WO2017143400A1

WO2017143400A1 - Implanted sensing system for joint replacements

Info

Publication number: WO2017143400A1
Application number: PCT/AU2017/050161
Authority: WO
Inventors: Michael Heimlich; Des BOKOR; Rajas Prakash KHOKLE; Karunanayake Pathirannahalage Asoka Priyathama Esselle
Original assignee: Macquarie University
Priority date: 2016-02-26
Filing date: 2017-02-24
Publication date: 2017-08-31
Also published as: AU2017224828A1

Abstract

A sensor and a system including a sensor for monitoring a prosthesis implanted in biological tissue of a prosthesis recipient are described. A sensor is implanted in the biological tissue proximate to the prosthesis. The sensor transmits a measurement signal towards the prosthesis and a receiver receives the measurement signal. An antenna transmits data dependent on the measurement signal to an external device. A data analyser analyses the transmitted data to obtain monitoring information comprising spatial position of the prosthesis relative to the one or more sensors. The system may comprise more than one sensor.

Description

Implanted Sensing System for Joint Replacements

Field of the invention

The present invention relates to systems and methods for monitoring the location and displacement of components of a joint replacement. In particular, the invention relates to a system including implanted sensors for sensing implanted joint components.

Background of the invention

It is estimated that several million joint replacements, for example for a hip, knee, ankle, shoulder, elbow or wrist, take place in the world every year. This number is likely to increase as the population ages and health care is extended to greater proportions of the world population.

Statistics from the Australian National Joint Replacement Registry (ANJRR) indicate that a substantial proportion of hip replacements, knee replacements and shoulder replacements require revision procedures after the initial joint replacement. This represents a significant cost to health systems and a considerable inconvenience to the patients involved.

Mechanical failure of a joint replacement is usually accepted when there is gross loosening and major wear of components of the replaced joint. The causes include mechanical imbalance which leads to loosening, polyethylene wear or failure to have bone ingrowth on the implant. In some cases there may be design faults with the prostheses.

There is a need for systems and methods for detecting the micro-movement of an artificial joint. Accurate monitoring may serve as an alert of loosening which may lead to joint failure. Such monitoring of the joint replacement may also facilitate optimal post-surgical rehabilitation. Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art. Summary of the invention

According to a first aspect of the invention there is provided a system for monitoring a prosthesis implanted in biological tissue of a prosthesis recipient, the system comprising: one or more sensors configured for implanting in the biological tissue proximate to the prosthesis, each sensor comprising:

• a measurement transmitter for transmitting a measurement signal towards the prosthesis;

• a receiver for receiving a reflected measurement signal;

· an antenna that transmits data dependent on the reflected

measurement signal; an external device configured to communicate with the one or more sensors from outside a body of the recipient, the external device receiving the data transmitted by the antenna; and a data analyser for analysing the transmitted data to obtain monitoring information comprising spatial position of the prosthesis relative to the one or more sensors.

The external device may be arranged to transmit an input signal to initiate operation of the one or more sensors and each sensor comprises power generation circuitry that obtains power to operate the sensor from the input signal.

The measurement transmitter may be arranged to transmit the measurement signal at a range of frequencies, selectable from an LF-HF range; a VHF range; a UHF range; and a mm-wave range.

The system may comprise a plurality of sensors coordinated by the external device to transmit measurement signals at a plurality of frequencies, wherein the data analyser is arranged to calculate monitoring information descriptive of intervening biological tissue located between the plurality of sensors and the prosthesis. The monitoring information may characterise calcification of the intervening biological tissue or fluid build-up in the intervening biological tissue.

The input signal transmitted by the external device may comprise information specifying the frequency of the measurement signal. The prosthesis may comprise at least one of:

• a femoral prosthesis for a hip replacement;

• an acetabular prosthesis for a hip replacement;

• a femoral prosthesis for a knee replacement;

• a tibial plate prosthesis for a knee replacement;

· a humeral prosthesis for a shoulder replacement;

• a glenoid prosthesis for a shoulder replacement;

• a spinal fusion fixation prosthesis;

• an elbow joint replacement;

• an ankle replacement;

· a wrist joint replacement;

• a finger joint replacement;

• a surgical plate, screw or rod.

The sensor may comprise a cylindrical housing configured for insertion into a hole drilled into bone tissue of the recipient. According to a further aspect of the invention there is provided a method of monitoring a prosthesis implanted in biological tissue of a prosthesis recipient, the method comprising: implanting a sensor in the biological tissue proximate to the prosthesis, the sensor comprising a measurement transmitter, a receiver and an antenna; transmitting an input signal from an external device to the sensor from outside a body of the recipient to initiate operation of the sensor; transmitting, from the measurement transmitter, a measurement signal towards the prosthesis; receiving, at the receiver, a reflected measurement signal; transmitting data to the external device via the antenna, wherein the data is dependent on the reflected measurement signal; and analysing the transmitted data, in a data analyser, to obtain monitoring

information comprising spatial position of the prosthesis relative to the sensor.

The sensor may be implanted during the implantation of the prosthesis. This may comprise drilling a guide hole in bone tissue of the recipient for use in the implantation of the prosthesis; and inserting the sensor in the guide hole for subsequent monitoring of the prosthesis. The sensor may be implanted in the recipient's biological tissue subsequent to an index procedure in which the prosthesis was implanted.

