The invention relates to an apparatus for promoting bone repair or integration of a prosthesis in a bone. The apparatus may be used, for example, for promoting repair of a fracture of a bone. Alternatively, the apparatus may be used for promoting the integration of a bone with a prosthesis used to repair or replace part of a bone; such as in joint replacement therapy.

Supports securable to a fractured bone for holding the fractured bone in a desired configuration for repair of the fracture are well known. The support may be, for example, an intramedullary nail which is inserted in the medullary cavity of a long bone or a plate for fixing adjacent an external surface of a bone. The support may also be an external support (i.e. located external to the patient's body) fixed to a bone using pins or the like passing through the patient's skin. Hence, as used herein, and unless a location is specified or implied by the context, when a support is said to be "secured to" or "securable to" a bone, the support may be located, when secured, in any suitable manner relative to the bone, including inside the bone, adjacent to the bone or spaced from the bone.

In addition, it is known to provide an intramedullary nail with a plurality of strain gauges attached to the nail at respective different positions of the nail. This is sometimes referred to as an "instrumented intramedullary nail". The strain gauges are used to detect strain occurring on deformation of the intramedullary nail. Deformation of the nail may occur if the fractured bone is subjected to a bending, compressive, tensile or torsional stress. It has been suggested that, as the fracture heals, the bone becomes more rigid and, in turn, the stresses applied to the intramedullary nail and the subsequent deformation of the nail are reduced. It has been proposed that measurement of strain in an instrumented nail can be used as a measure of fracture healing, with reducing strain experienced by the nail being indicative of bone healing.

Instrumented nails are described in:

Faroug , McCarthy ID, Meswania J, Janna S, Wilson D, Taylor SJG. 2011. Strain response of an instrumented intramedullary nail to three-point bending. Journal of Medical Engineering & Technology, 35:5, 275-282;

Schneider E, Michel MC, Genge M, Zuber K, Ganz R, Perren SM. 2001. Loads acting in an intramedullary nail during fracture healing in the human femur. Journal of Biomechanics, 34, 849- 857; Wilson DJ, Morgan L, Hesselden KL, Dodd JR, Janna SW, Fagan MJ. 2009. A single-channel telemetric intramedullary nail for in vivo measurement of fracture healing. J Orthop Trauma, 23:10, 702-709.

Prostheses are also well known for repairing or replacing bone. For example, in a joint replacement operation, a bone is repaired by replacement of a damaged or diseased part of the bone with a prosthesis. By way of a more specific example, in a hip joint replacement operation, the rounded head, or ball, of the femur is replaced by a ball of a prosthesis. A joint replacement prosthesis may include a support in the form of a prosthesis stem. The stem is inserted into an aperture in the bone to be repaired and serves to anchor the prosthesis in place.

In accordance with the invention, there is provided apparatus for promoting bone repair or integration of a prosthesis in a bone, comprising: a support securable to a bone; a plurality of strain gauges attached to the support at respective different positions of the support, the strain gauges detecting strain on deformation of the support; a controller; a signalling system operatively connected to the strain gauges and operatively connected to the controller for providing to the controller signals indicative of respective strains detected by the strain gauges; a functional electrical stimulation device for applying a functional electrical stimulation to one or more muscles; the controller being operatively connected to the functional electrical stimulation device so that the controller controls the application of functional electrical stimulation dependent on the signals provided by the signalling system.

In one embodiment, the apparatus is adapted for treating a fracture of a bone. In this case, the support is securable to a fractured bone for holding the fractured bone in a desired configuration for repair of the fracture. Treatment of a bone fracture using the current apparatus may improve the repair outcome of the bone fracture. For example, treatment using the apparatus may shorten the repair time of a bone fracture. Alternatively, treatment using the apparatus may stimulate repair of a fracture that would not ordinarily repair. Also, treatment using the apparatus may increase the strength of a repaired fracture.

In another embodiment, the apparatus is adapted for promoting the long-term integration of a prosthesis in a bone. In this case the support of the apparatus may be the stem of a joint replacement prosthesis.

In the following general discussion, unless clearly incompatible with one case or another, the preferred embodiments of the invention disclosed below apply both to the case where the invention is adapted for the treatment of a fractured bone, and to the case where the invention is adapted for promoting the integration of a prosthesis in a bone.

