Tibial component for a replacement knee prosthesis and total knee prosthesis incorporating such a component.

This invention relates to a tibial component for a replacement knee prosthesis and to a total knee prosthesis incorporating such a tibial component.

It is known to provide shock absorbers in total joint prostheses for example as shown in European Patent Application No. 83033081 (Publication No. 0 466 926) in which impact absorbers are provided on the tibial component. The impact absorbers are provided across the tibial component to provide shock absorbing for both the medial and lateral bearing components.

It is also known to provide menisci, that is to say, moving plastic components between typically a metal tibial tray and a metal femoral component that are not locked in place as normal plastic tibial condyles are. Some examples are shown in United States Patent Specification No. 4 586 933, United States Patent Specification No. 4 085 466, United States Patent Specification No. 4 340 978 and United Kingdom Patent Application No. 7937163 (Publication No. 2 061 730), British Patent Specification No. 1 534 263 and European Patent Application No. 80103598.1 (Publication No. 0 121 421). In all these constructions both the medial and lateral condylar bearing components are arranged as menisci which can move.

An analysis by the present Applicants of removed total knee components (removed for any reason whatsoever, pain, loosening, breakage, wear) has shown that damage occurs to a much greater degree in the medial compartment of the tibial bearing surface than the lateral. Biomechanical publications have for some time claimed that the loading in the medial condyle is very much higher than in the lateral condyle.

According to one aspect of the present invention therefore a tibial component for a replacement knee prosthesis comprises a tibial tray for connection to a suitably prepared tibia, said tray carrying fixed lateral and medial condylar bearing components but only said medial component having a shock absorber located beneath it.

In a preferred embodiment a stiffening element is provided between the shock absorber and the fixed medial condylar bearing component.

Similarly, an analysis of used knee implants and biomechanical studies all show that as rotation occurs in the tibia that the fore and aft sliding occurs to a greater degree in the lateral compartment than it does in the medial.

According to another aspect of the present invention therefore a tibial component for a replacement knee prosthesis comprises a tibial tray for connection to a suitably prepared tibia, said tray carrying a fixed medial condylar bearing component and a movable lateral meniscal bearing component which can slide with anterior and posterior movement in relation to said tray.

If desired some degree of medial or lateral movement can also be allowed and in plane rotation relative to the said tray. This has the advantage that it affords a lower constraint on the articulation, reducing the loads that can be transferred via it to the tibial tray and ultimately the bone fixation interface, and allowing the bearing to adopt the optimum position for maximum contact area with the condyles of the femoral component.

The fixed medial bearing component can be provided with a shock absorber beneath it and if desired a stiffening element can also be provided between the shock absorber and the medial bearing component.

Again, the movable lateral meniscal bearing component can be provided with a shock absorber beneath it which can again have the stiffening element.

The invention also includes a total knee prosthesis for surgical replacement of a knee joint incorporating a tibial component as set forth above.

The invention has various advantages. Firstly, during normal human motion the femur slides, rolls and glides around the tibia, but to achieve this normal motion with a prosthesis means that the contact between the metal femoral shell and the plastic tibial bearing surface is very small. There cannot be close conformity between the geometry of the femoral condyle and tibial condyle. If there were close conformity then the fore and aft sliding could not occur except by the femoral bearing surface riding over the rim of the plastic. However, when there is point contact between the two then the wear rate and potential for fracture of the plastic can be relatively high.

Secondly, as the fore and aft movement occurs in a highly conforming or semi-conforming design the fore and aft shear forces acting on the tibial component can be very high, causing it to become loose within the bone. Therefore, the introduction of prosthetic meniscii allow there to be close conformity between the femoral component and the mating plastic surface and yet the plastic component is free to slide around and not impart high horizontal forces on the tibial fixation interface. These horizontal forces are therefore transmitted from bone to bone not through the prosthesis but via the interconnecting ligaments.

Unfortunately, having two meniscii, as suggested by the prior art documents, free to move around, the ligaments can have greater loads placed on them than is normal in the original anatomical joint. Furthermore, the implant is more likely to dislocate if one of the plastic meniscii pops out. The invention described here by fixing the medial condyle and allowing the lateral condyle to move is claimed to be a better compromise. There is not such freedom for the totally unconstrained movement provided by a double meniscal joint. Because little fore and aft sliding is required on the medial compartment there can be greater surface conformity (in the limit partial sphere on partial sphere) designed into the prosthesis. The lateral meniscal compartment can also have a highly conforming bearing design. Some of the horizontal shear forces can be transmitted by the medial fixed condyle. It is expected this design of knee will be less prone to loosening and demanding on the ligaments than an implant with two sliding plastic meniscii.

