WO2005104945A2

WO2005104945A2 - Measurement of laxity of human joints

Info

Publication number: WO2005104945A2
Application number: PCT/GB2005/001725
Authority: WO
Inventors: Andrew Arthur Amis; Anthony Michael James Bull
Original assignee: Imperial Innovations Limited
Priority date: 2004-05-04
Filing date: 2005-05-04
Publication date: 2005-11-10
Also published as: WO2005104945A3; GB0409859D0

Abstract

Apparatus and methods for quantitatively determining laxity and kinematics of human and animal joints are described. An anterior draw force appliance facilitates application of a constant displacement force during movement of a joint such that the force is independent of position of the moving limb. A rotation appliance facilitates application of constant torque to a joint during to movement of the joint so that torque is independent of position of the moving limb. A non-invasive, surface mount technique for coupling position sensors to the bones, of a joint provides for accurate position sensing of the bones during joint flexure. An instrumented splint enables application of quantifiable abduction moments and torque moments to a is joint. A control system enables measurement of six degrees of freedom of the bones of a joint during joint flexure.

Description

MEASUREMENT OF LAXITY OF HUMAN JOINTS

The present invention relates to systems for the measurement of laxity in human or animal joints.

The following description primarily discusses measurement systems relating to the human knee, which has been an important focus of studies relating to joint instability. However, it will be recognised that the principles discussed are readily applicable and adaptable to other joints, such as the shoulder, ankle or elbow.

The knee is often afflicted by ligament injuries, and these are very important for subjects such as professional sports players, especially footballers. The principal injury is a rupture of the anterior cruciate ligament (ACL). When this has been damaged, the tibia has greater than normal mobility in relation to the femur, and sudden movement within this pathologically expanded 'envelope of laxity' can lead to the leg 'giving way'. This occurs principally during 'pivoting' movements, when the subject is mrning, with the foot stationary on the ground. This results in a relative rotation between the femur and tibia.

If the ACL is damaged, surgeons can identify an abnormal increase in knee laxity (i.e. bone-bone displacement in response to a given level of translational force or rotational torque) by means of two standard clinical tests.

1. The anterior draw test, hi this test, the knee is held at 90 degrees flexion, and the examiner is able to displace the proximal tibia (i.e. just ^• below the knee) anteriorly (forwards) an abnormally large distance. 2. The Lachman test. This is similar to the anterior draw test, but is carried out at 20 degrees knee flexion.

While these tests may provide objective measurements of abnormal laxity (which is defined by reference to the patient's other, normal, leg), they are static tests and do not correlate closely with the patient's symptoms.

There is a 'dynamic' test that is recognised to be the closest approximation to 'giving way': the 'pivot-shift' test. In the pivot-shift test, the knee is flexed from an extended posture, while the examiner also applies a valgus (abducting) moment. (This is when the foot is levered out away from the centreline of the body, holding the knee towards the centreline). This manoeuvre tenses the ligaments crossing the medial aspect (towards the centreline) and compresses the lateral (the side away from the centreline) joint surfaces of the knee togetlier. In me absence of me ACL, this allows the lateral aspect of the tibia to move anteriorly when the knee starts to flex. This movement is a combination of anterior translation of the tibia, plus internal rotation (that tends to rotate the leg so that the foot points towards the centreline). At approximately 35 degree knee flexion, the tibia suddenly falls back into its correct place in relation to the femur.

Although this test is accepted to be the best match for actual symptoms, it is highly subjective. The movements and loads are different between examiners as they attempt to elicit the 'shift' movement and there is no method known for quantifying it.

The present invention seeks to provide a system for applying forces and moments to an articulating joint, particularly in the leg, and for measuring tibio-femoral relative motions that will allow this clinical test to be quantified. The present invention provides apparatus and method according to each one of the appended independent claims.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows a perspective view of a tibial inteπial-external rotation appliance according to one aspect of the present invention; Figure 2 shows a schematic underside plan view of a patient's foot installed within the appliance of figure 1 ; Figure 3 shows a schematic plan view of a first embodiment of an anterior draw force appliance according to another aspect of the present invention; Figure 4 shows a perspective view of a first embodiment of a mounting assembly for mountmg position and/or rotation sensing devices onto a patient's leg according to another aspect of the present invention; Figure 5 shows a side view of an instrumented splint for applying abduction/adduction moments to a patient's leg; Figure 6 shows a schematic perspective view of a human leg and sensor arrangement useful in explaining a control system of the present invention; Figure 7 is a schematic representation of a first embodiment of a force application assembly for measuring tibiofemoral anterior-posterior laxity at a fixed angle of knee flexion according to another aspect of the invention; Figure 8 is a schematic representation of a connector foraiing part of a second embodiment of a force application assembly allowing easy alteration of a force applied by the handle to the assembly; Figure 9a is a schematic plan view of a second embodiment of an anterior draw force appliance according to an aspect of the present invention; Figure 9b is a schematic plan view of a third embodiment of an anterior draw force appliance according to an aspect of the present invention; Figures 10 and 11 are schematic representations of a second embodiment of a mounting device for mounting position and/or rotation sensing devices onto a patient's leg according to an aspect of the present invention; and Figure 12 is a schematic representation of a third embodiment of a mountmg assembly for mounting position and or rotation sensing devices onto a patient's leg according to an aspect of the present invention.

In order to provide a comprehensive system for quantitatively establishing laxity and kinematics of an articulating joint, the present invention provides a number of appliances or devices including:

1. an anterior draw force appliance;

2. a tibial internal-external rotation appliance; 3. means for mounting position and/or rotation sensing devices onto a leg or other limbs connected by an articulating joint; 4. an instrumented splint for applying known abduction moments with or without other torque or force to the limbs connected by an articulating joint; 5. a force applicator assembly for measuring tibiofemoral anterior- posterior laxity at a fixed angle of knee flexion; and 6. a control system for controlling, measuring, computing and displaying the output of this system.