A plurality of sensors may be implanted proximate to the prosthesis, such that there is intervening biological tissue between the sensors and the prosthesis.

The data analyser may analyse transmitted data from the plurality of sensors to generate monitoring information descriptive of the intervening biological tissue.

The external device may initiate operation of the or each sensor for a period of time to obtain time-varying monitoring information, for example the operation of the or each sensor may be initiated during the recipient's rehabilitation and the time-varying monitoring information may comprise forces and moments experienced by the prosthesis during movement by the recipient.

The method may comprise modifying a post-operative mobilisation of the recipient dependent on the monitoring information.

The method may comprise identifying a prospective mechanical failure of the prosthesis based on the monitoring information and replacing the prosthesis. According to a further aspect of the invention there is provided a sensor configured for implanting in the biological tissue proximate to the prosthesis, the sensor comprising: a magnetic field transmitter for transmitting a magnetic field towards the prosthesis; a receiver for receiving and measuring the magnetic field, wherein the receiver is a magneto-resistive or magneto-impedance sensor; communications circuitry comprising an antenna that transmits data external to the sensor dependent on the measured magnetic field.

According to another aspect of the invention there is provided a method of monitoring a prosthesis implanted in biological tissue of a prosthesis recipient, the method comprising: implanting a sensor in the biological tissue proximate to the prosthesis, the sensor comprising a magnetic field generator, a magneto-resistive or magneto-inductance sensor and an antenna; transmitting an input signal from an external device to the sensor from outside a body of the recipient to initiate operation of the sensor; transmitting, by the magnetic field generator, a measurement signal towards the prosthesis; receiving the measurement signal at the magneto-resistive or magneto- inductance sensor; transmitting data to the external device via the antenna, wherein the data is dependent on the received measurement signal.

As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings

Fig 1A is a schematic diagram illustrating a knee replacement having multiple implanted sensors and an external powering and measurement device.

Fig 1 B is a schematic diagram illustrating functional blocks of the implanted sensors of Fig 1A.

Fig 2 illustrates a hip replacement in which multiple sensors are placed in proximity to the hip prosthesis.

Fig 3 illustrates a knee replacement in which multiple implanted sensors are placed in proximity to the components of the knee replacement. Fig 4A illustrates a shoulder (reverse) replacement in which multiple sensors are implanted in proximity to the prostheses of the shoulder replacement.

Fig 4B illustrates a shoulder (anatomic) replacement in which multiple sensors are implanted in proximity to the prosthesis of the shoulder replacements. Figs 5A and 5B illustrate a spinal fusion prosthesis in which multiple sensors are implanted in a patient's vertebra proximate to the screws and rods of the spinal fusion prosthesis.

Fig 6 is a schematic diagram of a computing device that may be used in the system of Figure 1 . Fig 7 is a flowchart illustrating a method of monitoring an implanted prosthesis.

Detailed description of the embodiments

Embodiments of the present invention relate to an implantable, contactless, wireless sensor with RF telemetry for sensing the location and displacement of a joint prosthesis. Fig 1 A is a conceptual diagram illustrating the monitoring system with reference to a knee replacement. As described below, the system may also be applied to other types of prostheses. Fig 1A illustrates a knee 300 where a joint replacement has occurred. The inserted prosthesis, for example a knee replacement (metal) plate, includes a metal plate 310. Multiple sensors 12 are inserted in corresponding multiple places in the patient's bone 302 near the metal plate 310. The sensors 12 may be positioned in guide holes created during the knee replacement procedure. The sensors 12 are arranged to measure the location, for example by measuring one or more of any translation, pitch or roll of the prosthesis from a current or earlier position and orientation, and potentially other properties of the prostheses including the plate 310. The sensors 12 are also configured to wirelessly communicate with an external powering and measurement device 10.

Sensors

The structure of the sensors 12 is further illustrated in Fig 1 B. The sensors are encased in a housing 14 having an outer surface made of a bio-compatible material. The sensor 12 includes an antenna 16. The antenna 16 receives input signals 52, for example radio frequency or inductive coupling signals, from the external powering and measurement device 10, which may pass through the clothing and part of the body or bio-logical material 50 of the patient. The antenna 16 also transmits an output signal 54 to the external powering and measurement device 10, the signal 54 also passing wirelessly through the body and clothing 50 of the patient.

The sensor 12 has a power generation capability 18. The power generation capability 18 may be implemented by a power generator that derives the power required by the sensor 12 from the input signal 52, as received by the antenna 16. Such an arrangement avoids the need to implant a battery in the sensor 12. The power generated from the input signal 52 may be stored, for example in local capacitors, and is used to power the operation of the antenna 16 and the other components 20, 22 of the sensor 12.

Communication and control circuitry 20 controls the operation of the sensor 12. The communication and control circuitry 20 is in functional communication with the power generation capability 18, antenna 16 and measurement circuitry 22 of the sensor 12. The communication and control circuitry 20 responds to instructions contained in the input signal 52. The communication and control circuitry 20 also acts to transmit information in the output signal 54 via the antenna 16. The measurement circuitry 22 includes a measurement transmitter for transmitting a measurement signal 56 and a receiver for receiving a reflected signal 58.