The invention can be performed in various ways and some embodiments will now be described by way of example and with reference to the accompanying drawings in which : <ul id="ul0001" list-style="none"><li>Figure 1 is a diagrammatic isometric exploded view of a tibial component according to the invention and provided with a movable lateral meniscal bearing component;</li><li>Figure 2 is a part cross-sectional view through the medial side of the tibial tray;</li><li>Figure 3 is a view similar to Figure 1 but showing an alternative construction embodying a shock absorber;</li><li>Figure 4 is a diagrammatic isometric view of an alternative form of shock absorber embodying a stiffening element;</li><li>Figures 5, 6, 7 and 8 are cross-sectional elevations through further alternative shock absorber constructions; and,</li><li>Figure 9 is a view similar to Figure 1 of a construction embodying fixed lateral and medial condylar bearing components and incorporating a shock absorber.</li></ul>

As shown in Figure 1 a tibial component for a replacement knee prosthesis comprises a tibial tray 1 provided with an optional fixation peg, indicated by broken lines 2, for connection to a suitably prepared tibia. The tray 1 carries a fixed medial condylar bearing component 3 and a movable lateral meniscal bearing component 4 which can slide with anterior and posterior movement in relation to the tray 1, and can be allowed some freedom for medial and lateral movement and for rotation in the plane of the top of the tray 1. The uni-condylar meniscus provided by the bearing component 4 is fully floating on the upper surface 5 of the tray 1 but the medial bearing component 3 is held in position by an anterior posterior slot 3 which engages a rail 7 on the tray 1. The other side of the bearing component 3 is provided with a groove 8 which can be snapped beneath a detent 9 on the tray. Thus, the bearing component is first fitted onto the rail 7 and then snapped into position on the other side.

The upper surface 5 of the tibial tray 1 is polished and is provided with an intracondylar retaining wall 10.

The bearing surface on the bearing component 4 exactly matches the distal surface of the femoral component - be that spherical or toroidal or some other form generated by driving the sectional form around the sagittal plane section of the femoral form. The bearing surface on the fixed component may also be toroidal or spherical or some form equivalent to the distal form of the femoral component geometrically modified to introduce the required lack of conformity to allow the components to move by rotation gliding and sliding to reproduce the physiological kinematics of the knee.

The bearing components are made from ultra-high molecular weight polyethene.

The construction shown in Figure 3 is somewhat similar to Figure 1 and the same reference numerals are used to indicated similar parts although in this Figure the lower part 2 of the tray is not shown. As will be seen from Figure 3 the fixed medial bearing component 3 is of a reduced thickness to accommodate a shock absorber 11 which is of substantially the same plan form as the component 3 but is made from a suitable compliant material such as rubber or a foamed elastomer.

The use of the interpositional compliant materials forms a shock attenuation effect on the medial bearing component 3.

Figure 4 shows a modified construction of shock absorber indicated by reference numeral 12. In this construction a stiffening element 13 is provided on the upper surface. This stiffening element is in the form of an interpositional metal tray which, when in position, is between the shock absorbing elastomeric layer and the ultra-high molecular weight polyethylene of the bearing 3. The advantage of this feature is the protection of the plastic bearing from off-set support loads and excessive deformation during articulation, which could compromise the fatigue strength and wear life of the component. A further benefit is that the metal interpositional tray provided by the stiffening element 13 allows the assembly of the shock absorbing and bearing components inter-operatively by the surgeon.

Some alternative constructions for shock absorbers incorporating a stiffening element 13 are shown in Figures 5, 6, 7 and 8. In Figure 5 the stiffening element 13 is held by adhesive or by being arranged to be a press fit into a depression 14 in the upper surface of the element. Figure 6 shows a construction in which the stiffening element 13 is again located in a depression 14 but in this case the side edges 15 are undercut. This Figure also illustrates the use of voids 16 in the elastomeric material.

Figure 7 shows a somewhat similar construction to Figure 6 but in this case the shock absorber is made by a foamed elastomer indicated by reference numeral 17.

Figure 8 shows an enhanced interlocking arrangement between the metal stiffening element 13 and the elastomer by countersunk holes, for example indicated by reference numeral 18, or slots.

In the constructions shown in Figures 3 to 8 a shock absorber is only provided beneath the fixed medial bearing component 3 but if desired shock absorbers of a similar type could also be provided beneath the menisci bearing component 4. Thus a construction can be designed so that there can be shock absorbers in both the medial compartment and the lateral compartment and if the surgeon wishes not to use a lateral meniscus then he could have a medial shock absorber and a lateral compartment without either sliding or shock absorption.

As mentioned above it will be appreciated that the material of the shock absorber can incorporate voids or foaming or other features, for example shear loading arranging or other isovolumetric deformation mechanisms to adjust the compliance of this inter-layer.

Figure 9 shows a construction in which similar reference numerals are used to indicate similar parts as in Figure 7 but with this arrangement the lateral bearing component 20 is of similar construction to the medial fixed bearing component 3. The bearing component 20 is also intended to be fixed in the tray 1 and is therefore also provided with a slot 6 and groove 8. The tray 1 has an additional detent 9 and rail 7 to allow the fixed lateral bearing component 20 to be snapped into position in a similar manner to the medial bearing component 3. It will be seen however that the bearing component 20 is thicker than the bearing component 3 because it does not have a shock absorber 11 beneath it and both bearing components are therefore fixed but a shock absorber is only provided beneath the medial component.

The construction of the shock absorber can be as described with regard to Figures 4 to 8.