Each of these aspects will be described in turn. 1. Anterior draw force appliance

In order to apply a constant displacing force to the tibia while the knee is flexing-extending, it is necessary to have a displacer that is not position- sensitive, because an examiner's hand will not necessarily follow the leg movement exactly.

With reference to figure 3, there is shown a draw force appliance 30 for applying such a constant displacing force to the tibia while the knee is flexing-extending.

The draw force appliance 30 comprises a limb engagement member which preferably takes the form of a large hook- like extension 31 that may be located behind the proxmial tibia 32 in order to pull the tibia anteriorly, as indicated by arrow 33. The hook-like extension is linked to a pulling handle 34 via a 'constant-force spring' 35. The spring 35 may be of any suitable type such that the force applied to the tibia via the hook 31 remains constant even when the distance of the examiner's hand to the leg alters during the test (as mdicated by arrow 36). In other words, the spring 35 applies a force that is independent of its extension. The function of the spring may alternatively be provided by any other suitable 'constant force member' or device that enables application of a constant force independent of its extension, e.g. a suitably controlled pneumatic or hydraulic device.

The pulling handle 34 is preferably coupled to the hook 31 by way of an articulating joint 37 that allows rotation of the handle 34 relative to the hook 31. Preferably, the rotation comprises rotation at least about an axis that is orthogonal to the axis of the limb that is being pulled, thus compensating for any change in relative orientation of the limb during the pulling motion. In another embodiment, the rotation may provide two rotational degrees of freedom, i.e. the articulating joint 37 is of the type known as a universal joint.

5 Thus, in a general sense, the draw force appliance 30 provides a constant displacing force to a limb (e.g. the tibia) coupled to a body (e.g. the femur) by an articulating joint (e.g. the knee) such that the draw force applied to the limb is substantially constant and independent of displacement of the pulling handle relative to the hook over a range of distances commensurate l o with a normal range of movement of the limb .

Conventional draw force devices always use elastic deflections and so are force-position sensitive, relying on the examiner taking a reading when the force sensor indicates a chosen force level by either optical or audible 15 means. That approach is not suitable for a moving limb.

The anterior draw force appliance 30 has particular application to a new method for examining the knee laxity across the range of flexion-extension, as described herein. A key problem is that measurements of laxity are 0 force-sensitive because of the low stiffness of the stabilising tissues such as ligaments. Thus, gravity force may affect readings. In a preferred manoeuvre, the patient lies supine and the hip and knee are flexed simultaneously. The entire leg starts out horizontal, then the thigh flexes upwards, but the knee flexes downward at the same rate, thereby ensuring 5 the lower leg remains horizontal. This means that there is a constant posterior draw force acting across the knee joint, due to the weight of the lower leg. Typically, this is about 45 N, and typically lies in the range 30 to 60 N. That gives the posterior limit of the envelope of anterior-posterior laxity. When the draw force device is applied and tensed, typically to 100 0 N, this hnposes a net anterior draw force across the knee of (100 - 45) = 55 N. This, too, remains constant across the range of motion, to give the anterior limit of the envelope of laxity.

A second embodiment of a draw force appliance is illustrated schematically in Figure 9a and is designated generally by the reference numeral 90. The draw force appliance 90 comprises a limb engagement member which preferably takes the form of a large hook like extension 91 that may be located behind the proxmial tibia 92 in order to pull the tibia anteriorly as indicated by arrow 98.

The hook like extension 91 is linked to a pulling handle of the type shown in Figure 3 and identified by the reference numeral 34, via a "constant force spring" (not shown) similar to that shown in Figure 3 and identified therein by the reference numeral 35.

The appliance 90 further comprises an inner moving component 94 onto which a leg may rest when mstalled in the appliance 90. The inner moving component 94 is able to move independently of the hook 91 due to the presence of bearings such as rollers 100 positioned between the inner movmg component 94 and an inner surface 95 of the hook 91. The inner moving component 94 is thus freely moving.

This means that the leg can rotate freely about an axis located at the centre of an arc defined by the inner component 94. In this embodiment of the anterior draw force appliance 90, a leg mstalled in the appliance is able to rotate freely when it is pulled anteriorly in the direction of arrow 98 by the hook 91.

A third embodiment of a draw force appliance is illustrated schematically in Figure 9b and is designated generally by the reference numeral 900. The draw force appliance 900 comprises a limb engagement member which preferably takes the form of a ring 902 within which a leg 901 of a person may be positioned. The ring 902 comprises a hinge 903 which allows the ring to be opened to allow insertion of the leg 901. The ring further comprises a snap lock 904 which serves to close the ring once a leg has been positioned therein. Any other suitable type of closure may however be used. The ring is supported by a block 907 and is attached to the block by means of a roller 906. The block 907 is applied to a handle (not shown) via a "constant force spring" (not shown) similar to that shown in Figure 3 and identified therein by the reference numeral 35. By pulling on the handle, a load 905 may be applied to the leg 901.

The ring 902 is able to move relative to the block 907 due to the presence of the roller 906. This means that a leg 901 mstalled in the device 900 is able to rotate freely when it is pulled anteriorly in the direction of arrow 905.

In an alternative embodiment, the draw force appliance may comprise a "ring within a ring" device that is able to clamp around the tibia, or which is fed over the foot in order to allow free rotation of the leg. The device may further comprise an inflatable cuff within the device which may inflate around a leg installed in the device. This will result in an applied force being transmitted evenly around the proximal tibia.

The anterior draw force appliances 30, 90, 900 may additionally be used i association with a device that is intended to provide a measurement of tibiofemoral anterior-posterior laxity at a fixed angle of knee flexion. Such a measurement may be obtained conventionally using a Lachman or anterior draw test. Known such devices include the KT-1000 made by Medlvletric Co, and the Rolimeter marketed by Aircast Co.

