Patent Publication Number: US-2020275879-A1

Title: Radiographic Discernable Sensors and Orthopedic Applications for Same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. patent application Ser. No. 15/751,623, having a 35 U.S.C. § 371(c) date of Feb. 9, 2018, which is the United States national entry of International Patent Application No. PCT/US2016/046707, having a filing date of Aug. 12, 2016, which claims filing benefit of U.S. Provisional Patent Application Ser. No. 62/204,111, having a filing date of Aug. 12, 2015, all of which are incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under Grant No. GM103444, awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     Over 28 million musculoskeletal injuries are treated annually in the U.S., including 2 million fracture fixation surgeries. Of these, tibia fractures are the most common long bone fracture. Unfortunately, such fractures are frequently associated with complications (delayed union, non-union, and infection), particularly for severe trauma such as is often sustained in combat. For instance, non-union is a significant complication (approximately 100,000 injuries, 5% of all fixation surgeries in the U.S. go on to non-union), with even higher rates for severe trauma. Infection at the site of orthopedic surgery is an on-going issue and, while the incidence has been reduced due to improvements in both surgical and post-operative procedures, its prevalence is still unacceptably high. Such complications can lead to long-term or even permanent disability or death and are responsible for significant direct and indirect health care costs. 
     While a variety of supporting implants and adjunct therapies are available to caregivers, a crucial issue leading to complication is the inability to directly evaluate health and healing of the local area. Physicians routinely acquire X-ray images as part of diagnosis and evaluation, and while these images can show the hardware and fracture callus, they do not measure mechanical properties of the fracture and cannot detect early-stage infection. Three-dimensional CT images are better at indicating bone density and determining if union has occurred, but are expensive, expose the patient to significant radiation doses (typically around 300× more than a standard X-ray), and are imperfect, especially when allografts are used. For some fracture types, dynamic X-ray images can be acquired to measure bone motion with and without external load in order to assess fracture stability. In practice however, this is highly challenging. For example, even in spine fusion, where spinal processes are clearly evident and can move significantly, it has been determined that inter-observer variation of spinous process movement can be about 1.5 mm, with differences as large as 3.5 mm, compared to a recommended bone fusion cutoff criteria of less than 1 mm. 
     When considering localized infection, external visualization techniques provide little or no clues, particular in early stages of infection. Unfortunately, infection at orthopedic trauma sites are generally not diagnosed until after the infection has spread and symptoms have become systemic. 
     The lack of widely applicable tests to assess bone health, such as load-bearing state and early stage infection, presents a major challenge for physicians and patients. Infection at implant sites can require additional surgery or can even become life-threatening when diagnosis is delayed. Weight-bearing before the fracture callus is sufficiently strong carries risk of re-fracture and/or hardware failure. On the other hand, unnecessary delay in weight-bearing can hamper rehabilitation and is highly costly in terms of lost days of activity. Studies have shown that when the fractured bone has at least 25% of the bending stiffness of intact bone, weight-bearing rarely leads to re-fracture or hardware failure. For externally fixed devices, percutaneous pins can be directly loaded to assess stiffness. When testing is carried out and this 25% threshold is used, the majority of patients begin weight-bearing an average of 2.3 weeks earlier than average. Load testing on externally fixated devices can likewise identify patients with delayed and non-union for weight-bearing restrictions and additional interventions. 
     Most orthopedic surgeries involve internal fixation, which require either a percutaneously connected gauge or remote measurements to assess load-bearing capabilities during healing. A percutaneously connected strain gauge is impractical for patients and presents a number of safety challenges. A variety of remote interrogation methods based upon implanted wireless devices, ultrasound, vibrational analysis, and other approaches have been examined for non-invasive measurement of strain on orthopedic implants, but these generally require significant development, as well as equipment and/or expertise, currently unavailable to most caregivers. 
     What are needed in the art are passive implantable sensors for use in conjunction with orthopedic implants that can be easily read by conventional non-invasive methods to assess local conditions and bone health at the local site. In particular, what are needed are passive sensors capable of assessing strain under load and/or early stage signs of infection and thereby to determine a current state of bone health. For instance, a passive sensor locatable on bone fixation devices that can assess health and healing in the local area of an orthopedic implant by use of conventional radiography methods would be of great benefit. 
     SUMMARY 
     According to one embodiment, an implantable strain sensor is described. A strain sensor can include an indicator that includes a first end that is configured for fixation to an orthopedic implant (e.g., integral to, directly or indirectly bonded or otherwise attached to, etc.). For instance, a first end can be affixed in a sensor housing that is attached to an orthopedic implant such as a fixation device, a joint replacement component, a dynamic hip screw, etc. The indicator can also include a second end. During use, the second end can exhibit motion relative to the implant in response to a load placed on the implant either directly or indirectly. 
     According to another embodiment disclosed is an implantable chemical sensor that includes an indicator. In this embodiment, the indicator can include an analyte-sensitive material and a radiographically discernable component that is configured to exhibit motion in response to an analyte interaction with the analyte-sensitive material that leads to a dimensional change in the analyte-sensitive material. 
     In addition, a sensor (a chemical sensor or a strain sensor) can include a scale that is located in proximity to the discernable structure such that the location of this structure (e.g., the end of an indicator rod) with respect to the scale is radiographically discernible, e.g., the end of the indicator can overlay the scale in a radiographic image. 
     Also described are orthopedic implants such as fixation plates, screws, rods, or pins, and the like directly or indirectly held in conjunction with a sensor. 
     Methods for determining health of tissue by use of the sensors are also described. For instance, a method can include placing orthopedic tissue (e.g., a bone) under load. The tissue can have affixed thereto an orthopedic fixation component (e.g., a fixation plate, rod, screw, etc.) that carries a sensor. A method can also include examining the sensor via a radiographic imaging process while the tissue is under the load. Through examination of the sensor, the location of the second end of the indicator in relation to the scale can be discerned. 
     In one embodiment, a method can determine chemical conditions in the local area of an implant. For instance, upon interaction with a predetermined analyte (e.g., a pathogen or a pathogen determinant), a radiographically discernable component of the sensor can move in relation to the tissue and/or the implant due to, e.g., the swelling of the indicator of the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       A full and enabling disclosure of the present subject matter, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures in which: 
         FIG. 1A  schematically illustrates a broken bone affixed to a supporting device. 
         FIG. 1B  schematically illustrates the broken bone of  FIG. 1A  under a load. 
         FIG. 10  schematically illustrates the broken bone of  FIG. 1A  under a load after an amount of healing. 
         FIG. 2  illustrates one embodiment of a sensor as disclosed herein under no load (top panel) and under a load (bottom panel). 
         FIG. 3  illustrates another embodiment of a sensor as disclosed herein under no load (top panel) and under a load (bottom panel). 
         FIG. 4  illustrates representative examples of scale designations as may be utilized with a sensor. 
         FIG. 5  illustrates a Vernier-type scale for a sensor as disclosed herein that incorporates moiré fringes. 
         FIG. 6  illustrates a sensor in which the sensor incorporates a hydraulic amplification system. 
         FIG. 7  illustrates a sensor in which the sensor incorporates a lever-based amplification system 
         FIG. 8  illustrates a sensor in which the sensor incorporates a hydraulic amplification system. 
         FIG. 9  illustrates another embodiment of a sensor has disclosed herein under no or low load (left panel) and under a greater load (right panel). 
         FIG. 10  illustrates another embodiment of a sensor. 
         FIG. 11  illustrates another embodiment of a sensor. 
         FIG. 12  illustrates an anterior spinal interbody device (ACDF or ALIF) spine insert including a sensor. 
         FIG. 13  illustrates an anterior spinal interbody device (ACDF or ALIF) spine sensor including a sensor following implant. 
         FIG. 14A  illustrates a finite element analysis (FEA) of the movement of a load indicator rod comparing the load indicator rod location on a sensor under no load (top) and under a load (bottom). 
         FIG. 14B  provides photographs of the movement of a load indicator rod comparing the load indicator rod location on a sensor under no load (top) and under a load (bottom) on a sawbone tibia model. 
         FIG. 14C  provides radiographs of the movement of a load indicator rod comparing the load indicator rod location on a sensor under no load (top) and under a load (bottom) on a sawbone tibia model. 
         FIG. 15  presents radiographic images of a human tibia cadaver model under no load (left column) and under a 400 Newton (N) load (right column). Images include those of a plated fractured tibia (top panels) and a plated intact tibia (lower panels). 
         FIG. 16  schematically illustrates a top, front, and side view of a sensor. 
         FIG. 17  schematically illustrates a sensor as illustrated in  FIG. 16  affixed to an orthopedic fixation plate. 
         FIG. 18  illustrates radiographs of a plated fractured tibia under increasing load. The magnified images show the load indicator rod and the internal scale of the sensor. 
