WO2001037733A9 - Implant loosening and fracture healing evaluation apparatus and method - Google Patents

Abstract

A system (30) is provided which non-invasively evaluates the condition of the interface between an implant (32) and bone using acoustic energy. Variations in the acoustic energy received from the implant (32) are analyzed to provide an indication of proper healing or loosening. Similar principles are applied to assess the extent of healing associated with bone fractures. In a related embodiment, the system utilizes built-in sensors (162) to detect the load imparted on an implant to evaluate non- invasively the amount of healing/loosening which occurs in relation to an implant. According to another particular embodiment, a signal source (38) is utilized to transmit a signal through the region of interest such as a fracture site (150) in order that healing may be evaluated.

Description

TITLE: IMPLANT LOOSENING AND FRACTURE HEALING

EVALUATION APPARATUS AND METHOD

Cross Reference to Related Application This application claims priority under 35 USC §119 to United States provisional application number 60/167,174, filed on November 23, 1999.

Technical Field

The present invention relates generally to non-invasive diagnoses of medical implants and bone fractures, and more particularly to the non-invasive diagnosis of implant loosening and fracture healing.

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 and wear between different parts of the implant, between the implant and the skeletal frame, etc. Such wear can result in the loosening of the implant at the interface between the implant and the skeletal frame. In addition, abrasion can result in the formation of wear debris particles in the area of the implant which can lead to complications such as osteolysis. Thus, 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 loosening over time. 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 is costly and painful to the patient. Furthermore, implants such as intramedullary rods are oftentimes inserted in a broken bone to aid in the healing of the break and to provide strength. In instances such as this, it is desirable to know the degree at which the healing is occurring. Also, it is desirable to know whether the intramedullary rod is functioning and healing is progressing properly. Again, however, it is highly undesirable to have to perform invasive surgery in order to evaluate the extent of healing and/or the condition of the intramedullary rod.

Even more generally, it is oftentimes desirable to non-invasively analyze the condition of a bone fracture even in the absence of an intramedullary rod or other type implant. While conventionally x-rays have been used to make such evaluations, it has been necessary to wait approximately six weeks or so in order for there to be sufficient healing to be detected by x-rays. Additionally, extended exposure to x-rays can possibly lead to further complications. It would be desirable to be able to evaluate healing of a bone fracture without necessarily having to wait approximately six weeks and/or resort to using potentially harmful x-rays.

Summary of the Invention

An apparatus and method are provided for non-invasive detection of loosening of medical implants such as artificial joints for hips, knees, shoulders, elbows, etc. In one embodiment, a focused ultrasound transducer placed in contact with the body insonifies the implant with ultrasound. As a result of such insonification, acoustic energy is reradiated from the implant back towards the transducer. Signal analysis of the reradiated energy enables one to assess the condition of the interface between the implant and bone. According to another embodiment, a sensor is included in an intrarmedullary rod or other implant for monitoring a condition of a bone fracture. The sensor measures, for example, the stress or load placed on the implant to provide an indication of how well the fracture is healing. In addition, or in the alternative, the sensor can measure how well the implant is secured to the bone, etc. According to still another embodiment, the sensor is instead used to more directly monitor the degree of healing based on detecting the amount of mineralized tissue which forms at the fracture. Various non-invasive techniques may be used to couple the information from within the body of the patient to a computer or processor for analyzing the data. Such techniques include, but are not limited to, acoustic, electromagnetic, magnetic, optical, thermal, etc., as will be appreciated based on the disclosure below.

In accordance with a first aspect of the invention, a system and method are provided for non-invasively evaluating a condition of an interface between a structure implanted within a living animal and a bone within the living animal to which the structure is intended to be adhered. The system includes an acoustic energy source which insonifies the structure by transferring acoustic energy to the structure from outside the living animal. In addition, the system includes an acoustic transducer located outside the living animal which receives acoustic energy produced by the structure as a result of being insonified and outputs a signal representative of the received acoustic energy. An analyzer is provided which evaluates the output signal based on a predefined criteria to assess a degree to which the structure is adhered to the bone.