Such a device could be used for joints other than the knee, to enable the laxity of a joint to be measured in a fixed posture and hi a chosen direction relative to the skeleton.

2. Tibial internal-external rotation appliance

Tibial internal- external (I/E) rotation takes place around the long axis of the tibia. Figures 1 and 2 show a suitable tibial internal-external rotation appliance 10. The ankle complex does not allow much rotational laxity if the foot is held in a fixed posture, so it is convenient to apply the tibial torque via the foot. A splint 11, 21 is mounted below the foot and secured to it by bmdmg with Velcro tape, flexible bindings or shnilar attachment means 12. The splint 11, 21 has a medial side plate 13, 23 that locates alongside the medial aspect 14, 24 of the foot and big toe. The splint 11, 21 may include a proximal extension 15 formed as a gutter that fits behind the Achilles tendon and calf to control ankle position.

The internal- external torque is applied via a socket 16, 26 fixed to the underside of the splint that is on the extended axis of the tibia, i.e. below the centre of the ball of the heel. This socket 16, 26 is used to accommodate the driving spigot 17 from a suitable torque device 18. As shown, a suitable torque device 18 is an electric drill with an adjustable torque lhniter that is set to the desired level. In a preferred embodiment, this is typically 5 Nm, and typically in the range 3 to 8 Nm. The direction of rotation of the drill 18 can be reversed, thus giving both internal and external torques as indicated by arrow 29 in figure 2. Thus, in a general aspect exemplified in figures 1 and 2, a rotation appliance provides a means for applying rotational forces across an articulating joint of a human or animal body by way of a lhnb engagement member for rigidly engaging with a l nb which is comiected to the body by way of the articulating joint. In the example given, the limb engagement member comprises the foot and ankle splint 11, 21 as described, the limb is the lower leg and the joint is the knee.

In another general aspect exemplified in figures 1 and 2, a rotation appliance comprises a splint that rigidly engages a patient's foot in fixed orientation relative to the lower leg and has a coupling member, extending axially outward from a base plate of the splint substantially on the axis of the lower leg when a foot is installed i the splint. The coupling member facilitates coupling of a torque device adapted to apply a rotational force of predetermined constant torque to the splint about the lower leg axis.

hi an alternative embodiment of the aspect of the invention illustrated hi Figures 1 and 2, the splint 11, 21 further comprises a rotational measurement device such as an inclinometer illustrated schematically in Figure 2 by the reference numeral 24. The rotational measurement device 27 is attached to the base plate 28 of the splint 11, 21.

The rotational measurement device allows internal-external tibial rotation to be measured without the need to use external instrumentation.

The output of the rotational measurement device may be displayed graphically on, for example, a computerised system, which system may illustrate the rotation of a patient's foot relative to the torque applied to the foot. Through use of such a rotational measurement device, it is possible to measure rotation of the foot without the need to use external mstrumentation such as the control system according to an aspect of the present invention, and described in detail hereinbelow.

The inventors are not aware of any previous devices used to measure internal-external tibial rotation accurately, or to apply known torques while the knee is flexing. The tibial internal-external rotation appliance of the present invention incorporates a load limiter, to ensure substantially constant load regardless of the effects of moving the leg during measurements, rather than relying on a fixed relative position to maintain a fixed torque.

Mounting position and/or rotation sensing devices onto the leg

The most accurate measurements come from direct bone-bone instrumentation. Since this requires use of invasive techniques, it is generally only suitable for the surgical scenario, and may be the method of choice for intra-operative measurements. Conventional techniques use purpose-made sockets that are a close fit for electromagnetic sensors which are mounted onto the leg using 2 2 mm diameter threaded wires that are drilled in to the bones. One sensor is mounted onto each of the femur and tibia.

The present invention recognises that for normal clinical examination an accurate surface-mounted system is desirable. Figure 4 illustrates an exemplary system 40. This currently uses splints, a femoral splint 41 over the femur, and a tibial splint 42 over the tibia. The splints 41, 42 are moulded to fit the contours of the leg, with suitable sensors 43 mounted onto them. The sensors 43 may be any suitable type of position and/or rotation sensors using any suitable techniques for detennining relative or absolute position in space and/or relative or absolute rotation in space. Various types of sensor exist that use electromagnetic, magnetic, inertial or optical techniques. The splints 41, 42 are secured to the leg using Velcro tapes 44, bindings or shnilar attachment means.

The inventors are not aware of prior use of splint-mounted motion sensors, although they are sometimes attached to flexible items such as clothing or gloves. The use of a rigid splint mounted configuration offers significant improvements in accuracy that is highly desirable during measurements taken on the moving leg.

Particular features of the rigid splints 41, 42 include areas moulded so that the splints grip over bony prominences, such as the greater trochanter of the femur 45, or the medial and/or lateral condyles of the femur 46, over the anterior ridge of the tibia 47, and over the medial and/or lateral malleolar prominences at the ankle 48. These features reduce splint-bone motions in use. The actual extent of the splint may vary for different uses. Thus, the distal extension to the femoral condyle 46 may be omitted if it will interfere with work at the knee.

It will be understood that the sensors may be of any suitable type adapted to provide one or both of a spatial position or rotational attitude relative to a reference object. In one arrangement, the sensor may provide absolute spatial position, i.e. relative to an invariant reference coordinate system (e.g. the room in which the patient is located). In another arrangement, the sensor may provide a position relative to. another sensor or object which is itself movmg or moveable. In another arrangement, the sensor may provide absolute rotational attitude relative to an invariant coordinate system. In another arrangement, the sensor may provide a rotational attitude relative to another sensor or object which is itself moving or moveable.