         FIG. 19A  schematically illustrates a bone having a 1 cm osteotomy and affixed support plate including a sensor as described. 
         FIG. 19B  illustrates a sensor for the bone of  FIG. 19A  under no load and under a 400 N load. 
         FIG. 19C  illustrates an intact bone with support plate affixed. 
         FIG. 19D  illustrates a sensor for the intact bone of  FIG. 19C  under no load and under a 400 N load. 
         FIG. 20  illustrates a femur including a titanium dynamic hip screw affixed thereto (left), and a femur including a stainless steel dynamic hip screw affixed thereto (right). 
         FIG. 21  illustrates a sensor as may be incorporated in a cannula of an implant (e.g., a dynamic hip screw). 
         FIG. 22A ,  FIG. 22B ,  FIG. 22C , and  FIG. 22D  illustrate end views of a dynamic hip screw including a sensor as illustrated in  FIG. 21 . 
         FIG. 22E  provides a graph showing displacement of an indicator rod of the sensor of  FIG. 21  under increasing load. 
         FIG. 23  illustrates top views (left) and side views (right) of an increasingly swollen hydrogel indicator as may be utilized in a chemical sensor as described herein. 
         FIG. 24  graphically illustrates the swelling ratio of a hydrogel with pH. 
         FIG. 25  illustrates a chemical sensor prior to (left) and following (right) swelling of a hydrogel indicator. 
         FIG. 26  illustrates another chemical sensor affixed to an orthopedic implant in the absence of swelling of the hydrogel indicator (left) and following swelling of the hydrogel indicator (right). 
         FIG. 27  illustrates a chemical sensor affixed to an orthopedic implant. 
         FIG. 28  illustrates a chemical sensor capable of determining presence of multiple analytes. 
         FIG. 29  illustrates a plated tibia fracture with a magnified view of a sensor as described herein under no load and under a 100 N load. 
         FIG. 30A  illustrates a bone under increasing loads with a supporting plate and sensor affixed thereto. 
         FIG. 30B  illustrates the relationship between the displacement of the load indicating rod and the maximum callus strain of the bone of  FIG. 30A . 
         FIG. 31A  graphically illustrates the response of a sensor with variation in load. 
         FIG. 31B  graphically illustrates the response of a sensor with variation in callus stiffness. 
         FIG. 31C  graphically illustrates the response of a sensor with variation in compression. 
         FIG. 32  illustrates a chemical sensor at two different pH levels. 
         FIG. 33  illustrates the repeatability of the response of the sensor of  FIG. 32   
         FIG. 34A  illustrates the fixation of a chemical sensor on a cadaver tibia. 
         FIG. 34B  illustrates the variation of the sensor of  FIG. 34A  upon expansion. 
         FIG. 34C  provides radiographs of the sensor of  FIG. 34A  upon expansion. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present invention. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the disclosed subject matter, one or more examples of which are set forth below. Each embodiment is provided by way of explanation of the subject matter, not limitation thereof. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present disclosure without departing from the scope or spirit of the subject matter. For instance, features illustrated or described as part of one embodiment, may be used in another embodiment to yield a still further embodiment. 
     In general, the present disclosure is directed to devices and methods for determining bone health. More specifically, disclosed are sensors that can be utilized in conjunction with orthopedic implants that can provide information with regard to the ability of orthopedic tissue (e.g., bones, ligaments, tendons, etc.) to support a load and/or the presence of infection or other health issue in the area of an implant. Beneficially, the sensors are passive and can be read by use of standard radiographic imaging techniques (e.g., X-ray, computed tomography (CT) scanning, etc.) that are already taken as part of a standard patient evaluation. Moreover, the sensors can be affixed to known implantable support devices (e.g., tibial plates, spinal inserts, screws, rods, pins, etc.) without excessive modification of the implants so as to non-invasively monitor the implant to provide a quantitative assessment of one or more characteristics in the local area of the implant. 
     In one embodiment, the passive sensor can be a strain sensor that can be utilized to determine when a damaged bone, joint, or soft orthopedic tissue is sufficiently healed to allow safe weight-bearing upon the limb. While the bulk of this application discusses utilization of disclosed sensors in conjunction with bones, it should be understood that disclosed sensors can be utilized in conjunction with any orthopedic implant and any orthopedic tissue including, without limitation, bone, ligament, tendon, meniscus, etc. The strain sensors are based upon the transition of load from a supporting implant to the orthopedic tissue (e.g., bone) as the tissue heals and becomes stronger. For instance, at initial fixation, a fractured bone can take little or no load without possibility of further injury. As such, most or all of a load (e.g., an axial load in the case of a long bone) placed on the bone will be carried by the supporting orthopedic implant structure. 
     To illustrate,  FIG. 1A  schematically illustrates a fractured bone  10  with an orthopedic fixation device  12  affixed to the bone  10 . As shown, the fixation device  12  spans the fracture  14  and supports the bone  10 . As shown at B, the fixation device  12  is off of the axis of the bone  10 . When an axial load  16  is placed on the bone, e.g., during standing or walking, the fractured bone  10  cannot support the load  16  and at least a portion of the load  16  will be translated to a load  18  on the fixation device  12  causing the fixation device  12  to bend. Though illustrated as a compressive axial load, disclosed strain sensors can indicate transference of a load from a bone to an attached orthopedic implant and/or direct load on an orthopedic implant for any load applied in any direction and any degree of freedom including any one or any combination of compression, extension, and torsion loads. Exemplary loads can include, without limitation, axial compressive loads, axial extension loads, transverse bending loads, transverse compressive loads, axial torsion loads, and/or compressive, extensive, or torsion loads applied at any angle to an axial direction of a bone or sensor. 
     As the bone  10  heals, it becomes strong enough to carry more and more of the load  16 . For instance, as shown at  FIG. 10 , following a period of healing (for instance, when the fractured bone has about 25% or more of the bending stiffness of intact bone), an axial load  16  placed upon the bone  10  can be carried by the bone  10  with little or none of the load translating to the fixation device  12 . As such, the fixation device  12  will exhibit little or negligible bending. As is known, some degree of load placed on the bone can stimulate bone growth, and as the bone heals, bending of the fixation device will decrease as the bone heals and shares more of the load. For instance, animal models that examined plate strain via percutaneous access showed decrease during walking as the bone healed. 
     The passive strain sensors have been designed around the concept of the load transference from an orthopedic fixation device to the surrounding tissue that takes place during normal healing. The sensors can be utilized to determine the distribution of a load between a bone and a fixation device and, as such, can provide an indication of the bone health and level of healing. The passive sensors can be mounted to orthopedic devices so as to non-invasively serve to quantify implant bending/deflection using standard radiography. In some embodiments, the strain sensors can amplify the normal deflection of an orthopedic implant to provide clear indication of the load transfer to an implant. This can provide quantitative thresholds for safe weight-bearing during rehabilitation so as to not only prevent complications due to load-bearing too soon in the healing process but also to stimulate bone growth through increased load-bearing as the healing process progresses. Through use of the sensors, a caregiver can identify the point at which a fracture has sufficiently healed so that safe weight-bearing and rehabilitation may begin, as well as recognize when healing is insufficient for such weight-bearing, in order to reduce complications from premature weight-bearing and for the early detection of possible complications such as fracture nonunion. 
     Beneficially, the sensors are passive and, as such, require no internal energy source or active interrogation for examination. The sensors can be easily read with standard radiography available in all hospitals and many clinics and can be incorporated into existing implant constructs to provide robust, durable performance for use in conjunction with any orthopedic tissue. The sensors can be incorporated with fixation devices in a wide variety of orthopedic applications, in addition to long bone fixation applications including, without limitation, spine fusion, hip fixation, total or partial joint replacement of any joint, and others to provide mechanical and mechanically-transduced orthopedic measurements. Implantable orthopedic components of interest include, without limitation, cervical spine plates, spinal fusion devices, dynamic hip screws, compression plates, locking plates, screws, intramedullary rods, etc. 
     One embodiment of a strain sensor  20  is illustrated in  FIG. 2 . A sensor  20  can include an indicator  22  and an internal scale  24 . The internal scale  24  can be used to quantify bending of a fixation device as an accurate and reproducible characterization of fracture healing via radiographs. The indicator  22  of a strain sensor can be a load indicator and can include a first end  26  and a second end  27 . A first end  26  of the indicator  22  can be configured so as to be directly or indirectly joined to an implantable orthopedic fixation device. For instance, in the embodiment illustrated in  FIG. 2 , the first end  26  of the indicator  22  is held by a housing  28  that can be attached to an orthopedic fixation device, e.g., a fixation plate  12 . The first end  26  of the indicator  22  can be directly attached to the orthopedic device or to a housing that is, in turn, attached to an orthopedic device. In any case, the first end can be attached such that there is little or no relative motion between the orthopedic implant and the first end  26  of the indicator  22  when the bone is placed under a load, even in those cases in which the full load is transferred from the bone to the orthopedic implant. Attachment mechanisms can be any suitable mechanisms as is known in the art, including as a monolithic component of the device as formed or via attachment following formation. Attachment mechanisms can include utilization of adhesives, screws, pins, melt bonding, etc. 
