FEMORAL EXTENSION PROSTHETIC COMPONENT AND TIBIAL SHELF PROSTHETIC COMPONENT FOR UNICONDYLAR MENISCAL BEARING KNEE PROSTHESIS AND PROSTHESIS INCLUDING SUCH A COMPONENT

Abstract

A femoral extension prosthetic component for use in a unicondylar meniscal bearing unicompartmental knee replacement is described. The femoral component comprises a securing element and a bearing surface, the securing element adapted to abut and be connected to a distal end of a femur bone, and the Dearing surface adapted to abut a meniscal bearing. The femoral component includes an entrapment portion adapted to extend beyond the meniscal bearing. Tibial components and meniscal bearings are also described, as well as methods of using such components.

Claims

1. (canceled)

2. A unicondylar meniscal bearing for a unicompartmental knee replacement, the meniscal bearing comprising: a first concave bearing surface configured to bear against a femoral prosthetic component; a second concave bearing surface opposed to the first concave bearing surface and configured to bear against a tibial prosthetic component; and a side wall separating the first concave bearing surface from the second concave bearing surface, wherein the side wall is relatively thicker at an outer edge of the meniscal bearing and relatively thinner in a center of the meniscal bearing.

3. The meniscal bearing of claim 2, further comprising a retention groove formed in a portion of the side wall.

4. The meniscal bearing of claim 3, wherein the retention groove is complementary to a retaining flange of the tibial prosthetic component.

5. The meniscal bearing of claim 3, wherein the retention groove comprises an arcuate track that substantially follows a curvature of the second concave bearing surface.

6. A system for a unicompartmental knee replacement comprising: a femoral prosthesis component attachable to a femur; a tibial prosthesis component attachable to a tibia and having a retaining flange projecting from a free end of a medial wall; and a meniscal bearing insertable between the femoral prosthesis component and the tibial prothesis component, the meniscal bearing component comprising: a first bearing surface configured to bear against the femoral prosthetic component, a second bearing surface opposed to the first bearing surface and configured to bear against the tibial prosthetic component, and a side wall separating the first bearing surface from the second bearing surface, wherein the side wall has a retention groove formed in a portion thereof, and wherein the meniscal bearing and the tibial prosthesis component are configured such that the retaining flange enters the retention groove on movement of the meniscal bearing relative to the tibial prosthesis component towards the medial wall.

7. The system of claim 6, wherein both the first bearing surface and the second bearing surface are concave in shape.

8. The system of claim 6, wherein the retaining flange projects laterally from the medial wall and includes an overhang surface configured to contact a distal surface of the side wall that forms the retention groove to prevent partial medial dislocation of the meniscal bearing.

9. The system of claim 6, wherein the medial wall is arched over a length thereof from a first end to a second end.

10. The system of claim 6, wherein the side wall is relatively thicker at an outer edge of the meniscal bearing and relatively thinner in a center of the meniscal bearing.

11. The system of claim 6, wherein the retention groove comprises an arcuate track that substantially follows a curvature of the second bearing surface.

12. A system for a unicompartmental knee replacement comprising: a femoral prosthetic component configured to be secured a femur; a tibial prosthesis component configured to be secured a tibia and having a retaining flange projecting from a free end of a medial wall, wherein the tibial prosthesis component has a bearing surface and an opposing bone interfacing surface, and wherein the bearing surface is convex in shape; and a meniscal bearing configured to be inserted between the femoral prosthetic component and the tibial prosthesis component, the meniscal bearing component comprising: a first bearing surface configured to bear against the femoral prosthetic component and having a concave shape, a second bearing surface opposed to the first bearing surface and configured to bear against the tibial prosthetic component and having a concave shape, and a side wall separating the first bearing surface from the second bearing surface, wherein the side wall has a retention groove formed in a portion thereof, and wherein the meniscal bearing and the tibial prosthesis component are configured such that the retaining flange enters the retention groove on movement of the meniscal bearing relative to the tibial prosthesis component towards the medial wall.