Without being limited to any mode of action, it is hypothesised that healing of bone fractures is stimulated by bringing the fracture into compression; that is to say bringing the fracture surfaces on opposite sides of the fracture into direct or indirect contact with one another. Intermittent or cyclic contact is preferred rather than a continuous compressive force. Use of the current apparatus may bring fracture surfaces into contact with one another, or it may increase the area of the fracture surfaces that are in contact with one another. In either case, the contact may be intermittent or cyclical. For example, strains sensed by the strain gauges may indicate that the support is experiencing deformation, which may in turn indicate that the fractured bone is experiencing deformation such that part or all of the fracture surfaces are not in contact with one another across the fracture. The signals provided by the signalling system to the controller provide information about deformation experienced by the support, and indirectly about deformation of the supported bone, and in turn the controller directs the functional electrical stimulation device to apply stimulation to an appropriately chosen muscle, which causes the stimulated muscle to apply a mechanical force which has the effect of bringing the fracture surfaces into contact, or increasing the area of the fracture surfaces which are in contact. Hence, in a preferred embodiment, the apparatus is adapted to apply a functional electrical stimulation to a suitably chosen muscle to increase the area of fracture surface of the bone in contact across the fracture.

In another preferred embodiment, the apparatus is adapted to apply a functional electrical stimulation to a suitably chosen muscle so as to reduce or eliminate a deformation experienced by the support.

Preferably the controller monitors signals provided by the signalling system after application of a functional electrical stimulation and uses the signals provided after said application to modify the functional electrical stimulation as necessary. The modification may comprise increasing or decreasing the intensity of the functional electrical stimulation or ceasing the functional electrical stimulation. The controller might be designed to include a delay so that the system does not respond to very short term deformations of the support. However, it should be able to react quickly if an excessively high loading of the support is detected.

Where the apparatus is adapted for treating a fractured bone, the support may be any support capable of holding a fractured bone in a desired configuration for repair of the fracture. Preferably, the support will be a support for a long bone, in which case the support may be an intramedullary nail, a plate for affixing adjacent to an external surface of a long bone, or an external fixation device. The most preferred type of support is an intramedullary nail.

The strain gauges may be, for example, high sensitivity foil strain gauges. Alternatively the strain gauges may be ultra-small semiconductor strain gauges. Both of these types of strain gauges are commercially available. Other strain gauges of suitable size and sensitivity may be used.

Each strain gauge may be attached to the support in any manner which allows the strain gauge to detect a strain when the support is deformed in such a manner as to cause a strain in a region of the support adjacent the strain gauge. In this way, if a region of a support adjacent to a strain gauge experiences compression, the adjacent strain gauge will detect a compressive strain. Similarly, if the region is in tension, the adjacent strain gauge will detect a tensile strain. By way of example, the strain gauges may be attached to the support using an adhesive, such as a cyanoacrylate glue (e.g. Super Glue™) or an epoxy glue.

The strain gauges may be simply attached to an external surface of the support. However, and in particular where the support is an intramedullary nail, it is preferred to situate each strain gauge within a recess in an external surface of the support. For example, a strain gauge may be adhered to an inner surface of such a recess. Each strain gauge may be accommodated in a different respective recess. Alternatively, a recess may accommodate a plurality of strain gauges, in which case the strain gauges in a single recess may be oriented in different directions to detect different types of strain (e.g. compressive, tensile, torsional). Assemblies comprising a plurality of strain gauges in different orientations are commercially available.

Each strain gauge may be provided with a sealing covering to prevent contact between the strain gauge and biological fluids. For example, each strain gauge may be covered with a potting compound, such as a biocompatible resin, to prevent contact between the strain gauge and biological fluids. Alternatively, the strain gauge may be covered with a metal cover (e.g. titanium) welded to the support. Other methods of sealing the strain gauge to prevent contact between the strain gauge and biological fluids may be used. When a strain gauge is located within a recess in a support, the recess can be filled with a potting compound or covered with a metal cover welded to the support. It is particularly preferred to use a metal cover as a sealing covering over a recess. This is because the recess tends to reduce the strength of the support and this is counteracted in part by the metal cover.

The strain gauges are attached to the support at respective different positions of the support. In this way, the strain gauges collectively provide information indicative of the nature of deformation experienced by the support. In a simple example, a support may have a first strain gauge attached to a first side and a second strain gauge attached to a second side opposite to the first side. If the first strain gauge detects a compressive strain and the second strain gauge detects a tensile strain, the strain gauges collectively indicate that the support is experiencing bending with the first strain gauge closer to the interior (concave side) and the second strain gauge closer to the exterior (convex side) of the bend. If both strain gauges indicate compression, the support may be in simple compression and if both strain gauges indicate tension the support may be in simple tension. Alternatively if one gauge senses a higher compressive strain than the other, the support may be experiencing combined bending and compression.

