Tibial element for a replacement knee prosthesis

This invention relates to a tibial element for a replacement knee prosthesis of the kind comprising a tibial tray provided with a bearing component having medial and lateral compartments.

Reconstruction of the painful arthritic knee is now a commonplace operation throughout the world with results reported to be of similar success to hip replacements. However, some significant problems relating to knee surgery remain. Among these are :- <ul id="ul0001" list-style="none" compact="compact"><li>Wear and damage of articulating surfaces</li><li>Repeatability and ease of achieving a stable balanced reconstruction</li></ul>

In recent years there has been a return to the original concepts of more conforming and constrained knee replacements (i.e. Total Condylar) since concerns as to the long term survivorship of "flat" bearing forms have been raised. Greater congruency between metal and plastic components leads to larger contact areas which are believed to reduce the stresses transmitted to the plastic material. High stress levels are believed to initiate microscopic cracks within the plastic leading to the long term breakdown of the material or wear. Therefore, any reduction in the stress experienced by the plastic is beneficial. However, with increased conformity comes the risk of reduced range of motion from over-constraint and tibial fixation concerns with unloading of the soft tissue structures.

The advent of sliding Meniscal devices sought to combine increased tibio-femoral congruency whilst restoring normal joint function by allowing the tibial insert to move relative to the base plate.

There are now various meniscal systems available on the market, some with good long-term success, for example as set out in <ul id="ul0002" list-style="none"><li><b>Pappas MJ, Buechel FF</b> The New Jersey Low-Contact-Stress Knee Replacement System : Biomechanical Rationale and Review of the First 123 Cemented Cases. Rrch Orthop Trauma Surg 1986; 105:197-204</li><li><b>Goodfellow JW, O'Connor J</b> Clinical Results of the Oxford Knee. Clin. Orthop. 1986; 205:21-42</li></ul> However, problems with this type of device have also been reported in the form of insert dislocation and fracture.

The object of the present invention is to overcome some of the difficulties referred to above.

EP-A-0 634 155 shows a knee prosthesis which employs a plastics meniscus component supported on a metal platform for sliding movement. Control means are provided which act between the tray and the bearing component to provide free posterior and anterior movement of the lateral compartment of the bearing component which is greater than any allowed free posterior and anterior movement of the medial compartment in relation to the tray. This movement is controlled by an abutment at the edge of the tray.

In EP-A 0 634 156 a cannular replacement knee prosthesis is shown which operates in a similar manner to that shown in EP-A-0 634 155.

According to the present invention a tibial element for a replacement knee prosthesis comprising a tibial tray provided with a bearing component having medial and lateral compartments, and including control means acting between said tray and said bearing component which provides free posterior and anterior movement of the lateral compartment which is greater than any allowed free posterior and anterior movement of the medial compartment in relation to the tray said control means comprising a track in the base of the bearing component and guide means carried on the tray characterised in that said track is curved and said guide means include a guide and control abutment both carried on the tray and located in tandem within the track.

Thus, there may be no relative free posterior movement and anterior movement of the medial compartment or some may be allowed.

The control means acting between the tray and the bearing compartment may allow free rotational movement of said lateral compartment in relation to the tray about a pivotal axis centred within the medial compartment.

With this arrangement the control means may also be constructed to act to allow restricted anterior and posterior movement of the pivotal axis.

With the rotation centred within the medial compartment there is a mimicking of the natural knee physiology. The articulation of the bearing component ensures high conformity with the femoral condyles within the walking cycle and full conformity with the tibial base plate at all times. The centre of rotation being within the medial compartment ensures that the more heavily loaded condyle is always fully supported by the tibial tray thereby eliminating the chance of the bearing component tipping, dislocating or jamming on the edge of the tray.

The invention, for the provision of the asymmetric movement, is intended to provide good post-operative stability of the joint, provide a low sensitivity to surgical technique and soft tissue quality and provide mechanisms to avoid the likelihood of insert dislocation and other bearing damage.

In a preferred construction the tray is standard for both left and right knees and the bearing components are handed.

With this arrangement the tray can be substantially symmetrical about a vertical axis.

The medial compartment being less mobile than the lateral compartment, the medial side can be made larger than the lateral compartment so that the lateral compartment avoids soft tissue impingement during its greater degree of travel.

With this construction the guide can be in the form of a projecting boss.

The guide can be provided by part of fastening means which act to secure an attachment element, for example a stem, to the lower part of the tray.

The guide may also provide means for securing the bearing component to the tray.

Preferably the means for securing the bearing component to the tray are independently operable and can be releasable.

The control abutment can be made integral with the tibial tray.