     In one embodiment, the first end  26  of the indicator  22  can be fixed to a portion of the fixation plate  12  that is expected to exhibit a large movement distance when the bone to which it is attached is placed under load. For instance, when considering a long bone fixation plate  12 , the sensor  20  can be located on the fixation plate  12  such that the first end  26  of the indicator  22  is near or at the end of the fixation plate  12 . As such, when the bone to which the plate  12  is attached is placed under a load (as illustrated in  FIG. 1B ), and when a portion of that load  18  is transferred to the fixation plate  12  causing the plate  12  to bend ( FIG. 2 , bottom panel), the first end  26  of the indicator  22  can be at or near the locale of maximum movement distance. 
     The second end  27  of the indicator  22  is free to move relative to the fixation plate  12  and the bone to which the sensor is attached. For instance, if the sensor  20  includes a housing  28 , the second end  27  can be located such that it is not fixed to the housing  28 . 
     A housing  28  can be of any suitable design and material so as to protect the sensor and not interfere with sensor function and radiographic imaging. For instance, a housing can be made of rigid or semi-rigid biocompatible and implantable formation materials, and at least that portion of a housing  28  that covers a scale  24  can be radiographically transparent. In one embodiment, a non-immunogenic, radiographically transparent material can surround at least the second end  27  of the indicator  22  to allow the second end  27  to move relative to the fixation plate  12  while also forming a barrier between the second end  27  and surrounding tissue and allowing the second end  27  to be observed via radiographic imaging. 
     The lower panel at  FIG. 2  demonstrates the reaction of the sensor  20  under a load  18 . As shown, the housing  28  and the first end  26  will move with the fixation plate  12  as it bends under the load  18 . However, the second end  27  is free to move with respect to the fixation plate  12  and will not bend with the fixation plate  12 . Thus, there will be a relative motion between the fixation plate  12  and the second end  27 . The internal scale  24  is also fixed to the housing  28  and/or the fixation plate  12  and is located such that the relative motion between the fixation plate  12  and the second end  27  can be discerned on the internal scale  24 . 
     In the embodiment of  FIG. 2 , the indicator  22  is in the form of an elongated pin or rod having an aspect ratio (length divided by width) of greater than 1. At least the second end  27  of the indicator  22  can be formed of a radio-opaque material such as, e.g., tungsten, so as to be clearly visible during use. 
     The direction of movement of the indicator of the sensor is not particularly limited. For instance, with the lever-type indicator of  FIG. 2 , the direction of motion of the second end  27  of the sensor  20  can be generally parallel to the direction of the load  18 . This is not a requirement, however, and in other embodiments, the second end of the load indicator can move in other directions. For example, the sensor  120  illustrated in  FIG. 3  includes a housing  128 , a scale  124 , and an indicator  122 , the second end of which  127  moves in a direction generally normal to the direction of the load  18 . As illustrated, the load indicator  122  includes a first end  126  that is configured to be held directly or indirectly to the fixation plate  12  (e.g., via the housing  128 ). Upon placement of a load  18  on the sensor  120 , the first end  126  is compressed. The compression of the first end  126  causes the second end  127  to elongate and thereby to move in a direction generally parallel to the axial direction of the indicator  122  and normal to the direction of the load  18 . 
     The capability of elongation of the second end  127  can be provided by formation materials of the indicator  122 . For instance, the indicator  122  can include an extendable and radiographically visible material, e.g., an elastomeric solid, a gel, or a liquid that can extend at the second end  127  upon compression of the first end  126  such that the extension is radiographically discernible. The fluid or gel can serve as a transducer to convert motion in one direction to motion in another direction, as is well known in hydraulic and pneumatic actuation systems, optionally with amplification. A radio-opaque fluid may be used, (e.g. a solution with a high concentration of a high atomic element such as mercury, gallium, galistan, water solutions with rare earth nitrates, and dispersions of radio-opaque micro- or nanoparticles such as, but not limited to, gold nanoparticles, tungsten nanoparticles, bismuth nanoparticles, and gadolinium oxysulfide microphosphors. In addition to a radio-opaque fluid or gel, a radiolucent fluid may be used to push a radio-opaque marker such as, but not limited to, a tungsten pin or tantalum bead. Elastomers including, but not limited to, polydimethyl siloxane (PDMS), acrylic elastomers, and others can also be used in place of a hydraulic or pneumatic system: When load is applied in one direction due to applied load, elastomers generally expand in other directions, a phenomenon known as the Poisson effect. For example, when a spherical PDMS gel containing radio-opaque gadolinium oxysulfide microphosphors is pressed between two flat surfaces, it becomes roughly disc shaped with a diameter that increases with increasing load, and this change in diameter of a sensor indicator can be observed using radiography. 
     As illustrated in  FIG. 2  and  FIG. 3 , a sensor can include an internal scale  24 ,  124  that can be used to indicate relative motion between the second end  27 ,  127  of the indicator and the fixation plate  12 . The scale of a sensor can be located and arranged so as to clearly discern the position of the second end of an indicator with respect to the fixation device to which the sensor is attached. For example, in the embodiments of  FIG. 2  and  FIG. 3 , the scales  24 ,  124  are rotated with respect to one another so as to more clearly discern the position of the second end  27 ,  127  of the indicators  22 ,  122 , respectively. This rotation is indicated in order to fully encompass the direction motion of the indicators  22  (i.e., lever-type indicator) and  122  (extendible-type indicator). 
     In the embodiments of  FIG. 2  and  FIG. 3 , the internal scale  24 ,  124  is provided as a series of round markings in/on the housing  28 , 128 . For instance, the markings can be formed of a material that is radiographically discernable from the surrounding material of the housing. By way of example, a scale  124  can include a series of markings (e.g., circles) formed of a radio-opaque material such as tungsten that are surrounded by a radiographically transparent material. It should be understood, however, that the form of the internal scale is not critical, and any scale form, shape, or material that is radiographically discernible from the surrounding material is encompassed herein. For instance,  FIG. 4  illustrates several different scale configurations including circular apertures  2 , trenches  4 , linear markings  6 , etc. Any scale that can demonstrate relative motion between a movable segment of a sensor indicator and the orthopedic fixation device to which the sensor is affixed is encompassed herein. In addition, in one embodiment a scale  124  can include predetermined markings (e.g., circles, bars, lines, etc.) that are set at a physiologically relevant threshold so as to determine safe or unsafe weight-bearing when the affixed bone is placed under a load. 
     The dimensions of the indicator and/or the scale can be suitable so as to provide a radiographically determinable indication of movement of the attached orthopedic device. For instance, a relatively long lever-type indicator such as that of  FIG. 2  can amplify the movement distance of the orthopedic device so as to more accurately determine the distance of motion of the orthopedic device and more accurately determine the load on the orthopedic device. In one embodiment, the indicator may be tapered to provide both a relatively thick rigid base and a finer tip for pointing at the scale. 
     In some embodiments, the scale may have finely designed features (e.g., holes) to allow visualization of small motions; for instance, by use of a Vernier-type scale based upon moiré fringes as illustrated in  FIG. 5 . Other amplification techniques based upon gears, levers, pulleys, hydraulics, pneumatics, etc. can be used. For instance, an extendable-type indicator can incorporate a relatively small cross-sectional area for the extendable material, and a relatively small motion of an attached orthopedic device can be amplified to a relatively long extension of the indicator. By way of example, when considering a hydraulic system using an essentially incompressible fluid, in order to conserve fluid volume, the displacement of the fluid within a tube is generally the ratio between the area of the piston or region pressing on the fluid to the area of the fluid in the tube (e.g. if a 10 mm 2  area piston moves 200 μm as it pushes fluid through a 1 mm 2  tube, the fluid in the tube would move 2 mm). The same principle also applies to fluids being pushed by a bulb instead of a piston. 
     Amplification of the distance of motion of an orthopedic device at the sensor indicator can improve accuracy of the devices. By way of example,  FIG. 6 ,  FIG. 7 , and  FIG. 8  illustrate exemplary amplification systems. In the system of  FIG. 6 , the system can include an extendable material  7  that can be a component of an indicator that also includes a lever arm  21  as described above. Upon relative motion between the lever arm  21  of the indicator and an orthopedic device (not shown in  FIG. 6 ) due to a force  18 , the extendable material  7  can be extended, and the change in location of an end of the indicator (e.g., the end of the extendable material  7 ) can be radiographically discernable.  FIG. 7  illustrates another embodiment in which the indicator  22  includes a series of lever arms  23 , the response of which to a force  18  on the sensor can amplify the distance that is moved by the orthopedic implant (not included in  FIG. 7 ). In the embodiment of  FIG. 8 , a sensor including an extendable material  7  is illustrated. In this embodiment, when the sensor is placed under a load  18 , the extendable material  7  retracts. Any combination of materials and geometries as are known in the art can be utilized to amplify a radiographically detectable signal of a sensor. Of course, amplification of a signal is not a requirement of a sensor. In general, the ratio of distance of motion of an orthopedic implant to distance of motion of a second end of an indicator can be from about 1:1 to about 1:100, from about 1:2 to about 1:50, or from about 1:3 to about 1:10 in some embodiments. In some embodiments, a fine and course scale can be used. Of course, the size of the sensor components will also depend upon the size and shape of the particular fixation device used with the sensor. 