Attaching a greater number of strain gauges to the support, at respective different positions, may allow determination, with greater precision, of the nature and position of a deformation experienced by the support and hence the bone.

In one preferred embodiment, the support is elongate (such as an intramedullary nail for treating a bone fracture) and strain gauges are positioned at a plurality of axial positions along the length of the support. At each axial position along the length of the support, a plurality of strain gauges are positioned spaced from each other angularly around the axis of the support. For example, strain gauges may be positioned at three or four axial positions along the length of the support. At each axial position, three strain gauges may be positioned spaced from each other around the circumference of the support by about 120 ^° . In an alternative preferred embodiment, a plurality of strain gauges are positioned at different axial positions along the length of the support, with only one strain gauge at each axial position and with each strain gauge being angularly spaced around the support compared to the preceding strain gauge such that the strain gauges lie on a spiral path extending around and along the support.

In another preferred embodiment, the support is an elongate support (such as a support for holding a fractured long bone). The strain gauges are positioned such that the signals provided by the signalling system allow detection of bending of the support, in a plane parallel to the length of the elongate support, and discriminate between bending in first and second opposite senses within the plane. By first and second opposite senses, it is meant that in one of the senses, the concave side of the bend is on one side of the support in the plane and in the other one of the senses, the concave side of the bend is on the other side of the support in the plane. When the support is holding a fractured long bone, the plane may correspond to the anterior-posterior plane of the bone for detecting bending in the anterior-posterior plane. Alternatively, the plane may correspond to the medial-lateral plane of the bone for detecting bending in the medial-lateral plane. In this embodiment, the signals produced by the signalling system may not necessarily be able to detect or distinguish bending in a plane perpendicular to the said plane.

In an even more preferred embodiment, the signals provided by the signalling system allow detection of bending of the support, in each of two mutually perpendicular planes both parallel to the length of the elongate support, and discriminate for each plane between bending in first and second opposite senses within said each plane. In this case, when the support is holding a fractured long bone, the signals may allow detection of bending in each of the anterior-posterior and medial- lateral planes of the supported bone, and for each plane, allow determination of the sense of the bend.

The controller preferably comprises a microprocessor. The controller functions automatically directing the functional electrical stimulation device to apply functional electrical stimulation in a manner determined by the controller based on the signals provided by the signalling system.

The apparatus includes a signalling system that is operatively connected to the strain gauges and operatively connected to the controller for providing to the controller signals indicative of respective strains detected by the strain gauges. The signalling system comprises all of the components which serve to provide to the controller signals indicative of respective strains detected by the strain gauges.

A preferred type of strain gauge detects strain by undergoing changes in electrical resistance. For example, when a foil strain gauge is subjected to a compressive strain it exhibits a reduced resistance and when it is subjected to a tensile strain it exhibits an increased resistance. The signalling system may include electrical circuitry, for example circuitry such as Wheatstone bridges, for generating, from changes in resistances of strain gauges, output indicative of strains experienced by the strain gauges. Preferably, the signalling system comprises a respective Wheatstone bridge for each strain gauge - each strain gauge being incorporated into the associated Wheatstone bridge in a known manner so that the Wheatstone bridge produces an output (such as voltage or current difference) indicative of the strain experienced by the strain gauge.

The signalling system may also comprise other components necessary for providing to the controller signals indicative of respective strains detected by the strain gauges. For example, the signalling system may include a power source, such as a rechargeable battery. The power source may be used, for example, for energising Wheatstone bridges and/or other components of the signalling system. The signalling system may include a voltage regulator for regulating the output voltage of the power source. The signalling system may also include one or more amplifiers, one or more analogue-to- digital converters, and/or a multiplexer.

The signalling system preferably includes a wireless connection - that is to say a connection that allows communication without wires or other physical connection. This is because the controller is generally located externally to a patient's skin. In this case, some components of the signalling system will be provided on the support while other components will be located externally to the patient's skin. While is it possible to use a wired connection passing across the patient's skin, a wireless connection is greatly preferred. In a preferred embodiment, the signalling system includes a first coil attached to the support and a second coil for location externally to the patient's skin. Information, for example information indicative of strains detected by the strain gauges, may be passed between the two coils by inductive coupling. The two coils may also be used to transfer power across the skin, for example to charge a rechargeable battery located on the support or for energising components of the signalling system provided on the support. In another preferred embodiment, the signalling system has a first pair of coils for passing information across the patient's skin in an outward direction and a second pair of coils for passing power across the patient's skin in an inward direction.

