Abstract:
A system for detecting wear debris particulate from a medical implant within the body of a living animal is provided. The system includes an acoustic transmitter for transmitting acoustic energy from outside the body to a soft tissue region proximate the medical implant containing wear debris particles; an acoustic receiver located outside the body to detect resultant acoustic energy generated by the wear debris particles and produce a received signal indicative thereof; a processor for processing the received signal to evaluate at least one parameter associated with the wear debris particles; and an output for indicating the at least one parameter.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 USC §119 to U.S. provisional application No. 60/194,996, filed on Apr. 5, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to non-invasive diagnoses of medical implants, and more particularly to an ultrasound technique for the in-vivo detection of particulate wear debris from medical implants. 
     BACKGROUND OF THE INVENTION 
     Various types of medical implants have been developed over the years. In many instances, such implants enable humans to live longer, more comfortable lives. Implants such as pacemakers, artificial joints, valves, grafts, stents, etc. provide a patient with the opportunity to lead a normal life even in the face of major heart, reconstructive, or other type surgery, for example. 
     It has been found, however, that the introduction of such medical implants can sometimes lead to complications. For example, the human body may reject the implant which can ultimately lead to osteolysis or other types of complications. Alternatively, the implant may malfunction or become inoperative. 
     In the case of some implants such as artificial joints, the implant is subjected to everyday motion, stress and strain. This often leads to abrasion between different parts of the implant, between the implant and the skeletal frame, etc. Such abrasion results in the formation of wear debris particles in the area of the implant which can lead to complications. For example, in the case of an artificial hip joint, wear debris particles from the acetabular cup may build up over time. These wear debris particles can trigger a response of the human body immune system. Because the wear debris particles typically consist of artificial materials which are nonbiodegradable, the immune system attacks on the particles fail. This leads to further increases in immune system enzyme concentration and ultimately resorption of bone by the tissue, a process called osteolysis. The patient can experience a loose joint and pain. 
     It is desirable therefore to be able to monitor the condition of a medical implant, particularly in the case of an implant which is subject to the generation of wear debris particulate. On the other hand, it is highly undesirable to have to perform invasive surgery in order to evaluate the condition of the implant. Such invasive surgery is not only time consuming, but also costly and painful to the patient. 
     In view of the aforementioned shortcomings, there is a strong need in the art for an apparatus and method for detecting and evaluating wear debris particulate associated with a medical implant, particularly with respect to an artificial joint. Even more particularly, there is a strong need for an apparatus and method which can evaluate wear debris simply, reliably and non-invasively. Having the capability to detect such debris at an early enough stage would allow physicians to intervene with pharmaceuticals or otherwise before significant bone deterioration or other complications occur. 
     SUMMARY OF THE INVENTION 
     An apparatus and method are provided for the in-vivo detection of particulate wear debris from medical implants such as artificial joints for hips, knees, shoulders, elbows, etc. According to the invention, a focused ultrasound transducer placed in contact with the body insonifies a region of the body suspected of containing wear debris particulate. Such wear debris particulate may have linear dimensions on the order of 0.1 to 10 microns, for example. The particles are detected by the same or different transducer when cavitation events represented by bursts of scattered ultrasound with amplitudes orders of magnitude higher than background noise levels are received from the particles. 
     Applicants have found both a particle size and a concentration effect on the amplitude and number of the cavitation events. The cavitation events are believed to be linear and to result from small irregularities in the particle/liquid interface which trap microscopic volumes of gas that serve as cavitation nuclei. Computer analysis of the signal strength (i.e., amplitude) and number of cavitation events and the use of a lookup table or neural network, for example, provide a measurement of particulate distribution (size and concentration) in the vicinity of the implant. 
     According to the present invention, a system is provided for detecting wear debris particulate from a medical implant within a body of a living animal. The system includes an acoustic transmitter for transmitting acoustic energy from outside the body to a soft tissue region proximate the medical implant generating wear debris particles; an acoustic receiver located outside the body to detect resultant acoustic energy scattered by the wear debris particles and produce a received signal indicative thereof; a processor for processing the received signal to evaluate at least one parameter associated with the wear debris particles; and an output for indicating the at least one parameter. 
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an environmental view illustrating a system for non-invasively detecting wear debris particulate associated with a medical implant, such as an artificial hip, in accordance with the present invention; 
     FIG. 2 is a sectional view illustrating an artificial hip-joint to be analyzed in accordance with the present invention; 
     FIG. 3 is a sectional view of the artificial hip-joint of FIG. 2 including wear debris particulate to be analyzed in accordance with the present invention; 
     FIG. 4 is a block diagram of a wear debris detection system in accordance with the present invention; 
     FIGS. 5,  6  and  7  represent data indicative of different particle sizes detected in accordance with the present invention; 
     FIGS. 8 and 9 represent data indicative of the manner in which the number of cavitation events corresponds to particle concentration in accordance with the present invention; and 
     FIG. 10 is a flowchart representing the operation of the system for detecting wear debris particulate in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. 