       FIG. 9  illustrates another example of a sensor  220  that includes an indicator  222  formed of an extendable material. In this embodiment, the indicator  222  can be a radiographically discernible liquid, solid, or gel as discussed above confined to a reservoir  221  at a first end  226  of the indicator  222  and a track  223  along which the extendable material can travel. During use, the sensor  220  can be located in conjunction with an implantable orthopedic device such that upon motion of the orthopedic device (e.g., bending as shown in  FIG. 1B ), a portion of the orthopedic device contacts the reservoir  221  at the first end  226  and forces the indicator  222  along the track  223 . The distance moved by the second end  227  of the indicator  222  (as indicated by the arrows  230  on the left and right panels of  FIG. 5 ) can be radiographically discernible. A scale  224  can be included on a sensor  220  as a series of markings, as shown. Alternatively, another component of the sensor  220  can be utilized as the scale. For instance, the turnings of the track  223  can be used as an internal sensor scale to indicate the distance moved by the second end  227  of the indicator  222 , and this movement distance can be correlated to a load on the orthopedic implant to which the sensor is attached. 
       FIG. 10  illustrates another embodiment of a sensor  320  that includes an extendable indicator  322 . In this embodiment, the sensor  320  can include a piston  321  that can transfer a load  18  to an extendable indicator  322  held in a track  323 . For instance, as illustrated in  FIG. 11 , a sensor  320  can be located adjacent to an implantable orthopedic device  312  and a load  18  transferred to the device  312  from a bone to which the device  312  is attached can be determined through examination of the sensor  320 , and in particular, through determination of the distance of motion of the second end  327  of the indicator  322  when the device  312  is under the load  18 . 
     As discussed above, an orthopedic implant for use with disclosed sensors is not limited to a fixation plate for a long bone. In one embodiment, a strain sensor can be utilized in conjunction with a spinal surgical instrumentation system, including those of spinal fusion operations and arthroplasty (artificial disc) operations. Such systems may include, by non-limiting example, anterior cervical discectomy and fusion (ACDF) systems, Anterior Lumbar interbody systems (ALIF), posterior instrumentation, artificial disc placement systems, and laterally placed anterior lumbar interbody spacers. 
       FIG. 12  illustrates a fusion assembly  412  for use in an anterior spinal fusion (lumbar or cervical) that can be utilized in conjunction with a strain sensor  420 . An assembly  412  can include a cage  410  and a bone graft  414  held within a lumen  411 . During use, a surgeon determines the proper size cage  410  to fill the intervertebral space ( FIG. 13 ). The surgeon then shapes the bone graft  414 , typically an allograft (or autograft), to fit the lumen  410  of the cage  410 . 
     An anterior interbody (lumbar or cervical) fusion operation includes surgical exposure of two or more cervical vertebrae (e.g., vertebrae  422 ,  424  of  FIG. 13 ), removal of the disc between the two (discectomy and decompression), placement of an interbody assembly  412  between the vertebrae  422 ,  424  to maintain a desired spacing between the vertebrae allowing fusion of the two bodies by bone graft incorporation of the bone graft  414 . Screw and plate stabilization is often added to improve fusion rates. 
     By use of a sensor  420  implanted in conjunction with the assembly  412 , a caregiver can follow rehabilitation and fusion of the bone graft  414  with the adjacent vertebrae  422 ,  424 . Prior to fusion of the bone graft  414 , a load placed on the assembly  412  can cause extension or compression of the distance between the vertebrae  422 ,  424 , and this variation can be translated as an increase or decrease of strain on the assembly  412 . By examining the sensor  420 , and determining the location of the second end of the indicator (e.g., as illustrated in  FIG. 10  and  FIG. 11 ) when the assembly  412  is placed under a load, a caregiver can be informed as to the level of fusion of the bone graft  414 . Specifically, early on in the healing process, when there is little or no fusion of the bone graft, the sensor can indicate a large response to an applied load. As the surgical site heals and the bone graft  414  fuses, the load will be increasingly taken by the implant and the fused graft, and the sensor  420  will exhibit a smaller response. 
       FIGS. 14A, 14B, and 14C  illustrates a portion of a lever-type sensor in which the scale  524  includes a series of radiographically discernible circular markings. Upon attachment to a fixation device  512 , the second end  527  of the load indicator  522  is located in conjunction with the scale  524  such that the location of the second end  527  with respect to the scale  524  is discernable. In particular, when under a load if the second end  527  moves relative to the fixation device  512  (i.e., the fixation device bends or flexes), the change in position of the second end  527  from the zero point can be discerned by observation of the scale  524 . 
       FIGS. 14A, 14B, and 14C  include views obtained from different imaging techniques of the second end  527  and scale  524  of a sensor lever-type indicator  522  under no load (top panels) and under a load (bottom panels) causing deflection of the fixation device  512  that is attached to both the sensor and a sawbone tibia model. The imaging techniques include an FEA simulation at  FIG. 14A , a photograph at  FIG. 14B , and a radiograph at  FIG. 14C . As can be seen, under no load, the second end  527  of the indicator  522  is aligned with the bottom-most opening of the scale  524  (i.e., a predetermined zero point). When the tibia is placed under a load, the second end  527  moves relative to the scale  524  and the second end  527  is now aligned with the second of the five openings of the scale  524 . Thus, it can be determined that at least a portion of the load on the tibia has transferred to the fixation device  512 . 
     A sensor can be attached to an orthopedic implant by any means and at any convenient location provided that the scale can be read radiographically and the sensor does not interfere with bone healing.  FIG. 15  illustrates one exemplary mounting of a sensor on a long bone fixation device  612 . As shown, the sensor is mounted such that the lever-type load indicator  622  is attached to the surface of a plate fixation device  612 . Using radiography, the position of the second end  627  of the indicator  622  can be seen with respect to the scale  624  and the movement due to the load transference to the implant  612  can be observed. As shown, the scale  624  indicates no relative motion between the second end  627  of the load indicator  622  and the scale  624 , when an intact human cadaver tibia, is loaded with a 400 N force (bottom panels). In contrast, when examining a load on an unstable fracture (upper panels), significant relative motion of the second end  627  of the load indicator  622  can be seen by examination of the scale  624  between a no-axial load position and a 400 N axial load on the bone (½ body weight load). 
     There is no particular arrangement of components or mounting orientation required for a sensor. For instance, in the embodiments of  FIGS. 14A-140 and 15 , the indicator second end is observable behind the scale. However, a scale of a sensor can be formed so as to be visible from any one or multiple views of the indicator or off-set to a greater or lesser degree. For instance, the scale can be a part of a track filled by the indicator as discussed above. Alternatively, a portion of an implant can be used as the scale (e.g., the threads on a screw, bolt, hole, etc.) or markings on an implant component. For instance, a scale can be located within or formed along the edge of an aperture of the orthopedic implant (e.g., within an unused screw hole). In such an embodiment, the second end of the indicator can pass above the scale, and the scale can be read by an overhead view of the aperture. 
     In any case, the sensor components can be located such that they do not interfere with proper function of the implant and such that they do not cause irritation at the implant site. For instance, a sensor can be located at one side of a compression plate and optionally can utilize unused screw holes. In general, a sensor can have a relatively small profile and can either be set within the profile of the associated orthopedic implant or can extend to a small degree beyond the surface of the orthopedic implant. This can be particularly beneficial when considering a sensor for use in conjunction with an implant configured for location deep within tissue. 
     One embodiment of a low-profile sensor is illustrated in  FIG. 16 . As shown, the sensor can include a radio-opaque indicator  722  that includes a first end  726  and a second end  727 . The second end  727  being arranged such that its location is discernable with respect to a scale  724  that includes a series of markings formed as apertures in a radio-opaque material. Accordingly, relative motion between the second end  727  and the scale  724  can be observed when a load is sufficient to cause bending of the attached implant. 