According to a second aspect of the invention, a system and method are provided for non-invasively evaluating a condition of a bone fracture within a living animal. The system includes an acoustic energy source which insonifies the bone fracture by transferring acoustic energy to the bone fracture from outside the living animal. An acoustic transducer is located outside the living animal which receives acoustic energy produced by the bone fracture as a result of being insonified and outputs a signal representative of the received acoustic energy. An analyzer evaluates the output signal based on a predefined criteria to assess a degree to which the bone fracture is experiencing healing.

In accordance with a third aspect of the invention, a system and method are provided for non-invasively evaluating a degree of healing associated with a structure implanted within a living animal whereby the degree of healing is exhibited by a change in load imparted on the structure under predefined conditions. The system includes a sensor forming part of the structure for sensing the load imparted on the structure, and means for non-invasively interrogating the sensor to obtain information indicative of the load. Also included is an analyzer for evaluating the information based on predefined criteria in order to evaluate the degree of healing.

According to a fourth aspect of the invention, a system and method are provided for evaluating a region of interest within a living animal. The system includes a signal source located within the living animal in proximity to the region of interest for emitting a signal which passes through the region of interest. The system further includes a receiver unit located outside the living animal along a path of the signal which has passed through the region of interest, the receiver unit receiving the signal. An analyzer is included for analyzing the received signal based on a predefined criteria in order to ascertain a characteristic of the region of interest.

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 the adherence/loosening of a medical implant (e.g., an artificial hip) with respect to a bone structure to which it is intended to be adhered to, in accordance with the present invention;

Fig. 2 is a block diagram of the broadband acoustic analyzer used in accordance with the present invention;

Fig. 3A is a partial schematic view representing the source/detector unit in physical proximity to the artificial hip-joint in order to evaluate loosening in accordance with the present invention;

Fig. 3B is a block diagram of the source/detector unit in accordance with the present invention;

Fig. 4 is a block diagram of the broadband frequency response of the source/detector unit in accordance with the present invention; Fig. 5 is a sectional view illustrating an artificial hip-joint for which loosening is to be analyzed in accordance with the present invention;

Fig. 6A and 6B represent an exemplary comparison of the signals received by the acoustic analyzer in the case of a relatively tight interface and loose interface, respectively, between the implant and the skeletal structure in accordance with the present invention;

Figs. 7A and 7B represent the use of the broadband acoustic analyzer to evaluate the degree of healing of a bone fracture in accordance with the present invention;

Fig. 8 illustrates a non-invasive system for evaluating healing in the case of an intramedullary rod implanted within a fractured bone in accordance with the present invention;

Figs. 9A and 9B illustrate schematically exemplary sensors which may be used in accordance with the embodiment of Fig. 8 according to the present invention; Fig. 10 illustrates another embodiment of the system of Fig. 9 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 evaluating the adherence/loosening of a medical implant with respect to a bone to which it is intended to be adhered is generally designated 30. In accordance with the present invention, the system 30 evaluates the condition of 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, intramedullary (IM) rod or other type orthopedic implant which is intended to be secured, at least in part, to a portion of the skeletal frame of the patient 34. It will be appreciated, however, that the implant 32 can be any other type of device which is subject to being joined to one or more bones of the patient 34. The system 30 includes an acoustic analyzer 36 for remotely and non- invasively evaluating the boundary conditions between the implant 32 and the bone(s) to which it is attached in order to determine whether the implant 32 has become/remains loose or is properly attached. 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 source/detector unit 38 serves to insonify the implant 32 with acoustic energy to cause a reradiation of acoustic energy by the implant 32 in conjunction with the bone to which is it to be attached. The degree at which the implant 32 is attached to the bone (e.g., securely versus loosely) will determine at what frequencies and amplitudes the acoustic energy is reradiated.