Preferably, each splint provides one or more sensors such that all six degrees of freedom of the splint can be monitored over time, i.e. position in space relative to three Cartesian axes and rotational attitude relative to the three Cartesian axes.

In a general aspect exemplified in figure 4, the appliance provides for surface mounting of position and/or orientation sensors to a limb of a human or animal body by way of a rigid splint that attaches to the surface of a lhnb of the body. The rigid splint has binding means for effecting attachment to the lhnb such that the splint is substantially immovable relative to a bone within the limb. At least one sensor is rigidly positioned on the splint, which sensor is adapted for providing a spatial position and/or rotational attitude relative to a reference object.

Referring to Figures 10 and 11, a second embodiment of an assembly 110 for mounting position and/or rotation sensing devices onto a leg is illustrated schematically. The system 110 comprises a clamp 111 which, in use, clamps onto the leg at locations where bones are positioned close to the skin surface of the lhnb. The clamp 111 is in the form of an arch like frame 112 that, in use, is mounted around the knee. The frame 112 has attached thereto clampmg devices 113 and 114 in the form of clamping pads which define a clampmg axis, extending between the devices 113, 114, and which have been designed to interlock with the shape of the knee and specifically to bridge over the medial 115 and lateral 116 epicondyles of the femur 117. Ends 118 and 119 of the clampmg devices 113, 114 locate in recesses between the epicondyles and tendinous structures that pass nearby. The clamping devices 113, 114 engage with the knee by means of a threaded rod 120 which enables the clamping devices 113, 114 to be moved towards one another as appropriate to clamp around the knee, as shown in Figure 11.

The assembly 110 may further comprise a mounting rod 121 which serves to prevent the frame 112 from moving around the clamping axis of the clampmg devices 113, 114 while the clamping devices 114, 115 are appropriately located at the knee. The mounting rod 121 is connected to the frame 112 via an adjustable mounting block 122. The block 122 may be adjusted in a medial-lateral direction relative to the knee by means of a clampmg screw 123 located in a slot 124 in the frame 112.

The mounting rod 121 further comprises an adjustable wedge 125 which allows the block 122 and rod 121 to be inclined to match the sloping orientation of the axis of the femur in relation to the transverse clamping axis.

In an alternative embodiment of the aspect of the invention shown in Figures 10 and 11, the block 122 may be adjusted in a medial-lateral direction relative to the knee by means of alternative adjustment means such as a universal joint. The universal joint may be locked into a desired configuration after the assembly 110 has been mounted on the limb.

The rod 121 is shaped to include a bend 140 to allow one end portion 126 of the rod 121 to lie flat along a surface of the leg 127. The rod 121 may mcorporate a slot 128 that may accommodate a strap 129 which serves to hold the rod 121 in place on the leg 127.

The rod 121 engages with the mountmg block 122 at an angle 130. This means that the rod is provided with an inclined axis of rotation that is intended to intercept with the elongate axis of the femur at an mtersection 131. The intersection 131 lies in the region 132 where the rod 121 is in contact with the leg.

By means of the assembly 110, the mass of muscles positioned around the femur is able to rotate in relation to the bone without disturbing the mounting of the frame 122, since any movement of the mass of muscle is .accommodated through rotation of the rod 121 in the mounting block 122.

In an alternative embodiment, the rotational movement of the mass of muscles positioned around the femur may be accoimnodated by an alternative bearing/swivel arrangement to that shown between rod 121 and block 120, to allow similar freedom of motion between the muscles and the clamp 112 secured onto the femur 117. The rotation movement may be accommodated for example by altering the axis of rotation of the joint in the assembly 110.

A motion sensor 133 may be mounted on the frame 112.

Referring now to Figure 12, a further embodiment of an assembly for mounting position and/or rotation sensing devices is designated generally by the reference numeral 150.

The system 150 is adapted to allow a sensor 151 to be positioned on a leg 152 of a person at a location close to the ankle 153 of a person.

The system 150 comprises a clamp in the form of a frame 154 which is generally arch shaped and comprises clamping devices 155, 156 that allow the frame 154 to be positioned around the ankle 153 of a person. The motion sensor 151 is mounted on a plate 157 that is strapped to the leg 152 so that it is held against the relatively flat surface of the skin on the anterio-medial aspect just above the ankle 153.

The plate 157 is linked to the frame 154 by means of an adjustable-plus - locking ball or universal type joint mechanism 158. The clamping devices 155, 156 are shaped so that the frame 154 is clamped onto the bony prominences of the medial and lateral malleoli 159,. One of the clamping devices 155 is fixed, while the other clamping device 156 is movable via a threaded rod 161. The frame 154 is therefore clamped onto the ankle by screwing the clamping device 156 into contact with a malleolus. This results in both clamping devices 155, 156 interlocking with a malleolus.

In an alternative embodiment, the motion sensor 151 may be attached to the frame 154. This obviates the need for the plate 157 and linkage mechanism 158.

Alternatively, the motion sensor may be positioned separately from the frame 154, and attached thereto by means of a universal joint, for example.

Instrumented splint to apply known abduction/ adduction moments to the leg

Figure 5 shows an instrumented splint 50 for applying predetermined abduction/adduction moments to the leg. This allows an internal/external tibial rotation torque to be applied and measured at the same time as an abduction/adduction moment is applied. A foot splint 51 similar to that described in connection with figure 1 includes an instrumented handle 52 that locates into the socket 53 below the heel. It may take the form of a T- bar handle 52, to allow the examiner to push / pull sideways (for the abduction/adduction moment) and to twist it (for the internal/external rotation torque). Another variant could include a freely-rotating spindle 54 that prevents the examiner from applying internal/external torque during abduction/adduction movement. The base of the T-bar would be instrumented, typically via arrays of strain gauges 55 to provide electrical output.