     Referring initially to FIG. 1, a system for non-invasively detecting wear debris particulate from a medical implant is generally designated  30 . In accordance with the present invention, the system  30  detects and analyzes wear debris particulate associated with a medical implant  32  which is implanted in a living animal such as a human patient  34 . As is discussed in more detail below, the medical implant  32  can be any of a wide variety of different types of devices including, for example, an artificial joint, etc. In the preferred embodiment, the implant  32  is an artificial hip-joint, although it will be appreciated that the implant  32  can be any other type of device which is subject to the generation of wear debris particulate. 
     The system  30  includes an acoustic analyzer  36  for remotely and non-invasively analyzing the implant  32  in order to detect the presence of wear debris particulate (e.g., with respect to size and/or concentration). The analyzer  36  in the exemplary embodiment includes an acoustic transmitter/receiver unit  38  which is positioned outside the patient  34  in close proximity to the implant  32 . As will be discussed in more detail below, the transmitter/receiver unit  38  serves to excite the soft tissue region around the implant  32  with acoustic energy. The acoustic energy is used to evaluate the response of the region suspected of containing wear debris particulate. In particular, the transmitter/receiver unit  38  receives acoustic signals radiated/scattered back by the region suspected of containing wear debris particulate in response to the excitation. Such signals are then processed by the analyzer  36  to detect a parameter of interest (e.g., particle concentration and/or size, etc.). 
     The transmitter/receiver unit  38  is coupled via an electrical cable  40  to the main circuitry  42  included in the analyzer  36 . The main circuitry  42  includes suitable circuits for driving the transmitter/receiver unit  38  as described below, and for processing the output of the transmitter/receiver unit  38  in order to provide an output to an operator (e.g., via a display  44 ). In addition, or in the alternative, the output may be linked to a local area network (LAN), the Internet, etc., so that the results may be provided to a remote location if desired. 
     FIG. 2 shows a typical artificial hip joint  100  which may be analyzed in accordance with the present invention. The hip joint  100  includes a hemispherical acetabular cup  102  implanted in the acetabulum of the pelvic bone  104 . A shaft  106  is inserted into a space formed in the central portion of the femur  108  after removing the marrow existing in that portion of the femur  108 . A spherical femur head  110  is fixed to an upper end of the shaft  106 , and is pivotally fitted in the acetabular cup  102 . 
     A soft tissue membrane forms a sack  112  which surrounds the joint  100  in the area where the femur head  110  is pivotally fitted in the acetabular cup  102 . As is known, the sack  112  contains primarily synovial fluid  114  which serves to lubricate the joint  100 . The joint is then surrounded by muscle tissue and skin as represented at  116 . 
     A patient having an artificial hip joint  100  is typically able to walk in much the same manner as with a conventional hip. When the patient walks, the femur head  110  rotates and translates within the acetabular cup  102 . During such movements, however, the femur head  110  generates interfacial friction with the acetabular cup  102  which is typically made of ultra-high molecular weight polyethylene or some other inert material. This results in an abrasion of the acetabular cup  102 , and as a result fine polyethylene or other inert material wear debris particles are generated. 
     As is discussed below in relation to FIG. 3, these wear debris particles are contained within the sack  112  of synovial fluid  114 . The particles are problematic in that they generate osteolysis by moving from the sack  112  into the space  124 , for example. The abrasion of the acetabular cup  102  results in a reduction in the life of the joint  100 . Moreover, gone unchecked the hip joint  100  may become dislocated due to excessive mobility in the shaft  106 . Such dislocation causes the patient to feel pain. Furthermore, there is an increase in hospital expense. Additional details regarding a typical hip joint  100  and the associated wear particulate may be found in U.S. Pat. No. 5,725,597, the entire disclosure of which is incorporated herein by reference. 
     FIG. 3 represents the hip joint  100  after wear debris particulate  120  has built up in the synovial fluid  114 . The wear debris particulate  120  may include particles of different sizes and concentrations, depending upon the materials used to manufacture the joint  100  and the extent of wear, for example. Typical particles  120  found in the synovial fluid  114  may range in size, for example, from 0.1 micron to 10 microns and higher with particles in the 0.4 to 1.0 micron range provoking the largest response from the immune system. 
