as occurs in the muscular-skeletal system. The measurement data can be analyzed to achieve accurate, repeatable, high precision and high-resolution measurements. The system disclosed herein enables the setting of the level of precision or resolution of captured data to optimize trade-offs between measurement resolution versus frequency, including the bandwidth of the sensing and data processing operations, thus enabling a sensing module or device to operate at its optimal operating point without compromising resolution of the measurements. This is achieved by the accumulation of multiple cycles of excitation and transit time instead of averaging transit time of multiple individual excitation and transit cycles. The result is accurate, repeatable, high precision and high-resolution measurements of parameters of interest in physical systems. 
     Measurement using elastically compressible capacitors enables high sensitivity and high signal-to-noise ratio. The time-based measurements are largely insensitive to most sources of error that may influence voltage or current driven sensing methods and devices. The resulting changes in the transit time of operation correspond to frequency, which can be measured rapidly, and with high resolution. This achieves the required measurement accuracy and precision thus capturing changes in the physical parameters of interest and enabling analysis of their dynamic and static behavior. 
     Furthermore, summing individual capacitive measurements before dividing to estimate the average measurement value data values produces superior results to averaging the same number of samples. The resolution of count data collected from a digital counter is limited by the resolution of the least significant bit in the counter. Capturing a series of counts and averaging them does not produce greater precision than this least significant bit that is the precision of a single count. Averaging does reduce the randomness of the final estimate if there is random variation between individual measurements. Summing the counts of a large number of measurement cycles to obtain a cumulative count then calculating the average over the entire measurement period improves the precision of the measurement by interpolating the component of the measurement that is less than the least significant bit of the counter. The precision gained by this procedure is on the order of the resolution of the least significant bit of the counter divided by the number of measurement cycles summed. 
       FIG. 31  illustrates a prosthetic component  3100  having a plurality of sensors in accordance with an example embodiment. In general, there is need for short-term intra-operative sensored prosthetic components that support the installation of a prosthetic joint and prosthetic components. Similarly, there is a need for the prosthetic joint to include sensors to monitor the joint long-term. Prosthetic component  3100  can be used as a trial prosthetic component or as a permanent prosthetic component for long-term use in the body. Prosthetic component  3100  is illustrated as a tibial prosthetic component in the example. Prosthetic component  3100  can be adapted for use in hip, knee, shoulder, spine, ankle, elbow, toe, hand, or bone implants. Prosthetic component  3100  comprises a structure  3102 , a structure  3104 , interconnect  3106 , load pads  3108 , and electronic circuitry  3110 . 
     Prosthetic component  3100  typically comprises a metal such as titanium, titanium alloy, cobalt, cobalt alloy, steel, or a steel alloy. The material is suitable for handling the loading produced by the muscular-skeletal system on the joint. Alternatively, the prosthetic component  3100  can be formed of a polymer material. One such suitable material is PEEK (polyether ether ketone). PEEK is a semi-crystalline thermoplastic that has high tensile strength and is resistant to thermal, aqueous, or biological degradation. PEEK can be molded to form the complex shapes required for a prosthetic component. PEEK is light-weight and can be fastened to bone by gluing. PEEK components can be welded together to form a hermetic seal. PEEK has a further benefit that it is transmissive to signals used for communication or for sensor detection. 
     Structure  3102  includes at least one support surface. As shown structure  3102  includes a support surface  3112  and a support surface  3114 . The support surfaces  3112  and  3114  receive an insert  3116 . Insert  3116  includes an articular surface  3118  and an articular surface  3120  that support movement of the joint. Articular surfaces  3118  and  3120  respectively overlie support surfaces  3112  and  3114 . A force, pressure, or load applied to articular surfaces  3118  and  3120  apply a corresponding force, pressure, or load to support surfaces  3112  and  3114 . A lightly loaded region resides between support surface  3112  and  3114 . In one embodiment, a housing  3122  is formed in structure  3102  in the lightly load region. Housing  3122  includes a cavity for receiving electronic circuitry  3110  that controls measurement activity of prosthetic component  3100 . 
     Structure  3104  includes at least one feature that couples to bone. In the example, the proximal end of the tibia is prepared to receive structure  3104 . A stem  3124  can be inserted into the medullary canal of the tibia. The stem  3124  aligns and supports structure  3104  to the tibia. Structure  3104  can be glued to the tibia to securely fasten prosthetic component  3100  in place. Alternatively, structure  3104  comprising PEEK or a metallic structure can include points supporting bone growth. Structure  3104  would include features that anchor bone and provide bone growth hormone. Bone can grow into and around the prosthetic component fusing structure  3104  to the tibia. Utilizing bone growth for fastening can also be used in conjunction with glue or other bonding agent. 
     In one embodiment, three sensors comprise a sensor array. There is a sensor array for each knee compartment. Each sensor array is used to measure the load and position of load of a knee compartment. An articular surface  3118  of insert  3116  corresponds to a first knee compartment of prosthetic component  3100 . Similarly, an articular surface  3120  of insert  3116  corresponds to a second knee compartment of prosthetic component  3100 . A force, pressure, or load applied to articular surfaces  3118  and  3120  is respectively applied to a support surface  3112  and a support surface  3114  of structure  3102 . The support surfaces  3112  and  3114  transfer the force, pressure, or load to a corresponding sensor array. The load pads  3108  are at predetermined locations corresponding to articular surfaces  3118  and  3120 . Load pads  3108  transfer the force, pressure, or load at the predetermined location to the underlying sensors for measurement. Thus, the force, pressure, or load magnitude and the position of applied force, pressure, or load can be calculated from measurements by the three sensors in the first and second knee compartments. The position of load can be translated back to position on articular surfaces  3118  and  3120 . The sensors overlie a support surface  3126  and a support surface  3128  of structure  3104 . Support surfaces  3126  and  3128  respectively correspond to the first and second knee compartments. In one embodiment support surfaces  3126  and  3128  are rigid under loading. 
     Sensors for measuring load can be devices such as an ultrasonic waveguide, piezo-resistive sensor, mems sensor, strain gauge, polymer sensor, mechanical sensor, and capacitive sensor. In the example, the form factor of prosthetic component  3100  limits the height of the sensor. In a passive prosthetic component (e.g. having no sensors) the structure is formed as a single device. The thickness of the support surfaces is approximately 2 millimeters. In general, the combined thickness of support surfaces  3112  and  3114  coupled to support surfaces  3126  and  3128  can be maintained at 2 millimeters or less with the sensor therebetween. Thus, the sensor requires a form factor that is substantially less than 2 millimeters thick. In one embodiment, the sensor is an elastically compressible capacitive sensor. The area of the sensor is determined by the load range to be measured and the compressible range where the sensor remains elastic. As disclosed hereinabove a measurement technique can be applied that is sensitive to small changes in capacitance that allows measurement accuracy, precision, and repeatability. In one embodiment, the elastically compressible capacitors are formed in the interconnect  3106 . 
     Electronic circuitry  3110  can be fitted in the cavity formed by housing  3122  of structure  3102 . In one embodiment, the cavity is formed in an unloaded or lightly loaded area of prosthetic component  3100 . The unloaded or lightly loaded region of housing  3122  is between the support surfaces  3112  and  3114 . Thus, electronic circuitry  3110  is protected from impact forces and loading that occurs under normal operation of the joint. Interconnect  3106  and the sensors therein couple to electronic circuitry  3110 . Interconnect  3106  include interconnect that couples the sensors to electronic circuitry  3110 . Cavities  3130  are formed on a surface of structure  3104 . Cavities  3130  support interconnect  3106  coupling from support surfaces  3126  and  3128  of structure  3104  to electronic circuitry  3110 . Cavities  3130  provide a pathway for interconnect  3106  into housing  3122 . 
     In general, structure  3102  couples to structure  3104  to form prosthetic component  3100 . In one embodiment, structures  3102  and  3104  are welded together around the periphery to form a hermetic seal. Electronic circuitry  3110 , sensors, and interconnect  3106  are housed within prosthetic component  3100  and hermetically sealed from an external environment. Alternatively, structure  3102  and  3104  can be glued or mechanically fastened together to maintain hermeticity. The structure  3102  and  3104  can further include a seal or O-ring that prevents the ingress or egress of gas, liquids, or solids. 
     Interconnect  3106  respectively couple to support surface  3126  and surface  3128  of structure  3104 . As mentioned previously, load pads  3108  couple each sensor to a respective location on support surface  3112  and support surface  3114 . In the example, load pads  3108  bound an area in each knee compartment that corresponds to articular surfaces  3118  and  3120  of insert  3116 . The force, pressure, or load applied to articular surface  3118  and  3120  is respectively transferred to surface  3112  and  3114  of structure  3102 . It should be noted that surface  3112  and surface  3114  are compliant and not rigid. Each surface has sufficient compliance that allows the underlying sensors to compress. In one embodiment, surface  3112  and surface  3114  is thinned or made thin to achieve compliance. The combined thickness of surfaces  3112  or  3114  of structure  3102  and surfaces  3126  or  3128  of structure  3104  can be approximately 2 millimeters. Surface  3112  and  3114  of structure  3102  can be less than 1 millimeter thick to be made compliant. Alternatively, support structures  3112  and  3114  can comprise a material that is compliant such as a polymer material. 