More particularly, the source/detector unit 38 receives acoustic signals radiated/scattered back by the implant 32 and corresponding bone structure in response to the excitation. Such signals are then processed by the analyzer 36 to detect the degree of looseness. For example, such information may be trended over time to determine whether the implant has become loose or is tightening over time as the frequency and/or amplitude response to the acoustic energy excitation changes. The source/detector 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 source/detector unit 38 as described below, and for processing the output of the source/detector 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.

Referring now to Fig. 2, the acoustic analyzer 36 in accordance with the exemplary embodiment is illustrated in more detail. The source/detector unit 38 preferably is a hand-held sized device which is held by a doctor, nurse or medical assistant outside the body of the patient 34 in close proximity to the implant 32. Since the system 30 is non-invasive, the source/detector 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 a data processing and control circuit 52 which is programmed to carry out the various control and computational functions described herein. More particularly, the circuit 52 provides a control signal on control bus 54. The control signal controls the frequency (within the acoustic frequency band) at which the source/detector 38 excites the implant 32 by emitting acoustical energy while positioned in close proximity to the implant 32 as shown. In addition, the control circuit 52 provides a control signal on bus 54 in order to control whether the source/detector 38 is transmitting acoustic energy or receiving acoustic energy reradiated/reflected from the implant 32 in response to being excited.

The source/detector 38 receives acoustic energy from the implant 32 based on the mechanical transfer function of the implant 32, and converts the energy into an electrical signal on line 56. In the case of an implant 32 which is intended to be adhered to a bone or adhere to a bone during the healing process, it will be appreciated that the mechanical transfer function of the implant 32 will be a function of the degree of adherence at the interface between the implant 32 and the bone. For example, the frequencies, phase and/or amplitudes at which the acoustic energy is reradiated from the implant 32 will vary as a function of how securely or how loosely the implant 32 is adhered to the bone. Thus, signal analysis of the reradiated energy is used by the present invention to assess the condition of the interface between the implant 32 and the bone.

In the exemplary embodiment, the signal on line 56 is input to a signal conditioning circuit 58 which conditions the signal prior to being input to the data processing and control circuit 52. As is discussed more fully below, the data processing and control circuit 52 processes and analyzes the signal on line 56 in order to assess the condition of the interface between the implant 32 and the bone. For example, the excitation signal from the source/detector 38 is used to induce a mechanical resonance in the device 32. The source/detector 38 then detects the response of the device 32 to such excitation by analyzing, for example, any harmonics which are present as determined by the acoustical energy radiated by the resonating device 32. Alternatively, the circuit 52 may analyze the decay time associated with the mechanical resonance in response to excitation by the source/detector 38. Additionally, or in the alternative, the circuit 52 may analyze other properties of the acoustic signal reradiated and/or reflected by the device 32 in response to the excitation signal (e.g., changes in the Fourier Transform or frequency spectrum of the received signal). Features such as changes in the frequency spectrum of the received signal may be trended over time to evaluate the healing process associated with an implant 32 as the bone is intended to become more adhered. For example, different and/or higher frequency components may result over time with respect to the same frequency excitation signal as the bone adheres better to the implant 32. Conversely, different and/or lower frequency components may result over time in the case where an implant 32 becomes loosened due to wear, etc., as mentioned above. The scope of the present invention is intended to encompass any and all such correlations which may be found between the parameter of interest, the acoustic excitation and the response of the implant 32. The circuit 52 may be configured using known techniques to perform such analyses in order to provide an output indicative of such changes. For example, if the frequency spectrum of the acoustic energy reflected/reradiated changes beyond a predefined threshold, the circuit 52 can output on the display 44 an indication that the implant 32 has loosened.

In order to acoustically excite the implant 32 over a significant portion of its mechanical transform function frequency range, a broad band source/detector 38 is preferred. This provides for the greatest range of response and excitation of the implant 32. Conventional ultrasound transducers with more limited bandwidth can also be used, although preferably after those frequencies in the mechanical transfer function of the device 32 having significant correlation to bone adherence have been identified (for example, empirically).