     The sizes and materials of the various components of a sensor can vary depending upon the particular design of the sensor and the orthopedic implant to which the sensor will be attached. For instance, the cross-sectional dimension of the scale features (e.g., the diameter of the circular markings of the scale  724  of  FIG. 16 ) can generally be from about 0.05 mm to about 1 mm, such as about 0.5 mm in some embodiments, and can be spaced apart from one another at a similar distance (e.g., from about 0.5 mm to about 1 mm) so as to be clearly discernible during imaging. Smaller increments can generally be detected using higher resolution/magnification X-ray imaging setups and/or moiré patterns and/or image analysis algorithms. The second end  727  of the load indicator can likewise be sized so as to be clearly discernible when viewed against the scale  724 . For instance, a second end  727  of an indicator  722  can have a width of from about 0.5 mm to about 1 mm. As discussed above, the location and dimensions of an indicator can be designed to amplify the motion distance of the orthopedic implant and more clearly discern the load transferred to the orthopedic implant. As such, a lever-type indicator (or track of an extendible indicator) can be quite long, e.g., from about 2 cm to about 20 cm in some embodiments, or even longer in some embodiments. 
     The materials utilized to form a sensor can be chosen in order to clearly discern the relative motion of the second end of the indicator against the scale. For instance, at least the second end of the indicator and the scale markings (e.g., the circles) can be formed of a radiographically discernible material, e.g., tungsten, tantalum, stainless steel, etc. 
     When included, a housing  728  of a sensor can be radiographically transparent or opaque, depending upon the nature of the indicator and scale materials, the arrangement of the various sensor components, the implant type to which the sensor is to be fixed, and so on, as would be evident to one of skill in the art. 
       FIG. 17  illustrates the sensor of  FIG. 16  attached to a bone fixation plate  712 . As shown, the sensor is attached so as to present a relatively low profile against the plate  712  while still being capable of being read by imaging the plate  712  from the proper orientation. Generally, the second end  727  and the scale  724  can be protected from the environment of the surrounding implant area; for instance, by use of a radio-lucent formation material. By way of example, all or part of a sensor can be surrounded by a radio-lucent polymeric material. 
     A sensor can include a calibration/adjustment mechanism to properly align and zero the indicator and scale upon implant. For instance, and with reference to  FIG. 17 , a sensor can include an adjustment set screw  729  (or equivalent) that can be used to adjust the initial zero position of the second end  727  of an indicator  722  in relation to the scale  724  upon attachment of the orthopedic implant to adjacent bones when under no excessive load. For example, the second end  727  of the indicator  722  may be adjusted and/or the scale  724  can be moved to zero the sensor at initial implant of the orthopedic device. Of course, a zeroing adjustment can be carried by any useful means and mechanisms. 
     A sensor can be formed as a unitary part of an orthopedic fixation device at manufacture or can be mounted to a pre-existing orthopedic fixation device. For instance, a sensor can be added onto a fixation device following formation via any suitable attachment means. By way of example, the embodiment of  FIG. 17  can be attached to an existing fixation plate by sizing the housing  728  to fit on the implant  712 . Following proper location on the implant  712 , the housing  728  can be locked into place by use of, e.g., a C-clamp, plate-on-plate attachment with screws into threaded screw holes in a locking plate, or attachment in a recess below the screw holes of the fixation plate  712 . 
       FIG. 18  illustrates increasingly magnified views of a sensor utilized in conjunction with a broken tibia model. As shown, as the axial load on the tibia increases from 0 N to 300 N, the amount of load transferred to the attached fixation plate will likewise increase as the broken tibia is unable to carry little if any of the axial load. The load transferred to the attached fixation plate will manifest itself by causing the fixation plate to bend. The second end of the load indicator can be clearly seen in conjunction with the scale in the bottom-most panels. As shown, as the load increases, the location of the second end of the load indicator moves across the scale from the right-most hole in the first panel (Hole  1 ) to the center hole in the last panel (Hole  3 ). 
     A FEA simulation of strain and pin displacement for a fixation plate with a radiographic indicator is shown in  FIGS. 19A-19D . The strain on the construct with a 1 cm osteotomy (commonly used as an experimental model for unstable/comminuted fractures) under 400 N of axial compression load (at about ½ body weight) is shown in  FIG. 19A . Due to the osteotomy, the load will be concentrated on the plate, and the sensor can detect plate bending by a change in the position of the second end relative to the scale.  FIG. 19B  shows the position of the second end under 0 and 400 N loads. 
     The simulation was repeated with an intact plated bone (representing bone before fracture or after complete healing).  FIG. 19C  shows the fixation device and sensor attached to the bone.  FIG. 19D  illustrates minimal strain on the plate from load sharing with the bone, as even under a 400 N load there is little or no displacement of the second end of the load indicator. Although a mid-shaft osteotomy is shown in  FIGS. 19A-19D , the same underlying mechanical principles can apply to proximal fractures, as well as other orthopedic applications. 
     Of the two million fracture fixation surgeries each year in the U.S., 330,000 are for hip fractures. If the bone does not heal rapidly enough, the screws and other hardware can fail, which usually requires costly revision surgery and significant risk of morbidity and mortality. The most common hardware failure (75%) is from screw cutout with other significant failures from plate pullout, infection, and screw bending. 
     In one embodiment, disclosed sensors can be incorporated internal to an orthopedic implant rather than external as illustrated above. This may be a particularly beneficial approach in certain orthopedic applications such as joint replacements and hip implants, e.g., in repair of an intertrochanteric hip fracture, for measurement of load and excessive motion of one or more components of the implant. 
     In one such embodiment, a sensor can be incorporated within a dynamic hip screw. In this embodiment, the sensor can be utilized to radiographically indicate biomechanical bending of the screw in order to determine the extent of load sharing between the joint and the dynamic hip screw so as to determine whether the fracture is healed enough for weight-bearing. 
       FIG. 20  illustrates a femur including a titanium dynamic hip screw (left) and a femur including a stainless steel dynamic hip screw (right) as examples of dynamic hip screws that can be implanted in conjunction with a sensor as described herein. In this embodiment, in which the orthopedic implant is designed for placement within bone tissue, it can be beneficial to incorporate the sensor within the implant; for instance, within the cannula of a dynamic hip screw. 
       FIG. 21  illustrates a sensor  820  that can be incorporated within the cannula  811  of a dynamic hip screw  830 . As shown, the sensor  820  includes an indicator  822  that includes a first end  826  and a second end  827 . In this embodiment, the indicator  822  is a lever-type indicator. The scale  824  can be at the terminal portion of the hip screw  830  and under no load (top panel); the second end  827  of the indicator  822  is centered within the cannula  811 . Under load (bottom panel), the second end  817  of the indicator  822  will move relative to the scale  824  (i.e., the outer ring of the hip screw  830 ), and the location of the second end  827  against the scale  824  can inform the caregiver that the dynamic hip screw is bending under the load applied to the hip. 
       FIG. 22D  presents a side view of a dynamic hip screw implanted within a fractured femur and several end views ( FIG. 22A - FIG. 22C ) of the dynamic hip screw  830  that includes a sensor within the cannula. Dynamic hip screws are designed to slide but not rotate, with physiological bending directed along one axis. As shown in  FIG. 22A - FIG. 22C , the second end  827  of the indicator can have some degree of play within the cannula  811  and will move relative to the hip screw when the screw is placed under load. The first end  826  of the indicator  822  can be fixed within the dynamic hip screw; for instance, by threading, adhesive, melt bonding, etc. 
     By use of the disclosed sensors, bending of the dynamic hip screw can be observed directly by reference of the location of the second end  827  to a scale  824  that can be formed in the end of the hip screw or simply by utilizing the edge of the cannula  811  as the scale. In one embodiment, a scale  824  can be formed into or attached to the end of the dynamic hip screw to more clearly determine the load amount and direction. For instance, a scale can include markings on a cap (e.g., a radiolucent cap) that can be placed over the open end of the dynamic hip screw. This can not only provide a detailed scale but can also prevent bone in-growth into the cannula of the dynamic hip screw. Of course, a cap can be provided to cover an end of a dynamic hip screw that does not include scale markings thereon, as well. 
     By observation of the scale  824  in an end view of the dynamic hip screw, the alignment of the second end  827  within the cannula  811  can be seen, and excessive bending of the screw  830  can be evident. For example, in the illustrated embodiment under a load of 1 Newton (N), the second end  827  is essentially centered within the cannula  811 . As the load increases to 500 N, the second end  827  has moved off-center toward one edge of the cannula  811  and at a greater load of 1000 N, the second end  827  is pushed against the side of the cannula  811 . The graph shown in  FIG. 22E  presents the displacement distance of a cannulated tungsten rod with load for two trials. The rod was zero-centered within the cannula of a 90 mm lag screw for the trials. 
     Use of disclosed strain sensors can reduce complications from re-fracture or hardware failure by restricting weight-bearing until a bone is sufficiently healed. The quantitative metric within the devices can identify patients with delayed and non-union fractures so that adjunctive therapies may be used. The sensors can also provide information such that normally-healing patients can begin weight-bearing when a fracture sufficiently heals, generally earlier than current treatment protocols. These capabilities can result in reduced healthcare costs, earlier return to work, and improved quality of life. 