13. The system of claim 12, wherein the retaining flange projects laterally from the medial wall and includes an overhang surface configured to contact a distal surface of the side wall that forms the retention groove to prevent partial medial dislocation of the meniscal bearing.

14. The system of claim 12, wherein the medial wall is arched over a length thereof from a first end to a second end.

15. The system of claim 12, wherein the side wall is relatively thicker at an outer edge of the meniscal bearing and relatively thinner in a center of the meniscal bearing.

16. The system of claim 12, wherein the retention groove comprises an arcuate track that substantially follows a curvature of the second bearing surface.

Description

DETAILED DESCRIPTION OF THE DRAWINGS

[0096] There now follows by way of example only a detailed description of the present invention with reference to the accompanying drawings in which;

[0097] FIG. 1 shows a known unicompartmental unicondylar meniscal bearing knee prosthesis;

[0098] FIG. 2a shows a cross-sectional front view of an embodiment of a femoral component of a knee prosthesis in accordance with the first aspect of the invention;

[0099] FIG. 2b shows a cross-sectional front view of a further embodiment of a femoral component of a knee prosthesis in accordance with the first aspect of the invention;

[0100] FIG. 3 shows cross-sectional front view of an embodiment of a tibial component of a knee prosthesis in accordance with the second aspect of the invention;

[0101] FIG. 4a shows a side view of an embodiment of a tibial component of a knee prosthesis in accordance with the third aspect of the invention;

[0102] FIG. 4b shows a cross-sectional from view of further embodiment of a tibial component of a knee prosthesis in accordance with the third aspect of the invention;

[0103] FIG. 5a shows a side view of an embodiment of a tibial component of a knee prosthesis in accordance with the fourth aspect of the invention;

[0104] FIG. 5b shows a cross-sectional front view of an embodiment of a tibial component of a knee prosthesis in accordance with the fourth aspect of the invention;

[0105] FIG. 6a shows a front view of an embodiment of a tibial component of a knee prosthesis in accordance with the fifth aspect of the invention;

[0106] FIG. 6b shows a diagram that illustrates a calculation of the inclination of the bearing when the bearing both substantially contacting the bearing surface of the tibial component and contacting the medial wall;

[0107] FIG. 7a shows a cross-sectional front view of an embodiment of a tibial component of a knee prosthesis in accordance with the sixth aspect of the invention;

[0108] FIG. 7b shows a side view of an embodiment of a meniscal bearing of a knee prosthesis in accordance with the sixth aspect of the invention;

[0109] FIG. 8a shows a first embodiment of the tibial component of the seventh aspect of the invention;

[0110] FIG. 8b shows a second embodiment of the tibial component of the seventh aspect of the invention;

[0111] FIG. 8c shows a third embodiment of the tibial component of the seventh aspect of the invention;

[0112] FIG. 9 shows cross-sectional front view of an embodiment or a tibial component and meniscal bearing in accordance with the eighth aspect of the invention;

[0113] FIG. 10 shows a flow chart illustrating the method of surgery of the ninth aspect of the invention.

DETAILED DESCRIPTION

[0114] FIG. 1 shows a known knee prosthesis 1 comprising a femoral component, or femoral extension, 2, a meniscal bearing 3 and a tibial component 4. The femoral component is adapted to be connected to a femoral condyle of the femur bone and comprises a bearing surface 5 and securing element 6. The meniscal bearing comprises a bi-concave disc of plastics. The tibial component 4 is adapted to be connected to the tibial plateau of the tibia bone. The tibial component 4 has a tibial shelf 4 which comprises a shelf having a convex bearing surface 7 on one side and a connection surface 8 on an opposed side. The connection surface 8 includes a securing element 10. The bearing surface 7 includes an inner wall 11 that extends axially from the periphery thereof. The prosthesis 1 shown in FIG. 1 is adapted to be secured to the lateral side of the left knee, and so in the example shown the inner wall is a medial wall.

