Tibial component for a replacement knee prosthesis

Abstract: A tibial component of a replacement knee prosthesis comprising a tibial tray (1) for connection to a suitably prepared tibia, the upper surface (2) of said tray carrying one or more bearing components (6) characterised by means for altering the position of the tibia relative to its co-­ operating femur by alternatively altering one or more of the following variables : medio-laterial offset, antero-­ posterior offset, antero-posterior tilt, tibial-rotation or tibial condyle depression profile.

Description: This invention relates to a tibial component for a replacement knee prosthesis and can be used with the natural condyle bearing surfaces of a co-operating femur or with artificial condyles provided by a femoral prosthesis. Tibial components comprising a metal tibial tray with removable and interchangeable synthetic plastics material bearing surface components of different thicknesses are available, the alternative bearing surfaces being provided so that the correct ligament tension can be made. The femoral component of a surface replacement knee prosthesis is positioned and orientated mainly relative to the bone itself. There are three inner position variables : forward/backward, side to side, and vertically and there are three orientation variables : flexion/extension, varus/valgus, and rotation about the femoral shaft centre line, there is also the profile of the tibial condyle depression which affects the stability and locus of knee motions. The tibial component is similarly positioned and orientated relative to the tibia. When the tibial and femoral components have been installed the only way a surgeon can at present adjust the knee is by choosing a bearing surface of different thickness to create the desired ligament tension or by removing the components and retrimming the bones, thus for example, the Journal of Bone and Joint Surgery, Volume 63-A No. 5 discloses a knee prosthesis having tibial plateaus which are designed as universal thick, extra thick, and wedged right and left to allow for correction of instability or alignment problems. Such tibial plateaus allow for variations in thickness to alter the ligament tension and provide for variations in medio-lateral tilt. EP-A-0 135 319 suggests the use of a single bearing component of wedge shape which can alter the position of the tibia relative to the femur by altering the antero-posterior offset by moving the position of the bearing component. A disadvantage with this construction is that the cruciate ligaments are necessarily removed which would cause instability in the joint. It would be expected that the femoral component would wobble and slide limited only by the collateral ligaments which are not well orientated to limit such unstable motion. With the present construction, as described herein, the posterior cruciate can be kept intact. The suggestions set forth in the arrangements referred to above cannot be used however for the numerous other variables which arise, for example, tibial rotation or antero-posterior tilt or combinations of these variations or other variations. If a surgeon therefore finds he has made a mistake, or changes his mind about the optimum knee alignment there is no simple adjustment to be made. Furthermore, existing knee designs force a compromise in the horizontal positioning of both femoral and tibial components. The fixation must lie on or close to the outer cortical walls of the bone but this may not suit the ideal position for the bearing surface. The present invention is intended to provide means for allowing a surgeon to alter the knee alignment during the operation for various variations which have not previously been possible. According to the present invention a tibial component of a replacement knee prosthesis comprises a tibial tray for connection to a suitably prepared tibia, the upper surface of said tray carrying one or more bearing components each of which is located in a predetermined fixed position relative to said tray and including two or more alternative bearing components one or more of which can be selected and located into its fixed predetermined position on the tray, said bearing components having alternative located bearing surfaces to receive the natural or artificial condyles of the co-operating femur and which alter the position of the tibia relative to its co-operating femur characterised in that alternative bearing components are provided having alternative shaped and/or located bearing surfaces to alter the position of the tibia relative to its co-operating femur by alternatively altering one or more of the following variables : Anterior-posterior offset, medio-lateral offset, anterior-posterior tilt, tibial rotation. Means can also be included for altering a combination of any two or more of the offsets or tilts. A standard neutral bearing component is also preferably included in case no alteration is required. In one preferred embodiment the tray carries a single bearing component having two shaped bearing surfaces, the variations being provided by alternative single bearing components having bearing surfaces of different shapes and/or positions thereon. In another embodiment according to the invention the tray carries two bearing components, each having a shaped bearing surface to co-operate with a condyle on the co-operating femur, each component being selectable independently of the other from at least two alternative components having a bearing surface of different shape and/or position thereon. Thus once the surgeon has implanted the bone fixation components on the femur and the tibia measurements or X-rays or computer assisted tomography scans can be produced and trial bearing components can be selected to check for correct joint movement and soft tissue tension. A definitive implantable bearing component can then be installed. The invention can be performed in various ways but one embodiment will now be described by way of example and with reference to the accompanying drawings in which : Figure 1 is a cross-sectional front elevation of a standard neutral tibial component for a replacement knee prosthesis; Figure 2 is a cross-sectional side elevation of the component shown in Figure 1; Figure 3 is a plan view of the component shown in Figure 1; Figure 4 is a cross-sectional front view of an alternative bearing component shaped to provide medio-laterial offset; Figure 5 is a cross-sectional side view of a