GB2167193A - Joint operation simulator - Google Patents

Abstract

Apparatus for simulating the real- life operation of a bearing such as a hip joint prosthesis so that the wear characteristics of the material can be assessed, in which the lower component 16 of the bearing is mounted eccentrically on the planet 8 of an epicyclic gear so that as the gear is rotated the bearing, being relatively fixed in a horizontal plane, bobs up and down. The upper component 11 of the bearing, however, is fixed on a rotatable frame 13 including a series of weights 15 and a cam follower 18, so that the weights produce across the bearing a force which, by the bobbing of the bearing and inertial effects, varies in a cyclical manner except for a pre-determined period in the rotation of the gear, when the cam follower engages a cam 10 rotating with the gear to remove the action of the weights. <IMAGE>

Description

SPECIFICATION Joint operation simulator This invention relates to apparatus to enable the wear characteristics of joints or linkages to be measured in cases where the operation of the joint or linkage involves the abutment of two relatively moving surfaces.

The abrasion which takes place between such surfaces depends not only on the amount of speed of their relative movement, but also factors such as the pressure applied on the linkage, the nature of any lubricant or other fluid present in the linkage and the temperature. Whenever the suitability for such joints of new materials is investigated, then whenever the wear is to be monitored by continuous operation of the linkage it is important that the actual operating conditions, such as those mentioned above, should be reproduced as closely as possible.

This invention therefore consists of apparatus for reproducing repetitively and continuously the operating cycle of a linkage consisting of two mating bearing surfaces, comprising an epicyclic gear, means for driving the planet thereof, means for connecting one of the bearing surfaces to an eccentric point on the planet, means for applying a force between the bearing surfaces, and means for removing substantially the effect of said force during a desired part of the operating cycle.

The means for removing the effect of said force may conveniently comprise a cam surface driven within the ring gear, said surface engaging for part of its length with a support for weights arranged to exert a force across the bearing surface when the cam is not engaged.

One particular application of such apparatus is in the investigation of potential materials for clinical prostheses, such as hip joints: such devices necessarily have to operate without failure within the patient for a decade or longer; the debris arising from abrasion between the mating surfaces will either remain near the prosthesis or will be dispersed throughout the body, in either case with potentially harmful effects; and the stress applied to the hip joint-and hence the prosthesis-varies during the walking stride pattern from near zero (while the particular leg is off the ground) to two peaks both considerably in excess of the body weight.Various new materials, such as steeis, ceramics and composites, are proposed from time to time for use in such prostheses, and for the reasons given above some assessment of their wear characteristics is required before clinical trials can be approved. It is believed that this assessment can be carried out with the apparatus forming the subject of this patent application, wherein the apparatus can be activated continuously for a time period of, for example, 10 days, to simulate the amount of walking undertaken by the average patient over a normal period of several months.The apparatus is capable of reproducing the gait characteristics (stress pattern throughout the cycle, flexion/extension, abduction/adduction and internal and external rotation); the apparatus may conveniently include means for retaining the prosthesis under test within a flexible bag containing a proteinacious physiological serum solution similar to that found in vivo, and the temperature of the serum and hence the prosthesis may be maintained without difficulty by thermostatic control at 37"C.

The connection between at least one of the components of the prosthesis and the epicyclic gear may conveniently include a transducer and strain gauge for measuring the force between the two components of the prosthesis and the torque applied by the relative movement, and these two parameters together with the temperature may be recorded continuously throughout the operation of the apparatus.

The prosthesis components may be cemented by a conventional means into mounts readily insertable into the apparatus, so that the components may undergo more than one test cycle without undue disturbance and so that their dimensions may be determined readily at the end of each operating period.

By way of example, one embodiment of the invention will now be described with reference to the drawings, of which Figure 1 is a perspective partly cut-away view of an apparatus adapted for the operation of a ball-and-socket hip joint prosthesis, whose wear characteristics are subsequently to be measured, and Figure 2 is a similar view of part of the same apparatus, more detailed and at an enlarged scale.

With reference to Fig. 1, the apparatus is mounted on a rigid frame 1 in the lower part of which is mounted a mains-powered motor 2 actuated by a control box 3 for rotating a vertical shaft 4. The shaft 4 drives a horizontal beam 5 mounted so as to rotate diametrically of the ring gear 6 of an epicyclic combination mounted within a housing 7 on the frame 1. A planet gear 8 having a gear ratio of 1:2 with the ring gear 6 is mounted via a vertical stud 9 mounted eccentrically on the beam 5. The beam 5 has formed intergrally with it a cam 10 protruding upwardly from the gear housing 7 around a proportion of a circular arc concentric with the ring gear 6.

The socket portion 11 of the prosthesis (see Fig. 2) is cemented into a holder which in turn is mounted within a housing 12 fixed securely to a rocking frame 13 rotatable around a horizontal shaft mounted on the frame 1.

The rocking frame 13 includes a horizontal portion 14 on which may be mounted a desired number of weights 15, the arrangement being such that the gravitational force on the weights 15 is translated by rotation into a forward force exerted by the socket portion of the prosthesis in the direction of the ball portion 16. Extending downwardly from the portion 14 is a holder 17 for a cam follower 18 engageable with the cam 10. The effect of this is that as the beam 5 and cam are rotated by the motor, the cam engages with the cam follower during part of its rotation and relieves the socket portion 11 of the force exerted by the weights 15. By suitable choice of either cam profile or the height of the cam follower 18, the proportion of time during which the force is relieved may be set to correspond with that portion of the gait cycle during which a typical person's leg is off the ground.