     In the example, the load applied to each sensor can be calculated. The load magnitude corresponds to the combination of the three individual measurements. The position of applied load can be calculated from the load magnitudes measured at the fixed positions of the sensors. Electronic circuitry  3110  includes multiple channels of input/output circuitry, timing circuitry, conversion circuitry, logic circuitry, power management circuitry, transmit and receive circuitry. Electronic circuitry  3110  can further include memory for storing software programs to operate or control a measurement process. In one embodiment, an ASIC is used to combine the analog and digital circuitry in a low power solution. The ASIC reduces the form factor of electronic circuitry  3110  allowing it to fit within the housing  3122  of structure  3102 . Electronic circuitry  3110  can include the circuitry described herein and the disclosures incorporated by reference. Electronic circuitry  3110  includes transmit circuitry and an antenna for transmitting data from the sensors to a remote system. Electronic circuitry  3110  can further include receive circuitry to receive information and programming instructions from the remote system. The remote system can be a portable device with a display for reporting the data. The remote system can transmit the data to a database for further review and analysis. 
       FIG. 32  illustrates a cross-sectional view of structure  3102  in accordance with an example embodiment. The cross-sectional view is of the lightly loaded area between the first and second knee compartments. The view includes a portion of housing  3122  overlying electronic circuitry  3110 . Housing  3122  protects and isolates electronic circuitry from an external environment. 
       FIG. 33  illustrates prosthetic component  3100  and insert  3116  in accordance with an example embodiment. Structure  3102  is coupled to structure  3104 . In one embodiment, a hermetic seal  3302  is formed that couples structures  3102  and  3104 . Structures  3102  and  3104  can have position and alignment features that support assembly. The periphery of structures  3102  and  3104  can be in proximity to one another around the entire perimeter. In one embodiment, structures  3102  and  3104  comprise an alloy of steel or titanium. A hermetic seal  3302  is formed by welding structure  3102  to structure  3104 . The weld is circumferential to prosthetic component  3100  sealing the sensors and electronic circuitry from an external environment. Welding joins the metals of structure  3102  to structure  3104  forming a contiguous structure. The sensors and electronic circuitry  3110  are isolated from an external environment completely enclosed within prosthetic component  3100 . The weld is formed whereby little or no pressure is applied to the sensors. Any offset due to coupling structures  3102  and  3104  can be compensated for during device calibration. Prosthetic component  3100  is suitable for use as a long-term implant for providing periodic data on joint status. A similar approach could be performed if the structures were formed of PEEK. Alternatively, other approaches using adhesives, mechanical coupling, and seals can be used to join structures  3102  and  3104  together to form a hermetic seal. 
     Insert  3116  fits into the tray of prosthetic component. The tray of prosthetic component  3100  can have one or more features for retaining insert  3116 . Insert  3116  typically comprises a polymer material such as ultra-high molecular weight polyethylene. Articular surfaces  3118  and  3120  interface with another prosthetic component (not shown) of the joint. In the example, articular surfaces  3118  and  3120  would interface with the condyle surfaces of a femoral prosthetic component. Muscles, tendons, and ligaments motivate the prosthetic joint whereby articular surfaces  3118  and  3120  allow movement of the components in relation to one another. Insert  3116  can be a passive component or include one or more sensors. 
       FIG. 34  illustrates electronic circuitry  3110  coupled to interconnect  3106  in accordance with an example embodiment. Electronic circuitry  3110  can include one or more connectors for coupling to interconnect  3106 . In one embodiment, the sensors are elastically compressible capacitive sensors. The capacitors are formed underlying load pads  3108  in interconnect  3106 . Referring briefly to  FIGS. 21-25 , the sensor structure is described. Load pads  3108  can comprise a non-conductive material or a conductive material. In the example, load pads  3108  are rigid and non-compressible to transfer the force, pressure, or load to the underlying capacitor. A non-conductive load pad can comprise a polymer material. In one embodiment, load pads  3108  comprise a conductive metal such as copper or copper alloy that is plated onto the surface of interconnect  3106 . The conductive load pads  3108  electrically couple to the underlying plate of the capacitor. 
     In one embodiment, the capacitors can be formed on or in a flexible polyimide substrate. The load pads, capacitors, and interconnect can be formed accurately and repeatably using lithographic techniques. The polyimide substrate can be made very thin suitable for fitting within a prosthetic component. The capacitor is operated within a range where it is elastically compressible. Each capacitor underlying load pads  3108  are similar to capacitor  2100  of  FIG. 21 . Capacitor  2100  comprises 3 capacitors mechanically in series and 2 capacitors electrically in parallel. The force, pressure, or load is applied across capacitors  2204 ,  2206 , and  2208 . In one embodiment, capacitor  2204  is not electrically in the circuit because both plates of capacitor  2204  are coupled in common. Electrically, the sensor capacitor comprises capacitors  2206  and  2208  that are electrically coupled in parallel. In one embodiment, a plate of capacitor  2206  and a plate of capacitor  2208  are coupled to ground. The grounded plates  2112  and  2116  are respectively between an external environment of the interconnect and plates  2108  and  2110 . Similarly, the interconnect from capacitor  2100  to the electronic circuitry has a similar topology. Grounded interconnects  2124  and  2126  are between the external environment and the signal carrying interconnect  2122  that couples to plates  2108  and  2110 . Thus, parasitic coupling is minimized by the shield. Furthermore, any parasitic capacitance is constant and not variable. 
     The capacitive magnitude and changes in magnitude can be accurately measured using the circuitry and method disclosed in  FIGS. 18 and 19 . Referring briefly to  FIGS. 33 and 34 , a force, pressure, or load is applied to articular surface  3118  and  3120 . The force, pressure, or load is transferred from articular surfaces  3118  and  3120  respectively to support surfaces  3112  and  3114 . As mentioned previously, support surfaces  3112  and  3114  are compliant such that the force, pressure, or load is transferred through load pads  3108  to the underlying sensors. The sensors are supported by support surfaces  3126  and  3128 , which are rigid and non-compliant. A force, pressure, or load applied to a sensor capacitor compresses the structure. The dielectric layer between the capacitor plates is compressed. The capacitance value of the sensor capacitor is related to the thickness of the dielectric layer. Thus, measuring the capacitance and changes in capacitance can be related to the force, pressure, or load applied thereto. 
     A repeating signal is applied to the sensor capacitor. In general, the sensor is charged and discharged between predetermined voltage levels within a time period. The time period of a single waveform of the repeating signal is a measurement cycle. The time period of the measurement cycle corresponds to the capacitance of the capacitor. The waveform or signal is repeated a predetermined number of times. A measurement sequence comprises the repeated waveform for the predetermined number of times. An elapsed time of the measurement sequence is measured. The elapsed time is the time required to generate the predetermined number of waveforms. The force, pressure, or load is maintained during the measurement sequence. The measured elapsed time of the sensor capacitor is correlated to the force, pressure, or load measurement. The relationship between capacitance and force, pressure, or load is known. In one embodiment, each capacitive sensor can be measured against known force, pressure, or load values after assembly of the prosthetic component. The measurements can be stored in memory that is part of the electronic circuitry housed in the prosthetic component. Further refinement can be achieved by using calibration techniques or algorithms during final testing of each capacitor that can take into account interpolation between measurements and non-linear compression of the dielectric. The measurement resolution can be increased by this technique if the force, pressure, or load is substantially constant over the increased number of predetermined number waveforms. Moreover, the resolution supports measurement where the capacitance changes are relatively small over the force, pressure, or load measurement range. 
       FIG. 35  illustrates an assembled prosthetic component  3100  in accordance with an example embodiment. Prosthetic component  3100  comprises structure  3102  coupled to structure  3104 . Prosthetic component  3100  houses electronic circuitry and sensors. A hermetic seal  3302  couples the structure  3102  to structure  3104 . In one embodiment, hermetic seal  3302  is a contiguous weld around the periphery. As mentioned, weld does not load or lightly loads the sensors underlying the support surfaces. In the example, prosthetic component  3100  is a tibial prosthetic component. Structure  3102  includes a tray for receiving an insert having at least one articular surface. The tibial prosthetic component can be a single or dual compartment device. Structure  3104  includes a stem  3124  for coupling to bone. In the example, stem  3124  couples to the tibia. 
       FIG. 36  illustrates a partial cross-sectional view of prosthetic component  3100  in accordance with an example embodiment. The cross-sectional view is in a region near the periphery where hermetic seal  3302  couples structures  3102  and  3104  together. In the example, a sensor  2100  is included. The sensor  2100  is formed in interconnect  3106 . A load pad  3108  is formed on sensor  2100 . Interconnect  3106  further illustrates shielding of the sensor to minimize signal coupling and parasitic capacitance. 