bearing component incorporating antero-posterior offset; Figure 6 is a cross-sectional front elevation of a bearing component incorporating medio-lateral tilt; Figure 7 is a cross-sectional side elevation of a bearing component incorporating antero-posterior tilt; Figure 8 is a plan view of a bearing component incorporating tibial rotation; Figure 9 is a cross-sectional front elevation of a known kind of bearing component of different thickness to adjust the height of the joint; Figure 10 is a cross-sectional front elevation of a bearing component incorporating tibial condyle depressions of different profile; Figure 11 is a plan view of an alternative construction; Figure 12 is a cross-sectional side elevation of the bearing component shown in Figure 4 but located in its tray in the opposite direction; Figures 13, 14 and 15 are plan views of an alternative tray shape; Figure 16 is a cross-sectional front view of an alternative construction; Figure 17 and 18 are cross-sectional side views of the construction shown in Figure 16 with alternative bearing inserts; Figure 19 is a cross-sectional side view of the construction shown in Figure 16; Figures 20, 21 and 22 are plan views of the construction shown in Figures 16 and 19 with alternative bearing inserts; and Figure 23 is a side elevation of an insert incorporating two variables. Figures 1, 2 and 3 show a tibial component of a replacement knee prosthesis comprising a tibial tray 1 having an upper tray like portion 2 and means for connecting the tray to a suitably prepared tibia provided by a spigot 3. Trays of this kind are well known in themselves and usually have raised tray walls 5. Mounted in the tray portion 2 is a bearing component 6 provided with two shaped bearing surfaces 7 which are in the form of depressions and which accommodate the condyles of the co-operating femur. Such condyles might be the natural condyles or artificial if provided on a co-operating femoral prosthesis component. The bearing component 6 may be a press fit in between the walls of the tray 5 or can be a snap fit or be held in place by any other arrangement for example, screws of pegs or other means and it may be desirable for it to be subsequently removable after fitting. The component is usually made from a synthetic plastics material. Configuration and location of the bearing surfaces dictate the optimum knee alignment. It is known to provide a bearing component as shown in Figure 9 in which the same reference numerals are used to indicated similar parts. In Figure 9 the depth of the standard component is indicated by broken line 8 and it will be seen that the thickness of the component as shown in Figure 9 is somewhat greater. After a tibial component of the kind shown in Figure 1 has been installed one way a surgeon can adjust the knee prior to the present invention is by replacing the bearing component 6 as shown in Figure 1 by a bearing component 9 as shown in Figure 9 to create the desired ligament tension. The other alternative is by removing the components and retrimming the bones. If therefore a surgeon finds he has made a mistake or changes his mind about the optimum knee alignment there is no simple adjustment to be made. It has however also been proposed to provide alternative variations in medio-lateral tilt, and a bearing component for such a purpose for use in the present construction is shown in Figure 6, the bearing surfaces being indicated by reference numeral 15 and the original standard bearing surfaces being again indicated by reference numeral 7. The present invention however provides a number of alternative bearing components which can be inserted into the tray 2 to replace the standard component 6 thus permitting a relative adjustment of the tibia relative to the femur. Figure 4 shows a bearing component 10 of similar overall shape to the component 6 in which the bearing surfaces indicated by reference numeral 11 are provided with medio-lateral offset. The position of the standard bearing surfaces are indicated in broken lines by reference numeral 7. This medio-lateral offset is very important for establishing knee alignment. Figure 5 shows a bearing component 12 for use in the present construction in which the bearing surfaces 13 have antero-posterior offset which is an important variation in providing optimum knee alignment. Once again the position of the standard bearing surface is indicated in broken lines by reference numeral 7. Figure 7 shows a bearing component 16 in which the bearing surfaces 17 incorporate antero-posterior tilt. The standard bearing surfaces 7 are again indicated by a broken line. The bearing component 18 shown in Figure 8 is provided with bearing surfaces 19 to provide for tibial-rotation to accommodate for foot pointing, pigeon toes or wide spread toed and again is an important aspect of optimum knee alignment. Figure 10 shows a bearing component in which the bearing surfaces 21 are of a different profile to the standard bearing surfaces, again indicated by reference numeral 7. The bearing surfaces 21, which provide the tibial condyle depression, affect the stability of the knee and the locus of knee motion. Thus they may have different shapes affecting for example the height of the outer rim at different locations and the profile slope to provide the desired effect. As shown in Figure 10 another alternative profile is shown by broken lines 22 which provide a deeper depression with higher rims. A kit for setting up a tibial component according to the invention can incorporate any of the alternative bearing components shown in Figures 4,5,7, and 8 of the drawings and preferably includes at least two of the components shown in Figures 4 to 10. A standard component 6 may of course also be incorporated. Perhaps the most important alternative bearing components are those shown in Figures 6 and 7 to accommodate medio-lateral tilt and antero-posterior tilt as these two variations tend to be the most common. Further or alternative bearing components can be provided which incorporate simultaneously two of the alternative bearing surface configurations. Thus components can be provided which incorporate not only medio-lateral offset as shown in Figure 4 but also tibial rotation as shown in Figure 8. The same component can even be provided in different thicknesses as shown in Figure 9. It will be appreciated that there are many combinations which can be provided and which can be used for special requirements. Figure 11 is a plan