Fig. 3 provides a perspective view of the source/detector 38 in relation to an artificial hip-joint type implant 32. As shown in Fig. 3, the source/detector 38 includes a two-dimensional (mxn) array 60 of miniature acoustic devices 62. Each device 62 is made up of an electro-acoustic transducer such as a piezoceramic device. In a transmit or excite mode, each device 62 is responsive to an electrical driving signal so as to emit an acoustic wave. Conversely, in a receive mode each device is designed to receive an acoustic wave and convert the received wave into an electrical signal. The level of the signal is based on the intensity of the received wave. Although the preferred embodiment utilizes an array 60 of piezoceramic devices 62, other type devices can also be used without departing from the scope of the invention. The devices 62 are arranged in a generally planar array. The active faces of the devices 62 are oriented in a common direction so as to be directed downward towards the artificial hip-joint 32. A housing 64 (shown in cut-away) provides a protective enclosure for the source/detector 38, with an acoustic window provided in the housing 64 to allow acoustic waves to be emitted and received by the devices 62.

As is illustrated in Fig. 4, an electrical input/output 66 of each device 62 in the array 60 is hardwired together with the others in parallel. The input/outputs 66 are selectively connected via a switch 68 to either the output of a voltage controlled oscillator (VCO) 70 or a received signal line 54. During a transmit or excite mode, a control signal on line 74 from the circuit 52 (Fig. 2) causes the switch 68 to couple the output of the oscillator 70 to the input/output 66 of each of the devices 62. At the same time, the circuit 52 provides a control voltage on line 76 to control the frequency of the VCO 70.

The VCO 70 preferably is an oscillator which is designed to produce an output signal at any frequency within the acoustical range of 50 kilohertz (kHz) to 10 megahertz (MHz). Furthermore, it is desirable that each of the devices 62 provide a generally uniform response throughout the range. However, with existing piezoceramic devices 62 currently available, each device has a generally narrow band of operation (e.g., on the order of ±5% about its center operating frequency f_op). Consequently, the array 60 in the present invention is made up of devices 62 selected with different operating frequencies f_op uniformly distributed across the broadband acoustical range of 50 kHz to 10 MHz. As a result, the composite response of the devices 62 is generally uniform as represented in Fig. 4. In this manner, the array 60 is able to transmit and detect acoustic energy at any of a broad range of frequencies at which the artificial hip-joint implant 32 is to be excited or at which the implant 32 emits acoustic energy in response to excitation. The operating frequencies f_op of the devices 62 are selected so that at least one device 62 is responsive to the excitation signal from the VCO 70 in order to emit an acoustic signal at each frequency. Similarly, at least one device 62 is responsive in the receive mode to detect the respective frequencies reradiated/reflected by the implant 32.

In a further preferred embodiment, the devices 62 with the different operating frequencies f_op are spatially distributed within the array 60. Such spatial distribution preferably is selected so that the respective operating frequencies will be uniformly distributed across the array 60 and the overall frequency response of any region within the array 60 will be the same as the other. For example, regions 80 and 82 each preferably contain a sufficient number of devices 62 with selected operating frequencies to exhibit the same response curve shown in Fig. 4. Therefore, it will be appreciated that the overall array 60 will function as a broadband source/detector generally independent of the particular region (e.g., 80 or 82) which is positioned immediately adjacent the implant 32. The array 60 therefore will be operative throughout the entire acoustic frequency band of interest.

Fig. 5 shows a typical artificial hip-joint 32 which may be analyzed in accordance with the present invention. The hip joint 32 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 32 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 32 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.

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 intojhe space 124, for example. The abrasion of the acetabular cup 102 results in a reduction in the life of the joint 32. Moreover, gone unchecked the hip joint 32 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 32 and the associated wear particulate may be found in U.S. Patent No. 5,725,597, the entire disclosure of which is incorporated herein by reference.