[0115] FIG. 2a shows an anteroposterior view of a femoral extension prosthetic component 20 comprising a securing element (not visible) and a bearing surface 21. The securing element is adapted to abut and be connected to a distal end of a femur bone. The bearing surface 21 is adapted to abut a meniscal bearing 22. The femoral component 20 includes an inwardly extending entrapment portion 23 (shaded region) adapted to extend inwardly beyond the meniscal bearing 22. In this example the entrapment portion extends medially, and so the entrapment portion 23 decreases the size of the gap between the femoral component and medial wall of the tibial component. The entrapment portion 23 increases the width of the femoral component 20 at least 10%. The entrapment portion 23 projects substantially 5 mm medially beyond the meniscal bearing when the knee is in the extended position as shown in the Figures.

[0116] The entrapment portion 23 includes an entrapment bearing surface 24 which forms a continuation of the bearing surface 21. Thus, the convex curve of the bearing surface 21 is continued. by the entrapment bearing surface 24. The entrapment bearing surface 24 acts to prevent the meniscal bearing 22 lifting from the tibial prosthetic component 26 and dislocating over the medial wall 25. The presence of the entrapment portion 23 causes the bearing surface 21, including the bearing surface 24, to be asymmetric and, in particular, the apex of the combined bearing surface is off-centre towards the lateral side. If a lateral entrapment portion is present, the apex may be off-centre towards the medial side. If both a medial and lateral entrapment portion is present the apex may be central.

[0117] The entrapment portion 23 is sized and arranged to be aligned with the medial wall 25 of the tibial component 26 when in use and the knee is extended.

[0118] The meniscal bearing 22 comprises a bi-concave disc having first concave bearing surface 27 adapted to bear against the femoral component 20, and a second concave bearing surface 28 opposed to the first concave bearing surface 27 and adapted to bear against a convex bearing surface 29 of the tibial shelf component 26.

[0119] FIG. 2b shows an anteroposterior view of a further embodiment of the femoral prosthetic component 20. The same reference numerals have been used for the same parts, as present in FIG. 2a. In this embodiment, the entrapment portion 23 comprises a projection 30 that extends from the femoral component adjacent the bearing surface 21 on the inner, in this case medial, side. The projection 30 extends from the lower medial edge of the femoral component and is arranged to extend towards the medial wall 25 of a tibial component 26 when in use. The lower edge of the projection 30 may be arranged to extend to the same level as the apex of the bearing surface 21. The projection 30 is secured by a longitudinal side to the femoral component 20 adjacent to the bearing surface 21 and first extends medially and then distally. The projection 30 defines a concave region 31 at the point it joins the bearing surface 21 between it and the bearing surface 21. The concave region 31 retains the meniscal bearing 22 when it moves medially and prevents it lifting over the medial wall 25 of the tibial component 26.

[0120] In particular, the concave region is arranged to receive a rim 33 of the meniscal bearing 22 (i.e. the edge formed between the side wall and the concave bearing surface arranged to abut the femoral component).

[0121] Figure shows an anteroposterior view of a tibial shelf prosthetic component 40 comprising a shelf 40 having a connection surface 41 and convex bearing surface 42. The connection surface 41 is adapted to abut and be connected to a proximal end of a tibia bone. The convex bearing surface 42 is adapted to abut a concave face of a meniscal bearing 43. The tibial shelf 40 includes an inner wall, in this case medial wall 44, extending from the perimeter of the bearing surface over a portion of the inner, medial, side. The medial wall 44 has a laterally facing inclined side 45 adjacent to the bearing surface 42. The tibial shelf 40 also includes an outer, lateral, wall 44 extending from the convex bearing surface 42 of the lateral peripheral side. The lateral wall 44 substantially a mirror image of the medial wall 44 and includes a medially facing inclined side 45. The lateral wall 44 acts to retain the meniscal bearing on the tibial shelf when it moves in the lateral direction. It also inhibits placement of the femoral component and the bearing too far laterally, reducing the risk of medial dislocation.