view showing the combination of medio-lateral offset and tibial rotation. The dimensions have been exaggerated to make them more clear. As will be seen from the drawings the tray is substantially rectangular in plan view and in certain circumstances it is possible to provide variations by reversing the bearing component 6. Thus Figure 12 shows the same component as Figure 4 but inserted in the opposite way so that the medio-lateral offset, which is shown to the right in Figure 4, now appears to the left. This reversal is possible due to the symmetrical shape of the tray and can be employed with the other inserts. In Figures 13, 14 and 15 three plan views are shown of an alternative tray shape. In each case there is an insert 30 carried in a flanged tray 31. The bearing surfaces 32 are shown in Figure 13 in a neutral position, in Figure 14 in a toe in position, and in Figure 15 in a toe out position. Once again any of these constructions could also incorporate one or more of the other configurations, for example, antero-posterior offset and alternative depths. It will be appreciated that with the constructions referred to above a considerable number of variations and different sizes and configurations can be produced and Figures 16 to 23 show an alternative construction which provides easy variations but at reduced cost in as much that the range of inserts can be reduced. In this type of construction the tibial tray 1 is replaced by a component having an upper tray-like head 40 connected to a spigot 41. The upper surface of the tray is provided with two spaced apart depressions 42, 43 which are generally of oval shape in plan and into which a pair of bearing components 44, 45 can be snapped into place. Other arrangements could be employed, for example screws or pegs, or other means, to hold the bearing components 44, 45 in place. Once again the components will usually be made of a synthetic plastics material. Figures 16, 19 and 20 show standard inserts in which the bearing surfaces are in neutral or standard positions. Figure 17 shows inserts which are shaped to provide medio-lateral tilt to one direction and Figure 18 shows the tilt in the other direction. The tilt may be provided by either a taller insert 46 in place of the standard insert 44 or by a shorter insert 47 in place of the standard insert 45. It is possible to thus employ a standard insert 44, 45 on one side and a modified insert on the other and thus various combinations can be produced as required. In Figures 21 and 22 the bearing surfaces are shaped and located to provide for tibial rotation. Thus Figure 21 shows toe in with appropriate inserts 48 and 49. The amount of toe in could be reduced by using a standard surface 45 in place of 49. Similarly in Figure 22 the insert 49 can be placed in the position of insert 48 and the insert 48 replacing the insert 49 to provide a construction for toe out. Figure 20 shows inserts 44 and 45 providing the neutral position. A range of inserts can be provided for medio and lateral offset and the other variations referred to with regard to the first type of construction but with this arrangement it will be appreciated that the number of inserts required can be reduced as they can usually be rotated to the opposite position due to the oval shape of the depressions 43 to enable them to be placed not only on opposite sides of the tray 40 but in reversed positions fore and aft. Once again each insert 44 and 45 can incorporate two or more of the variations and Figure 23 shows an insert 50 which shows in broken lines the position of the bearing surface as, for example, in the neutral insert 45. In this insert however the bearing surface 51 is not only provided with antero-posterior offset but also with antero-posterior tilt.

Claims: A tibial component of a replacement knee prosthesis comprising a tibial tray (1,40) for connection to a suitably prepared tibia, the upper surface of said tray carrying one or more bearing components (6) each of which is located in a predetermined fixed position relative to said tray and including two or more alternative bearing components one or more of which can be selected and located into its fixed predetermined position on the tray (1,40), said bearing components (6) having alternative located bearing surfaces to receive the natural or artificial condyles of the co-operating femur and which alter the position of the tibia relative to its co-operating femur characterised in that alternative bearing components (10,16,18,20,44,45,46,47) are provided having alternative shaped and/or located bearing surfaces to alter the position of the tibia relative to its co-operating femur by alternatively altering one or more of the following variables : Anterior-posterior offset, medio-lateral offset, anterior-posterior tilt, tibial rotation. A tibial component of a replacement knee prosthesis as claims in claim 1 characterised by including means (10,12,14,16,18,20,44,45,46,47) for altering a combination of any two or more offsets or tilts. A tibial component of a replacement knee prosthesis as claims in claims 1 to 2 characterised in that a standard neutral bearing component or components (6,44,45) are included. A tibial component of a replacement knee prosthesis as claims in claim 1, claim 2 or claim 3 characterised in that the tray (1) carries a single bearing component (6) having two shaped bearing surfaces (7), the variations being provided by alternative single bearing components having bearing surfaces of different shapes and/or positions thereon. A tibial component of a replacement knee prosthesis as claimed in claim 4 characterised in that said single bearing component (6) provides variations in two or more variables. A tibial component of a replacement knee prosthesis as claimed in claims 1 to 3 characterised in that the tray (40) carries two bearing components (44-47), each having a shaped bearing surface (7) to co-operate with a condyle on the co-operating femur, each component (44-47) being selectable independently of the other from at least two alternative components having a bearing surface of different shape and/or position thereon. A tibial component of a replacement knee prosthesis as claimed in claim 6 characterised in that bearing components (44,45) are also included to alter tibial condyle depression profile.

Inventors: LAWES PETER, LAWES, PETER
Applicants: HOWMEDICA, HOWMEDICA INTERNATIONAL INC.
Application year: 1989
Main classification: A61F2
IPC codes: A61F2