In one arrangement, the sensor 12 operates only when interrogated by the external powering and measuring device 10 which may, for example, be a hand-held unit that is brought near to the body of the patient in the vicinity of the replaced joint. As the sensor 12 is dormant until interrogated, this arrangement minimises power dissipation in the patient's tissue near the sensor. This avoids local heating and is thought to extend the lifespan of sensor 12. Using wireless communication for the input and output signals 52, 54 eliminates the need for feed-through wires for powering telemetry, thus enhancing patient mobility.

In one arrangement, the measurement circuitry 22 is a homodyne millimetre- microwave interferometric analyser. Such analysers are described, for example, in C. Nguyen and S. Kim, "Theory, Analysis and Design of RF Interferometric Sensors", Springer, 2012. In microwave interferometry techniques, an RF power signal is split using, for example, a Wilkinson Power Divider. The power signal is split into a reference signal and another measurement signal, which is transmitted by the sensor 12 as the measurement transmission 56. The measurement transmission 56 transmits through surrounding biological material 60 and is reflected by local components such as the knee replacement plate 310, and other surfaces which have significantly different dielectric constants and electrical conductivity to the biological material 60. Thus, metals, plastics, Teflon™ and other materials which have low conductivity and low dielectric constants could be sensed using transmitted signal 56. The reflected signal 58 is detected by the measurement circuitry 22. The reflected signal 58 is compared with the reference signal. The relative changes in path length, even with displacements of the prosthesis as small as 10 pm, affect the phase of the reflected signal and thus enable measurement and monitoring of the displacement of the prostheses.

In another arrangement, the measurement circuitry 22 is a magnetic field sensor. In one embodiment, a single loop is used to generate as well as receive the magnetic field. The motion of the monitored implant changes the characteristics of the loop, such as inductance, resistance and Q value. These changes are measured by measuring circuitry. Change in resistance may be measured by monitoring voltage level, whereas change in inductance may be measured by monitoring the change in frequency by resonating the loop with a capacitor. Q may be calculated indirectly from inductance and resistance values by using expression Q=col_/R, where ω is angular frequency and L and R are inductance and resistance respectively.

In another embodiment the magnetic field sensor includes a magnetic field generator loop and a magnetic field receiver. The magnetic field generator loop may be optimised to achieve higher sensitivity and/or larger range of operation for the particular installation. The geometry of the loop may be optimised to generate maximum magnetic fields. It may also be optimised for creating focused field configuration. The number of turns, width, spacing between turns and shape of the loop are the parameters to be optimised. Splitting the loop and driving them in proper phase may focus the fields. This may have an added advantage of increasing the sensitivity and decreasing the influence of other sensors. The magnetic field receiver may, for example, be a giant magneto-resistor (GMR), tunnel magneto-resistor (TMR) or giant magneto-impedance element (GMI). The motion (translation, pitch and/or roll) of a monitored implant changes the magnetic field created by the transmitting loop. The GMR/TMR/GMI elements change their resistance/impedance in response to the change in magnetic field. In one embodiment these elements are arranged in a Wheatstone Bridge form to convert the change in resistance/impedance to a change in voltage, which is measured by the measurement circuitry 22. Separating the transmitter and receiver gives an added advantage of dynamically controlling the sensitivity and range of operation. The dynamic control may be achieved by changing the amount of current flowing through the loop and by changing the frequency of operation. GMR/TMR/GMI sensors may be independently optimised for low power consumption, higher sensitivity, saturation field strength, low hysteresis and output linearity.

In some embodiments, adjacent sensors 12 or alternatively a sensor 12 with two sets of measurement circuitry 22, are arranged so that the generated field is orthogonal between the two transmitters. In this way, substantial isolation may be achieved between the measurements. In certain embodiments the isolation may be 60 dB or more even while operating at same frequency. A still higher isolation may be achieved by operating different sensors at different frequencies and employing filters in measurement circuitry. Further, using two sets of measurement circuitry 22, one tuned at adjacent sensor frequency and one at self-frequency may improve the isolation and actively reduce the cross-talk noise. This in turn may improve the detection capabilities, in terms of range, sensitivity, power consumption and resolution.

The operation of multiple sensors around a joint replacement may be coordinated by software running external to the body. For example, the coordinating software may run on the device 10. Alternatively, the device 10 may communicate with a broader system, for example via an intranet or the Internet, and the device 10 may channel commands from the broader system to the sensors 12. Software running on to the device 10 or the broader system post-processes the information received from the sensors to create data and images, which may be time-varying. Each sensor has a unique identifier and may be a transmitter, receiver or both transmitter and receiver of the measurement signals 56, 58. Instructions contained in the signal 52 may determine whether a sensor is required to act as a transmitter (i.e. signal 56) a receiver (i.e. signal 58) or both.