A further embodiment of the instrumented splint as shown hi Figure 5 would use a constant weight hanging off the foot splint 51 shown in Figure 5. This embodiment would further comprise a connector to allow the weight to be attached to the foot splint around the socket 53. Such an embodiment would provide a constant level of abduction/adduction moment at the knee if the patient were to lie on one side with the leg free below the knee. The patient or examiner would then flex and extend the knee, moving the weighted foot splint 51 in an approxhnately horizontal plane. The opposite moment would then be applied by taming the patient onto the other side and repeating the movement of the knee. Combined abduction/adduction and internal- external limb rotation moment could be applied by off-setting the weight anterior or posterior to the axis of the tibia.

In a general aspect exemplified by figure 5, the instrmnented splint comprises an appliance for applying abduction/adduction moments across an articulating joint of a human or animal body comprising a lhnb engagement member for rigidly engaging with a lhnb which is connected to the body by way of the articulatmg jomt and a handle coupled to the limb engagement member. The handle may be rotatable relative thereto about an axis defined by the limb to which the lhnb engagement member is attached (i.e. the longitudinal axis of the limb). The handle generally enables translational motion of the lhnb engagement member by movement thereof to impose a bending moment on the lhnb. This causes angulation of the joint being examined (in this case, the knee). At least one sensor provides as an output, a measurement of translational force bemg applied to the lhnb engagement member by the handle.

5. A force applicator assembly for measuring tibiofemoral anterior- posterior laxity at a fixed angle of joint flexion.

A force applicator assembly according to an aspect of the present invention is designated generally by the reference numeral 70 and is illustrated schematically in Figures 7 and 8.

The assembly 70 comprises a frame in the form of an elongate portion 81 attached to a leg 80 of a person by means of a strap 82. The elongate member 81 comprises a contact pad 83 that makes contact between the elongate member 81 and leg 80 at a point towards the ankle of the person. The assembly 70 further comprises a joint movement sensor 73 that is able to measure relative motion between bones at a joint h a human or anhnal body. In the illustrated example, the joint movement sensor 73 is adapted to measure relative motion between the femur and the tibia in the leg 80.

The assembly 70 further comprises a force application handle 71 and a constant force sprmg 72. The handle 71 may be moved in the direction of arrow 84 in order to apply a relatively constant force to the leg 80. The magnitude of the force applied to the leg 80 is determined by the constant force spring 72.

In the embodiment of the assembly 70 shown hi Figure 7, the handle 71 and constant force spring 72 are attached to the elongate member by means of a connector 85. Turning now to Figure 8, a connector forming part of a second embodiment of a force applicator assembly according to the present invention is designated generally by the reference numeral 200. The connector 200 is connectable to the handle 71 shown in Figure 7 or the hook 31 shown in Figure 3. The connector 200 comprises a connector device 210 attachable by means of a connection member 220 to a limb loading apparatus such as frame 81 in Figure 7, or hook 31 in Figure 3. The connector device 210 comprises engagement devices 230, 240 each of which comprise a threaded rod 250, 260 or sliding pin, for example.

The handle 71 comprises a spring receiving portion 270 adapted to receive two constant force springs 280, 290. Each of the springs comprises a spring rod 300, 310 which extends into a respective engagement devices 230, 240.

By means of this embodiment of the present invention, it is possible to apply three different forces to the joint. The first force to be applied will be the load from spring 280. Spring 280 can then be disconnected by releasing engagement device 230, and spring 290 can be connected to the connector device 210 by engagement device 240. The load from spring 290 may then be applied to the joint. Sprmg 280 may then be reconnected to the connector device 210 by means of engagement device 230, and the load from both springs 280 and 290 may jointly be applied to the joint via connector 220.

One or more of the three possible forces may be applied to the leg. If the forces are applied sequentially, they may be applied in any order.

In other embodiments of the invention, more than two constant force members may be associated with handle 71. The constant force member may be connected to other equipment by any convenient means. For example, if the constant force members are pneumatic, then it would be necessary to alter the pressure applied.

In a general aspect exemplified in the arrangement of Figure 8, an apparatus for applying a constant displacing force to a lhnb comprises: a lhnb engagement member for engaging the lhnb and for supplying a draw force to the limb; a pulling handle coupled to the lhnb engagement member by way of a constant force member such that the force applied to the limb engagement member is substantially constant, the apparatus further comprising a connector for releasably connecting the handle to the limb engagement member, the handle further comprising a receiving portion for receiving the constant force member.

6. Control system for controlling, measuring, computing and displaying the output

This system may use any suitable commercially-available electromagnetic measurement system for position sensing. Alternatively, the system may use mounting devices 110, 150 shown in Figures 10, 11, 12. For example the 'Nest of Birds' from Ascencion Technology Inc, Burlington, VT, USA, or the Polhemus electromagnetic system, or any other system that enables electromechanical, optical or acoustic tracking of targets.

The control system of the present invention preferably allows the full six degrees-of-freedom (dof) of tibio-femoral relative motion to be measured and displayed. With reference to figure 6, femoral motion sensors 61 and tibial motion sensors 63 are respectively mounted either directly to the femur 62 and tibia 64 or more preferably skin-mounted usmg the splints 41, 42 as described with reference to figure 4.

In order to relate the output of the motion sensors 61 and 63 to lhnb alignment and anatomy, the control system preferably requires that the positions of the medial epicondyle 65 and the lateral epicondyle 66 of the femur 62 are digitised, establishing a medial-lateral axis 67 relative to the position of the femoral sensor. This may be carried out using a temporary third sensor (not shown) which is temporarily attached to or positioned adjacent to the medial epicondyle 65 and the lateral epicondyle 66 to establish fixed offsets for those positions from the femoral sensor 61.