Alternatively, forms of wireless connection other than inductive coupling may also be used.

In a particularly preferred embodiment, all components of the signalling system which are provided on the support are provided within one or more recesses on an external surface of the support and are provided with a sealing covering (such as a potting compound or welded metal cover) to prevent contact with biological fluids. In an alternative embodiment, some of the electronics are housed within the interior of a hollow support, such as a hollow nail.

The apparatus also comprises a functional electrical stimulation device for applying a functional electrical stimulation to a muscle. Functional electrical stimulation devices are well known. They utilise electrical currents to activate muscle causing contraction of the muscle.

In a preferred embodiment, the functional electrical stimulation device comprises a plurality of pads. The device is such that a functional electrical stimulation may be applied through any one of the pads, independently of the other pads. The controller determines which one of the pads is used to apply functional electrical stimulation, and the determination is dependent on the signals provided by the signalling system. In use, each pad may be affixed over a respective different muscle. The controller is "aware" of which muscle each pad is affixed over. For example, the pads may be labelled, or otherwise visually distinguishable, so that a medical practitioner is able to affix each pad over a predetermined muscle. Alternatively, the controller may be programmed, after the pads have been affixed over respective muscles, with information about which pad is affixed over which muscle.

In this way, the controller is able to direct the functional electrical stimulation device to stimulate an appropriate specific muscle, dependent on the signals provided to the controller by the signalling system. For example, where the apparatus is adapted for treatment of a fractured bone, if the signals provided to the controller indicate that the support (and the supported bone) is bent in a certain sense, the controller may stimulate a muscle that, when contracted, will exert a mechanical force tending to bend the support (and the supported bone) in the opposite sense. This may help to bring the fracture surfaces into contact with one another, or increase the area of contact between the fracture surfaces across the fracture. By affixing pads over different respective muscles, the controller is able to stimulate whichever one of the muscles (or more than one of the muscles) is best suited to bring the fracture surfaces into contact with one another across the fracture, or to increase the area of the fracture surfaces in contact with one another. The identity of the muscle that should be stimulated will depend, for example, on the position and posture of the patient (standing, sitting, lying etc) and on the loading being placed on the fractured bone. The controller determines automatically which muscle should be stimulated and the stimulation takes place automatically.

Similarly, in cases where the apparatus is adapted to promote the integration of a prosthesis in a bone, the provision of a plurality of pads, and the ability of the controller to selectively apply a functional electrical stimulation through any one of the pads, allows the apparatus to minimise bending of the support regardless of the position and posture of the patient.

The controller may vary the operation of the functional electrical stimulation device depending on feedback received from the signalling system. For example, if the controller causes a functional electrical stimulation to be applied to counteract a deformation of the support, and the signals received from the signalling system indicate that the deformation has not been sufficiently counteracted, the controller may direct the functional electrical stimulation device to increase the intensity of the functional electrical stimulation. Alternatively, if the controller causes a functional electrical stimulation to be applied to counteract a deformation of the support, and the signals received from the signalling device indicate that the deformation has been sufficiently counteracted, the controller may direct the functional electrical stimulation device to decrease or cease the intensity of the functional electrical stimulation. The following is a more detailed description of embodiments of the invention, by way of example, reference being made to the appended, schematic drawings in which:

Figure 1 represents the electronic components of apparatus for treating a fracture of a bone;

Figure 2 shows a support which forms part of the apparatus of Figure 1;

Figure 3 is a similar representation to Figure 2 showing the support during use of the apparatus;

Figure 4 shows various alternative arrangements of strain gauges that may be used in the support of the apparatus of Figures 1 to 3 and 5;

Figure 5 shows a second apparatus which includes a bone prosthesis and which is adapted to promote integration of the bone prosthesis in a bone.

Referring first to Figures 1 and 2, the apparatus includes a support which, in this embodiment, takes the form of an intramedullary nail 10. In Figure 2, the intramedullary nail 10 is shown in use within the medullary cavity of a human femur which has suffered a fracture. As is well known, the intramedullary nail 10 supports the fractured femur in a configuration suitable for repair of the fracture. In use, the intramedullary nail 10 is fixed to the femur 12 by fixing screws located at the proximal and distal ends of the intramedullary nail 10. The fixing screws and the method of fixing of the intramedullary nail 10 within the medullary cavity of the femur 12 are conventional and will not be described. The fixing screws are not shown in Figures 2 and 3 for the sake of clarity.