     In another embodiment, a sensor can be a chemical sensor that can be utilized to assess the local area of an implant for one or more analytes. For instance, an area can be assessed for the presence of an analyte that can indicate early stage infection at the implant site. 
     According to this embodiment, the indicator of a sensor can include a material that, in the presence of a targeted analyte, can exhibit a change that is detectable by radiographic imaging. For example, the indicator can exhibit a dimensional change upon interaction with the targeted analyte. The sensor can also include a scale as discussed above and optionally, a radiographically discernable component, such as an extended rod, channel, or the like, that can amplify and improve visualization of the dimensional change of the indicator in the presence of the analyte. 
     In general, an analyte-sensitive sensor can include a hydrogel portion that is chemically configured to vary its displacement volume according to changes in concentration of an analyte in the surrounding area. Hydrogels sensitive to analytes have been described; for instance, in U.S. Pat. No. 6,751,491 to Lew, et al.; U.S. Pat. No. 6,835,553 to In Suk Han, et al.; and U.S. Patent Application Publication No. 2009/0170209 to Machauf, et al.; all of which are incorporated herein by reference. As a component of a sensor, the hydrogel can be disposed within a housing as described previously. The housing, at least in the area of the hydrogel, can include a semipermeable membrane to prevent contact between the surrounding environment, while allowing the analyte to pass through the membrane and interact with the analyte-sensitive hydrogel. The housing and membrane may also be configured to prevent surrounding tissue from mechanically pressing on the hydrogel. 
     An analyte-sensitive hydrogel may include any analyte-sensitive material that can directly or indirectly modify the displacement volume of the hydrogel in response to a change in analyte concentration. For example, the analyte-sensitive material can directly bond with the analyte to modify the displacement volume of the hydrogel. Alternatively, the analyte-sensitive material can catalyze a reaction of the analyte, and the reaction product can directly or indirectly modify the displacement volume of the hydrogel. 
     By way of example and without limitation, in one embodiment, an analyte-sensitive material can be immobilized within a pH-sensitive hydrogel. As utilized herein, the term ‘pH-sensitive hydrogel’ refers generally to a hydrogel modified to contain pendant-charged groups on polymers of the hydrogel in proportions that produce an overall acidic or basic environment in the fluid within the gel. An analyte-sensitive material present in the hydrogel can bind to the targeted analyte or catalyze a reaction of the targeted analyte and thereby provide a charged product within the hydrogel. The charged product generated by activity of the analyte-sensitive material can cause the hydrogel to change its displacement volume (swell or shrink). For example,  FIG. 23  presents top views (left) and side views (right) of a pH-sensitive hydrogel made from polyacrylic acid in increasingly basic environments.  FIG. 24  graphically illustrates the swelling ratio of the hydrogel of  FIG. 23  with increasing pH. 
     A pH-sensitive hydrogel can include a copolymer synthesized from various types of methacrylate-derived monomers by free radical solution polymerization, as are known in the art. Such copolymers can be relatively tough, flexible polymers that are highly biocompatible and inert yet non-degradable in vivo. Synthesis conditions for pH-sensitive hydrogels have been well established. In one particular embodiment, acrylamide or dimethylacrylamide can be utilized as a polymer backbone, sodium acrylate as a pH-sensitive pendant group, and N,N′-methylene-bis-acrylamide as the cross-linker. To obtain pH sensitive hydrogel copolymers with desired properties, the ratios of the monomers and cross-linker can be varied as is known in the art. 
     The targeted analyte can include one or more compounds, the presence of which in the local area of an orthopedic implant can indicate infection. For instance, the analyte can include a pathogen or a determinant of a pathogen that can be, e.g., an endotoxin or exotoxin of the pathogen or a compound present in the local area due to an infection response of the host. Analytes of interest can include, without limitation, hydrogen ions (e.g., pH), quorum sensing molecules, glucose, lactic acid, carbon dioxide, proteins, proteases, antibiotics, drugs, etc. 
     In one particular embodiment, the targeted analyte of a sensor can be glucose, which can provide an indication of infection from a broad range of causative agents. Even at early stages, the presence of infection is generally accompanied by an inflammatory state. Cytokines released during inflammation can affect glucose metabolism directly or indirectly by augmenting glucoregulatory hormone secretion and curtailing insulin release. In particular, glucose can be shunted away from its normal sites of utilization to sites that assist in overcoming infection, and as such, increased levels of glucose in a local area can be an excellent broad-spectrum indicator of early-stage infection in the area. 
     One such glucose-sensitive chemical sensor can include an immobilized glucose oxidase that can detect the presence of glucose via the conversion of glucose to gluconic acid by the enzyme. The rate of gluconic acid formation is proportional to the glucose concentration in the hydrogel at the reaction location. A change in glucose concentration in the fluid surrounding the hydrogel can lead to a change in the pH value within the hydrogel due to the glucose oxidase-catalyzed production of gluconic acid. In particular, the gluconic acid product can protonate pH-sensitive pendant groups in the hydrogel and causes the hydrogel to swell or shrink, depending on the nature of the pendant groups. If the hydrogel contains basic pendant groups such as diethylaminoethyl methacrylate, it will swell when pH decreases. If it contains acidic pendant groups such as acrylic acid (AA), the hydrogel will shrink when pH decreases. 
     In another embodiment, an analyte-sensitive material can directly interact with the targeted analyte. For instance, a hydrogel can include analyte binding molecules and, upon binding, the hydrogel can exhibit a change in volume due to steric changes within the hydrogel or the presence of charge groups on the bound analyte (e.g., in the case of a pH-sensitive hydrogel). 
     In yet another embodiment, an analyte-sensitive hydrogel can include analyte-binding molecules and analyte analogue molecules immobilized therein, optionally in conjunction with charged pendant groups on the hydrogel polymer. In the absence of free analyte, the immobilized analyte-binding molecules can bind to the immobilized analyte analogue molecules, forming what are, in effect, cross-links within the hydrogel (generally non-covalent cross-links). As free analyte in the local area interacts with the hydrogel, binding competition can displace immobilized analyte analogue molecules with free analyte molecules, thus reducing the number of cross-links. This reduction in cross-linking causes swelling of the hydrogel. This type of sensitive hydrogel is based on a competitive mechanism and does not require oxygen-consuming enzymatic reactions. 
     In one embodiment, an analyte-binding hydrogel can contain immobilized phenylboronic acid or another glucose-binding molecule that can be covalently bonded to the hydrogel polymer, for instance via diols on the hydrogel polymer. For instance, a hydrogel can include one or more polyols, such as poly(vinyl alcohol) (PVA), with other pendant groups as necessary to achieve the desired sensitivity, response, and durability. As discussed above, an analyte-binding hydrogel can optionally include analyte analogue molecules, such as a D-sugar or other carbohydrate, that can bind the glucose-binding molecule with a non-covalent bond. The volume of this hydrogel can change in the presence of free glucose due to a competitive binding effect of the free glucose with the glucose-binding molecule in place of the glucose analogue molecule. When glucose concentration increases near the implant area, additional amounts of free glucose can diffuse into the hydrogel and displace the glucose analogue (e.g., a D-sugar) from the binding sites of the immobilized glucose-binding molecule. This can reduce the hydrogel cross-link density, and thus, the hydrogel can swell. 
     To produce a competitive binding glucose targeting hydrogel, glucose analogue molecule-conjugated and glucose-binding molecule-conjugated vinyl monomers can be synthesized. These conjugated monomers can then be co-polymerized with cross-linkers and either cationic or anionic monomers as discussed above. The cross-linker introduces a small number of permanent cross-links into the hydrogel in order to keep hydrogel integrity at all free glucose concentrations. 
     Examples of glucose-binding molecules can include, without limitation, lectins (e.g., Con A, glucokinase, xylose isomerase, and isolactin I), glucose antibody, concanavalin A, boronic acid, thiols, cell membrane receptors, cytosol receptors, nuclear receptors, heparin, DNA, RNA, polylysine, polyarginine triazine dye, Coomassie® Blue, Azure A, metal binding molecules including chelating agents, etc. Examples of glucose analogue molecules can include, without limitation, glucose antigen, glucose cofactor, glucose substrate, glucose inhibitor, D-sugar, carbohydrates, 1,2-cis-diol sugars, cysteine, metal ions (e.g., Ca, Mg, etc.), etc. 
     A sensor can also include a semi-permeable membrane that can protect the hydrogel such that analyte-containing body fluid is free to pass through the membrane and permeate the hydrogel. A membrane can generally have a thickness on the order of microns. In one embodiment, a membrane can be radiographically transparent. In addition, the membrane can be biocompatible and have sufficient stiffness to substantially prevent the hydrogel from swelling in undesirable directions and from escaping into the surrounding area. 