     Accordingly, two objectives of the present invention are to detect the size and concentration of the particles in order to assess the integrity of the joint  100 . For example, early detection of the presence and extent of wear debris particulate may allow for remedial action which is less costly, invasive, painful, etc. compared to allowing the buildup of particulate to continue undetected. 
     As shown in FIG. 3, the transmitter/receiver unit  38  is held against the body of the patient  34  in the vicinity of the hip joint  100 . The transmitter/receiver unit  38  is designed to transmit excitation pulses of acoustic energy towards a region within the synovial fluid  114  within the sack  112 . In between each excitation pulse, the transmitter/receiver unit  38  receives scattered bursts of acoustic energy which are created as a result of the excitation pulse generating cavitation events among the particulate  120  within the synovial fluid  114 . The different amplitudes of the scattered bursts have been found to track the size of the different wear debris particles. In addition, the frequency or number of events (i.e., the number of scattered bursts detected in response to an excitation pulse) has been found to be indicative of the concentration of the wear debris particles. Accordingly, the present invention is able to detect and analyze the presence of wear debris particulate by analyzing the acoustic energy which is scattered back towards the transmitter/receiver unit  38  from the region containing the synovial fluid  114 . 
     Referring now to FIG. 4, the acoustic analyzer  36  in accordance with the exemplary embodiment is illustrated in more detail. The transmitter/receiver unit  38  preferably is a hand-held sized device which is held by a doctor, nurse or medical assistant against the body of the patient  34  in close proximity to the implant  32 . The transmitter/receiver unit  38  may be a conventional single element or array type ultrasonic transducer used in medical or NDE applications as will be appreciated. It is understood that combinations of imaging transducers and single element transducers may be beneficial to the practice of this technology. 
     In this example, the implant  32  may be the artificial hip joint  100  of FIGS. 2 and 3. Since the system  30  is non-invasive, the transmitter/receiver unit  38  may be placed adjacent the implant  32  with the body of the patient (e.g., skin, muscle tissue, etc.), designated  50 , disposed therebetween. 
     The analyzer  36  includes an ultrasonic pulse generator and receiver circuit  52  which is programmed to carry out the various control and functions described herein. More particularly, the ultrasonic pulse generator and receiver circuit  52  controls the frequency, the amplitude and the length of the ultrasonic pulses generated by the transmitter and also amplifies the received acoustic signals scattered back towards the transmitter/receiver unit  38  (e.g., from the particulate  120  within the synovial fluid  114 ) in response to being excited. The frequency can be changed between 0.1 MHz to 30 MHz, for example. The acoustic pressure amplitude can be changed between 0.05 Mpa to 3 Mpa, for example. The circuit  52  selectively provides a pulse control signal on bus  54  in order to control the frequency, amplitude, etc. of the acoustic energy the transmitter/receiver unit  38  transmits towards the implant  32 . The acoustic energy is then scattered back towards the transmitter/receiver unit  38  (e.g., from the particulate  120  within the synovial fluid  114 ) in response to being excited by the excitation pulses. 
     The transmitter/receiver unit  38  receives acoustic energy scattered back from the wear debris particulate and converts the energy into an electrical signal on line  56 . The signal on line  56  is received by circuit  52  and is input to a signal conditioning circuit  58  which conditions the received signal prior to being input to a data processing circuit  60 . As is discussed more fully below, the data processing circuit  60  is programmed using conventional techniques to process and analyze the signal received on line  56  in order to determine a parameter(s) associated with the wear debris particulate. For example, the excitation signal from the transmitter/receiver unit  38  is used to create cavitation among the wear debris particles  120  within the synovial fluid  114 . The transmitter/receiver unit  38  then detects the acoustic energy scattered back towards the transmitter/receiver unit  38  due to such cavitation. More specifically, the data processing circuit  60  analyzes the amplitude and frequency of the cavitation events, for example. As is pointed out below in connection with FIGS. 5-9, the amplitude of the cavitation events has been found to track the size of the particles, and the number of events has been found to track the concentration of the particles. The principles of cavitation are explained more fully in Madanshetty et al.,  Acoustic Microcavitation: Its Active and Passive Acoustic Detection , J. Acoust. Soc. Am., Vol. 90, 1515-1526, 1991, for example, the entire disclosure of which is incorporated herein by reference. 
     FIG. 5 represents exemplary data showing the response of wear debris particulate having a diameter of 0.4 micron at a concentration of 10 7  particles/milliliter(mL). In FIG. 5, a single event is shown in response to an excitation pulse from the transmitter/receiver unit  38  at a given amplitude within medically accepted practice and a frequency of 2.25 MHz. The circuit  52  captures the response signal received by the transmitter/receiver unit  38  representing a number of events, and the particular event shown in FIG. 5 may be selected via signal processing to partition the respective events. As is shown in FIG. 5, the amplitude of the response signal is approximately 5×10 −2  volts. 