After the fixed offset positions 65, 66 have been established, the extended knee is then moved around the hip, allowing the hip centre 68 to be located relative to the femoral sensor 61. The hip centre plus two digitised points defines the coronal (frontal) plane of the femur 69. It also, by default, defines the tibial position in extension that is assumed to have a coincident medial-lateral axis with that of the femur in that extended knee posture.

Motion of both femur 62 and tibia 64 in space is measured respectively by the femoral and tibial sensors 61, 63 fixed to each. Appropriate transformations allow then relative motion to be calculated (e.g. knee flexion-extension and tibial I/E rotation). The transformations use the 'floating axis' system of Grood and Suntay (Trans. American Soc. of Mech. Eng, J. Biomech. Eng., 1983), that sets up three mutually perpendicular axes: the femoral medial-lateral axis 67, the tibial longitudinal axis 70 and a mutually-perpendicular 'floating' axis 71. The three axes are joined by revolute joints 72, 73 that allow both rotation and sliding. Thus, this system defines the full six degrees-of-freedom of tibial motion relative to the femur:

(i) medial-lateral translation (as indicated by arrow 74), (ii) flexion-extension rotation (about the medial-lateral translation axis, as mdicated by arrow 75),

(iii) anterior-posterior translation (orthogonal to axis 74, as indicated by arrow 76),

(iv) abduction-adduction rotation (about the anterior-posterior translation axis, as indicated by arrow 77),

(v) compression-distraction (orthogonal to axes 74, 76, as indicated by arrow 78), and

(vi) tibial internal-external rotation (about the compression-distraction axis 78, as mdicated by arrow 79).

The advantage of this system is that all six degrees of freedom of motion are defined in relation to only two digitised points, thus being very quick to use and to set-up.

An enhancement of this technique, that minimises the potential errors in digitising the anatomical landmarks, defines an "ideal kinematic axis" that defines the direction of the long axis of both the femur and the tibia to minimise varus/valgus angulation of the knee durmg a prescribed flexion/extension path and that minimises tibial internal/external rotation during a different prescribed flexion/extension path. The path used to nrinhnise tibial internal/external rotation should be for a mid-range of flexion (approximately 30° to 90°).

The origins of both femoral and tibial coordinate systems are defined as cooincident for a defined loading and position of the knee. For example, for a knee that has a posterior cruciate deficiency, the defined coincident loading position would be with an anterior drawer at 0° knee joint flexion. For a knee that has an anterior cruciate deficiency the defined loading condition would be with a posterior drawer at 0° knee flexion.

The location of the coincident origins can also be optimised as part of the "ideal kinematic axis". The location is optimised for a specific flexion range in order to niininiise the calculated compression/distraction movement. For an "intact" knee, the location is optimised for an axis that also minimises the anterior/posterior movement. The flexion range used to minhnise compression/ distraction is a mid-flexion range (approximately 20° to 90°). This range can be modified based on further experimental data. The flexion range used to minimise anterior/posterior movement is highly dependent on the loading condition.

An "ideal kinematic axis" is one which, by numerical optimization techniques, reduces the variability in the compression-distraction and medial-lateral translational degrees of freedom for the knee joint movement. Any change in loading condition can then be explained in terms of the change in all other degrees of freedom, including tibial internal-external rotation, tibial abduction-adduction, and anterior-posterior translation.

The numerical techniques required to ascertain such an "ideal kinematic axis" can rely on specific geometric information, such as articulatmg topology. However, ideally they can use the kinematic data from a specific loading condition. The concept described here uses a specific flexion- extension range of the knee joint, with a specific defined loading condition, that minimises movements in the aforementioned degrees of freedom. This system allows tibial laxity in relation to the femur 62 to be measured across a range of knee flexion, thus leading to an 'envelope of laxity', rather than isolated measurements at one or two chosen angles of knee flexion. This allows changes to be seen clearly that would otherwise not be detected by conventional tests.

The use of electromagnetic position / rotation sensors yields full six degrees of freedom motion data, allowing calculation of relative motions between the bones. The different components of motion may be transformed into clinically relevant output.

In a general aspect exemplified in the arrangement of figure 6, the system provides measurement of the relative movement of bones of an articulatmg jomt usmg a first position sensor coupled to a first bone and at least a second position sensor coupled to a second bone, the first and second bones being articulated by the joint at a distal end of the first bone and a proximal end of the second bone, and signal processing means for determining, from the sensor outputs, one or more of (i) relative medial-lateral translation of the distal end of the first bone and the proximal end of the second bone,

(ii) relative flexion-extension rotation of the distal end of the first bone and the proximal end of the second bone about the medial-lateral translation axis,

(iii) anterior-posterior translation of the proximal end of the second bone relative to the distal end of the first bone,

(iv) abduction-adduction rotation of the second bone about the anterior- posterior translation axis,

(v) compression-distraction translation of the second bone along its axis relative to the first bone, and (vi) rotation of the second bone about the compression-distraction axis. In a preferred configuration, the system is used such that external force(s) is applied to the joint according to at least one of types (i) to (vi), and resultant motion according to at least two others of the types (i) to (vi) above is 5 measured.

A further advantage of the system is that it allows the application of combined loading such as internal or external rotation torque with or without anterior or posterior draw force. This is an important combination, l o being the complex motion that results after ACL injury.

To be able to apply a combined loading and to measure the resulting bone- bone relative motion is a significant advance i the diagnosis of knee ligament injuries.

15 Although the knee is an important area of interest and application of the present invention, other joints may have then kinematics and/ or stability disturbed by injury, such as the ankle, shoulder and elbow. Aspects of the present invention can be utilised and/or adapted to evaluate envelopes of 0 laxity and ldnematics in other articulating joints, when assessing the severity of injury or the success of treatment.