[0122] It will be appreciated that the medial wall 44 and lateral wall 44 need not have inclined sides 45 and 45 and could alternatively extend perpendicular to the connection side 41. Alternatively, one of the medial wall 44 and lateral wall 44 could include an inclined side to receive the side wall of the meniscal bearing.

[0123] FIGS. 4a and 4b show a view of a tibial shelf prosthetic component 50 in a unicondylar meniscal bearing unicompartmental knee replacement. The tibial component 50 comprises a shelf 51 having a bearing surface 52 and an opposed connection surface 53. The bearing surface 52 is adapted to abut a bearing element 54, in use. The connection surface 53 adapted to be secured to a proximal end of a tibia bone 55. To aid the connection, the connection surface 53 includes a keel 56 that extends into a groove 57 that is made in the tibia bone 55.

[0124] The bearing element 54 is arranged to be located between the tibial component 50 and a femoral component 58 in a unicondylar meniscal bearing unicompartmental knee replacement. The femoral component 58 is secured to a femur bone 59. In this embodiment, the bearing element 54 has a concave lower surface 60 and a concave upper surface 61, which engages with a bearing surface 62 of the femoral component 58.

[0125] The tibial component 50 includes an inner wall, in this case medial wall 63, extending from the perimeter of the bearing surface 52 over a portion of the inner, medial side. The medial wall 63 is designed to abut the laterally facing surface 64 of the proximal end of the tibial bone 55.

[0126] In FIG. 4a, in addition to the tibial component 50, an extension portion 65 is shown. The extension portion 65 is adapted to be secured to a surface 66 of the proximal end of the tibia bone 55 inwardly adjacent (in this case medially adjacent) to the free end of the inner wall (medial wall 63). The extension portion 65 includes a securing part to secure it to the tibia 55. The extension portion 65 is elongate in the anteroposterior direction and includes an arched upper edge. In the embodiment of FIG. 4a, the medial wall 63 and the extension portion 65 form a stepped region to aid in retention of the meniscal bearing against medial dislocation. However, the extension portion 65 may include an inclined laterally facing side (with respect to the medial wall 63), which may extend over the free end of the medial wall 63.

[0127] FIG. 4b shows a further embodiment and the same reference numerals have been used. However, in this embodiment the extension portion 67 forms part of the inner (medial) wall 63 rather than solely being secured to the tibia 55. It will be appreciated that the extension portion 67 could be secured to the tibia in addition to forming part of the medial wall 63.

[0128] The medial wall 63 extends from the perimeter of the bearing surface 52 of the tibial component 51 over a portion of the medial side. The medial wall includes a base part that extends from the bearing surface in a direction substantially perpendicular to the plane of the tibial component 51. The extension portion 67 extends from the free end of the base part of the medial wall 63. The extension Portion 67 extends outwardly in a medial direction. The extension portion 67 is inclined relative to the medial wall 63 (and also to the shelf 51) at an angle of substantially 45?. The extension portion 67 increases the width of the tibial component 50.

[0129] FIG. 5a and FIG. 5b show medial and anteroposterior views of an embodiment of a tibial component 70 in accordance with the fourth aspect of the invention. The tibial component 70 has a tibial shelf 70 which comprises substantially D-shaped shelf having a convex bearing surface 71 on one side and a connection surface 72 on an opposed side. The connection surface 72 includes a securing element (not shown). The bearing surface 71 includes an inner wall, in this example, medial wall 74, that extends axially from the periphery thereof.

[0130] The medial wall 74 extends around approximately a third of the perimeter of the hearing surface 71. The medial wall 74 is also arched over its longitudinal direction. Thus, it rises from the bearing surface 71 at a first end 75, rises to a maximum height and then decreases in height to a second end 76.