The measurement signal 56 is tuneable and may be operated at a wavelength commensurate with the sensing appropriate to the specific application. For example, the measurement signal could be low UHF/VHF for applications involving relatively large internal distances, or the signal could be mm-wave for higher frequency applications involving smaller distances. The sensitivity and range are nonlinearly dependant on the standoff distance between the orthopaedic implant 310 and sensor. The non-linearity coefficient is frequency dependant. Consequently, different frequency signals may be employed to get an optimal sensitivity and range. In some embodiments the measurement signal 56 is in the 1 -100 MHz range (inclusive). In other embodiments the measurement signal 56 is in the 10-100 MHz range (inclusive). For example, the sensitivity within this frequency range may be highest for single turn loop. These embodiments may, for example, be suited to distances of about 10 mm. In other embodiments, the measurement signal 56 is in the 100-500 kHz range (inclusive). These embodiments may, for example, be suited to distances over about 25 mm with about 40% reduction in sensitivity at maximum range. In some embodiments different sensors monitoring the same prosthesis may operate at different frequencies, for example dependent on the distance of the sensor to the prosthesis and/or a required sensitivity for the individual sensors. In some embodiments the same sensor may operate at two or more frequencies or over frequency ranges, with the results selected and/or combined to provide a measurement or indication of whether or not a threshold amount of movement has been detected.

The frequency may be tuned before the sensor is implanted. Alternatively, the frequency may be adjusted dynamically, directly by the sensor or indirectly by the external powering and measurement device 10 telling the sensor to adjust the frequency. For example, control information may be transmitted in the input signal 52.

The sensors may be inserted during the joint replacement or in a later procedure in which, for example, the sensors 12 are inserted in holes drilled into the patient's bone near an existing prosthesis. A calibration process may be implemented, for example by the external powering and measuring device 10 and/or the sensor 12, which calibration process may generate output curves at different frequencies, for use in adjusting the raw measurement signals. External device

The external powering and measurement device (EPMD) 10 may be implemented using a smart phone or similar form-factor. The device 10 may, for example, be an app running on the smart phone of the recipient of the joint replacement. The device may than communicate the data to a clinician without the patient needing to present in person to the clinician. The wireless sensor 12 may be powered and queried by the device 10 on separate frequencies to avoid interference. The sensor 12 may use a third frequency for the actual measurement signals 56, 58.

The external powering and measurement device 10 may also be a dedicated monitoring unit used by clinicians. For example, an EPMD 10 may be temporarily attached to the knee of a recipient to test the loads applied to a knee replacement. The recipient may, for example, walk barefoot on a level surface while the time-varying output of the sensors 12 is captured and stored by the EPMD 10. This allows the clinician to assess the forces and moments experienced by the knee joint.

The EPMD 10, or software running on a distributed system communicating with the EPMD, may coordinate the operations of multiple sensors associated with a joint replacement.

If there are n sensors, the n sensing devices can be considered from the perspective of the coordinating software as an n x n array where the rows correspond to a subset, m, of the n devices transmitting at a given frequency. Each entry in the row then corresponds to the received phase and amplitude of the signal received by the n sensors at that frequency. For example, if sensor 1 transmits in a 4 sensor system, then a signal is received by sensors 1 through 4. The measurement process repeats the measurement at sensors 1 through 4, but with sensors 1 and 2 turned on, setting a different received signal, conceptually like an "interference pattern" at all 4 sensors. This is repeated for every combination of 2 transmitters, every combination of 3 transmitters, and then all 4. The method harnesses the fact that the measured value is a complex number and the measurement may be treated mathematically as a linear system. The superposition of linear systems is described, for example, in J. A. Svoboda, R.C. Dorf, "Introduction to Electric Circuits", Wiley, 9^th edition 2014. The interference pattern emerges as the linear combination of each of m sources at n receivers. The data processing adds up the components in a column by selecting the row elements that are considered as sources. The phase and amplitude of each row element in a column create the interference pattern.

Through a calibration/training routine early after surgery, this procedure gives a baseline picture in 2D of the state of the reflective surface (e.g. the metal part 310 of the replacement joint) relative to the n sensors. By sweeping over a number of frequencies, q, the coordinating process generates a q x n x n dataset which provides additional spatial information about the sensors relative to the reflective surface and the intervening material (e.g. biological material 60) between the sensors and the reflective surface. This may for example provide information on calcification, fluid build-up, etc.. The EPMD 10 coordinates all this and may include a data analyser to perform the postprocessing of the data to determine the 2D and material data. Alternatively, the postprocessing and storage of the data may be done using a data analyser implemented in a wider distributed system in data communication with the EPMD 10.

The system uses multi-port S-parameter measurement techniques, which are described, for example, in D. M. Pozar, "Microwave Engineering", Wiley, 4 edition 2012. In one arrangement, for example in the system illustrated in Fig 3, there are three sensors 12d proximate to the tibial plate 310. The EPMD 10 coordinates the following monitoring operations: at a first frequency, the system instructs each sensor 12d in turn to transmit a measurement signal;

all three sensors 12d act as receivers to receive the reflected measurement signals;

the process is repeated at different frequencies; and

the array of measurements from the sensors 12d is analysed to monitor the plate 310 and/or tissue between the sensors 12d and the plate 310.