     In one embodiment, a semipermeable membrane can be permeable to the passage of the analyte and any other materials utilized in the sensing application (e.g., glucose, oxygen, carbon dioxide, lactic acid, gluconic acid, etc.) and can be totally or substantially impermeable to the passage of blood clots, cells, and non-analyte proteins. In addition, a semipermeable membrane can be an inert, nontoxic material that maintains its integrity when implanted in humans. A suitable biocompatible semipermeable material, to minimize immune reactions and to prevent protein and cell absorption, can be selected from the group of polymers including, without limitation, cellulose acetate, methyl cellulose, polyvinyl alcohol, polypropylene, HEMA, tetra-acrylated poly(ethylene glycol) (PEG), and/or polyurethane. 
     There is an extensive body of knowledge on developing stimuli-responsive gels for chemical sensors, actuators (e.g., artificial muscles), and drug release materials. For example, a partial review of stimuli responsive materials is given in Journal of Controlled Release 190 (2014) 337-351. Many methods are available to detect different types of analytes include coupling pH-responsive gels to enzymes that generate acidic products (e.g., glucose and oxygen can be detected by encapsulating glucose oxidase and catalase into a pH-responsive gel to produce gluconic acid according to the concentration of glucose and oxygen present). Alternatively, molecular recognition interactions (e.g., antibody-antigen, lectin-carbohydrate, ionophore-ion, enzyme-substrate, DNA-complementary DNA, aptamer-substrate, etc.) can be used to affect gel swelling. The gel swelling can be measured based upon axial expansion, bending of a multilayered structure with different expansion properties in the structure (e.g., in cantilever-based chemical sensors, and similar to the working of bimetallic thermal sensors/actuators and paper-based hygrometers). Multi-layered structures can be formed by bonding two or more structures together, or allowing particles to settle within a polymer, varying the amount of cross-linking in the top of a polymer film compared with the bottom, etc. Multiple sensors can be used to increase specificity and account for interactions between multiple variables. For example, the combination of a pH sensor and a separate glucose sensor with glucose oxidase can be used to account for how the ambient pH affects the glucose sensor in order to estimate the two analyte concentrations. In addition, measuring multiple analytes may be more indicative of pathology (e.g., infection) than a single analyte. 
     A chemical sensor can be affixed to an orthopedic implant, similar to strain sensors as discussed above, but this is not a requirement of a chemical sensor, as the response of the chemical sensor does not depend upon the action of the implant or the bone. Accordingly, the indicator of the chemical sensor need not be directly or indirectly attached to the bone at any point but should be implantable in the local area of the implant. As such, it may be convenient to locate a chemical sensor on the implant. Alternatively, a chemical sensor can be located directly on a bone or on another structure in the general area. In any case, a chemical sensor can be located such that it can be examined radiographically to determine the presence of an analyte in the local area of the implant. In one embodiment, the analyte-sensitive material can function as the radiographically discernable material. For instance, a sensor as illustrated in  FIG. 9  can include an analyte-sensitive material in the reservoir  221  at the first end  226  of the indicator  22 . Upon contact with the analyte, the analyte-sensitive material can swell and extend along the track  223  of the sensor. In this embodiment, the sensor  220  can be configured to permit expansion of the hydrogel in substantially only one dimension along the track  223 . In order to be clearly discernable in the radiographic imaging, the track  223  can have a length that is greater than the width of the track, e.g., from about 5 to about 50 times the crosswise dimension(s). Upon interaction with the analyte and dimensional variation of the hydrogel held at the first end  226  of the indicator  222 , the hydrogel can be forced along the track  223 , and the motion of the indicator (e.g., the second end  227  of the hydrogel) can be discernable by radiographic imaging. 
     In another embodiment, the analyte-sensitive material can be in communication with a secondary component of the indicator that can amplify the volume variation of the analyte-sensitive material. For instance, and with reference to  FIG. 25 , a sensor  920  is illustrated that includes an analyte-sensitive hydrogel  921  held adjacent to a first end  926  of an indicator rod  922 . In the left panel, the analyte-sensitive hydrogel is illustrated in a neutral condition, e.g., not in the presence of the analyte. Following contact with the analyte of interest, the hydrogel  921  can swell, which can cause the indicator rod  922  to rotate, elongate, or otherwise alter the location of the second end  927  of the indicator rod  922 . The second end  927  of the indicator rod can be held in conjunction with a scale as discussed above, such that the location of the second end  927  with respect to the scale is radiographically discernible. 
       FIG. 26  illustrates a chemical sensor  1020  attached to a surface of an orthopedic implant  1012 , which is, in turn, attached to a bone model  1023 . The left panel illustrates the analyte-sensitive hydrogel prior to exposure to the analyte. In the right panel, the hydrogel has swollen due to exposure to the analyte, and the change in location of the second end  1027  of the indicator rod  1022  is clearly visible. When utilized in vivo, the second end  1027  of the indicator rod  1022  can be located in conjunction with a scale, as discussed previously, so as to quantify the distance moved by the second end  1027  of the indicator rod  1022 . This distance can indicate the presence or concentration of the analyte in the area of the sensor  1022 . 
     As with the strain sensors, a chemical sensor can be located at any convenient location in conjunction with an orthopedic implant in order that an end of the indicator and the scale are discernible without interfering with the function of the implant or causing undesirable irritation to surrounding tissue. For instance, in one embodiment illustrated in  FIG. 27 , a sensor  1120  can be located within an aperture (e.g., an unused screw hole) of an orthopedic implant. As can be seen, the sensor  1120  includes an analyte-sensitive material  1121  and an indicator rod  1122 . In addition, the sensor  1120  includes a scale  1124  in the form of markings around the circumference of the sensor  1120 . Upon interaction with an analyte, the analyte-sensitive material  1121  can exhibit a dimensional change that can be translated to the indicator rod  1122 . Upon radiographic imaging of the sensor, the location of the movable end  1127  of the indicator rod  1122  with respect to the scale  1124  can be determined, and this can provide information regarding the presence or concentration of the analyte in the local area of the orthopedic implant  1122 . 
     The analyte-sensitive component of an indicator, e.g., an analyte-sensitive hydrogel, can be a single piece of material or can include a plurality of pieces of material. In addition, multiple pieces of analyte-sensitive material can exhibit a detectable response in the presence of the same or different analytes. 
       FIG. 28  illustrates an embodiment of a chemical sensor that includes multiple segments of analyte-sensitive material  1221   a ,  1221   b , and  1221   c  affixed to an orthopedic implant  1212  that, in turn, is affixed to a bone  1223 . The different segments of analyte-sensitive materials can provide different functions. For instance, one of the segments  1221   a  can be utilized as the scale. For instance, segment  1221   a  can be a non-reactive hydrogel segment that is not sensitive to the presence of the analyte. A second segment  1221   b  can be sensitive to a particular analyte, e.g., glucose. For instance, the second segment  1221   b  can include specific analyte-binding molecules incorporated therein that are specific for an analyte of interest. A third segment  1221   c  can be sensitive to a different specific analyte or can exhibit a more general response. For instance, the third segment  1221   c  can exhibit a dimensional variation in response to a pH change, independent of the cause of the pH change. During use, a dimension (e.g., height) of the analyte-sensitive segments  1221   b ,  1221   c , can be compared to one another, as well as to the scale segment  1221   a , to provide information regarding the presence or concentration of one or more analytes of interest. 
     Of course, additional sensitive materials specific to other analytes of interest can also be included in a chemical sensor, and a sensor is in no way intended to be limited to determination of the presence or concentration of any particular number of analytes. Moreover, multiple sensors, including any combination of chemical and strain sensors, can be used in an area. For instance, a joint, bone, or general area including one or more orthopedic tissues can include one or more strain sensors optionally in conjunction with one or more chemical sensors. 
     The present disclosure may be better understood with reference to the Examples set forth below 
     Example 1 
     A lever-type strain sensor was formed and attached to a human cadaver tibia. The scale included 0.4 mm holes formed in a tungsten plate and separated from one another by a distance of 0.75 mm. A tungsten rod was used as the indicator. The rod was 0.89 mm in diameter and 80 mm in length. Using a C-arm X-ray system, the position of the tungsten rod relative to the 0.4 cm hole scale was evident and movement was clearly observed, while no motion was evident for loading an intact human cadaver tibia. The introduction of an unstable fracture caused significant motion under axial loading of ½ body weight ( FIG. 29 ). These results qualitatively agreed with the FEA simulation described above (although the cadaver experiments were more sensitive as a longer dial arm was used). The sensor did not affect plate mechanical properties. The sensor could detect changes in pretension (e.g., from subsidence), though the main purpose was to measure stiffness by detecting changes in position during stance loading. This easily read sensor moved four holes under a load of only 100 N, with readings repeatable under cyclic loading ( FIG. 29  at right). 