The ball portion of the prosthesis 16 is likewise cemented into a holder (not shown) which in turn is mounted within a seat 19.

The rear of the seat is attached to one side of a pressure transducer 20 whose other side is attached to s shaft 21 mounted within bearings 22 for free rotation within a cylindri cai housing 23, the common axes of the shaft 21 and housing 23 passing through the centre of the ball portion 16.

The housing 23 is formed integrally within and at right angles to the rear of a shelf member 24, whilst extending at right angles from the base portion of the shelf (and hence at right angles to the axis of the housing 23) is a shaft 25 whose axis also passes through the centre point of the ball portion 16 and is attached through universal bearings 26 to an eccentric point on the top surface of the planet gear 8.

The lower portion of the seat 19 is attached to the front portion of the shelf 24 by a frictional torque transducer 25, which consists of a beam extending horizontally in a vertical plane coupled with strain gauges; to counteract faise torque readings on rotation of the shaft 21, an adjustable counterbalance 27 is mounted at the top end of the seat 19. In order that engagement of the ball and socket portions is maintained whilst the effect of the weights 15 is removed by operation of the cam 10, a spring 28 is mounted between the top end of the shelf member 24 and the housing 22 so that a small constant force is exerted between the components of the prosthesis throughout the operation of the apparatus.

Completely free rotation of the shelf member 24 and the assembly mounted thereon is prevented by a rod 29 attached through a horizontal bearing 30 in the front portion of the shelf member 24 and arranged to reciprocate freely within a spherical bearing 31 mounted in a housing on the rigid frame 1.

A perspex tube 32 mounted on the base 19 surrounds the ball and socket portions, whilst during operation of the device a flexible watertight jacket (not shown) extends between the tube 32 and the housing 12; the jacket is sealed at both ends before a proteinaceous physiological serum solution is introduced into the tube 32 by a needle passing through a grommet in a small hole in the side of the tube (not shown). A resistance thermometer and heating elements (not shown) are mounted within the tube 32 and are controlled automatically to a pre-set temperature when activated at the control box 3. The actual temperature, together with readings given by the transducers 20 and 26 may be recorded continuously during the entire period of operation of the apparatus.

When the motor 2 is activated, the beam 5 rotates as explained above causing the planet gear 9 to rotate within the ring gear and hence causes the lower end of the shaft 25 to follow an elliptical locus the centre of which ellipse coincides with the centre of the ring gear. The centre of the ball and socket joint is arranged to be in line with the axis of the shaft 4, and therefore as the bottom end of the shaft 25 traces its elliptical path, then the closer that bottom end is to the centre of the ring gear (ie approaches the minor axis of the ellipse) then the more parallel will the shaft 25 become to the axis of the shaft 4 so that the bail portion 16 will be forced upwardly away from the plane of the ring gear 6.Conversely, as the lower end of the shaft 25 approaches the major axis of the ellipse, then the shaft 25 will become more inclined to the axis of the shaft 4 and the ball portion 16 will be allowed to fall under the action of the weights 15 acting through the socket portion 11. Thus, as the shaft 4 is rotated, then for every single rotation the ball portion 16 will twice force upwards the socket portion 11 against the action of the weights 15 and twice descend, reducing by inertia the load produced by the weights 15 on the bearing.

This pattern is however modified by the engagement of the cam follower 18 with the cam 10, this being arranged to occur during one of the two "troughs" of the load cycle.

The load detected at the transducer 20 thus approximates very closely to a typical human gait pattern in which the maximum load on the hip joint occurs at two parts of the gait cycle, immediately before and after the leg in question is off the ground. By the suitable positioning of the mounting for the lower end of the shaft 25, typical flexion and extension values of 21 and abduction and adduction of 5 and 11" respectively may be achieved, whilst the geometry of the shaft 29 may be set to produce internal and external rotation values of, say, 8". The number of weights 15 may be adjusted to simulate any range of typical patient; it has to be remembered that the peak load experienced may typically be in excess of three times the body weight although the load detected by the transducer 20 de pends not only on the amount of weights 15 but also on the speed of rotation of the shaft 4 (which controls the upward and downward acceleration of the ball portion 16). It is found that in practice, 10 days continuous operation of the apparatus at 30 rotations per minute simulates several months normal wear. After this period, the prosthesis may be removed for wear measurement and then replaced-using fresh serum-or a different prosthesis may be inserted. With the type of serum employed, it is nevertheless important to ensure that it does not deteriorate, and to reduce the risk of contamination by extraneous metallic ions and bacteria, it may be desirable to operate the apparatus in a clean area.

Claims (6)

1. Apparatus for reproducing repetitively and continuously the operating cycle of a linkage consisting of two mating bearing surfaces, comprising an epicyclic gear, means for driving the planet thereof, means for connecting one of the bearing surfaces to an eccentric point on the planet, means for applying a force between the bearing surfaces, and means for removing substantially the effect of said force during a desired part of the operating cycle.

2. Apparatus according to Claim 1 in which the means for removing the effect of said force comprises a cam surface driven within the ring gear, said surface engaging for part of its length with a support for weights arranged to exert a force across the bearing surface when the cam is not engaged.

3. Apparatus according to either preceding claim in which the epicyclic gear ratio is 2:1.

4. Apparatus according to any preceding claim including means for monitoring continuously during operation the torque and compression forces acting between the two mating surfaces.

5. Apparatus according to any preceding claim in which both of the mating surfaces are immersed in a fluid contained in a flexible jacket and are maintained at a desired temperature by thermostatic control means.

6. Apparatus for the simulation of operation of clinical prostheses, substantially as hereinbefore described with reference to the drawings.