Hip Replacement Fig 2 illustrates how the monitoring system may be implemented in a hip replacement. In the hip joint 200, the patient's femur 204 moves relative to the patient's hip bone 202. In the hip replacement, an acetabular prosthesis 208 is inserted in the patient's hip bone 202. A femoral prosthesis is inserted in the patient's femur 204. The femoral prosthesis includes a stem 206b oriented along the axis of the femur 204 and a metal ball 206a that is positioned in the socket defined by the acetabular prosthesis 208.

Multiple sensors 12b are placed near to the femoral prosthesis 206. Two sensors are shown in Fig 12, but a different number may also be used. For example, between one and four sensors may be associated with the prosthesis. In the illustrated example, there are two sensors 12b inserted into the top of the patient's femur 204. Multiple sensors 12a may also be placed near to the acetabular prosthesis 208. The sensors 12a may be placed in the hip bone 202 along an arc of 180 degrees around the lip of the socket defined by the acetabular prosthesis 208. Knee Replacement

Fig 3 illustrates in more detail the knee replacement shown schematically in Fig 1 . In the knee joint 300, the patient's femur 302, ending in the patella 304, moves relative to the patient's tibia 306. In the knee replacement, the femoral component 308 is inserted in and around the femur 302 and patella 304. A tibial plate 310 is positioned on the top surface of the patient's tibia 306. The tibial plate 310 has a stem that extends along the axis of the tibia 306.

A polyethylene plastic surface 312 is positioned between the femoral component 308 and the stemmed tibial plate 310.

Multiple sensors 12d are positioned in the tibia near to the tibial plate 310. In one arrangement three sensors are used. The sensors 12d may be placed, for example, within an arc of 180 degrees, on the front side of the tibia 306. During current knee replacement procedures, guide holes are drilled into the patient's tibia 306. The sensors may be inserted into these guide holes. In one arrangement, the sensors 12 have a circular shape with a diameter of 3mm and a length of 10-15mm. Typically, the sensors 12d will be located around 0.5 to 1 .5mm from the tibial plate 310. The sensors 12d may sense the distance to the tibial plate 310 to a resolution of better than 10 micrometre.

Multiple sensors 12c may also be placed adjacent to the femoral component 308. In some arrangements, sensors may be placed into the patella 304 to monitor the patella resurfacing prosthesis.

This distance may be monitored at determined intervals of time, for example daily or weekly, via the external powering and measurement device 10, which reports the data to a clinician.

Shoulder replacement Figs 4A and 4B illustrate two types of shoulder replacement. The original shoulder joint has a ball and socket configuration in which the socket is in the scapular 400 and the ball is formed by the end of the humerus 402. In the reverse shoulder replacement illustrated in Fig 4A, the glenoid prosthesis 404 inserted in the scapular 400 has a ball configuration, and the humeral prosthesis 406 forms a socket at the head of the patient's humerus 402. The humeral prosthesis 406 has a stem that is oriented along the axis of the humerus 402. Multiple sensors 12e are placed in the patient's humerus adjacent to the humeral prosthesis 406. Multiple sensors 12f may also be placed adjacent to the glenoid prosthesis 404, for example drilled in cylindrical holes in the patient's scapular 400. Fig 4B illustrates an anatomic shoulder replacement in which a plastic socket 420 is inserted in the patient's scapular 400. The humeral prosthesis 422 ends in a ball that cooperates with the plastic socket 420. The humeral prosthesis 422 includes a stem that is oriented along the axis of the patient's humerus 402.

Multiple sensors 12g are located adjacent to a humeral prosthesis 422. For example, as illustrated, three sensors are placed in an arc of approximately 180 degrees below the base of the hemispherical ball portion of the humeral prosthesis 422.

Multiple sensors may also be placed adjacent to the glenoid prosthesis 420. The three sensors 12h may be positioned in the scapular along an arc of approximately 180 degrees around the rim of the plastic socket 420. Spinal fusion prosthesis

Figs 5A and 5B illustrate the configuration of a spinal fusion prosthesis. Fig 5A is a rear view and a Fig 5B is a side view of the configuration. The figures show two adjacent vertebrae 502 and 504 of a recipient's spine. Two rods 506a and 506b are positioned to run substantially along the vertical axis of the spine. Screws 508 anchor the rods 506a, 506b into vertebrae 502 and screws 510 anchor the rods 506a, 506b into the vertebrae 504.

A bone graft 512 is positioned between the vertebrae 502, 504 in order to functionally fuse the vertebrae together. Multiple sensors 12 are positioned, for example in holes drilled in the patient's vertebrae 502, 504 proximate the spinal fusion screws and rods 506, 508, 510.

The sensors 12 may be interrogated to monitor the relative location and possible displacement of the components of the spinal fusion.

Advantages The implanted sensors 12 described herein are not required to be positioned in contact with the joint implants or prostheses. A consequence of this arrangement is that the sensors 12 may be used with most implanted components. Therefore, the arrangement is not dependent on particular products, manufacturers or physical sites in the body. The sensors 12 are not required to have any physical contact with the prostheses.

It is thus anticipated that the sensors 12 may be used without additional TGA/FDA assessment of the individual prostheses, for the case that the individual prostheses are used in conjunction with the sensors 12.