Such loosening of the shaft 106 from the femur 108 is detected in accordance with the present invention by analyzing changes in the acoustic energy reradiated/reflected by the joint 32 over time. Specifically, a baseline is established, for example, following a surgical procedure to install the joint 32. Such baseline may consist of the acoustic energy patterns (e.g., frequency spectrums) obtained from the joint 32 in response to insonification at one or more different frequencies in accordance with the procedures described above in connection with Figs. 1-4. Periodically the patient may be reevaluated by subjecting the joint 32 to insonification at the same baseline frequency or frequencies and comparing the results in order to assess the condition of the interface between the shaft 106 and the femur 108. As a result, the condition of the patient may be evaluated without necessitating surgery and/or exposure to x-rays or the like.

For example, Fig. 6A represents a frequency spectrum obtained using the analyzer 36 at a baseline frequency f1 following the installation of the hip-joint 32 in a patient. In this particular case, a biocompatible adhesive is used to secure the shaft 106 to the femur 108. Fig. 6B represents the frequency spectrum obtained by the analyzer 36 at the same baseline frequency f1 after loosening has occurred between the shaft 106 and the femur 108. The analyzer 36 detects the change in amplitude and distribution of the spectral energy based on predefined criteria determined, for example, empirically, and produces an output indicative of the occurrence of such loosening. As shown in Fig. 6B, the lower frequency content of the spectrum increases as compared to the spectrum of Fig. 6A. It may also be the case that it is desirable to evaluate the healing process associated with the installation of an implant 32. As the healing process progresses, the bone may become more rigidly secured to the implant. In such case, the frequency spectrum of Fig. 6B may be representative of the signal received by the analyzer 36 immediately following installation. The frequency spectrum of Fig. 6A is representative of successful healing occurring. Although described herein in the context of a hip-joint, it will be appreciated that the insonification approach described above may also be applied to other type implants intended to be secured to the skeletal frame without departing from the scope of the invention. Furthermore, the system 30 may also be used to evaluate healing of bone fractures using the same principles. For example, Fig. 7A illustrates the use of the system 30 to analyze a leg fracture. Specifically, the exemplary embodiment relates to a fracture site 150 of the femur bone 108. As is known, the healing process of a bone such as the femur 108 is characterized by soft tissue slowly being replaced by stiffer tissue. In the early stages of the healing process, the fracture site 150 experiences localized growth of soft tissue 152 as represented in Fig. 7A. The fracture site 150 is insonified externally using the source/detector 38 at one or more different acoustic frequencies in order to obtain a baseline frequency spectra. Assuming healing progresses properly, the soft tissue 152 will be replaced over time with stiffer material 154 at the fracture site 150 as represented in Fig. 7B. The stiffer material 154 at the fracture site 154 results in a change in the frequency spectra compared to the baseline, and the analyzer 36 detects such change based on predefined criteria obtained empirically, for example. As a result, the analyzer 36 provides an output indicative of whether satisfactory healing is occurring.

Fig. 8 illustrates another aspect of the present invention relating to the use of a skeletal implant 32 such as an artificial hip joint, intramedullary (IM) rod, etc. According to this aspect of the invention, one or more sensors are placed on the surface of the implant 32 in order to measure such parameters as the degree of healing of a bone fracture or the degree to which the bone implant is bonded or adhered to the surrounding bone.

In the example of Fig. 8, a serious bone fracture such as the fracture of the femur 108 may involve the placement of an IM rod 160 axially within the bone 108. The IM rod 160 includes a sensor 162 secured to the surface of the sensor 162. The sensor 162 includes one or more strain gages (not shown) designed to detect one or more bending moments of the IM rod 160. The sensor 162, as will be explained in more detail below with respect to Figs. 9A and 9B, is designed such that the sensor 162 may be interrogated non- invasively from outside the body using electromagnetic, magnetic, acoustic, or virtually any other form of coupling. In this manner, healing of the fracture may be interrogated without surgery or resort to x-rays.