     The cross-section illustrates placement of sensor  2100  of prosthetic component  3100  for load sensing. Support surface  3128  of structure  3104  supports sensor  2100 . In the example, support surface  3128  is rigid. Conductive region  2116  is a plate of the capacitor formed in interconnect  3106 . Interconnect  2126  couples conductive region  2116  to the electronic circuitry  3110 . Conductive region  2116  and interconnect  2126  couples to support surface  3128 . In the example, conductive region  2116  and interconnect  2126  are coupled to ground. Conductive region  2116  acts as a shield to prevent signal or parasitic coupling to conductive regions  2110  and  2108  of sensor  2100 . Similarly, interconnect  2126  acts as a shield for interconnect  2124  to prevent signal or parasitic coupling. In one embodiment, support surface  3128  comprises a conductive material such as metal. Thus, structure  3104  is coupled to ground by way of conductive region  2116  and interconnect  2126 . Structure  3104  acts as a shield for preventing signal or parasitic coupling to the capacitive sensors. 
     Support surface  3114  of structure  3102  is supported by load pad  3108  and sensor  2100 . Load pad  3108  distributes the load to sensor  2100 . Support surface  3114  is compliant to loading placed thereon. In one embodiment, support surface  3114  made thin to allow flexing. In general, support surface  3114  deflects a short distance over the entire load range. Sensor  2100  can elastically compress approximately 20% of the total dielectric thickness. In one embodiment, compression of sensor  2100  is limited to 10% or less of the total dielectric thickness. For example, a capacitor as disclosed herein can compress approximately 0.00254 millimeters over the load range of a typical prosthetic component load sensor. In one embodiment, a stack three capacitive sensors in series, lamination material, and insulating material would yield a total compression under maximum loading of approximately 0.0076 millimeters. Thus, support surfaces  3112  or  3114  do not flex significantly over the entire load range. Load pad  3108  couples to a known location on support surface  3114 . The known location also relates to a point on the articular surface of the insert. The known location of each of the sensors is used to determine where the load is coupled to the articular surface by comparing the measured load magnitudes. Although a single sensor is shown, the other sensors formed in interconnect  3106  are similarly coupled to structures  3102  and  3104 . Hermetic seal  3302  couples structures  3102  and  3104  together. Hermetic seal  3302  can be a weld that melts and joins the material of structures  3102  and  3104 . 
     Conductive region  2112  is a plate of the capacitor formed in interconnect  3106 . Interconnect  2124  couples conductive region  2112  to electronic circuitry  3110 . In the example, conductive region  2112  and interconnect  2124  are coupled to ground. Conductive region  2108  and conductive region  2110  are plates of the capacitor formed in the interconnect  3106 . Conductive regions  2108  and  2110  of sensor  2100  are coupled in common by via  2120 . Interconnect  2122  couples the conductive regions  2108  and  2110  to the electronic circuitry. Interconnect  2122  carries a signal from the electronic circuitry to sensor  2100  to measure the capacitor. Conductive region  2112  is separated from conductive region  2108  by dielectric layer  2104 . Similarly, conductive region  2116  is separated from conductive region  2110  by dielectric layer  2106 . Conductive regions  2108  and  2110  are separated by a dielectric layer  2102  but as mentioned previously are coupled in common. In one embodiment, dielectric layers  2102 ,  2104 , and  2106  comprise polyimide. Other dielectrics such as silicon dioxide, silicon nitride, mylar, and other polymers can be used. Interconnect  3106  and sensor  2100  can be formed by deposition, plating, and lithographic techniques on the substrate. 
     The capacitor of sensor  2100  comprises three capacitors mechanically in series. A force, pressure, or load applied to support surface  3114  compresses the three capacitors. A first capacitor comprises conductive region  2112 , dielectric layer  2104 , and conductive region  2108 . A second capacitor comprises conductive region  2108 , dielectric layer  2102 , and conductive region  2110 . A third capacitor comprises conductive region  2108 , dielectric layer  2106 , and conductive region  2116 . Electrically, the capacitor of sensor  2100  comprises the first and third capacitors coupled in parallel. The first and third capacitors have conductive regions  2108  and  2110  coupled in common. Similarly, conductive regions  2112  and  2116  of the first and third capacitors are coupled to ground. Conductive region  2112  and  2116  respectively shield conductive region  2108  and  2110  from coupling and parasitic capacitance external to interconnect  3106 . Similarly, interconnect  2124  and  2126  shield interconnect  2122  from signal coupling and parasitic capacitance external to interconnect  3106 . 
     Structures  3102  and  3104  can comprise a conductive material. For example, titanium, cobalt, and steel alloys are conductive materials used to manufacture prosthetic component  3100 . Placing interconnect  3106  on support surface  3128  couples conductive region  2116  and interconnect  2126  to structure  3104 . Conductive region  2116  and support surface  3128  are coupled in common to ground. Similarly, load pad  3108  can comprise a conductive material. In one embodiment, a material such as copper or copper alloy can be deposited or plated to the surface of interconnect  3106 . Load pad  3108  is coupled to conductive region  2112  and interconnect  2124 . Support surface  3114  is coupled to conductive region  2112  and interconnect  2124  by load pad  3108 . As mentioned previously, conductive region  2112  and interconnect  2124  are coupled to ground. Thus, structure  3102  and  3104  are coupled to ground. Alternatively, structures  3102  and  3104  can be coupled to ground via an alternate path other than sensor  2100 . In one embodiment, the electronic circuitry and sensor  2100  are housed in prosthetic component  3100 . Structures  3102  and  3104  form a shield that isolates electronic circuitry  3110  and sensor  2100  from parasitic coupling and parasitic capacitance in the external environment. The design further incorporates the internal shields built into the capacitor that prevents or minimizes parasitic coupling and parasitic capacitance external to interconnect  3106 . Although a capacitive sensor is used in the example, the load sensor in prosthetic component  3100  can comprises one of a strain gauge, mems device, piezo-resistive sensor, mechanical sensor, polymer sensor, and ultrasonic sensor. 
       FIG. 37  illustrates structure  3102  in accordance with an example embodiment. Structure  3102  of prosthetic component  3100  when installed in a joint region of the patient includes at least one region having exposure external to the joint. The view shows housing  3122  of structure  3102  that includes a transmissive region  3702 . In one embodiment, transmissive region  3702  comprises glass, PEEK, plastic, or polymer. Transmissive region  3702  can be bonded to an opening in a wall of housing  3122  that comprises a steel alloy, titanium, cobalt, an alloy, or metal. In one embodiment, housing  3122  houses electronic circuitry. Alternatively, part of or all of structure  3102  can comprise a polymer such as PEEK, which is transmissive to some of the spectrum. In one embodiment, transmissive region  3702  is transmissive to sensor signals and communication signals. For example, signals can be blocked when structure  3102  comprises a conductive material and the conductive material is grounded. Prosthetic component  3100  can act as a shield to the electronic circuitry and sensors housed within the device. Transmissive region  3702  can be transmissive to signals such as acoustic, ultrasonic, radio frequency, infrared, and light. Transmissive region  3702  has exposure to regions around and in proximity to the joint region. In one embodiment, window  3702  can be used to monitor the synovial fluid that resides in and around the joint. 
     Sensors can also be located at or near transmissive region  3702 . The sensors can be mounted with electronic circuitry  3110 . Electronic circuitry  3110  can comprise one or more pc boards having interconnect and connectors. Integrated circuits, ASIC devices, a power source, communication circuitry, digital logic, converters, power management, and other systems can be coupled together in a small form factor. In one embodiment, an ASIC combines many of the features to minimize form factor and to lower power consumption. Sensors and communication circuitry are located on electronic circuitry  3110  in proximity to transmissive region  3122  allowing transmission and reception of signals. A directional antenna can be placed in proximity to transmissive region  3702  to send and receive information to a remote system. 
     In general, sensors can be used to monitor the synovial fluid that is in proximity to the joint region. Synovial fluid is a natural lubricant found in a muscular-skeletal joint. Synovial fluid is found in joints such as the elbow, knee, shoulder, hip and others. Synovial fluid comprises mucin, albumin, fat, epthelium, and leukocytes. The lubricant also nourishes the avascular articular cartilage. Synovial fluid cushions joint impact and reduces friction as bone and cartilage contact one another over the range of motion. Synovial fluid can also carry oxygen and other nutrients to cartilage and other areas of the joint. Similarly, synovial fluid acts as transport to remove waste materials from the joint region. The synovial fluid remains in and around the joint. The synovial fluid can be retained by a synovial membrane that holds the lubricant in place. 