The intramedullary nail 10 is provided with twelve strain gauges. Only eight of these strain gauges 14a, 14b, 16a, 16b, 18a, 18b, 20a, 20b are visible in Figure 2. The remaining four strain gauges that are not visible in Figure 2 are located behind the plane of Figure 2 (and one of these four is visible in Figure 1 at 14c). The twelve strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b are positioned on the intramedullary nail 10 at four axial positions along the length of the intramedullary nail 10. Hence, looking at Figure 2, three strain gauges 14a, 14b (and also 14c) are located at a first axial position which is closest to the proximal end of the intramedullary nail. The three strain gauges 14a, 14b, 14c located at the first axial position are spaced from one another angularly around the axis of the intramedullary nail 10 by 120°. Similarly, three strain gauges 16a, 16b (and one not shown) are located at a second axial position along the length of the intramedullary nail 10. The three strain gauges 16a, 16b located at the second axial position are also spaced from on another angularly around the axis of the intramedullary nail 10 by 120°. In a similar manner, three stain gauges 18a, 18b (and one not shown) are located at a third axial position along the length of the intramedullary nail 10 and three stain gauges 20a, 20b (and one not shown) are located at a fourth axial position along the length of the intramedullary nail. At each one of the third and fourth axial positions, the strain gauges 18a, 18b, 20a, 20b are spaced from one another around the axis of the intramedullary nail 10 by 120°. As seen in Figure 2, the fourth axial position, corresponding to strain gauges 20a, 20b (and one not shown), is located towards the distal end of the intramedullary nail 10.

Each strain gauge 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b is located in a respective recess that has been machined into the exterior surface of the intramedullary nail 10. Each recess is approximately 1 or 2mm deep.

As seen in Figure 1, each strain gauge 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b is a high sensitivity foil strain gauge. Each strain gauge is glued to the base of the corresponding recess using cyanoacrylate glue. Each strain gauge 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b is orientated with its axis generally parallel to the axis of the intramedullary nail 10 so that the strain gauge is positioned for sensing compressive and tensile strains in the direction of the axis of the intramedullary nail 10.

As will be described in more detail below, each one of the strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b is associated with electronic circuitry which is also located in the corresponding recess. The recesses which house the strain gauges reduce the strength of the intramedullary nail 10. This is undesirable. However, each recess is covered with a respective metal plate which is welded to the surface of the intramedullary nail 10. The covering plates are not shown. The covering plates serve to seal the recesses and prevent contact between the strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b, and the associated electronics, with biological fluid when the intramedullary nail 10 is in use within the medullary cavity of a bone. In addition, the plates have the effect of strengthening the intramedullary nail 10 and so counteract the weakening effect of the recesses.

Referring now to Figure 1, the apparatus also includes a microprocessor 22 and a signalling system which is shown collectively at 24. The signalling system 24 serves to provide the microprocessor 22 with signals which are indicative of the respective strains sensed by the strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b. The signalling system 24 will now be described in more detail.

As is well known, when a foil-type strain gauge experiences a compressive strain, the electrical resistance of the strain gauge reduces. Conversely, when the strain gauge experiences a tensile strain, the electrical resistance of the strain gauge increases. One function of the signalling system 24 is to convert variations in electrical resistance undergone by the strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b into electrical signals that can be read by the controller 22. In order to achieve this, the signalling system 24 includes a respective Wheatstone bridge for each strain gauge 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b. In Figure 1, for the sake of clarity, only three of the strain gauges 14a, 14b, 14c, are shown together with the three corresponding Wheatstone bridges 26a, 26b, 26c. The remaining nine strain gauges and the remaining nine Wheatstone bridges are omitted for clarity.

Each one of the twelve strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b is incorporated into the corresponding Wheatstone bridge 26a, 26b, 26c in a known manner. When each Wheatstone bridge 26a, 26b, 26c is energised, variations in the electrical resistance of the corresponding strain gauge 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b cause the associated Wheatstone bridge 26a, 26b, 26c to produce an output indicative of the strain experienced by the strain gauge. The output may be, for example, an electrical voltage or an electrical current.

Each Wheatstone bridge 26a, 26b, 26c is located in the same recess as the corresponding strain gauge 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b.