As will be appreciated, immediately after the IM rod 160 is installed (i.e., prior to any appreciable amount of healing), no bending loads can be carried by the broken bone 108. All such loads are carried by the IM rod 160. This gives rise to a maximum strain value detected by the sensor 162 which corresponds to zero healing. As healing proceeds and callus and mineralized tissues form around the fracture, less load will be carried by the IM rod 160 and lower strain values will be detected by the sensor 162. Through controlled testing by a physician, the amount of load carried by the IM rod 160 can be evaluated over time based on the values obtained by the sensor 162. Thus, a physician is able to determine whether satisfactory healing is occurring based on the rate and/or degree of load bearing shift from the IM rod 160 to the bone 108.

In a similar manner, other bone implants (e.g., femoral, tibial, etc.) may be assessed with respect to their degree of bonding to the bones in which they are implanted. For example, a sensor 162 having one or more strain gages may be secured to the shaft 106 of the hip-joint implant 32 of Fig. 5 to measure the load carried by the implant 32. The initial value measured by the sensor 162 after a successful implant procedure should be essentially constant if there is no deterioration at the interface between the bone and the implant. If the interface fails (i.e., there is a loosening of the implant), less load in bending will be transferred to the implant 32. Thus, by periodically interrogating the sensor 162, loosening of the implant may be detected.

Fig. 9A represents an exemplary construction of the sensor 162 which may be interrogated non-invasively using a variable impedance loading effect. Such a sensor 162 is described in detail in commonly assigned, copending United States patent application Serial No. 09/275,308 to Spillman, et al., entitled "Remotely Interrogated Diagnostic Implant Device with Electrically Passive Sensor", filed March 24, 1999, and in corresponding PCT International Published Application WO0056210 A. The entire disclosure of each of these applications is incorporated herein by reference. As shown in Fig. 9A, the sensor 162 includes a strain gage whose impedance (e.g., resistance) varies as a result of the amount of strain exerted thereon. Such variation in impedance changes the resonant frequency of the LRC circuit formed by the sensor 162. Using the techniques described in the aforementioned 09/275,308 and WO0056210 A applications, an exciter unit 38' (Fig. 8) which is inductively coupled to the sensor 162 provides the analyzer 36 with data indicative of the measured strain. The analyzer is configured to process the measured information to produce an output indicative of a change in measured load in order to detect, for example, healing of the fracture or loosening of the implant. Fig. 9B illustrates another possible embodiment of the sensor 162. Such embodiment relies on telemetry techniques to transmit a signal to the analyzer 36 outside the body of the patient. For example, the load measured by a strain gage in the sensor 162 serves to change the oscillation frequency of an RF oscillator 170 included in the sensor 162. An antenna 172 serves to transmit the RF signal output by the oscillator 170 to the analyzer 36 which in turn determines the measured load based on the frequency of the received signal. Details on the use of RF telemetry to non-invasively obtain information from medical implants can be found, for example, in United States Patent No. 5,807,258, the entire disclosure of which is incorporated herein by reference. Fig. 10 illustrates yet another variation in accordance with the present invention. In this case, the implant such as an IM rod 160 includes a signal source 178 secured thereto. The signal source 178 may be as simple as an oscillator and transducer designed to transmit an acoustic, electromagnetic, infrared, or other type signal with a predefined power output. In this case, however, care is taken to make sure that the signal source 178 is located in line with the area of interest and a receiver element 38" for receiving the signal from the signal source 178.