The invention can be performed in many ways but one embodiment will now be described by way of example and with reference to the accompanying drawings in which : <ul id="ul0003" list-style="none"><li>Figure 1 is a part cross-sectional side elevation of a tibial element according to the present invention;</li><li>Figure 2 is an exploded view of the construction shown in Figure 1 with the bearing component removed;</li><li>Figure 3 is a cross-sectional side elevation of the bearing component shown in Figure 1;</li><li>Figure 4 is a plan view from below of the bearing component shown in Figure 3;</li><li>Figure 5 is a diagrammatic representation showing how the bearing component is placed in position on the tibial tray;</li><li>Figure 6 is a part cross-sectional side view showing the bearing component in the position shown in Figure 5 and about to be pushed into position;</li><li>Figures 7 and 8 are views similar to Figure 6 showing the bearing component in sequential loading positions of assembly;</li><li>Figure 9 is a view similar to Figure 6 showing the bearing component after loading and in an anterior position;</li><li>Figure 10 is a view similar to Figure 9 showing the bearing component after loading and in a posterior position;</li><li>Figure 11 is a diagrammatic plan view illustrating the range of movement of the bearing component on the tray;</li><li>Figure 12 is a view similar to Figure 9 but showing a construction in which the bearing component is not removable; and,</li><li>Figure 13 is a diagrammatic plan view of the bearing component in position on the tibial tray and showing additional features which can be incorporated.</li></ul>

As shown in Figures 1 and 2 a tibial element for a replacement knee prosthesis comprises a tibial tray 1 on which is carried a bearing component 2 having medial and lateral compartments respectively 3 and 4 and which are best shown in Figure 4. The upper surfaces of the compartments 3 and 4 are shaped to provide bearing surfaces 5 and 6. The tray itself is standard for both left and right knees and is substantially symmetrical about a vertical axis.

Fastening means are provided which act to secure an attachment element in the form of a stem to the lower part of the tray. These are in the form of a screw 7 having an enlarged boss shaped head 8. The lower part of the boss bears against a flange 9 on the tray and enters a tapered opening 10 where it acts to retain a stem 11 which has a co-operating tapered spigot 12 by engaging a screw thread 13 in a socket 14.

The lower surface of the tray can be provided with shaped engagement features 15 intended for engagement with the proximal sub-condylar area of the tibia of the patient and the general construction of the connection between the tray and the stem can, for example, be as set forth in the Applicants' European Patent Application <b>0 552 950</b> (H.42).

The upper surface of the tray 1 is provided with a control abutment 16 the function of which will be defined hereunder. It will be seen that the boss 8 of the screw 7 projects upwardly and the upper end is provided with a flange 17. When in position the upper surface 18 of the boss 8 is substantially horizontally in line with the upper surface 19 of the abutment 16. As is most clearly shown in Figures 1 and 2 the side of the abutment 16 adjacent the boss 8 is cut away to provide a recess 20 within which the flange 17 of the boss 8 is located when the screw 7 is in position.

As shown in Figures 3 and 4 the bearing component 2 has medial 3 and lateral 4 compartments and can be made from any suitable bearing material, for example ultra high molecular weight polyethylene. The lower surface 21 of the bearing component 2 is shaped to provide a curved track 22 which is most clearly shown in Figure 4. This curved track 22 is provided by a recess 23 which is formed with a peripheral inwardly protruding securing flange 24 around its edges. At the anterior end of the recess 23 the flange is deeper and is indicated by reference numeral 25.

Above the flange 24 the recess 23 is shaped to provide two horizontally extending grooves, the lower groove being indicated by reference numeral 26 and an upper groove 27 above the deeper portion 25 of the flange 24. The anterior end of the upper groove 27 is in the form of a radiused portion 28 so that the groove is closed at this end. The posterior end of the upper groove also has a radiused portion 29.

The posterior ends 31 of the lower groove 26 are open and emerge out of the side wall of the bearing component 2.

At the point where the lower posterior groove 26 meets the upper anterior groove 27 there is an enlargement provided by a circular vertically extending well 32, the diameter of which is equal to the horizontal distance extending between the base of the groove on either side of the recess 23.

The posterior end of the upper groove 27 is closed by a wall 33.

The horizontal distance between the base of the grooves 26 and 27 and indicated by arrows 26a is slightly more than the diameter of the flange 17 on the boss 8, the vertical depth of the lower posterior groove 26 is slightly greater than the vertical thickness of the flange 17 and the vertical depth of the upper anterior groove 27 is slightly greater again.

Figures 5 to 10 show how the bearing component 2 is placed in position and located on the tibial tray 1.

In the position shown in Figures 5 and 6 the bearing component is inserted by pushing its posterior side towards the boss 8. At this position, as shown in Figure 6, the lower surface 21 of the bearing component is raised above the upper surface 35 of the tray so that the open ends 31 of the lower posterior groove 26 engage over the flange 17 of the boss 8.