As seen in Figure 1, the signalling system 24 includes a first circuit 28 provided with a first induction coil 32, and a second circuit 30 provided with a second induction coil 34. The first circuit 28 and the first induction coil 32 are provided on the intramedullary nail 10 and are conveniently situated within a recess (not shown) machined into the surface of the intramedullary nail 10. The recess may be covered and sealed with a plate welded to the intramedullary nail 10. The second circuit 30 and the second induction coil 34 are, in use, located externally to a patient's skin. The first circuit 28 and the second circuit 30 communicate with one another, across the skin of a patient, by inductive coupling between the first and second coils 32, 34.

The first circuit 28 serves two main purposes. Firstly, the first circuit 28 energises the Wheatstone bridges 26a, 26a, 26c so as to allow the Wheatstone bridges to produce electrical outputs indicative of the strains experienced by the associated strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b. In order to energise the Wheatstone bridges 26a, 26b, 26c, the first circuit 28 includes a rechargeable battery (not shown). This rechargeable battery energises the Wheatstone bridges 26a, 26b, 26c, via a voltage regulator which is also included within the first circuit 28. In order to charge the rechargeable battery of the first circuit 28, electrical power is fed from the microprocessor 22 to the external second circuit 30, and power is transmitted across the patient's skin by inductive coupling between the first and the second coil 32, 34.

The second purpose of the first circuit 28 is to receive the electrical outputs of the Wheatstone bridges 26a, 26b, 26c, to convert the outputs into convenient electrical signals and to transmit the electrical signals across the patient's skin to the second circuit 30 using inductive coupling between the first and second coils 32, 34.

The second circuit 30 receives the electrical signals transmitted by the first circuit 28 and passes these electrical signals, which are indicative of the strains experienced by the strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b, to the microprocessor 22.

The first and second circuits 28, 30 may be configured in any manner suitable for performing the purposes outlined above. By way of example, one suitable configuration is shown in Figure 2 on page 851 in the article by Schneider et al. cited above (Journal of Biomechanics 34 (2001), pp 849- 857). In Figure 2 of Schneider, the components within the box labelled "implant" correspond to the first circuit 28 of the current invention and the components outside of the box correspond to the second circuit 30 of the current invention. In Schneider, each strain gauge (and associated electronic) is connected to a multiplexer which samples the outputs of all of the strain gauges. The output of the multiplexer is fed into an amplifier which, in turn, has its output connected to an analogue to digital convertor. The digital signals produced by the analogue to digital convertor are fed into a parallel/serial convertor and then into a transmitter connected to an inductive coupling coil. In Schneider, the strain gauges, the multiplexer, the amplifier, the analogue to digital convertor, the parallel/serial convertor and the transmitter are all energised by a power source which is inductively coupled to the circuit comprising these components. The power is fed to the components via a voltage regulator. A more thorough description of the electronics in Schneider is given on page 851 of Schneider.

The apparatus also includes a functional electrical stimulation device 36. The functional electrical stimulation device 36 is of known type and provides an electrical stimulation to muscles in order to cause the muscles to contract. Figures 1 to 3 show a pad 38 that is connected to the functional electrical stimulation device 36. The functional electrical stimulation device 36 applies an electrical stimulus to a muscle via the pad, the pad being placed on the skin over the muscle to be stimulated. Although only one pad 38 is shown, the functional electrical stimulation device 36 has a plurality of pads 38 so that each pad can be placed on the skin over a respective muscle. The functional electrical stimulation device 36 is capable of energising each pad 38 independently of the other pads 38.

The microprocessor 22 receives electrical signals from the second circuit 30. In turn, the microprocessor 22 directs the functional electrical stimulation device 36 to apply functional electrical stimulation to appropriate muscles. The microprocessor 22 choses the muscle that is to be stimulated. The muscle is chosen with the objective of bringing into contact respective regions of the opposing fracture surfaces at an area, or areas, of a fracture where the opposing fracture surfaces are not in contact. It is preferred that, at all areas of a fracture, regions of the opposing fracture surfaces, at that area, are brought into contact on an intermittent or cyclical basis. The operation of the apparatus will now be explained, making use of a simple example, and referring to Figures 2 and 3.

As shown in Figures 2 and 3, the femur 12 in which the intramedullary nail 10 has been inserted has suffered a simple transverse fracture which is indicated at 40. In Figures 2 and 3, the proximal and distal fragments 42, 44 of the femur 12 are shown separated by a gap at the fracture 40 which is exaggerated for purposes for clarity. At the fracture 40, the proximal fragment 42 of the femur 12 has a proximal fracture surface 46, and the distal fragment 44 of femur 12 has a distal fracture surface 48. The proximal and distal fracture surfaces 46, 48 face one another across the fracture 40 in Figures 2 and 3.