[0131] The medial wall 74 comprises a base part 77 (proximate to the tibial shelf 70) and an extension part 78. The base part 77 extends from the bearing surface 71 and is of the same material as the remainder of the tibial shelf 70, which may be a cobalt chromium alloy. The maximum height of the base part 77 is typically 5 mm. The extension part 78 extends from a free end 80 of the base part 77. The extension part 78 increases the height of the medial wall 74 a further 2-3 mm at its centre point. The extension part 78 is of a flexible material, such as plastics (typically polyethylene). In particular, the extension part 78 is of a more flexible material than the base part 77 and is sufficiently flexible not to cause damage to the intercondylar notch or surrounding bone when installed in a knee. The extension part 78 increases the height of the medial wall 74 to reduce the chance of the bearing element 3 moving such that a partial medial dislocation can occur. The extension part 78 may form up to half of the total height of the medial wall 74. The extension part being of plastics is advantageous as if the extension part 78 was of metal, it could contact the metal femoral part 2 which could lead to the production of metal particles. The metal particles could cause major damage inside the knee.

[0132] The free end 80 of the base part 77 is stepped such that an inner edge 81, adjacent to the bearing surface 71, higher than an outer edge 82, adjacent to the periphery of the tibial shelf 70. The extension part 78 is compression moulded onto the free end 80. The extension part 78 is of substantially the same length as the base part 77. The extension part 78 is arched along its length and thus it rises from the base part 77 adjacent to the first end 75, where it is of nominal height. It reaches a maximum height above the base part 77 at a centre point 81 along its length and then decreases in height to point adjacent to the second end 76, where it is of nominal height.

[0133] In use, the tibial component 70 is secured to the tibia, the femoral component 8 is secured to the femur and a bearing is inserted therebetween. The bearing is biconcave and is adapted to slide between the femoral component 2 and tibial component 70. The increased height of the medial wall 74 provided by the extension part 78 helps reduce the chance of partial medial dislocation of the bearing element and obviates the risk of grains of metal debris being produced by metal components rubbing together within the knee.

[0134] FIG. 6a shows an anteroposterior view of a tibial shelf prosthetic component 90 comprising a shelf 91 having a connection surface 92 and a convex bearing surface 93. The connection surface 92 is adapted to abut and be connected to a proximal end of a tibia bone. The convex bearing surface 93 is adapted to abut a concave face of a meniscal bearing 94. The tibial shelf 91 includes an inner in this case medial wall extending from the perimeter of the bearing surface over a portion of the inner, medial side. The medial wall 95 has aa laterally facing side 96 adjacent to the bearing surface 93. The laterally facing side 96 is inclined to an axis 97 of the tibial component. The axis 97 of the tibial component extends perpendicular to the connection surface 92.

[0135] The laterally facing side 96 is inclined such that it extends from a medially inward position adjacent to the bearing surface 93 to a medially outward position adjacent to a free end of the medial wall 95. This inclination is selected to improve how the tibial component interacts with a meniscal bearing. The meniscal bearing 94 comprises a bi-concave disc having a first concave bearing surface 98 adapted to bear against a femoral extension (not shown in FIG. 6), and a second concave bearing surface 99 opposed to the first concave bearing surface 98 and adapted to bear against the convex bearing surface 93 of the tibial shelf 91. The second concave bearing surface 99 is complementary to the convex bearing surface 93. The first and second bearing surfaces 98, 99 are separated by a side wall 100.

[0136] In FIG. 6a, the meniscal bearing 94 is shown adjacent to the medial wall 95 bearing against the medial side of the bearing surface 93. It will be appreciated that in the Figure, the components are shown with a slight gap between them for clarity. The inclination of the laterally facing side 96 substantially matches the inclination of the side wall 100 of the meniscal bearing 94 when it is bearing against the convex bearing surface 93 and is adjacent to the medial wall 95. In particular, the angle 101 formed between the side wall 100 and the second concave bearing surface 99 is substantially the same as the angle 102 formed between the inclined laterally facing side 96 and the convex bearing surface 93.

[0137] Accordingly, contact between the tibial component 90 and the meniscal bearing 94 occurs between the laterally facing side 96 of the medial wall 95 and the side wall 100, of the meniscal bearing 94, rather than just between the free end of the medial wall and the side wall 100. This increased area of contact will increase the resistance to movement of the meniscal bearing 94 up the medial wall 95. Furthermore, the wider distribution of force will help prevent wear on the side wall 100 of the meniscal bearing 94.