Claims

1. A rotation appliance for applying rotational loads across an articulating joint of a human or animal body comprising: a lhnb engagement member for rigidly engaging a lhnb which is connected to the body by way of the articulating joint; and a torque device adapted to apply a rotational force of predetermined constant torque to the lhnb engagement member about an axis defined by the lhnb to which the lhnb engagement member is attached while allowing translational motion of the limb engagement member and the torque device.

2. The rotation appliance according to Claim 1 further comprising a coupling member for coupling the torque device to the lhnb engagement member.

3. The rotation appliance of Claim 1 or Claim 2 in which the limb engagement member comprises a splint coupled to the lhnb by way of attachment means.

4. The rotation appliance of Clahn 3 in which the splint comprises a base plate for engagement with the sole of a foot and a side plate for engagement with a side of the foot.

5. The rotation appliance of Clahn 4 in which the coupling member extends axially outward from the base plate.

6. The rotation appliance of Clahn 4 in which the side plate is a medial side plate.

7. The rotation appliance of any one of Claims 5 or 6 further including a proximal extension that is adapted to engage the Achilles tendon and calf to control ankle position of the foot.

8. The rotation appliance of any one of Claims 1 to 7 in which the torque device comprises an actuator with torque limiter.

9. A rotation appliance for applying rotational loads across the knee comprising: a splint for rigidly engaging a patient's foot in fixed relative orientation relative to the lower leg; and a coupling member, extending axially outward from a base plate of the splint substantially on the axis of a lower leg when a foot is installed in the splint, the coupling member for coupling to a torque device adapted to apply a rotational force of predetermined constant torque to the splint about the lower leg axis.

10. The rotational device of any one of Claims 1 to 8, further comprising a rotational measurement device for measuring rotation of the limb engagement member.

11. The rotational device of Claim 10, wherein the measurement device comprises an mclinometer.

12. The rotational device of Clahn 11 or Claim 12 further comprising an output device for providing an output of rotation of the limb engagement member versus torque applied to the limb engagement member.

13. A method of applying rotational loads across an articulatmg joint of a human or animal body comprising the steps of: attaching a limb engagement member that rigidly engages with a lhnb which is connected to the body by way of the articulating joint; attac hig a torque device adapted to apply a rotational load of predetermined constant torque to the limb engagement member about an axis defined by the limb to which the lhnb engagement member is attached; and applying constant torque to the joint while moving the joint through a range of normal motion.

14. An appliance for surface mounting of position and / or orientation sensors to a limb of the human or animal body comprising: a rigid device for attachment to the surface of a limb of the body; attachment means for effecting said attachment such that the rigid device is substantially immovable relative to a bone within the limb; and at least one sensor rigidly positioned on the device, the sensor adapted for providing a spatial position and/or rotational attitude relative to a reference object.

15. The appliance of Claim 14 wherein the rigid device comprises a splint, and the attachment means comprises binding means.

16. The appliance of Claim 15 further including at least one moulded region having a profile adapted to confonn to a bony prominence on the lhnb.

17. The appliance of Clahn 16 adapted for use on the human leg, the moulded region bemg adapted to confonn to one or more of the greater trochanter of the femur, the medial condyle of the femur and the lateral condyle of the femur.

18. The appliance of Claim 16 adapted for use on the human leg, the moulded region being adapted to conform to one or more of the anterior ridge of the tibia, the medial malleolar prominence at the ankle and the lateral malleolar at the ankle.

19 The appliance of Claim 14 wherein the rigid device comprises a clamp.

20. The appliance of Claim 19 wherein the attachment means comprises one or more clamping devices adapted to interlock with the bone, at least one of the clampmg devices being moveable relative to other of the clampmg devices.

21. The appliance of Clahn 20 adapted for use on the human leg, the one or more clamping devices adapted to interlock with one or more of the medial epicondyle of the femur, the lateral epicandyle of the femur, the medial malleoli of the ankle and the lateral malleoli of the ankle.

22. The appliance of any of one of Claims 19, 20 or 21 further comprising a mountmg rod having a first end and a second end, the mounting rod being adjustably connected to the clamp at or close to the first end.

23. The appliance of Claim 22, further comprising a strap attachable to the mounting rod at or close to the second end, the strap being attachable to the human leg.

24. The appliance of Claim 21 further comprising a plate attachable to the clamp, and to the human leg, the at least one sensor being mounted on the plate.

25. A draw force appliance for applying a constant displacing force to a limb coupled to a body by an articulating joint comprising: a lhnb engagement member for engaging a posterior portion of the lhnb and for supplying an anterior draw force to the limb; and a pulling handle coupled to the lhnb engagement member by way of a constant force member such that the force applied to the limb engagement member is substantially constant and independent of displacement of the pulling handle relative to the lhnb engagement member over a range of distances commensurate with a nonnal range of movement of the limb.

26. The draw force appliance of Claim 25 in which the handle is coupled to the lhnb engagement member by way of a pivot that allows rotation of the handle relative to the lhnb engagement member.

27. The draw force appliance of Clahn 26 in which the pivot facilitates relative rotation of the handle relative to the lhnb engagement member about an axis that is orthogonal to the axis of the limb that is being pulled by the lhnb engagement member, thus compensating for any change in relative orientation of the limb during a pulling motion.

28. The draw force appliance of any one of Clahns 25 to 27 further comprising a moving component onto which a lhnb may rest when installed in the appliance.

29. The draw force appliance of any one of Claims 25 to 28 adapted for use on a knee joint, where the lhnb is the lower leg (tibia) and the body is the upper leg (femur), and in which the draw force is an anterior force applied to the back of the lower leg.