For example, in the case of an IM rod 160 inserted in a fractured bone 108, it is desirable to monitor healing which occurs at the fracture site 150. It is at this fracture site 150 that soft tissue will form initially and gradually change into hard tissue as discussed above. The signal source 178 is placed on or in the implant rod 160 so as to be in line with the fracture site 150 and the receiver element 38", with the fracture site 150 disposed between the signal source 178 and the receiver element 38". In this manner, the signal emitted by the signal source 178 will pass through the region of interest (e.g., the fracture site 150) prior to being received by the receiver element 38". By analyzing changes in the received signal over time (i.e., trending), the invention makes it possible to evaluate the extent/degree of healing, etc. occurring in the region of interest. As a particular example, the present invention makes it possible to trend changes in the amplitude of the received signal. In the case of the fracture site 150, as the soft tissue hardens over time the amplitude of the signal received by the receiver element 38" will change. This information may be correlated to previously obtained empirical data so as to enable physicians to assess the degree of healing.

The invention is not limited to a particular type of signal source 178. The signal source 178 may be an acoustic source using a piezo-electric crystal, an RF source using an oscillator and small antenna, an optical or infrared source using an light-emitting diode (LED), etc. Any of these sources, or a combination of them, can be located inside the implant and interrogated after their signal passes through the region of interest. For prosthetic implants (e.g., hip implants), one or more such embedded signal sources can serve to alert medical personnel of an incipient loosening event, or of a lack of interpenetration of the implant by new bone cells. In the case of a hip replacement, locating an acoustic source in the implant between the ball and femoral shaft can be used to measure the presence of wear debris particles in the soft tissue thereat. The signal source 178 may be powered internally or remotely using known non- invasive techniques for powering electrical devices implanted within a patient. Thus, a further description thereof has been omitted for sake of brevity. 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.

Claims

What is claimed is:

1. A system for non-invasively evaluating a condition of an interface between a structure implanted within a living animal and a bone within the living animal to which the structure is intended to be adhered, the system comprising: an acoustic energy source which insonifies the structure by transferring acoustic energy to the structure from outside the living animal; an acoustic transducer located outside the living animal which receives acoustic energy produced by the structure as a result of being insonified and outputs a signal representative of the received acoustic energy; and an analyzer which evaluates the output signal based on a predefined criteria to assess a degree to which the structure is adhered to the bone.

2. The system of claim 1 , wherein the analyzer evaluates a frequency spectrum of the output signal in order to assess the degree of adherence.

3. The system of claim 1 , wherein the analyzer employs trending of the output signal over time.

4. The system of claim 3, wherein the analyzer detects a change in the output signal in order to identify a loosening between the structure and the bone.

5. The system of claim 3, wherein the analyzer detects a change in the output signal in order to identify an increase in the adherence between the structure and the bone.

6. The system of claim 1 , wherein the acoustic transducer comprises a broadband acoustic transducer.

7. The system of claim 6, wherein the acoustic energy source comprises a broadband acoustic energy source.

8. The system of claim 1 , wherein the acoustic energy source also serves as the acoustic transducer.

9. The system of claim 1 , wherein the structure is an artificial hip- joint.

10. A system for non-invasively evaluating a condition of a bone fracture within a living animal, the system comprising: an acoustic energy source which insonifies the bone fracture by transferring acoustic energy to the bone fracture from outside the living animal; an acoustic transducer located outside the living animal which receives acoustic energy produced by the bone fracture as a result of being insonified and outputs a signal representative of the received acoustic energy; and an analyzer which evaluates the output signal based on a predefined criteria to assess a degree to which the bone fracture is experiencing healing.

11. The system of claim 10, wherein the analyzer evaluates a frequency spectrum of the output signal in order to assess the degree of healing.

12. The system of claim 10, wherein the analyzer employs trending of the output signal over time.

13. The system of claim 12, wherein the analyzer detects a change in the output signal in order to identify a change in an amount of hard tissue formed at a fracture site of the bone fracture.