[0138] FIG. 6b shows the determination of an appropriate angle 102 formed between the inclined laterally facing side 96 and the convex bearing surface 93. FIG. 6b shows a diagram which labels the dimensions of a tibial component having an inclined medial/lateral wall. The diagram shows the meniscal bearing in a first position 110 centrally, symmetrically located on the bearing surface. It also shows the bearing in a second position 111 on the medial side abutting the medial wall. For a convex bearing surface, the inclination from the vertical a of the bearing when the meniscal bearing is both substantially contacting the bearing surface of the tibial component and contacting the base part of the medial wall may be calculated using the formula

y?x cos ??(h+x sin ?+?{square root over (R.sup.2?y.sup.2)}) tan ?=0

where
y is the distance from the central eminence of the domed bearing surface to the laterally facing side of the medial wall,
x is the half-width of the meniscal bearing,
h is the height of the medial wall from the medial edge of the bearing surface and
R is the radius of the convex bearing surface.

[0139] FIG. 7a shows an anteroposterior view of an embodiment of a tibial component 120 and a meniscal bearing in accordance with the sixth aspect of the invention. The tibial component 120 comprises a tibial shelf 121 having substantially D-shaped shelf having a convex bearing surface 122 on one side and a connection surface 123 on an opposed side. The connection. surface 123 includes a securing element 124. The bearing surface 122 includes an inner wall, in this case medial wall 125, that extends axially from the periphery thereof. The medial wall 125 extends around approximately a third of the perimeter of the bearing surface 122. The medial wall 125 is also arched over its direction. Thus, it rises from the bearing surface 122 at a first end (not visible), rises to a maximum height and then decreases in height to a second end

[0140] The medial wall 125 includes a laterally extending retaining flange 127 that projects from a free end 12A of the medial wall 125 over a portion of its length. The retaining flange 127 comprises a detent portion 129 and an inclined deflection surface 130.

[0141] The bearing 131, as shown in FIG. 7b, substantially has the shape of a bi-concave disc. In particular, the bearing 131 comprises a first concave bearing surface 132 and a second concave bearing surface 133 opposed to the first concave bearing surface 132, the bearing surfaces 132 and 133 being separated by a side wall 134. The first concave bearing surface 132 is adapted to bear against the femoral component 2. The second concave bearing surface 133 is adapted to bear against the bearing surface 122 of the tibial shelf 121. A retention groove 135 is formed in a portion of the side wall 134 that is complementary to the retaining flange 127. The retention groove 135 comprises an arcuate track that substantially follows the curve of the second concave bearing surface 133, through the centre of the bearing element 131, as can be seen in the lateral view of the bearing in FIG. 7b.

[0142] In use, tibial component 120 is secured to the tibia, the femoral component 2 is secured to the femur and the bearing element 131 is inserted therebetween. The retention groove 135 of the bearing element 131 is substantially aligned with the retaining flange 127. On movement of the bearing element 131 towards the medial wall 125, the retaining flange 127 enters the retention groove 135. If the bearing element 131 were to lift, such as in the event of a partial medial dislocation, the detent portion 128 will engage the retention groove 135 and prevent it lifting further from the bearing surface 122. Thus, the detent acts to prevent partial medial dislocation of the bearing element 128. The position of the retention groove 135 as advantageous as it as located below the upper surface of the meniscal bearing. This ensures that the retaining flange 127 does not interfere with a femoral component of the knee prosthesis when the components are in use. The deflection surface 130 acts to direct a bearing that has lifted past the retaining flange 127 back onto the bearing surface 122.

[0143] FIG. 8a shows a first embodiment of the seventh.

