KNEE JOINT PROSTHESES

Abstract: A knee joint prosthesis in which the tibial component (11) has a bearing surface which defines a recess (13) which is tapered inwardly towards its base when viewed in cross section, the meniscal component (17) has (i) a tibial bearing surface (19) that is appropriately conical to engage the tibial component bearing surface, and (ii) a femoral bearing surface (21) for engaging a femoral component, and the femoral component (1) is appropriately tapered on its bearing surface (5) to engage the relevant bearing surface of the meniscal component. A peg (45) on a patellar component can be received in the groove (9) in the bearing surface of the femoral component. The groove can be wider at its upper end than towards its other end. A groove in the bearing surface of the femoral component defined by walls which extend into the groove by a distance that is greater than the thickness of the component adjacent to the groove.

Description: KNEE JOINT PROSTHESES This invention relates to knee joint prostheses and to components of such prostheses. GB-A-2120943 discloses a knee joint prosthesis of the non- constrained type, in which relative movement of femoral and tibial components is controlled by soft tissue. The prosthesis comprises a tibial component for attachment to the tibia of a patient, a femoral component for attachment to the femur, and a meniscal component which is located in use between the tibial and femoral components. The bearing surface of the femoral component is curved, being generated by revolving a revolving a straight line about one or more axes which are parallel to the medial-lateral axis of the knee. The corresponding bearing surface of the meniscal component is generated similarly by revolving a straight line. The bearing surface of the tibial component defines a conical recess, and the corresponding bearing surface of the meniscal component is similarly conical. A patella, when present, slides over the bearing surface of the femoral component. The knee prosthesis disclosed in GB-A-2120943 allows flexing movement between the femur and tibia by virtue of relative movement between the femoral and meniscal components. The prosthesis allows twisting movement between the femur and tibia by virtue of relative rotational movement between the tibial and meniscal components. In one aspect, the present invention provides a knee joint prosthessiiss wwhhiicchh ccooπmmprriisseess :: (a) a tibial component having a bearing surface which defines a recess which is tapered inwardly towards its base when viewed in cross-section; (b) a meniscal component having (i) a tibial bearing surface that is appropriately conical to engage the tibial component bearing surface, and (ii) a femoral bearing surface for engaging a femoral component, which defines a recess which is tapered inwardly towards its base when viewed along the anterior-posterior axis, at least towards its medial and lateral extremities; (c) a femoral component that is appropriately tapered on its bearing surface to engage the relevant bearing surface of the meniscal component. The conical shape of the recess in the tibial component as disclosed in GB-A-2120943 allows rotational movement between the tibia and femur, while providing a stable configuration for the tibial and meniscal components. However, there can be a risk of the meniscal component becoming disengaged from the recess in the tibial component in certain modes of articulation of the joint. This problem has been found to be overcome by providing, a femoral bearing surface on the meniscal component which defines a recess which is tapered inwardly towards its base when viewed along the anterior-posterior axis, at least towards the medial and lateral extremities of the recess, and by providing the femoral component with a bearing surface that is correspondingly tapered so that, when viewed along the anterior-posterior axis, the bearing surface appears to be cut away, at least towards the medial and lateral extremities of the component. The femoral component can engage the meniscal component more securely in this way. Previously, stability in the connection between the meniscal and tibial components in the prosthesis of GB-A-2120943 had been minimised by reducing the thickness of the meniscal component. This lead to weakness in the meniscal component at its edges. The construction proposed in the present invention avoids that disadvantage, providing stability at the menisco- tibial interface while allowing also an acceptable thickness of material in the meniscal component even at its edges. Furthermore, the tapered configuration of the femoral component bearing surface allows the component to be formed with a deep groove between its condylar portions without having to remove a large quantity of bone. This can facilitate secure engagement between the femoral component and a peg provided on the femoral bearing surface of the meniscal component. The portions of the femoral bearing surface of the meniscal component which define the tapered portion of the recess can be substantially straight when viewed in cross-section. The portions of the femoral bearing surface of the meniscal component which define the tapered portion of the recess are generally rounded in a concave sense. For example, the femoral bearing surface might have a parabolic portion or a spherical portion. The recess will generally rounded when viewed in cross-section along both the medial/lateral and anterior/ posterior axes. However, the configurations of the rounded surfaces when viewed along these axes will generally be different from one another. Preferably, the bearing surface of the femoral component is defined as a surface of revolution, defined about an axis parallel to the medial/lateral axis. This has the significant advantage of facilitating manufacture to produce a bearing surface which has an acceptable finish, for example to be manufactured with a geometrical accuracy of not more than about ■5 μm and a surface roughness of not more than about 0.05 μm R a (centre line average) . Preferably, the surface is defined by revolving a line about one or more axes. The shape of the line, the number and locations of the axes and the radius of the revolution are selected according to the configuration of the bearing surface. For example, the line might be straight or curved, or have portions which are straight and portions which are curved. For example, curved line (or portion of a line) might be a portion of a circle, an ellipse, or a parabola. Preferably, the recess in the femoral bearing surface of the meniscal component is