     There is a strong correlation between the health of a joint and the condition of the synovial fluid. Sensors that measure temperature, pH, color, turbidity, viscosity, glucose, and proteins can be used to analyze synovial fluid. The sensors can be used individually or in concert with one another to determine joint health. Prosthetic component  3100  includes one or more of the sensors for monitoring the joint. In the example, the joint is monitored for infection. Infection in a newly implanted joint is a critical problem. It is often difficult for a patient with a joint implant to determine if he or she has an infection. The surgery itself and joint rehabilitation can mask early signs of an infection. The prosthetic joint is an ideal place for an infection to grow without abatement. There are areas in the prosthetic joint that are isolated but have nutrients that can harbor bacteria and foster growth. Infection can lead to a substantial health risk, anti-biotic treatment, increased rehabilitation, long-term hospitalization, and substantial cost. If the infection is significant there is a scenario that requires the removal of the prosthetic joint. The patient is immobilized until the infection subsides and then a new prosthetic joint is implanted. The patient trauma under such circumstances can be significant. Prosthetic joint  3100  can detect infection local to the joint, notify a doctor or healthcare provider, or take appropriate action in a timely manner. 
     In one embodiment, temperature can be monitored. A temperature sensor  3704  can be mounted in proximity to transmissive region  3702 . Temperature sensor  3704  is coupled to electronic circuitry  3110  for receiving temperature data. In one embodiment, electronic circuitry  3110  has multiple I/O channels for coupling to sensors. Temperature sensor  3704  monitors the temperature of the joint. In one embodiment, temperature sensor  3704  measures the temperature of the synovial fluid. Measurements of the synovial fluid can occur periodically. 
     A temperature difference can be detected between a healthy knee and an infected knee. In the example, temperature sensor  3704  is calibrated to a normal temperature of the synovial fluid. The calibrations can occur periodically because the normal temperature will change depending on the patient condition. The absolute temperature and changes in temperature are monitored. A change in temperature from the norm can be an indication of an infection. In the example, temperature sensor can be a MEMS sensor, a thermocouple, thermistor or other temperature measuring device. 
     In one embodiment, pH can be monitored. A pH sensor  3706  can be mounted in proximity to transmissive region  3702  and coupled to electronic circuitry  3110  for receiving pH data. Similar to temperature, pH sensor  3706  can be initially calibrated to the normal pH and recalibrated periodically. A lower pH than the norm can indicate the presence of an infection. Measurement of absolute pH and differential pH over time can be used to detect an increase in bacteria. In general, a healthy knee has a pH of approximately 7.23. An infected knee has a pH of approximately 7.06. The device can be calibrated for specifics of an individual patient. The pH sensor can be a MEMS pH sensor, an implantable pH microsensor, electro-static pH sensor, or other pH measuring device. 
     In one embodiment, turbidity and color can be monitored. Turbidity is a measure of the cloudiness or haze due to the suspension of particles within a fluid. For example, synovial fluid becomes turbid as an infection grows. Bacteria, bacterial waste products, and white blood cells are but a few of the particulates that can be suspended in the synovial fluid. The turbidity increases as the infection worsens due to increased bacterial growth. Similarly, the color of the synovial fluid changes as an infection increases. For example, healthy synovial fluid is a relatively clear fluid. The synovial fluid changes color as the joint status changes from healthy to non-inflammatory, non-inflammatory to inflammatory, and inflammatory to septic. A non-inflammatory synovial fluid is a yellowish clear liquid that is indicative of joint related problems such as osteoarthritis. The synovial fluid will be viscous retaining its lubricating and damping properties. An inflammatory synovial fluid is yellowish in color. The inflammatory synovial fluid is hazy and not clear. It will also have lost some of its viscous properties having a watery consistency. The inflammatory synovial fluid can indicate problems such as rheumatoid arthritis or infection. Septic synovial fluid can be dark yellow to red in color. Moreover, septic synovial fluid is opaque. The synovial fluid can contain high counts of bacteria, fungus, white blood cells, and red blood cells. Measuring color, turbidity, or a combination of both can be used to determine joint health. 
     In the example, optical sensors such as a LED  3708  (light emitting diode) and photo-diode array  3710  can be used to measure color and turbidity. In one embodiment, the LED  3708  and photo-diode array  3710  are positioned behind transmissive region  3702 . LED  3708  and photo-diode array  3710  are housed within prosthetic component  3100  and can couple to or be part of electronic circuitry  3110 . As previously mentioned, transmissive region  3702  can be glass that is transmissive to light. LED  3708  can transmit white light directly to a photo-diode. The photo-diode can be part of photo-diode array  3710  or a separate device. The photo-diode can be used for calibration of LED  3708  and for detecting changes in the light or intensity output by the device. LED  3708  also illuminates a sample of synovial fluid. As shown, light emitted by LED  3708  is transmitted through transmissive region  3702  into the synovial fluid in proximity to prosthetic component  3100 . In one embodiment, three photo-diodes respectively having red, green, and blue optical filters detect light transmitted through the synovial fluid. Each photo-diode measures the relative contribution of red, green, and blue. The contribution can be ratiometrically compared with a calibration value corresponding to a measurement by the calibration photo-diode. The calibration value corresponds to the sum of red, green, and blue components of white light. More than one transmissive region can be used to send and receive light. Also, one or more barriers or transmissive regions can be used to direct the light into the synovial fluid and prevent direct light from LED  3708  from radiating onto photo-diode array  3710 . 
     Equations for the measurement can be as follows:
 
 r =red, g =green, b =blue, c =calibration  a)
 
Color=[ r,g,b ]/( r+g+b )  b)
 
Turbidity=( r+g+b )/3 c   c)
 
     The color measured by photo-diode array  3710  can be compared to known infection color data. Similarly, the turbidity measurements by photo-diode array  3710  can be compared against known turbidity color data. Both color and turbidity measurements can be taken by prosthetic component  3100 . Using both measurements in combination can provide data that allows further refinement of the prognosis thereby providing a better assessment and treatment methodology. Furthermore, taking periodic measurements and comparing the color and turbidity measurements can yield a rate of change. The rate of change can be used to determine if the infection is increasing or declining. Comparing measurements over time can be used to determine if the infection treatment is successful. Placing sensors in the prosthetic component has substantial benefits in preventing infection. Statistically most infections occur shortly after the joint implant or within the first few months after surgery. Infection is less likely to occur after the surgical wound has healed and rehabilitation of the joint has taken place. Pain due to the surgery and during rehabilitation can also mask infection symptoms. If an infection occurs, it will start as a local infection in proximity to the joint. A first benefit is that prosthetic component  3100  can identify an infection that is local to the joint before it has spread throughout the body. A second benefit is that treatment of the infection can be local to the joint region. A third benefit is that prosthetic component  3100  can also include an antibiotic that could be released in proximity to the joint. A fourth benefit is that prosthetic component  3100  can be in communication with a remote system and a database. The remote system can be provide notification to the patient to see a doctor. The remote system can also provide data to the doctor for analysis and treatment. 
     A method of long-term joint monitoring is disclosed using prosthetic component  3100 . The method can be practiced with more or less than the steps shown, and is not limited to the order of steps shown. The method is not limited to the example tibial prosthetic component example but can be used for hip, shoulder, ankle, elbow, spine, hand, foot, and bone. In a first step, electronic circuitry and one or more sensors are housed in a prosthetic component. In a second step, characteristics of synovial fluid are periodically measured in proximity to the prosthetic component. The characteristic can be used to determine the presence of an infection or other problem. Examples of measured characteristics are temperature, pH, color, turbidity, viscosity, glucose levels, and proteins. In a third step, measurements are compared. In one embodiment, measurements compared against one another to determine if a change has occurred. Furthermore, multiple measurements made over time can indicate a trend. In another embodiment, the measured characteristics can be compared against known or predetermined values that relate to infection or other problem being identified. 
     In a fourth step, a color of the synovial fluid is measured. In a fifth step, the color of the synovial fluid is compared against a known color range. In a sixth step, it can be determined if an infection is present. In one embodiment, the comparison yields a color similar to a known synovial fluid color. For example, clear synovial fluid is normal. A clear yellow synovial fluid can indicate inflammation and other problems. A hazy yellow synovial fluid can indicate the presence of bacteria or other problems. A synovial fluid having a red tint can indicate sepsis and blood in the synovial fluid. 
     In a seventh step, the relative contributions of red, green, and blue colors are measured. In an eighth step, a contribution of each color is ratiometrically compared to a sum of the relative contributions. A color of the synovial fluid can be determined by assessing the contributions of red, green, and blue colors. In a ninth step, a rate of change in color is determined. The rate of change in color can be used to determine the status of an infection. For example, once an infection is detected the rate of change corresponds to a decrease or increase in the infection. It can also be used to determine the effectiveness of treatment. After treatment the rate of change should indicate a decrease in the infection. 