[0144] aspect of the invention. The tibial component 140 has a tibial shelf 140 which is substantially D-shaped in plan view, having a bearing surface 141 on one side and a connection surface 142 on an opposed side. The connection surface 142 includes a securing element 143. The bearing surface 141 includes an inner wall, in this case medial wall 144, that extends axially from the periphery thereof. The medial wall 144 extends around approximately a third of the perimeter of the bearing surface 141. In this embodiment, the bearing surface 141 is curved in the medial-lateral direction only and thus appears barrel shaped, with the axis of the barrel extending in the anterior-posterior direction. The peak of the ridge formed by the curved surface thus extends in the anterior-posterior direction and is shown by dashed line 145. The bearing surface 141 is substantially flat in the anterior-posterior direction. In this embodiment, the peak of the ridge 145 extends through the centre of the bearing surface 141, but it could be offset to the medial or lateral sides.

[0145] FIG. 8b shows a second embodiment of the seventh aspect of the invention. The same reference numerals have been used for the same features as those shown in FIG. 8a. This embodiment differs from that of FIG. 8a as the surface 141 is not curved in medial-lateral direction. Instead, it is curved in the anterior-posterior direction only. Thus, the bearing surface 141 is barrel shaped but with the axis of the barrel extending in the medial-lateral direction. The peak of the ridge formed by the curved surface thus extends in the medial-lateral direction and is shown by dashed line 146. The bearing surface 141 is substantially flat in the medial-lateral direction. In this embodiment, the peak of the ridge 146 extends through the centre of the bearing surface 141, but it could be offset to the anterior or posterior sides.

[0146] FIG. 8c shows a third embodiment of the tibial component 140 of the seventh aspect of the invention. Once again the same reference numerals have been used for the same features as those shown in FIGS. 8a and 8b. In this embodiment, the bearing surface 141 has a first function of curvature in the anterior-posterior direction and a second function of curvature in the medial-lateral direction. Thus, the surface may be a quadric surface. The first function of curvature is different to the second function of curvature. will be appreciated that the functions of curvature could be the same depending on the configuration of the knee replacement components. The ridge 127 of the anterior-posterior curve and the ridge 128 of the medial-lateral curve intersect at the centre of the bearing surface 141. It will be appreciated that the apex of the surface could be offset from the centre of the bearing surface 141.

[0147] FIG. 9 shows a diagrammatic anteroposterior view of a meniscal bearing 150 of the eighth aspect of the invention located between a femoral component 151 and a tibial shelf component 152. The meniscal bearing 150 comprises a bi-concave disc having a first concave bearing surface 153 adapted to bear against the femoral extension component 151, and a second concave bearing surface 154 opposed to the first concave bearing surface 153 and adapted to bear against a convex bearing surface 155 of the tibial shelf component 152. The first and second bearing surfaces 153, 154 are separated by a side wall 156. The side wall 156 is thicker on the outer, in this case lateral, side than it is on the inner, in this case medial, side of the meniscal bearing 150. In this embodiment, this difference in thickness is at least 1 mm. The thicker lateral side wall 156 will provide resistance to medial dislocation as the femoral component 151 will have to separate from the tibial component 152 a substantial distance before the thicker side wall 156 can pass between them. This reduces the risk of medial dislocation.

[0148] FIG. 10 shows a flow chart of the method of performing knee arthroplasty. Step 160 comprises preparing a femur bone to receive a femoral component. Step 161 comprises preparing a tibia bone to receive the tibial component. Step 162 comprises securing the femoral component to the femur bone. Step 163 comprises securing the tibial component to the tibia bone. Step 164 comprises inserting the bearing between the femoral component and the tibial component. It will be appreciated that the steps can be performed in a different order. For example, step 160 followed by step 162 and then step 161 followed by step 163 or vice versa. If the tibial component of FIG. 4a or FIG. 4b is used, it may be necessary to resect part of the tibia 55 to accommodate the extension portion 65 or 67. Therefore, the tibia 55 may be prepared with a flat or inclined surface 66 complementary to the extension portion 65 or 67 respectively.

[0149] It will be appreciated that the use of the terms medial and lateral in describing the components shown in the Figures is by way of example only, and is not intended to limit the invention to the embodiments shown.