Referring now to Figure 2, the arrow 50 represents a downward force applied to the femur 12 at the hip joint. The downward force represented by the arrow 50 causes the femur 12 (when the patient is in a certain position) to bend in the medial-lateral plane: with the interior or concave side of the bend being at the medial side 52 of the femur 12 (the right hand side in Figures 2 and 3).

This bending of the femur 12 has the following effect on the contact between the proximal and distal fracture surfaces 46, 48. Specifically, at the medial side 52 of the fracture 40 (right hand side in Figures 2 and 3), the proximal and distal fracture surfaces 46, 48 are in contact with one another. This may also be referred to as being in compression and is represented by the letter "C" in Figure 2. However, at the opposite lateral side 54 of the fracture 40 (left hand side in Figures 2 and 3), the proximal and distal fracture surfaces 46, 48 are separated from one another. This may also be referred to as being in tension and is represented by the letter "T" in Figure 2.

Without being limited to any mode of operation, it is considered that fracture healing is stimulated when the fracture surfaces 46, 48 contact one another (ideally with intermittent or cyclical contact). Accordingly, in the scenario above, the current apparatus may be used to try to maximise the area of the fracture surfaces 46, 48 over which there is contact, by bringing the fracture surfaces 46, 48 into contact with one another at the lateral side 54 of fracture 40 (as well as at the medial side 52). The way this is achieved is as follows.

The bending of the femur 12 described above (with the interior or concave side of the bend lying at the medial side 52) produces a corresponding bend in the intramedullary nail 10. This bending deformation of the intramedullary nail 10 is sensed by the strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b provided on the intramedullary nail 10. For example, in the current situation, the strain gauge shown in Figure 2 at 16b will experience a compressive strain, whilst the stain gauge shown in Figure 2 at 16a will experience a tensile strain. The outputs from the associated Wheatstone bridges are fed to the first circuit 28, as described above, and signals produced by the first circuit 28 are passed across the patient's skin by inductive coupling to the second circuit 30. The second circuit 30 provides the signals to the microprocessor 22. The microprocessor 22 analyses the signals and determines, for each one of the strain gauges 14a, 14b, 14c, 16a, 16b, 18a, 18b, 20a, 20b, whether the strain gauge is in tension, in compression or not experiencing any strain. In the current situation, the microprocessor 22 determines that the intramedullary nail 10 is bent with the interior or concave side of the bend at the medial side 52 of the femur 12.

The microprocessor 22 then directs the functional electrical stimulation device 36 to stimulate a muscle that is appropriate for applying a mechanical force that will tend to counteract the bending deformation experienced by the intramedullary nail 10. In the current circumstances, the microprocessor 22 will cause functional electrical stimulation of the muscle 56 that is shown in Figures 2 and 3 and which is located on the lateral side 54 of the femur 12. The functional electrical stimulation is applied via the pad 38 shown in Figures 2 and 3, the pad being placed on the skin over the muscle 56. As shown in Figure 3, the functional electrical stimulation causes the muscle 56 to contract and this tends to counteract the force represented by the arrow 50 so as to straighten the femur 12 and, and in turn, straighten the intramedullary nail 10. As represented in Figure 3 by the letters "C" for "compression", the straightening force applied by the muscle 56 causes the proximal and distal fracture surfaces 46, 48 to be brought into contact with one another at the lateral side 54 of the fracture 40. Hence, the proximal and distal fracture surfaces 46, 48 are now in contact with one another at both the medial and lateral sides 52, 54 of the fracture.

If the patient subsequently changes position, the deformation experienced by the femur 12 and the intramedullary nail may change and, as a consequence, the processor 22 may direct the functional electrical stimulation device 36 to stimulate another muscle using another pad 38. This is automatic.

It may not be necessary to keep all areas of the fracture surfaces 46, 48 in contact with one another constantly throughout the healing process. Instead, it may be sufficient for the apparatus to apply intermittent functional electrical stimulation so that all areas of the fracture surfaces 46, 48 are brought into contact with one another at least intermittently. Again, without being limited to any mode of action, it is hypothesised that intermittent contact between fracture surfaces will stimulate repair of a fracture. By way of explanation, it is considered that contact between fracture surfaces stimulates fracture healing and the laying down of new bone. These processes may continue for some time, following contact between fracture surfaces, even after the load has reduced or the fracture surfaces have separated from one another again.

It will be appreciated that many variations may be made to the apparatus described above without departing from the scope of the invention as defined in the claims.