     A method of long-term joint monitoring is disclosed using prosthetic component  3100 . The method can be practiced with more or less than the steps shown, and is not limited to the order of steps shown. The method is not limited to the example tibial prosthetic component example but can be used for hip, shoulder, ankle, elbow, spine, hand, foot, and bone. In a first step, electronic circuitry and one or more sensors are housed in a prosthetic component. In a second step, a turbidity of synovial fluid is periodically measured in proximity to the prosthetic component. The turbidity can be used to determine the presence of an infection or other problem. Examples of other measured characteristics are temperature, pH, color, turbidity, viscosity, glucose levels, and proteins. In a third step, the turbidity measurements are compared to known turbidity measurements or a predetermined turbidity range. In one embodiment, comparing the periodic measurements determine if a change has occurred. Furthermore, multiple turbidity measurements taken over time can indicate a trend. In another embodiment, the measured characteristics can be compared against known or predetermined turbidity values that relate to infection or other problem being solved. In a fourth step, it can be determined if an infection is present. Turbidity is a measure of the cloudiness or haziness of a substance. For example, healthy synovial fluid is clear. Conversely, infected synovial fluid is hazy or cloudy due to the presence of bacteria. Moreover, the severity of the infection can be related to the number of particulates in the synovial fluid. The higher the number of particulates the worse the infection can be. 
     In a fifth step, the turbidity is compared against previous turbidity measurements. In a sixth step, a rate of change in turbidity is determined. In general, if the turbidity increases the infection or problem is worsening because healthy synovial fluid is clear. Alternatively, if treatment has been provided and the turbidity over time is decreasing than the patient health is improving. In a seventh step, data is wirelessly transmitted to a remote system. In one embodiment, the remote system is in proximity to the prosthetic component due to the limited range of transmission. The remote system can include a processor and graphic processor. In an eighth step, light is received through a transmissive region of the prosthetic component. Light is transmitted into the synovial fluid in proximity to the prosthetic component. The light illuminates the synovial fluid that is detected by a photo-diode array. Each diode of the photo-diode array can have a filter for filtering the incoming light through the transmissive region of the prosthetic component. 
       FIG. 38  illustrates prosthetic component  3100  and a remote system  3802  in accordance with an example embodiment. Remote system  3802  can be equipment, a tool, a computer, a note pad, a cell phone, a smartphone, or medical device. Data transmitted from prosthetic component  3100  is received by remote system  3802 . Similarly, remote system  3802  can transmit information to prosthetic component  3100  that supports operation and sensor measurement. Remote system  3802  can include logic circuitry, microprocessor, microcontroller, or digital signal processor. In the example, remote system  3802  is a laptop computer with a display. Remote system  3802  can include software for analyzing quantitative measurement data from prosthetic component  3100  and displaying the information for assessment. Remote system  3802  includes transmit circuitry, receive circuitry, or both for coupling to electronic circuitry  3110  of prosthetic component  3100 . Similarly, electronic circuitry  3110  includes transmit circuitry, receive circuitry, or both. In the example, electronic circuitry includes an ASIC having transmit and receive circuitry. In one embodiment, transmit and receive circuitry transmits through transmissive region  3702 . Alternatively, other transmissive regions can be added to prosthetic component  3100  for supporting antenna placement. Also, prosthetic component  3100  can be made from a polymer such as PEEK that allows transmission and reception of signals. In one embodiment, transmission of data to remote system  3802  is short range. The transmission range is typically less than 10 meters. In an installed prosthetic component, the RF transmission is made through tissue. The short transmission distance reduces un-authorized reception of data. In one embodiment, the data transmission is encrypted for security. The data can be decrypted by remote system  3802 . 
     In the example, housing  3122  includes electronic circuitry  3110  and a window  3702 . Window  3702  can be transmissive to signals such as acoustic, ultrasonic, radio frequency, infrared, and light. Window  3702  can comprise glass that is bonded to the steel, titanium, cobalt, alloy, or metal of the prosthetic component. Alternatively, part of or all of structure  3102  can comprise a plastic or a polymer such as PEEK, which is transmissive to some of the spectrum. Window  3702  is not blocked by other components of the prosthetic joint and has exposure to regions around and in proximity to the joint region. In one embodiment, window  3702  can be used to monitor a region in proximity to the prosthetic joint. Similarly, sensors can be fastened to structure  3102  or  3104  and exposed to the region. Window  3702  can be used to measure one or more parameters that relate to the health of synovial fluid. In the example, optical sensors are used to measure color and turbidity. Electronic circuitry  3110  couples to each of the sensors. In one embodiment, a channel is assigned to each sensor. The channels can be operated serially or in parallel. Logic circuitry in electronic circuitry  3110  controls when measurements are taken. The measurement data can be stored in memory on electronic circuitry  3110  until transmitted. The measurement data can be converted to a digital format. The quantitative parameter measurements can be used individually or in combination to determine a health issue. 
     A method of long-term joint monitoring is disclosed using prosthetic component  3100 . The method can be practiced with more or less than the steps shown, and is not limited to the order of steps shown. The method is not limited to the example tibial prosthetic component example but can be used for hip, shoulder, ankle, elbow, spine, hand, foot, and bone. In a first step, electronic circuitry and one or more sensors are housed in a prosthetic component. In a second step, synovial fluid in proximity to the prosthetic component is monitored. In a third step, a characteristic of the synovial fluid is measured. Examples of characteristics being measured are temperature, pH, color, turbidity, viscosity, glucose levels, and proteins. In a fourth step, data is sent to a remote system. The data can be wirelessly transmitted from the prosthetic component to the remote system. The remote system can include digital logic, a processor, a digital signal processor, a graphic processor, communication circuitry, or analog circuitry. In one embodiment, the transmission can be less than 10 meters due to power constraints of the signal and the medium in which it travels. For example, the transmission has to be sent through the multiple layers of tissue between the prosthetic component and the external environment. 
     In a fourth step, the data sent by the prosthetic component can be analyzed. The data can be analyzed by the remote system. The data can also be sent to other equipment, devices, computers, or a database. The data can be combined with other information or data to create a clinical database related to a study of the joint or prosthetic system. In a fifth step, a report is generated. The report is based on quantitative data provided by the sensors in the prosthetic component. In a sixth step, the report is sent to at least one entity. In general, the report uses quantitative data generated by the sensors in the prosthetic component. In the example, the sensor data can be an analysis of the synovial fluid in proximity to the joint. The report can lead to an action being taken. For example, detecting an infection or a condition such as arthritis can lead to treatment. The sensors can be used to monitor progress of the treatment. In a seventh step, temperature of the synovial fluid can be measured. In an eighth step, pH of the synovial fluid can be measured. In a ninth step, the color or turbidity of the synovial fluid can be measured. The report can be as simple as a status update on the sensor data to the patient or a detailed listing of all the parameters measured, trends, and analysis of the data sent to a health care provider such as a doctor, surgeon, or hospital. The entity can be broadly interpreted as anything or anybody that has rights to use the information. The report can be encrypted to maintain privacy of the information. Similarly, the sensor data can also include the load and position of load data. This sensor data can be used to address kinematic issues regarding the joint and how the patient is adapting to the prosthesis. 
       FIG. 39  is an illustration of electronic circuitry  3110  and structure  3104  in accordance with an example embodiment. Structure  3104  is a component of prosthetic component  3100  disclosed herein. Structure  3104  can includes a cavity  3902  for housing electronic circuitry  3110 . Electronic circuitry  3110  is placed vertically into cavity  3902 . Cavity  3902  extends into stem  3124  of structure  3104 . In general, the electronic circuitry can be housed within structure  3102 ,  3104 , or both. 
       FIG. 40  is an illustration of electronic circuitry  3110  and structure  3104  in accordance with an example embodiment. Structure  3104  can include a cavity  4002  for housing electronic circuitry  3110 . Electronic circuitry  3110  is placed horizontally into cavity  4002 . Cavity  4002  is centered between interconnect  3106  in a lightly loaded region of prosthetic component  3100 . Sensors such as temperature, pH, optical, glucose, and others can be mounted in housing  3122  and coupled to electronic circuitry  3110 . Cavity  4002  underlies housing  3122  and provides room to accommodate sensors for measuring in proximity to prosthetic component  3100 . Interconnect  3106  overlies support surface  3126  and  3128 . Each interconnect  3106  includes a sensor array and corresponds to a compartment of the knee. Sensors underlie load pad  3108  of interconnect  3106  for measuring a force, pressure, or load. Electronic circuitry  3110  can include accelerometers for providing positioning information of the joint. 
       FIG. 41  depicts an exemplary diagrammatic representation of a machine in the form of a system  4100  within which a set of instructions, when executed, may cause the machine to perform any one or more of the methodologies discussed above. In some embodiments, the machine operates as a standalone device. In some embodiments, the machine may be connected (e.g., using a network) to other machines. In a networked deployment, the machine may operate in the capacity of a server or a client user machine in server-client user network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. 