     FIG. 6 represents data corresponding to FIG. 5, but in this particular case the particulate has a diameter of 3.3 micron at the same concentration. In this case, the amplitude of the received signal for a given event is approximately 1.70×10 −1  volts. FIG. 7 represents similar data except that the particles have a diameter of 1.0 micron. In this case, the amplitude of the received signal for a given event is approximately 8×10 −2  volts. Thus, for a given event captured by the circuit  52  in the signal received by the transmitter/receiver unit  38 , it is shown that the amplitude of the event is related to the size of the wear debris particles. The circuit  52  together with the data processing circuit  60  may be programmed and/or configured using conventional techniques to capture and analyze the signal received by the transmitter/receiver unit  38  in order to evaluate the amplitude of the signal for each of the events contained in the received signal, as will be appreciated. The data processing circuit  60  automatically evaluates the amplitude of the events in order to determine the respective sizes of the wear debris particles. 
     FIG. 8 is a data histogram representing the number of events which are detected in the acoustic response signal received by the transmitter/receiver unit  38  following an acoustic excitation pulse, as a function of the concentration of particles. As is shown in FIG. 8, for particles having a diameter of 0.4 micron the number of events in the received signal tracks the concentration of particles in the fluid. In other words, the higher the concentration of particles the higher the number of events which are detected. FIG. 9 shows a similar correspondence between the concentration of particles and the number of events which are detected following an excitation pulse for particles having a diameter of 3.3 microns. 
     The data processing circuit  60  is programmed and/or configured using conventional techniques to analyze the signal received by the transmitter/receiver unit  38  in order to evaluate the number of events contained in the received signal for the different sized particles. The data processing circuit  60  can then automatically reach a determination as to the concentration of the respective sized particles. 
     Advanced signal processing methods such as neural networks, expert systems, digital transforms, etc. may be applied by the data processing circuit  60  in order to increase resolution of the apparatus in distinguishing between the different particle sizes and concentrations, as will be appreciated. 
     FIG. 10 represents the operation of the system  30  in accordance with the present invention. As shown in step S 1 , the acoustic analyzer  36  (FIG. 4) transmits an acoustic (e.g., ultrasonic) energy pulse via the transmitter/receiver unit  38  towards the medical implant  32  and the region believed to contain wear debris particulate. In step S 2 , the acoustic analyzer receives the acoustic energy scattered back from the particulate via the transmitter/receiver unit  38 . The signal conditioning circuit  58  conditions the received signal and the data processing circuit  60  stores the received signal. 
     The data processing circuit  60  in step S 3  then analyzes the received signal. For example, the received signal is processed to identify the different cavitation events which occur amongst the wear debris particulate in the fluid surrounding the joint. The amplitude of each event and number of events of each amplitude are then determined by the control circuit  52  in step S 3 . The data processing circuit  60  compares the amplitude of each event with predetermined values stored in a look up table in memory, for example, in order to ascertain the size of the particles. In addition, the data processing circuit  60  counts the number of events of each amplitude and refers to a set of predetermined values stored in a look up table in memory to ascertain a corresponding concentration of particles. Alternatively, the data processing circuit  60  may employ a neural network or other advanced signal processing in order to evaluate the different events in order to ascertain particle size, concentration, etc. 
     Next, in step S 4  the data processing circuit  60  processes the particulate data in order to produce an output such as the particle size and/or concentration as determined in step S 3 . In the case of performing several excitation pulses in step S 1 , the data processing circuit  60  in step S 4  may average or apply statistical methods to the particulate data obtained in response to each excitation pulse in order to produce an output. 
     In step S 5 , the data processing circuit  60  determines if the detection process is to continue (e.g., based on an operator input). If so, the process returns to step S 1 . Otherwise, the data processing circuit  60  outputs the data (e.g., the detected particle size(s) and/or concentration(s) in step S 6  and the process is completed. 
     Accordingly, the inventors have developed a manner for non-invasively detecting and analyzing wear debris particulate associated with a medical implant. 
     Moreover, the invention has utility in that it also may be used to monitor and characterize wear debris generated by medical implants in a simulation environment. For example, the present invention may be used to characterize wear debris generated by an artificial joint using a joint motion simulation device. The artificial joint is enclosed in a sack filled with saline or some other fluid to emulate an actual joint with a body, for example. The artificial joint is cycled according to a desired time period and/or number of repetitions. The joint is then analyzed using the above procedures in order to gain information on the various types, sizes and responses of the wear debris particulate occurring over time, e.g., with respect to different implant devices, etc. 
     Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.