For example, in the embodiment described above, each recess of the intramedullary nail 10 contains a single strain gauge orientated to detect strain in the same direction as the axis of the intramedullary nail 10. However, it would be possible for each recess to contain a plurality of strain gauges. Assemblies comprising a plurality of stain gauges in respective different orientations are commercially available. For example, Figure 4a shows a rosette arrangement of three strain gauges mounted as an assembly. Figure 4b shows a linear arrangement of three strain gauges again mounted as an assembly, but with a smaller width than the rosette in Figure 4a. Either of such assemblies may be mounted in a recess on the intramedullary nail 10. This may allow the intramedullary nail 10 to detect different types of deformations affecting the intramedullary nail 10. For example, the nail might then be able to detect torsional deformation.

In the embodiment described above, the strain gauges are foil strain gauges. However, semiconductor strain gauges of the type shown Figure 4c may also be used.

Parts of a second apparatus are shown in Figure 5. The second apparatus differs from the apparatus described above in that the second apparatus is not intended for treating a fracture of a bone. Instead, the second apparatus is adapted for promoting integration of a joint replacement prosthesis in a bone.

In a hip joint replacement operation, the ball of the natural hip joint, located on the femur, is replaced by the ball of a prosthesis. The prosthesis has a stem and the ball is connected to the stem. The ball and neck of the femur are removed and a suitable aperture is created in the proximal end of the femur. The stem of the prosthesis is inserted into the aperture. The stem is intended to become integrated or anchored within the femur so that the ball is held firmly in the desired spatial position in relation to the femur. In some replacement operations, the stem is held initially by frictional engagement with the femur. Over time, the bone and tissue around the stem conform to the stem and this increases the strength of integration of the prosthesis in the femur. It is desirable to minimise bending of the stem of the prosthesis until the stem has become firmly integrated in the femur. The second apparatus is adapted to minimise bending of the stem during this anchoring stage. The second apparatus includes a microprocessor, a signalling system, a functional electrical stimulation device and a plurality of pads for use with the functional electrical stimulation device. These components of the second apparatus are identical to the corresponding components 22, 24, 36, 38 of the apparatus described above with reference to Figures 1 to 4, and these components of the second apparatus will not be described in detail.

In addition, as seen in Figure 5, the second apparatus includes a hip joint replacement prosthesis 60 which has a stem 62, a ball 64 and a neck 66 connecting the stem 62 and the ball 64. The stem 62 is adapted to be inserted into an aperture formed in the proximal end 68 of a femur 70. The stem 62 extends down into the medullary cavity of the femur 70. The stem 62 serves to anchor the prosthesis 60 in the femur 70. The ball 64 replaces the natural ball of the femur 70.

The stem 62 is provided with nine strain gauges 72a, 72b, 74a, 74b, 76a, 76b. The nine strain gauges 72a, 72b, 74a, 74b, 76a, 76b are arranged at three axial positions along the length of the stem 62. Three of the strain gauges 72a, 72b are provided at a first axial position closest to the neck 66. Another three of the strain gauges 74a, 74b are provided at a second axial position approximately mid-way along the stem 62. A final three of the strain gauges 76a, 76b are provided at a third axial position near the distal end of the stem 62. At each axial position, the strain gauges 72a, 72b, 74a, 74b, 76a, 76b provided at that position are angularly spaced from one another by 120° around the axis of the stem. (It will be noted that three of the strain gauges are not visible in Figure 5 as they are behind the plane of the figure.)

Another three strain gauges 78a, 78b are provided in the neck 66 of the prosthesis 60. The three strain gauges 78a, 78b in the neck 66 are spaced angularly from one another around the neck by 120°.

The strain gauges 72a, 72b, 74a, 74b, 76a, 76b, 78a, 78b are connected to the signalling system of the second apparatus as described above for the first apparatus shown in Figures 1 to 4.

After joint replacement, the second apparatus monitors strains experienced by the strain gauges 72a, 72b, 74a, 74b, 76a, 76b, 78a, 78b in the same way as described above in respect of the first apparatus shown Figures 1 to 4. The microprocessor of the second apparatus receives the signals, provided by the signalling system; the signals being indicative of the strains sensed by the strain gauges 72a, 72b, 74a, 74b, 76a, 76b, 78a, 78b. In turn, the microprocessor assesses whether the stem 62 is undergoing any bending. If bending is detected, the microprocessor controls the functional electrical stimulation device to apply functional electrical stimulation, via one of the pads, to a muscle - the muscle having been chosen by the microprocessor to apply a mechanical force that will tend to reduce the bending experienced by the stem.