     The machine may comprise a server computer, a client user computer, a personal computer (PC), a tablet PC, a laptop computer, a desktop computer, a control system, logic circuitry, a sensor system, an ASIC, an integrated circuit, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. It will be understood that a device of the present disclosure includes broadly any electronic device that provides voice, video or data communication. Further, while a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. 
     System  4100  may include a processor  4102  (e.g., a central processing unit (CPU), a graphics processing unit (GPU, or both), a main memory  4104  and a static memory  4106 , which communicate with each other via a bus  4108 . System  4100  may further include a video display unit  4110  (e.g., a liquid crystal display (LCD), a flat panel, a solid state display, or a cathode ray tube (CRT)). System  4100  may include an input device  4112  (e.g., a keyboard), a cursor control device  4114  (e.g., a mouse), a disk drive unit  4116 , a signal generation device  4118  (e.g., a speaker or remote control) and a network interface device  4120 . 
     The disk drive unit  4116  can be other types of memory such as flash memory and may include a machine-readable medium  4122  on which is stored one or more sets of instructions (e.g., software  4124 ) embodying any one or more of the methodologies or functions described herein, including those methods illustrated above. Instructions  4124  may also reside, completely or at least partially, within the main memory  4104 , the static memory  4106 , and/or within the processor  4102  during execution thereof by the system  4100 . Main memory  4104  and the processor  4102  also may constitute machine-readable media. 
     Dedicated hardware implementations including, but not limited to, application specific integrated circuits, programmable logic arrays and other hardware devices can likewise be constructed to implement the methods described herein. Applications that may include the apparatus and systems of various embodiments broadly include a variety of electronic and computer systems. Some embodiments implement functions in two or more specific interconnected hardware modules or devices with related control and data signals communicated between and through the modules, or as portions of an application-specific integrated circuit. Thus, the example system is applicable to software, firmware, and hardware implementations. 
     In accordance with various embodiments of the present disclosure, the methods described herein are intended for operation as software programs running on a computer processor. Furthermore, software implementations can include, but not limited to, distributed processing or component/object distributed processing, parallel processing, or virtual machine processing can also be constructed to implement the methods described herein. 
     The present disclosure contemplates a machine readable medium containing instructions  4124 , or that which receives and executes instructions  4124  from a propagated signal so that a device connected to a network environment  4126  can send or receive voice, video or data, and to communicate over the network  4126  using the instructions  4124 . The instructions  4124  may further be transmitted or received over a network  4126  via the network interface device  4120 . 
     While the machine-readable medium  4122  is shown in an example embodiment to be a single medium, the term “machine-readable medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term “machine-readable medium” shall also be taken to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by the machine and that cause the machine to perform any one or more of the methodologies of the present disclosure. 
     The term “machine-readable medium” shall accordingly be taken to include, but not be limited to: solid-state memories such as a memory card or other package that houses one or more read-only (non-volatile) memories, random access memories, or other re-writable (volatile) memories; magneto-optical or optical medium such as a disk or tape; and carrier wave signals such as a signal embodying computer instructions in a transmission medium; and/or a digital file attachment to e-mail or other self-contained information archive or set of archives is considered a distribution medium equivalent to a tangible storage medium. Accordingly, the disclosure is considered to include any one or more of a machine-readable medium or a distribution medium, as listed herein and including art-recognized equivalents and successor media, in which the software implementations herein are stored. 
     Although the present specification describes components and functions implemented in the embodiments with reference to particular standards and protocols, the disclosure is not limited to such standards and protocols. Each of the standards for Internet and other packet switched network transmission (e.g., TCP/IP, UDP/IP, HTML, HTTP) represent examples of the state of the art. Such standards are periodically superseded by faster or more efficient equivalents having essentially the same functions. Accordingly, replacement standards and protocols having the same functions are considered equivalents. 
       FIG. 42  is an illustration of a communication network  4200  for measurement and reporting in accordance with an exemplary embodiment. Briefly, the communication network  4200  expands broad data connectivity to other devices or services. As illustrated, the measurement and reporting system  4255  can be communicatively coupled to the communications network  4200  and any associated systems or services. 
     As one example, measurement system  4255  can share its parameters of interest (e.g., angles, load, balance, distance, alignment, displacement, movement, rotation, and acceleration) with remote services or providers, for instance, to analyze or report on surgical status or outcome. This data can be shared for example with a service provider to monitor progress or with plan administrators for surgical monitoring purposes or efficacy studies. The communication network  4200  can further be tied to an Electronic Medical Records (EMR) system to implement health information technology practices. In other embodiments, the communication network  4200  can be communicatively coupled to HIS Hospital Information System, HIT Hospital Information Technology and HIM Hospital Information Management, EHR Electronic Health Record, CPOE Computerized Physician Order Entry, and CDSS Computerized Decision Support Systems. This provides the ability of different information technology systems and software applications to communicate, to exchange data accurately, effectively, and consistently, and to use the exchanged data. 
     The communications network  4200  can provide wired or wireless connectivity over a Local Area Network (LAN)  4201 , a Wireless Local Area Network (WLAN)  4205 , a Cellular Network  4214 , and/or other radio frequency (RF) system (see  FIG. 4 ). The LAN  4201  and WLAN  4205  can be communicatively coupled to the Internet  4220 , for example, through a central office. The central office can house common network switching equipment for distributing telecommunication services. Telecommunication services can include traditional POTS (Plain Old Telephone Service) and broadband services such as cable, HDTV, DSL, VoIP (Voice over Internet Protocol), IPTV (Internet Protocol Television), Internet services, and so on. 
     The communication network  4200  can utilize common computing and communications technologies to support circuit-switched and/or packet-switched communications. Each of the standards for Internet  4220  and other packet switched network transmission (e.g., TCP/IP, UDP/IP, HTML, HTTP, RTP, MMS, SMS) represent examples of the state of the art. Such standards are periodically superseded by faster or more efficient equivalents having essentially the same functions. Accordingly, replacement standards and protocols having the same functions are considered equivalent. 
     The cellular network  4214  can support voice and data services over a number of access technologies such as GSM-GPRS, EDGE, CDMA, UMTS, WiMAX, 2G, 3G, WAP, software defined radio (SDR), and other known technologies. The cellular network  4214  can be coupled to base receiver  4210  under a frequency-reuse plan for communicating with mobile devices  4202 . 
     The base receiver  4210 , in turn, can connect the mobile device  4202  to the Internet  4220  over a packet switched link. The internet  4220  can support application services and service layers for distributing data from the measurement system  4255  to the mobile device  4202 . Mobile device  4202  can also connect to other communication devices through the Internet  4220  using a wireless communication channel. 
     The mobile device  4202  can also connect to the Internet  4220  over the WLAN  4205 . Wireless Local Access Networks (WLANs) provide wireless access within a local geographical area. WLANs are typically composed of a cluster of Access Points (APs)  4204  also known as base stations. The measurement system  4255  can communicate with other WLAN stations such as laptop  4203  within the base station area. In typical WLAN implementations, the physical layer uses a variety of technologies such as 802.11b or 802.11g WLAN technologies. The physical layer may use infrared, frequency hopping spread spectrum in the 2.4 GHz Band, direct sequence spread spectrum in the 2.4 GHz Band, or other access technologies, for example, in the 5.8 GHz ISM band or higher ISM bands (e.g., 24 GHz, etcetera). 
     By way of the communication network  4200 , the measurement system  4255  can establish connections with a remote server  4230  on the network and with other mobile devices for exchanging data. The remote server  4230  can have access to a database  4240  that is stored locally or remotely and which can contain application specific data. The remote server  4230  can also host application services directly, or over the internet  4220 . 
     It should be noted that very little data exists on implanted orthopedic devices. Most of the data is empirically obtained by analyzing orthopedic devices that have been used in a human subject or simulated use. Wear patterns, material issues, and failure mechanisms are studied. Although, information can be garnered through this type of study it does yield substantive data about the initial installation, post-operative use, and long term use from a measurement perspective. Just as each person is different, each device installation is different having variations in initial loading, balance, and alignment. Having measured data and using the data to install an orthopedic device will greatly increase the consistency of the implant procedure thereby reducing rework and maximizing the life of the device. In at least one exemplary embodiment, the measured data can be collected to a database where it can be stored and analyzed. For example, once a relevant sample of the measured data is collected, it can be used to define optimal initial measured settings, geometries, and alignments for maximizing the life and usability of an implanted orthopedic device. 
       FIG. 43  is an illustration of a measurement device  4300  for measuring a force, pressure, or load of the muscular-skeletal system in accordance with an example embodiment. Measurement device  4300  can comprise or be integrated with equipment, tools, prosthetic components, or devices that couple to the muscular-skeletal system. Measurement device  4300  can include any of the packaging, circuits, power sources, sensors, system architecture, remote systems, or application integrated circuits, disclosed herein or incorporated by reference. Measurement device  4300  has a small form factor suitable for integration into an intra-operative or permanent prosthetic component. Measurement device  4300  includes telemetry for sending measurement data to a remote system for analysis, processing, display, or storage of information. In general, measurement device  4300  comprises a support structure  4302  and a support structure  4316 . Support structures  4302  and  4316  couple to the muscular-skeletal system. Support structures  4302  and  4316  form an enclosure or housing to house electronic circuitry  4306 , sensors, and circuit board  4308 . 