14. The system of claim 10, wherein the acoustic transducer comprises a broadband acoustic transducer.

15. The system of claim 14, wherein the acoustic energy source comprises a broadband acoustic energy source.

16. The system of claim 10, wherein the acoustic energy source also serves as the acoustic transducer.

17. A method for non-invasively evaluating a condition of an interface between a structure implanted within a living animal and a bone within the living animal to which the structure is intended to be adhered, the method comprising the steps of: insonifying the structure by transferring acoustic energy to the structure from outside the living animal; receiving from outside the living animal acoustic energy produced by the structure as a result of being insonified and outputting a signal representative of the received acoustic energy; and evaluating the output signal based on a predefined criteria to assess a degree to which the structure is adhered to the bone.

18. A method for non-invasively evaluating a condition of a bone fracture within a living animal, the method comprising the steps of: insonifying the bone fracture by transferring acoustic energy to the bone fracture from outside the living animal; receiving from outside the living animal acoustic energy produced by the bone fracture as a result of being insonified and outputting a signal representative of the received acoustic energy; and evaluating the output signal based on a predefined criteria to assess a degree to which the bone fracture is experiencing healing.

19. A system for non-invasively evaluating a degree of healing associated with a structure implanted within a living animal and whereby the degree of healing is exhibited by a change in load imparted on the structure under predefined conditions, the system comprising: a sensor forming part of the structure for sensing the load imparted on the structure; means for non-invasively interrogating the sensor to obtain information indicative of the load; and an analyzer for evaluating the information based on predefined criteria in order to evaluate the degree of healing.

20. The system of claim 19, wherein the sensor comprises at least one strain gage.

21. The system of claim 19, wherein the means for non-invasively interrogating the sensor comprises an inductive circuit within the sensor for presenting an electrical load which varies as a function of the sensed load, and means for inductively coupling to the inductive circuit from outside the living animal.

22. The system of claim 19, wherein the means for non-invasively interrogating the sensor comprises a transmitter within the sensor which transmits an electromagnetic signal to the outside of the living animal.

23. The system of claim 19, wherein the structure is an intramedullary rod implanted within a bone of the living animal.

24. The system of claim 23, wherein the analyzer evaluates the load imparted on the intramedullary rod under predefined conditions.

25. The system of claim 25, wherein the analyzer trends the load over time.

26. The system of claim 19, wherein the structure is an artificial hip- joint.

27. The system of claim 19, wherein the analyzer detects a change in the load to identify a loosening between the structure and a bone in the living animal to which the structure is to be adhered.

28. The system of claim 19, wherein the analyzer detects a change in the load to identify an increase in the adherence between the structure and a bone in the living animal to which the structure is to be adhered.

29. A method for non-invasively evaluating a degree of healing associated with a structure implanted within a living animal and whereby the degree of healing is exhibited by a change in load imparted on the structure under predefined conditions, the method comprising the steps of: sensing the load imparted on the structure using a sensor placed within the living animal; non-invasively interrogating the sensor to obtain information indicative of the load; and evaluating the information based on predefined criteria in order to evaluate the degree of healing.

30. A system for evaluating a region of interest within a living animal, the system comprising: a signal source located within the living animal in proximity to the region of interest for emitting a signal which passes through the region of interest; a receiver unit located outside the living animal along a path of the signal which has passed through the region of interest, the receiver unit receiving the signal; and an analyzer for analyzing the received signal based on a predefined criteria in order to ascertain a characteristic of the region of interest.

31. The system of claim 30, wherein the signal source comprises at least one of an acoustic, electromagnetic, optical and infrared signal source.

32. The system of claim 30, wherein the signal source is part of an intramedullary rod placed within the living animal.

33. The system of claim 32, wherein the region of interest comprises a fracture site and the signal passes through tissue at the fracture site.

34. A method for evaluating a region of interest within a living animal, the method comprising the steps of: locating a signal source within the living animal in proximity to the region of interest so as to emit a signal which passes through the region of interest; locating a receiver unit outside the living animal along a path of the signal which has passed through the region of interest such that the receiver unit receives the signal; and analyzing the received signal based on a predefined criteria in order to ascertain a characteristic of the region of interest.