The sensors 12 can potentially be used with a wide range of products (existing or yet to be developed) including, knee, hip, shoulder, elbow, ankle, wrist and finger joint replacements as well as surgical plates, screws and rods used in general orthopaedics, fractures and spinal surgery. The systems and methods described herein provide surgeons implanting a prosthesis with the ability to monitor the prosthesis during the course of its life. This is thought to offer advantages in optimising early rehabilitation of the patient, and in addition monitoring the performance of the prosthesis. Such monitoring may enable providers of prostheses to detect potential problems with particular products at an earlier stage than might otherwise be the case.

The system also allows for implantation of the sensors into bone at a later point after the index joint replacement. This could be done as a day surgery case using x-ray control and local or light general anaesthesia in some cases. The sensors may be used to identify potential loosening of a joint implant where radiological findings are inconclusive.

Rehabilitation

The system described herein may be used to monitor a joint replacement following surgery. Sensors may be interrogated to monitor the dynamic loads placed on a prosthetic-bone interface, for example in a knee replacement, to access how the patient is using the replacement and detecting early any problems that might arise.

The system enables clinicians to measure the micro motion of the implant. It is thought that motion exceeding a threshold of 50 pm leads to decreased bone ingrowth. Micromotion greater than 150 pm may result in very little or no bone ingrowth. If excessive initial motion is detected, the clinician may modify the patient's post-operative mobilisation to allow for better bone ingrowth.

The ability to monitor motion at a resolution of 10 pm or less allows clinicians to identify characteristic patterns of impending failure by measuring loads, micro-motion and wear during the life of a prosthesis, before an obvious failure occurs. Computer system

The present invention is necessarily implemented using electronic devices. The electronic device is, or will include, a computer processing system. For example, the external powering and measuring device 10 or a distributed system communicating with the device 10 may be implemented using a computer processing system. Figure 6 provides a block diagram of one example of a computer processing system 100. System 100 as illustrated in Figure 6 is a general-purpose computer processing system. It will be appreciated that Figure 6 does not illustrate all functional or physical components of a computer processing system. For example, no power supply or power supply interface has been depicted, however system 100 will either carry a power supply or be configured for connection to a power supply (or both). It will also be appreciated that the particular type of computer processing system will determine the appropriate hardware and architecture, and alternative computer processing systems suitable for implementing aspects of the invention may have additional, alternative, or fewer components than those depicted, combine two or more components, and/or have a different configuration or arrangement of components.

The computer processing system 100 includes at least one processing unit 102. The processing unit 102 may be a single computer-processing device (e.g. a central processing unit, graphics processing unit, or other computational device), or may include a plurality of computer processing devices. In some instances all processing will be performed by processing unit 102, however in other instances processing may also, or alternatively, be performed by remote processing devices accessible and useable (either in a shared or dedicated manner) by the system 100.

Through a communications bus 104 the processing unit 102 is in data communication with one or more machine-readable storage (memory) devices that store instructions and/or data for controlling operation of the processing system 100. In this instance system 100 includes a system memory 106 (e.g. a BIOS), volatile memory 108 (e.g. random access memory such as one or more DRAM modules), and nonvolatile memory 1 10 (e.g. one or more hard disk or solid state drives). System 100 also includes one or more interfaces, indicated generally by 1 12, via which system 100 interfaces with various devices and/or networks. Generally speaking, other devices may be physically integrated with system 100, or may be physically separate. Where a device is physically separate from system 100, connection between the device and system 100 may be via wired or wireless hardware and communication protocols, and may be a direct or an indirect (e.g. networked) connection. Wired connection with other devices/networks may be by any appropriate standard or proprietary hardware and connectivity protocols. For example, system 100 may be configured for wired connection with other devices/communications networks by one or more of: USB; FireWire; eSATA; Thunderbolt; Ethernet; OS/2; Parallel; Serial; HDMI; DVI; VGA; SCSI; AudioPort. Other wired connections are, of course, possible.

Wireless connection with other devices/networks may similarly be by any appropriate standard or proprietary hardware and communications protocols. For example, system 100 may be configured for wireless connection with other devices/communications networks using one or more of: infrared; Bluetooth; Wi-Fi; near field communications (NFC); Global System for Mobile Communications (GSM), Enhanced Data GSM Environment (EDGE), long term evolution (LTE), wideband code division multiple access (W-CDMA), code division multiple access (CDMA). Other wireless connections are, of course, possible.

Generally speaking, the devices to which system 100 connects - whether by wired or wireless means - allow data to be input into/received by system 100 for processing by the processing unit 102, and data to be output by system 100. Example devices are described below, however it will be appreciated that not all computer- processing systems will include all mentioned devices, and that additional and alternative devices to those mentioned may well be used. For example, system 100 may include or connect to one or more input devices by which information/data is input into (received by) system 100. Such input devices may include physical buttons, alphanumeric input devices (e.g. keyboards), pointing devices (e.g. mice, track pads and the like), touchscreens, touchscreen displays, microphones, accelerometers, proximity sensors, GPS devices and the like. System 100 may also include or connect to one or more output devices controlled by system 100 to output information. Such output devices may include devices such as indicators (e.g. LED, LCD or other lights), displays (e.g. CRT displays, LCD displays, LED displays, plasma displays, touch screen displays), audio output devices such as speakers, vibration modules, and other output devices. System 100 may also include or connect to devices which may act as both input and output devices, for example memory devices (hard drives, solid state drives, disk drives, compact flash cards, SD cards and the like) which system 100 can read data from and/or write data to, and touch-screen displays which can both display (output) data and receive touch signals (input).