Further movement in the direction of the arrow 36 in Figure 7 shows that the bearing component 2 now has to be tipped to allow the flange 17 on the boss 8 to pass upwardly through the well 32 and into the upper anterior groove 27. The tipping movement is caused due to the upper surface 19 of the abutment 16 engaging the wall 33 at the end of the upper groove 28, and the flange 17 passing upwards in the well 32.

Further posterior movement achieves the position shown in Figure 8 in which the anterior side of the flange 17 of the boss 8 has entered the upper groove 27 but the posterior corner 37 is still engaging the lower corner of the wall 33.

Due to the resilient nature of the material from which the bearing component is made, UHMWPE, the bearing component can now be snapped downwards by resiliently deforming the end of the wall 3 over the corner 37 of the abutment 16 to the position shown in Figure 9 where the bearing component is in its most anterior position. It will be seen that the abutment 16 now engages within the curved portion 29 at the posterior end of the upper groove 27 and the wall 33 prevents further anterior movement.

The bearing component can however move in a posterior direction until the boss 8 engages the anterior end of the recess 23 as shown in Figure 10. The flange 17 acting in the upper groove 27 prevents vertical removal of the bearing component and its horizontal movement on the tray 1 is controlled by the control abutment 16 and boss 8 which are located in tandem in the curved track 22 and act as guide means.

Thus the boss 8 provides a guide and with its flange 17 provides independently operable means for securing the bearing to the tray, the bearing component being a resilient snap fit into the guide which can be releasable.

The abutment 16 and boss 8 which provide the guide means and which are in tandem together act as control means between the tray and the bearing component to allow free posterior and anterior movement of the lateral compartment 4 which is greater than the small amount of free posterior and anterior movement of the medial compartment 3 in relation to the tray 1.

Figure 11 shows the relative movement. The central position of the bearing component 2 on the tray 1 is indicated by solid line 40. The general axes of the tray 1 are indicated by broken lines 41 and 42. From these it will be seen that in plan view the tray 1 is symmetrical about the centre line 42 but the medial compartment 5 of the bearing component 2 is larger than the lateral compartment 6.

From this central position the maximum posterior movement of the bearing component is indicated by broken line 43 and it will be seen that the tibial compartment has rotated about a mobile axis 44 the locus of the movement of which is indicated by the lines and crosses 45. The locus of movement of a similar point on the lateral compartment 6 is indicated by crosses and lines 46 and the much greater range of movement will be apparent.

If desired the shape of the track 22 can be arranged so that there is virtually no relative free posterior movement and anterior movement of the medial compartment 5.

In the construction described and shown in the drawings the control means acting between the tray 1 and the bearing compartment 2 allow rotational movement of the lateral compartment 6 in relation to the tray 1 about the pivotal axis 44 centred on the medial compartment and the arrangement allows restricted anterior and posterior movement of this pivotal axis.

It will be appreciated that other means for controlling the movement of the lateral compartment could be employed, for example, the control means could be in the form of a pivot which provides an axis of rotation and which would be centred on the medial compartment. Such a pivot could even allow a restricted free posterior and anterior movement relative to the tray.

Figure 12 shows an alternative construction in which the bearing component 2 is not removable once it has been fitted. In this construction the abutment 16 is provided with a projecting lip 47 which can engage a co-operating lip 48 on the wall 33 to prevent the bearing component 2 from being tipped to allow the wall 33 to be resiliently deformed and sprung over the corner 37 of the abutment 16.

Figure 13 shows a construction in which a multi functional tibial tray is employed and which can be used for a construction as described above and as shown in the drawings or with one in which the bearing component is fixed in position in relation to the tray 1.

With this construction medial and lateral retaining means are provided in the form of abutments 50 and 51. Each abutment comprises an upwardly projecting portion 52 and a horizontally projecting flange 53. The bearing component for use in a fixed construction has an outwardly projecting flange 54 enclosed in a cut out segment 55 indicated by broken lines. The construction is such that as the bearing component is snapped into position the flanges 54 resiliently deform and pass below the flanges 53 so that the edges of each segment 55 locate the bearing component against posterior and anterior movement.

When such a tray is to be used with a sliding meniscal component of the kind described herein and with reference to the drawings the bearing component is cut away on each side along a line indicated by reference numerals 56 and 57 so that each relative movement between the tray and the bearing component is not impaired.

It will be appreciated that with the constructions described above in which the bearing component moves in relation to the tray that the bearing components have to be handed.

An advantage of the construction shown in the drawings is that if required the bearing component can be removed from the tray by reversing the loading procedure and replaced without disturbing the tray or the connection to the stem. Again, the construction allows stems of different sizes and shapes to be used as required by the surgeon.

The invention, through the provision of the asymmetric bearing component and the capture, provides good post-operative stability of the joint, provides a low sensitivity to surgical technique and soft tissue quality and provides a mechanism to avoid the likelihood of bearing component dislocation and other bearing damage.