defined by revolution of a line, especially a line that is the same over at least part of its length as that which is revolved to generate the femoral component bearing surface. In this way, accurate line contact can be maintained between the femoral and meniscal components throughout flexion of the knee joint. The radius about which the line is revolved can vary along the bearing surface. For example a relatively small radius can be appropriate at the posterior end of the bearing surface, and a relatively large radius at the anterosuperior end of the bearing surface. The recess within the femoral bearing surface of the meniscal component can be continuously concave across the entire surface. It can be convex over a portion only of its surface, for example because of a peg which extends from the surface to engage a groove in the bearing surface of the femoral component. Preferably, the femoral component has a groove in its bearing surface and the meniscal component has a peg protruding from its femoral bearing surface which can be received in the groove in the femoral component bearing surface, to move relative to the femoral component bearing surface as the knee is flexed, within the groove. Preferably, the thickness of the meniscal component, measured between the tibial and meniscal bearing surfaces, is substantially constant at least over a portion of the component, especially over portions towards the medial and lateral extremities of the component, away from any peg that is provided to protrude into a groove in the femoral component. The recess in the tibial component and the corresponding tibial bearing surface of the meniscal component are preferably conical in the sense that they are generated over at least a substantial portion of their respective areas by revolution of a line about the longitudinal axis of the tibia, thus facilitating rotational freedom of movement of the meniscal component (and the femoral component in contact with it) relative to the tibial component around that axis. The line may but need not necessarily be straight In another aspect, the invention provides a knee joint prosthesis which comprises: (a) a femoral component having a bearing surface through which loads are transferred, directly or indirectly, to a tibial component, the femoral component bearing surface having a groove formed in it extending generally in the anterior-posterior plane towards the upper anterior edge of the component; (b) a patellar component having a bearing surface for engaging the bearing surface of the femoral component, the patellar component bearing surface having a peg extending from it which can be received in the groove in the femoral component bearing surface; the groove in the femoral component bearing surface being wider at its upper end than towards the other end of the groove. As a knee joint is straightened, the patella moves upwardly over the femoral bearing surface, away from the interface with the tibia. As it approaches the upper reaches of the femoral bearing surface, it is caused by soft tissue which is connected to it to deviate laterally from the axis of the femur. This can be accommodated in a knee joint prosthesis by providing a curved groove for the patella to move in. A disadvantage of this arrangement is that increased inventory is required to accommodate left and right knees. Furthermore, the patella can be damaged if it becomes disengaged from the groove because of the damaging profile of the surface that it then contacts. The prosthesis of this aspect of the invention has the advantage that it can accommodate the tendency of a patella to deviate laterally when the knee is straightened. This is achieved by providing a wide groove across which the patella can move when towards the top of the groove. The groove in the femoral bearing surface can be flared laterally or, more preferably, both laterally and medially. Flaring the groove in both senses has the advantage that a single component can be used for both left and right knees. Preferably, the femoral component is tapered on its bearing surface, at least towards its medial and lateral extremities, to engage a load bearing surface which defines a recess which is tapered inwardly towards its base when viewed in cross- section along the anterior-posterior axis. For example, as discussed above in greater detail, the bearing surface of the femoral component can be substantially straight, or generally rounded in a convex sense, at least at its medial and lateral extremities. Preferably, the bearing surface of the femoral component is defined as a surface of revolution, especially by revolving a line about one or more axes. As discussed above in greater detail, the shape of the line, the number and locations of the axes and the radius of the revolution are selected according to the configuration of the bearing surface. Preferably, the bearing surface of the patellar component which engages the femoral component bearing surface is defined too as a surface of revolution, especially by revolution of a line. It is preferred that the line that is revolved is the same, at least over part of its length, as the line which is revolved to generate at least the upper part of the femoral component bearing surface. This has the advantage of providing line contact between the bearing surfaces of the femoral and patellar components. Correct passage of the patella down the groove as the knee is flexed can be facilitated by a number of features, separately or, as can often be preferred, in combination. For example, the edges of the groove, where the groove meets the bearing surface of the femoral component, can be rounded. The corners at the root of the peg can be rounded when the component is viewed along the groove in the femoral component, preferably to match the rounded edge of the groove. Preferably, the depth of the groove in the femoral component is greater than the depth of the peg on the patellar component, at least towards the lower end of the groove. Preferably, the depth of the groove in the femoral component at the upper end of the groove is approximately equal to the depth of the peg on the patellar component. This has the advantage that loads transmitted between the patellar and femoral components are transmitted through the peg and the base of the groove, in addition to transmission between the bearing surfaces outside the groove and peg regions. This can enable the patellar component to withstand high loads applied to it from above even when it is not fully supported by contact with the femoral component bearing surface because of widening of the groove