30. A method of applying a draw force to a lhnb coupled to a body by an articulatmg joint comprising the steps of: engaging a posterior portion of the limb with a lhnb engagement member of a draw force appliance; and applying an anterior draw force to the limb by way of a pulling handle coupled to the limb engagement member, by way of a constant force member such that the force applied to the limb engagement member is substantially constant and independent of displacement of the pulling handle relative to the lhnb over a range of distances commensurate with a nonnal range of movement of the lhnb.

31. An appliance for applying abduction/adduction moments across an articulating jomt of a human or animal body comprising: a limb engagement member for rigidly engaging with a limb which is connected to the body by way of the articulating j oint; a handle coupled to the lhnb engagement member and rotatable relative thereto about an axis defined by the lhnb to which the limb engagement member is attached, the handle enabling ti-anslational motion of the limb engagement member by movement thereof; and at least one sensor mounted between the handle and the limb engagement member to provide as output a measurement of translational force being applied to the lhnb engagement member by the handle.

32. The appliance of Claim 31 in which the handle is freely rotatable relative to the lhnb engagement member so that no torque can be applied to the limb engagement member about the axis defined by the lhnb.

33. The appliance of Claim 31 further including at least one sensor mounted between the handle and the limb engagement member to provide as output a measurement of torque applied to the lhnb engagement member about the axis defined by the limb.

34. The appliance of Claim 31, Claim 32 or Claim 33 in which the limb engagement member comprises a splint coupled to the limb by way of attachment means.

35. The rotation appliance of Claim 34 in which the splint comprises a base plate for engagement with the sole of a foot and a side plate for engagement with a side of the foot.

36. The appliance of Claim 35 in which the side plate is a medial side plate.

37. The appliance of Clahn 35 or Claim 36 further mcluding a proxhnal extension that is adapted to engage the Achilles tendon and calf to control ankle position of the foot.

38. An instrumentation appliance for applying a bending moment to the knee comprising: a splint for rigidly engaging a patient's foot in fixed relative orientation relative to the lower leg; a handle, extending axially outward from a base plate of the splint substantially on the axis of a lower leg when a foot is mstalled in the splint, the handle for applying a translational force to the splint relative to the lower leg axis; and at least one sensor mounted between the handle and the splint to provide as output a measurement of translational force bemg applied to the splint by the handle.

39. The appliance of Claim 38 further including at least one sensor mounted between the handle and the splint to provide as output a measurement of torque applied to the splint by the handle.

40. Apparatus for measuring relative movement of the bones of an articulatmg joint comprising: at least a first position sensor coupled to a first bone and at least a second position sensor coupled to a second bone, the first and second bones bemg articulated by the joint at a distal end of the first bone and a proxhnal end of the second bone; means for receiving position signals from the first and second sensors; signal processing means for detennining, from the sensor outputs, one or more of (i) relative medial-lateral translation of the distal end of the first bone and the proxhnal end of the second bone,

(ii) relative flexion-extension rotation of the distal end of the first bone and the proxhnal end of the second bone about the medial-lateral translation axis, (iii) anterior-posterior translation of the proximal end of the second bone relative to the distal end of the first bone,

(v) compression-distraction translation of the second bone along its axis relative to the first bone, and

(vi) rotation of the second bone about the compression-distraction axis.

41. The apparatus of Claim 40 fuirher mcluding means for mapping one or more of the movements (i) to (vi) as a function of relative flexure of the joint.

42. The apparatus of Clahn 40 or Claim 41 hi which the signal processing means is adapted to determine all of the movements (i) to (vi).

43. The apparatus of Claim 40 in which the first and second position sensors are respectively attached to the first bone and the second bone by way of surface mount appliances that rigidly affix the sensors to the surface of the body such that movement relative to the underlying bone is precluded.

44. The apparatus of any one of Claims 40 to 43 adapted for measurement of the relative movement of the femur and tibia of the human body to assess the laxity and kinematics of the knee joint.

45. A method for quantitatively detennining laxity and kinematics of an articulating joint comprising the steps of: coupling at least a first position sensor to a first bone and at least a second position sensor to a second bone, the first and second bones being articulated by the joint at a distal end of the first bone and a proximal end of the second bone; receivhig position signals from the first and second sensors during relative movement of the first and second bones; processing the position signals from the sensors to detennhie one or more of (i) relative medial-lateral translation of the distal end of the first bone and the proxhnal end of the second bone,

(ii) relative flexion-extension rotation of the distal end of the first bone and the proximal end of the second bone about the medial-lateral translation axis, (iii) anterior-posterior translation of the proximal end of the second bone relative to the distal end of the first bone, (iv) abduction-adduction rotation of the second bone about the anterior- posterior translation axis, (v) compression-distraction translation of the second bone along its axis relative to the first bone, and (vi) rotation of the second bone about the compression-distraction axis.

46. The method of Claim 45 including the step of defining an ideal Idnematic axis that defines the direction of the long axis from each of the first and second bows.

47. The rotation appliance of any one of Claims 1 to 9, further mcluding means for applying a translation force to the lhnb engagement member at the same time as the torque device applies the rotational force.

48. Apparatus for applying a constant displacing force to a lhnb, the apparatus comprising: a lhnb engagement member for engaging the lhnb and for supplying a draw force to the limb; a pulling handle coupled to the lhnb engagement member by way of a constant force member such that the force applied to the lhnb engagement member is substantially constant, the apparatus further comprising a connector for realeasably comiecting the handle to the limb engagement member, the handle further comprismg a receivhig portion for receiving the constant force member.

49. The apparatus of Claim 48 wherein the receivhig portion comprises a plurality of constant force members.

50. The apparatus of Clahn 49 wherein the connector comprises a selection portion for selectively connectmg one or more of the constant force members to the limb engaging member, thereby enabling the force applied to the lhnb engagement member to be varied.

51. Apparatus substantially as described herein with reference to the accompanying drawings.

52. A method substantially as described herein with reference to the accompanying drawings.