     In the example, measurement device  4300  is shown as a prosthetic component. The prosthetic component is an insert for a knee joint implant system. The insert fits between a femoral prosthetic component and a tibial prosthetic component. Measurement device  4300  measures a force, pressure, or load and the position of the force, pressure, or load on a surface. Support structure  4302  includes one or more surfaces that couples to the tibial prosthetic component. Support structure  4316  includes articular surfaces  4318  for coupling to and allowing articulation with the femoral prosthetic component. Measurement device  4300  is formed substantially dimensionally equal to a passive insert. Measurement device  4300  can also be integrated into the femoral prosthetic component or the tibial prosthetic component. Although shown as an insert for the knee, measurement device  4300  is suitable for integration into prosthetic components for the hip, spine, shoulder, ankle, elbow, bone, hand, and feet. As mentioned previously, measurement device  4300  can be integrated into tools or equipment that couple to the muscular-skeletal system for providing force, pressure, or load information such as a distractor, cutting jig, spacer, orthopedic screw, robot, or other tool. 
     Measurement device  4300  further comprises load plates  4304 , printed circuit board  4308 , electronic circuitry  4306 , load pads  4312 , sensor arrays  4310 , and load plates  4314 . Electronic circuitry  4306  couples to and controls measurement from sensor arrays  4310 . Electronic circuitry  4306  includes a power source such as a battery, capacitor, or inductor that can power the device while measurements are taken. Electronic circuitry  4306  further includes a transceiver and antenna for wireless communication to a remote system. In one embodiment, communication is short range, typically less than 10 meters. 
     Electronic circuitry  4306  is mounted on and coupled by metal traces formed on printed circuit board  4308 . In the example, printed circuit board  4308  comprises three sections. A first section of printed circuit board  4308  is located centrally between the second and third sections. Electronic circuitry  4306  is located on the first section of printed circuit board  4308 . The second and third sections of printed circuit board  4308  extend from the first section. The second and third sections of printed circuit board  4308  couple the sensors to electronic circuitry  4306 . In one embodiment, printed circuit board  4308  is a flexible and comprises a unitary substrate or circuit board. In other words, the first, second, and third sections of printed circuit board  4308  are formed as a single structure. This provides substantial benefits in both the assembly and cost of measurement device  4300 . In a further embodiment, the sensors are integrated into printed circuit board  4308  or substrate and more specifically in the second and third sections as sensor arrays  4310 . The sensors are elastically compressible capacitors that are formed in printed circuit board  4308  or an interconnect substrate as disclosed herein. As an alternative example, a piezo-resistive sensor can be formed within the printed circuit board by screen printing or using photolithographic techniques to create regions where the compressible conductive material is deposited. Sensor arrays  4310  can be placed between load bearing surfaces for measurement. Multiple sensors are used to determine force, pressure, or load magnitude and the position of force, pressure, or load applied to the surface of measurement device  4300 . As shown, each sensor array  4310  includes three sensors in the vertices of a triangular shaped area corresponding to the second and third sections of printed circuit board  4308 . The sensors underlie load pads  4312 . Load pads  4312  support load plates  4314  for directing a force, pressure, or load applied to articular surfaces  4318  to the sensors. Printed circuit board  4308  further includes a reference sensor  4330 . Reference sensor  4330  is similarly loaded to sensor arrays  4310  under a no-load condition. Reference sensor  4330  does not underlie articular surfaces  4318  and is located in a different region of measurement device  4300 . Reference sensor  4330  is used for test and calibration of sensor arrays  4310 . In one embodiment, reference sensor  4330  is formed identical to the sensors in sensor arrays  4310 . 
     Support structure  4302  includes alignment features  4324 ,  4326 , and  4328  for retaining and aligning printed circuit board  4308 . In the example, electronic circuitry  4306  on printed circuit board  4308  can be placed in a region of support structure  4302  having alignment features  4324 ,  4326 , and  4328 . Alignment features  4324 ,  4325 , and  4328  support placement of the first section of printed circuit board  4308  therein. Similarly, alignment features  4322  retain and align load plates  4304 , sensor arrays  4310 , and load plates  4314  to support structure  4302 . Alignment features  4322  precisely align sensors of sensor array  4310  to articular surfaces  4318  of support structure  4316  for determining position of applied force, pressure, of load to measurement device  4300 . In the example, alignment features  4324  retain and align support structure  4302  to support structure  4316 . Although not shown, support structure  4316  includes alignment features that couple to alignment features  4324 . Support structure  4302  further includes a peripheral channel  4320  that mates with a peripheral flange of support structure  4316  to support forming a hermetic seal that isolates electronic circuitry  4306  and sensor arrays  4310  from an external environment. 
       FIG. 44  is an illustration of support structure  4302  and load plates  4304  in accordance with an example embodiment. Support structure  4302  includes cavities that underlie articular surfaces  4318  for housing the sensing assembly. Load plates  4304  includes openings  4332  or cutouts for aligning and positioning to alignment features  4322  of support structure  4302 . In one embodiment, load plates  4304  are rigid structure that support and distribute loading over a large surface area. Load plates  4304  can comprise a metal or polymer material. In the example, load plates  4304  comprise stainless steel. 
     Sensors of sensor arrays  4310  are integrally formed in printed circuit board  4308 . The sensors are placed at the vertices of a polygon. In the example, the polygon is a triangle. The area of the polygon relates or corresponds to an area of articular surface  4318  of support structure  4316 . Sensor array  4310  measures the force, pressure, or load, and the position of the force, pressure, or load in at least the area of the polygon and the corresponding area of articular surfaces  4318 . 
       FIG. 45  is an illustration of support structure  4302  and printed circuit board  4308  in accordance with an example embodiment. The first section of printed circuit board  4308  is located centrally overlying support structure  4302 . Alignment features  4324 ,  4326 , and  4328  align and retain the first section of printed circuit board  4308  to a support surface of support structure  4302 . In one embodiment, electronic circuitry  4306  is located in a region of measurement device  4300  having little or no loading during a measurement process. Thus, electronic circuitry  4306  is protected from damage due to the high loads that can be placed on the device during use. 
     The second and third sections of printed circuit board  4308  correspond to sensor arrays  4310 . Sensor arrays  4310  include openings  4334  for receiving alignment features  4322 . Sensor arrays  4310  are placed overlying load plates  4322 . Openings  4334  couple through alignment features  4322  aligning the sensors to articular surfaces  4318 . In one embodiment, the entire surface of sensor arrays  4310  couples to the surface of load plates  4322  to distribute and spread the force, pressure, or load thereto. As mentioned previously, printed circuit board  4308  is flexible such that the second and thirds sections can be at a different height than the first section. Load pads  4312  which overlie the sensors are formed on the surfaces of the second and third sections of printed circuit board  4308 . In one embodiment, load pads  4312  can comprise a metal such as copper or gold. Load pads  4312  couple to a terminal or electrode of the corresponding sensor. In the example, load pads  4312  couple to a capacitor plate of the capacitive sensor. Alternatively, load pads  4312  can be non-conductive. Load pads  4312  can be coupled to shields that couple to ground for preventing parasitic coupling to the sensors. As disclosed herein, the sensors are elastically compressible capacitors formed in the first and second sections of printed circuit board  4308 . Load pads  4312 , the sensors, and interconnect on printed circuit board  4308  can be formed using photolithographic techniques that provide for accurate and repeatable device structures. An example of how the capacitive sensors are formed in a flexible substrate with interconnect and one or more ground shields to prevent parasitic coupling is disclosed herein. 
       FIG. 46  is an illustration of support structure  4302  and load plates  4314  in accordance with an example embodiment. Load plates  4314  are placed overlying sensor arrays  4310 . Load plates  4314  include openings  4336  for coupling to alignment features  4322 . Alignment features  4322  retain and align load plates  4314  in relation to sensor arrays  4310  and articular surfaces  4318 . Load plates  4314  are a rigid structure to distribute a force, pressure, or load to the sensors of sensor arrays  4310 . Load plates  4314  can comprise a metal or polymer material. In the example, load plates  4314  comprise stainless steel. In one embodiment, load plates  4314  have a planar surface that couples to load pads  4312  that are above the surface of sensor arrays  4310 . Alignment features  4322  allow vertical movement of load plates  4314  and sensor arrays  4310  when compressed or being compressed. 