       FIG. 30  at A illustrates a computer-simulated strain in callus for a 400 N axial compression load (½ body weight) on a plated tibia with a 1 cm tibial osteotomy and a spacer with varying stiffness relative to intact bone (darkness indicates the log of strain). At B is graphically shown a comparison of pin displacement with maximum callus strain. The non-zero y-intercept at B indicates some displacement when the callus is fully healed because the device reads the bending of the whole bone construct. As shown at B, movement of the second end was detected at about 0.5 mm at a maximum callus strain of about 1.5% (150 μm for a 1 cm fracture gap), a likely safe level for weight-bearing. Simulations with 20 N kicking/ground reaction forces showed an essentially identical displacement at a 400 N axial compression. 
     Additional response data for a strain sensor are graphically presented in  FIG. 31  including pin displacement (mm) with axial compression load (N) (left), pin displacement (mm) with callus stiffness as a percentage of intact bone (400 N axial compression load, BW/2) (center), and pin displacement under a bending load (mm) vs. pin displacement under an axial compression (mm) for a 400 N compression vs. a 35 N bending load with various fracture callus stiffness (right). 
     Example 2 
     A 1 mm tungsten rod was press fit into a threaded stainless steel cylinder. This threaded piece was then screwed into a titanium dynamic hip screw (Narang Inc.) ( FIG. 22A ) and imaged using projection radiography ( FIG. 22D ). The position of the pin could be easily discerned through the titanium screw using standard X-ray radiography with 82 kV applied to the X-ray tube ( FIG. 22D ). Generally, higher energy X-rays were able to penetrate more deeply through titanium or steel. The experiment was therefore repeated with a stainless steel screw, using a tube voltage of 100 kV, and the position could be determined via X-ray. Under bending loads, the pin moves relative to the screw providing a readout for amount of bending ( FIG. 22E ). 
     Example 3 
     To synthesize a chemical sensor including a pH-responsive polyacrylic gel, an aqueous solution was prepared by mixing the monomer, acrylic acid (AAc) (2.0 M), a chemical cross-linker, methylene bis acrylamide (MBAAm) (2.0 mol %), and a thermal initiator, ammonium per sulfate (APS) (0.15 mol %) together (the amount in mol % is related to the AAc monomer concentration). After homogeneous mixing, the precursor solution was poured into a reaction cell consisting of a ˜1.0 mm thick rectangular silicone rubber frame sandwiched between a pair of parallel flint glass plates. Polymerization reaction was effected at the temperature nearly 65° C. under nitrogen atmosphere in a vacuum oven. After prolonged, ˜6 hours thermal polymerization, the plate-like gel (about 40×40×1.0 mm 3 ) was obtained and carefully removed from the glass reaction cell and cut into specific dimensions (a disk of about 5.0 mm diameter and 2.0 mm thickness) using a metallic cutter. Then, the gels were immersed in water for spontaneous swelling. Water was changed every 24 hours for at least 3 days to remove all unreacted monomers in the gel. Finally, swollen disk-gel pieces were carefully incubated in a sealable container to carry out an experiment for measuring swelling-induced strain of the gel. 
     A gel-sensor placed in the built-in screw hole of an implanted orthopedic plate demonstrated abrupt changes in volume due to the occurrence of swelling and de-swelling when the appropriate pH buffer was used as stimuli. A small piece of water (pH˜5.5)-swollen gel (about 2.0 mm×2.0 mm×2.0 mm) was placed in the hole of an orthopedic implant fixed on a human bone. A thin (0.5 mm radius) tungsten rod was attached on the top of the implant such a way that the rod touched the surface of the swellable gel and could rotate like a dial when the gel volume changes due to swelling/deswelling. The set up and results of this experiment are presented in  FIG. 32 . In this figure, the x-ray images demonstrate the position of the free end of the tungsten dial before and after the addition of biological simulant buffer (enlarged X-ray images on the right shows clearly what occurred in the left images). Prior to the start of the swelling of the gel-sensor, the rod-end was gently touched by the plate surface, and the position of the rod-end, therefore, was settled in hole #1. When the spontaneous swelling was commenced by injecting high pH buffer (˜7.0), mass/volume of the gel-sensor increased significantly, and the tungsten dial, therefore, was displaced which is evidenced by clear visualization of the rod-end at the hole #4, or somewhere between #3, and #4. Later, when the buffer was changed into low pH (˜4.0) to induce de-swelling, the rod-end took position back to the hole #1.  FIG. 33  illustrates the repeatability of the sensor response through multiple cycles of pH. This simple experiment was a clear exhibition of the capability of the gel-sensor for internal biomedical devices. 
     Example 4 
     An orthopedic surgeon fixed an implant on a cadaver tibia and following, attached a gel sensor utilizing a pH-sensitive gel as described above in the implant ( FIG. 34  at (A)).  FIG. 34  at (B) presents a schematic diagram of the gel-sensor during use. X-ray images enlarged in  FIG. 34  at (C) illustrate the tungsten dial and the relative motion of the indicator when at high and low pH. 
     Example 5 
     The concentrations of reagents of a pH-sensitive hydrogel were varied to provide a thin hydrogel (200 μm thick) with fast response rate. In addition to the reagents described above, the gel included n-octyl acrylate to provide sensitivity at higher pH than was observed using only acrylic acid. After synthesis, it was verified that the sensor shrunk in response to acidic products from bacterial growth and re-swelled in response to incubation in pH 7.4 buffer. 
     To synthesize the pH-sensitive hydrogel film, a solution was prepared by combining the monomers, acrylic acid (AAc), and n-octyl acrylate (C8-A), the chemical cross-linker, poly(ethylene glycol) diacrylate 700 (PEGDA), and the photo-initiator, 2-oxoglutaric acid (OGAc). These reagents were mixed together in the following ratios: AAc+C8-A+PEGDA700+OGAc (10+5+1+0.1) (wt/wt %) in N, N-dimethyl formamide (DMF). After homogeneous mixing, the precursor solution was degassed by N 2  and was then poured into a simple reaction cell, where a rectangular frame of 100 μm thick silicone rubber was sandwiched between a pair of parallel flint glass plates. The spacer and glass plates were cleaned very carefully before use. Under an inert nitrogen atmosphere maintained in a glove box (CLEATECH®), a photo-polymerization reaction was performed by using UV lamps (360 nm) irradiation at a distance 10 cm from the both sides of the reaction cell at the temperature nearly 30-35° C. After a prolonged ˜6-hour polymerization, a piece of hydrogel film (size: about 40×40×0.1 mm 3 ) was obtained and was removed carefully from the customized glass reaction cell. Following, the gel pieces were immersed in sufficient water for spontaneous swelling. Water was changed every 24 hours for at least 3 days to remove all unreacted molecules in the hydrogel. Water-swollen gels were transferred and finally incubated into a phosphate buffer saline PBS (1×) at pH ˜7.4. To implement as a sensor, the PBS-swollen hydrogel films were cut into a disk shape using a metallic hole-punch (about 12.0 mm diameter and 0.2 mm thickness in this study). 
     The hydrogel-sensor, swollen in PBS 1× (diameter of ˜12.0 mm, and thickness of ˜0.2 mm), was sterilized by rinsing with ethanol-water mixture (70-30% ratio), exposed under UV, and finally incubated again in PBS 1× at 37° C. Next, a bacterium ( Staphylococcus epidermis , ATCC 35984) culturing media was prepared from sterilized tryptic soy broth (TSB) which contains 1.0 wt/vol % glucose. Following this, TSB media (˜25 mL, and pH ˜7.0) was mixed with some extent of bacteria and incubated in 37° C. prior to use. 
     A piece of the hydrogel-sensor (diameter of ˜12.0 mm, and thickness of ˜0.2 mm) was carefully taken out from PBS and was directly dipped into the bacterial suspension in TSB; the culture was kept in the incubator. After 24 and 48 hours, the culture was taken out, pH of the media was measured, and image of the hydrogel was recorded to determine the change in size/volume. One hydrogel sample was added to each centrifuge tube. After collecting the 48-hour data point, the hydrogel-sensor was returned to the PBS 1× solution (pH ˜7.4) at 37° C. and its restorable size was recorded. 
     The initial pH of the TSB media was 7.1, and the initial diameter of the hydrogel was 12.17 mm. After 24 hours, the pH was reduced to 4.68 due to bacterial growth and the hydrogel diameter was reduced to 6.35 mm. After 48 hours, the pH increased to 4.73 and the concomitant hydrogel diameter was measured 6.39 mm. After the completion of 48-hour incubation period, the hydrogel-sensor was rinsed in the PBS 1× of pH ˜7.4 three times, and allowed to expand due to swelling. After a short while (˜20-25 minutes), the diameter of the hydrogel-sensor was recovered into 11.76 mm, which is 0.41 mm less than the diameter of virgin hydrogel-sensor. 
     While certain embodiments of the disclosed subject matter have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the subject matter.