System 100 may also connect to communications networks (e.g. the Internet, a local area network, a wide area network, a personal hotspot etc.) to communicate data to and receive data from networked devices, which may themselves be other computer processing systems.

It will be appreciated that system 100 may be any suitable computer processing system such as, by way of non-limiting example, a desktop computer, a laptop computer, a netbook computer, tablet computer, a smart phone, a Personal Digital Assistant (PDA), a cellular telephone, a web appliance. Typically, system 100 will include at least user input and output devices 1 14 and (if the system is to be networked) a communications interface 1 16 for communication with a network 1 18. The number and specific types of devices which system 100 includes or connects to will depend on the particular type of system 100. For example, if system 100 is a desktop computer it will typically connect to physically separate devices such as (at least) a keyboard, a pointing device (e.g. mouse), a display device (e.g. a LCD display). Alternatively, if system 100 is a laptop computer it will typically include (in a physically integrated manner) a keyboard, pointing device, a display device, and an audio output device. Further alternatively, if system 100 is a tablet device or smartphone, it will typically include (in a physically integrated manner) a touchscreen display (providing both input means and display output means), an audio output device, and one or more physical buttons.

System 100 stores or has access to instructions and data which, when processed by the processing unit 102, configure system 100 to receive, process, and output data. Such instructions and data will typically include an operating system such as Microsoft Windows®, Apple OSX, Apple IOS, Android, Unix, or Linux.

System 100 also stores or has access to instructions and data (i.e. software) which, when processed by the processing unit 102, configure system 100 to perform various computer-implemented processes/methods in accordance with embodiments of the invention (as described above). It will be appreciated that in some cases part or all of a given computer-implemented method will be performed by system 100 itself, while in other cases processing may be performed by other devices in data communication with system 100.

Instructions and data are stored on a non-transient machine-readable medium accessible to system 100. For example, instructions and data may be stored on non- transient memory 1 10. Instructions may be transmitted to/received by system 100 via a data signal in a transmission channel enabled (for example) by a wired or wireless network connection.

Monitoring methods

Figure 7 is a flowchart illustrating the steps involved in one embodiment of a method 700 of monitoring a prosthesis implanted in biological tissue of a prosthesis recipient. Similar to the sensor arrangement depicted in Figure 1 b, the sensor used in this embodiment comprises a measurement transmitter, a receiver and an antenna. As such, the method comprises: a step 701 of implanting a sensor in the biological tissue proximate to the prosthesis; a step 703 of transmitting an input signal to the sensor from an external device outside a body of the prosthesis recipient, the input signal initiating operation of the sensor; a step 705 of transmitting, from the measurement transmitter, a measurement signal towards the prosthesis;

• a step 707 of receiving, at the receiver, a reflected measurement signal; a step 709 of transmitting data to the external device via the antenna, wherein the data is dependent on the reflected measurement signal; and

• a step 71 1 of analysing the transmitted data, in a data analyser, to obtain monitoring information comprising spatial position of the prosthesis relative to the sensor. An alternative embodiment of such a method may use a sensor comprising a magnetic field generator, a magneto-resistive or magneto-impedance sensor and an antenna. In such an alternative method, the transmission of the measurement signal towards the prosthesis in step 705 may be by the magnetic field generator, and the step of receiving 707 may be of the measurement signal at the magneto-resistive or magneto-inductance sensor.

It will be appreciated that one option for implanting the sensor is to implant it during the implantation of the prosthesis. In one embodiment, implanting the sensor involves drilling a guide hole in bone tissue of the recipient for use in the implantation of the prosthesis, and then inserting the sensor in the guide hole for subsequent monitoring of the prosthesis. In an alternative embodiment the sensor is implanted in the recipient's biological tissue subsequent to an index procedure in which the prosthesis was implanted.

In a further alternative embodiment, the method of monitoring the prosthesis involves the implantation of a plurality of sensors proximate to the prosthesis. In such an embodiment, the sensors are implanted such that there is intervening biological tissue between the sensors and the prosthesis. The data analyser is then able to analyse transmitted data from the plurality of sensors to generate monitoring information descriptive of the intervening biological tissue In some embodiments of the monitoring method, the external device initiates operation of the sensor, or each sensor in the case of a plurality of sensors being implanted, for a period of time to obtain time-varying monitoring information. The operation of the sensor, or each sensor, may be initiated during the recipient's rehabilitation such that the time-varying monitoring information comprises forces and moments experienced by the prosthesis during movement by the recipient.

It will also be appreciated that further embodiments of the method can include the step of modifying a post-operative mobilisation of the prosthesis recipient dependent on the monitoring information; and/or the steps of identifying a prospective mechanical failure of the prosthesis based on the monitoring information and replacing the prosthesis. It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

1 . A system for monitoring a prosthesis implanted in biological tissue of a prosthesis recipient, the system comprising: one or more sensors configured for implanting in the biological tissue proximate to the prosthesis, each sensor comprising: a measurement transmitter for transmitting a measurement signal towards the prosthesis;

a receiver for receiving a reflected measurement signal;

an antenna that transmits data dependent on the reflected measurement signal; an external device configured to communicate with the one or more sensors from outside a body of the recipient, the external device receiving the data transmitted by the antenna; and a data analyser for analysing the transmitted data to obtain monitoring

information comprising spatial position of the prosthesis relative to the one or more sensors.