     An underside of support structure  4316  illustrates regions  4338  and alignment features  4340 . Alignment features  4324  are inserted into alignment features  4340  when support structures  4302  and  4316  are coupled together. Alignment features  4324  and  4340  precisely align sensor arrays  4310  to articular surfaces  4318 . The position of the sensors of sensor array  4310  in relation to articular surfaces  4318  are used in the calculation to determine where a force, pressure, or load is applied to articular surfaces  4318 . Regions  4338  align to and couple with load plates  4314 . Support structure  4316  includes a peripheral flange  4342  that couples with peripheral channel  4320  of support structure  4302 . In one embodiment, flange  4342  of support structure  4316  is inserted into channel  4320  of support structure  4302  and bonded by an adhesive or glue. Channel  4320  can act as a glue channel for distributing glue peripherally prior to a fastening process. Flange  4342  is a barrier to the ingress or egress of foreign solids, liquids, or gas into and out of measurement device  4300 . Flange  4342  further provides a large bonding area for the adhesive to ensure a hermetic seal of measurement device  4300 . Sensor arrays  4310  are under no load or lightly loaded when support structures  4302  and  4316  are coupled together. 
     A force, pressure, or load applied to articular surfaces  4318  is coupled to load plates  4314 . Load plates  4314  distribute the load to the three sensors of sensor array  4310 . In the example, the sensors are elastically compressible capacitors. The capacitors generate a signal having a time period corresponding to the force, pressure, or load. The signal is received by electronic circuitry  4306 . The measurement process is disclosed hereinabove in more detail. The measurements can be stored in memory on measurement device  4300 . Alternatively, measurement data can be transmitted to a remote system. The remote system can calculate the force, pressure, or load magnitudes. The remote system can also calculate the position of the force, pressure, or load on the articular surface. The remote system can include a display to present the data in real time. 
       FIG. 47  is a cross-sectional view of measurement system  4300  in accordance with an example embodiment. Electronic circuitry  4306  and sensor arrays  4310  are housed in measurement device  4300 . The cross-sectional view illustrates channel  4320  and flange  4342 . As mentioned previously, channel  4320  and flange  4342  are formed around the entire periphery of support structures  4302  and  4316 . Channel  4320  and flange  4342  can also be respectively on support structure  4316  and support structure  4302 . Channel  4320  in a bonding process can be used to hold a quantity of adhesive or glue distributed around the entire periphery before bonding. Channel  4320  and flange  4342  provides a large surface area to which the adhesive or glue can bond. Flange  4342  also acts as a barrier to the ingress or egress of gas, liquids, and solids. 
       FIG. 48  is an illustration of an assembled measurement device  4300  in accordance with an example embodiment. In general, measurement device has a surface that is coupled to the muscular-skeletal system. A force, pressure, or load is applied to the surface by the muscular-skeletal system. The position of applied force, pressure, or load one the surface can be determined during the measurement process. As shown, measurement device  4300  is formed as an insert for joint replacement surgery. In one embodiment, measurement device  4300  includes a set of shims to adjust the height of the insert. Measurement device  4300  can be removed such that a different shim can be attached to measurement device  4300  to change the thickness or height of the device. A shim attaches to a major exposed surface of support structure  4302 . The total height of measurement device  4300  including the shim corresponds to a passive insert height that is available for permanent installation. As mentioned previously, measurement device  4300  configured for use in tools, equipment, measurement devices, and prosthetic components for measurement of the muscular-skeletal system. The articular surface  4318  couples to the muscular-skeletal system. The force, pressure, or load on articular surfaces  4318  can be measured by measurement system  4300 . The location where the applied force, pressure, or load on articular surface  4318  can also be determined from the more than one sensor measurements. The support structures  4302  and  4316  form a housing for electronic circuitry and sensors as disclosed herein. Measurement device  4300  can be a permanent or temporary device. Measurement device  4300  can transmit data wirelessly to a remote system for further processing, evaluation, display, or storage. 
       FIG. 49  is an illustration of measurement system  4300  coupled to a prosthetic component in accordance with an example embodiment. Measurement system  4300  is coupled to a tibial prosthetic component  4902 . Tibial prosthetic component  4902  is implanted into the tibia and provides a support surface for measurement system  4300 . Tibial prosthetic component  4902  distributes the force, pressure, or load applied to the insert over a large area thereby reducing the per unit area loading. A remote system  4904  is coupled to measurement system  4300 . Information, programming material, software, data, and measurement instructions can be sent from remote system  4904  to measurement system  4300 . In one embodiment, the transfer of data is wireless as disclosed herein. Similarly, measurement data from measurement system  4300  can be transmitted to remote system  4904 . Remote system  4904  can include logic circuitry, a processor, or a digital signal processor for further processing of the data. The data can be displayed on a display  4906  in real-time for viewing of the measurement data. 
       FIG. 50  is an illustration of measurement system  4300  in accordance with an example embodiment. Tibial prosthetic component  4902  is coupled to tibia  5002 . A femoral prosthetic component  5004  couples to femur  5006 . A portion of femoral prosthetic component  5004  couples to articular surfaces  4318  allowing articulation of the joint assembly. In one embodiment, the area of femoral prosthetic component  5004  coupling to articular surfaces  4318  is substantially smaller than the area of measurement system  4300  coupling to tibial prosthetic component  4902 . Thus, the force, pressure, or loading on articular surfaces  4318  is higher per unit area than the force, pressure, or load of measurement system  4300  coupling to tibial prosthetic component  4902 . Ligaments, tendons, and muscles hold the joint in place under tension. 
     Remote system  4904  receives data from measurement system  4300 . Remote system  4904  can wirelessly send bone and prosthetic component position information. For example, measurement system  4300  can include accelerometers for providing relative position information. The position information can be displayed on display  4906 . 
     In one embodiment, measurement system  4300  as a tool or as a prosthetic insert is used intra-operatively to support size selection and assess prosthetic component positioning. Measurement system  4300  can further be used for fine-tuning of the installation such as balancing of the joint through soft tissue tensioning. Measurement data of the load magnitude and position of applied load on articular surfaces  4318  can be measured and displayed on display  4906 . Moreover, the measurements can be made over the range of motion of the joint. The load magnitude measurements can be used to select an appropriate sized insert for the patient. An insert that is too thick will be tight in the joint. Conversely, an insert that is too thin will be too loose in the joint. A suitable range for joint loading can be provided from a database that is retrieved by remote system  4904  and downloaded to measurement system  4300 . Display  4906  can compare the patient measurements versus a known good value range. As mentioned, measurement system  4300  also measures position of load on each articular surface. Remote system  4904  can display position of load throughout the range of motion. Similarly, a range of loading over the range of motion for a correct installation can be retrieved from a database and compared to the patient measurements. All of the information and data on the installation can be recorded and stored as part of the patient file. The data can be further used anonymously as part of a larger database on joint installations. 
       FIG. 51  is a method  5100  of assembling a device for measuring a force, pressure, or load measurement device that couples to the muscular-skeletal system in accordance with an example embodiment. The device can also measure position of applied force, pressure, or load to a surface of the structure. The method description relates to and can reference  FIGS. 43-50 . The example disclosed herein is applicable to a device, equipment, tool, prosthetic component, or jig for coupling to the muscular-skeletal system. The steps of method  5100  are not limited to the order disclosed and can be practiced in a different sequence or with additional intervening steps. In general, method  4900  can also have a greater number of steps or a fewer number of steps than shown. 
     In a step  5102 , a unitary circuit board comprising electronic circuitry and a sensor array is coupled to a first support structure. In a step  5104 , a load plate is placed overlying the sensor array. The load plate couples to load pads raised above a surface of the sensor array. The sensors of the sensor array underlie the load pads. In a step  5106 , a second support structure is coupled to the first support structure. The first and second support structures form a housing for the electronic circuitry, unitary circuit board, and sensor array for isolation from an external environment. In one embodiment, a hermetic seal is formed. 
     In a step  5108 , a load plate is placed underlying the sensor array. In one embodiment, the load plate couples to a surface of the first support structure. The sensor array is placed overlying the load plate. In a step  5110 , the first support structure is glued to the second support structure. In one embodiment, the first support structure includes a peripheral channel around the entire periphery of the first support structure. The peripheral channel can retain adhesive or glue to support the gluing process. The second structure includes a peripheral flange. The peripheral flange fits into the peripheral channel for forming a barrier into the device. The peripheral flange provides increased area for bonding to the adhesive or glue. The gluing process forms a hermetic seal for the measurement device. 
     The present invention is applicable to a wide range of medical and nonmedical applications including, but not limited to, frequency compensation; control of, or alarms for, physical systems; or monitoring or measuring physical parameters of interest. The level of accuracy and repeatability attainable in a highly compact sensing module or device may be applicable to many medical applications monitoring or measuring physiological parameters throughout the human body including, not limited to, bone density, movement, viscosity, and pressure of various fluids, localized temperature, etc. with applications in the vascular, lymph, respiratory, digestive system, muscles, bones, and joints, other soft tissue areas, and interstitial fluids. 
     While the present invention has been described with reference to particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the invention.