2. The system of claim 1 wherein the external device is arranged to transmit an input signal to initiate operation of the one or more sensors and each sensor comprises power generation circuitry that obtains power to operate the sensor from the input signal.

3. The system of claim 1 or 2 wherein the measurement transmitter is arranged to transmit the measurement signal at a range of frequencies.

4. The system of claim 3 wherein the measurement signal frequency is selectable from at least one of: · an LF-HF range;

• a VHF range;

• a UHF range; and

• a mm-wave range.

5. The system according to any one of the preceding claims comprising a plurality of sensors coordinated by the external device to transmit measurement signals at a plurality of frequencies, wherein the data analyser is arranged to calculate monitoring information descriptive of intervening biological tissue located between the plurality of sensors and the prosthesis.

6. The system of claim 5 wherein the monitoring information characterises calcification of the intervening biological tissue or fluid build-up in the intervening biological tissue.

7. The system of any one of claims 3 to 6 wherein the input signal transmitted by the external device comprises information specifying the frequency of the measurement signal.

The system according to any one of the preceding claims wherein the prosthesis prises at least one of: a femoral prosthesis for a hip replacement;

an acetabular prosthesis for a hip replacement;

a femoral prosthesis for a knee replacement;

a tibial plate prosthesis for a knee replacement;

a humeral prosthesis for a shoulder replacement;

a glenoid prosthesis for a shoulder replacement;

a spinal fusion fixation prosthesis;

an elbow joint replacement;

a wrist joint replacement;

an ankle replacement;

a finger joint replacement;

a surgical plate, screw or rod.

9. The system according to any one of the preceding claims wherein the sensor comprises a cylindrical housing configured for insertion into a hole drilled into bone tissue of the recipient.

10. A method of monitoring a prosthesis implanted in biological tissue of a prosthesis recipient, the method comprising: implanting a sensor in the biological tissue proximate to the prosthesis, the sensor comprising a measurement transmitter, a receiver and an antenna; transmitting an input signal from an external device to the sensor from outside a body of the recipient to initiate operation of the sensor; transmitting, from the measurement transmitter, a measurement signal towards the prosthesis; receiving, at the receiver, a reflected measurement signal; transmitting data to the external device via the antenna, wherein the data is dependent on the reflected measurement signal; and analysing the transmitted data, in a data analyser, to obtain monitoring information comprising spatial position of the prosthesis relative to the sensor.

1 1 . The method of claim 10 wherein the sensor is implanted during the implantation of the prosthesis.

12. The method of claim 1 1 wherein implanting the sensor comprises: drilling a guide hole in bone tissue of the recipient for use in the implantation of the prosthesis; inserting the sensor in the guide hole for subsequent monitoring of the prosthesis.

13. The method of claim 10 wherein the sensor is implanted in the recipient's biological tissue subsequent to an index procedure in which the prosthesis was implanted.

14. The method of any one of claims 10 to 13 comprising implanting a plurality of sensors proximate to the prosthesis.

15. The method of claim 14 wherein the sensors are implanted such that there is intervening biological tissue between the sensors and the prosthesis.

16. The method of claim 15 wherein the data analyser analyses transmitted data from the plurality of sensors to generate monitoring information descriptive of the intervening biological tissue.

17. The method of any one of claims 10 to 16 wherein the external device initiates operation of the or each sensor for a period of time to obtain time-varying monitoring information.

18. The method of claim 17 wherein the operation of the or each sensor is initiated during the recipient's rehabilitation and the time-varying monitoring information comprises forces and moments experienced by the prosthesis during movement by the recipient.

19. The method of any one of claims 10 to 18 further comprising modifying a postoperative mobilisation of the recipient dependent on the monitoring information.

20. The method of any one of claims 10 to 19 further comprising: identifying a prospective mechanical failure of the prosthesis based on the monitoring information; and replacing the prosthesis.

21 . A sensor configured for implanting in biological tissue proximate to a prosthesis, the sensor comprising: a magnetic field transmitter for transmitting a magnetic field towards the prosthesis;

a receiver for receiving and measuring the magnetic field, wherein the receiver is a magneto-resistive or magneto-impedance sensor;

communications circuitry comprising an antenna that transmits data external to the sensor dependent on the measured magnetic field.

22. A method of monitoring a prosthesis implanted in biological tissue of a prosthesis recipient, the method comprising: implanting a sensor in the biological tissue proximate to the prosthesis, the sensor comprising a magnetic field generator, a magneto-resistive or magneto- impedance sensor and an antenna; transmitting an input signal from an external device to the sensor from outside a body of the recipient to initiate operation of the sensor; transmitting, by the magnetic field generator, a measurement signal towards the prosthesis; receiving the measurement signal at the magneto-resistive or magneto- inductance sensor; transmitting data to the external device via the antenna, wherein the data is dependent on the received measurement signal.