in that surface. Indeed, the width of the groove in the femoral component at the upper end of the groove can be greater than the width of the peg on the patellar component, measured from one side of the peg to the opposite edge of the patellar component. In another aspect, the invention provides a femoral component of a knee joint prosthesis which has a bearing surface through which loads are transferred, directly or indirectly, to a tibial component, the femoral component bearing surface having a groove formed in it extending generally in the anterior- posterior plane towards the upper anterior edge of the component to define two condylar portions and a space between them, the groove being defined medially and laterally by walls which extend into the space between the condylar portions, to an extent that is greater than the thickness of the condylar portions adjacent to the groove. Knee joint prostheses frequently include a peg which protrudes into a groove in the femoral component between two condylar portions, for example provided on the tibial component or, more commonly, on a meniscal component or a patellar component. The peg serves to control the relative movement of the femoral component and the tibial, meniscal or patellar component as the case may be. It can also minimise the instance of the femoral and tibial, meniscal or patellar components becoming separated. It is desirable for the peg to protrude into the groove in the femoral component as deep as possible to optimise the features mentioned above. However, it has been necessary in order to provide a deep groove to resect the femur significantly so that a femoral component can be fitted which provides a significant depth of material into which a groove can extend. It is recognised that it is desirable to keep the extent to which bone is resected to a minimum. The component of this aspect of the invention enables a deep groove to be provided for a peg to protrude into, while maintaining the degree of resection to a minimum. This is achieved by having the groove defined medially and laterally by walls which extend into the space between the condylar portions to an extent that is greater than the thickness of the condylar portions adjacent to the groove. Thus the depth of the femoral component at the medial and lateral walls of the groove is greater than the depth of the component adjacent to the groove. The walls can be provided by an insert that is provided in the groove or more preferably by working, especially machining, the material of the femoral component. A further advantage of providing deep walls to define a groove in a femoral component is that they can control flow of cement used in the attachment of a component to a resected femur, to reduce flow of cement (a) from the region between the resected end of the femur and the component, and (b) into the groove where it might interfere with the peg protruding into the groove. Preferably, the depth of the walls which define the groove is greater than the thickness of the condylar portions adjacent to the groove, at least in the posterior portion of the component through which load is transmitted to a tibial component when the knee joint is flexed. The bearing surface of the femoral component in which the groove is formed can have any of a number of configurations. For example, it might be generally planar when viewed along the anterior-posterior axis, for example as would be generated by revolution of a straight line. Alternatively, it might be generally rounded, as discussed above. Preferably, the groove in the femoral component bearing surface is wider at its upper end than towards the other end of the groove. Preferably, the edges of the groove are rounded where the groove meets the bearing surface of the femoral component. The invention also provides individual components of the knee joint prostheses of the invention. The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an exploded isometric view of a knee prostheses; Figure 2 is an isometric view of a femoral component which can be used in the prostheses shown in Figure 1; Figure 3 is an isometric view of another femoral component which can be used in the prostheses shown in Figure 1; Figure 4 is another view of the femoral component shown in Figure 3; Figure 5 is a sectional elevation of a knee joint prosthesis which includes femoral and patellar components; Figures 6a and 6b are sectional elevations through a femoral component at different points along the groove between its condylar portions to show the variation in width of the groove; Figures 7a and 7b are elevational views through another femoral component at different points along the groove between its condylar portions to show the variation in width of the groove; and Figures 8a and 8b are sectional elevations through the components shown in Figures 7a and 7b. Referring to the drawings, Figure 1 shows a knee joint prostheses which includes a metal femoral component 1 having two condylar portions 3. The femoral component has a bearing surface 5 on the outside of the condylar portions, and extending generally in the anterior-posterior plane towards the above anterior edge. There is a groove 9 between the condylar portions 3. A metal tibial component 11 has a recess 13 formed in its upper surface. The recess is tapered inwardly towards its base. The taper can extend to the base of the recess. However, the taper could extend only part way towards the base, so that the recess is frusto-conical when viewed in cross-section. The recess is defined by the bearing surface 14 of the tibial component. A stem extends downwardly from the apex of the recess, to be received in the cavity along the axis of the resected tibia. A meniscal component 17 has a tibial bearing surface 19 that is appropriately tapered to engage the bearing surface of the tibial component within the recess. The meniscal component has a bearing surface 21 which defines a recess which is tapered inwardly towards its base when viewed along the anterior- posterior axis, at least towards its medial and lateral extremities. The shape of the recess corresponds to that of the bearing surface 5 of the femoral component which it engages when the prostheses has been implanted. Figure 2 shows the end of a femoral component which can be used in a knee joint prostheses of the type shown in Figure 1. Figure 2 shows in more detail the dome-shape which is imparted to the bearing surface 5 of the component . The bearing surface can be seen to be tapered. It is defined by substantially straight lines when the component is viewed along the anterior- posterior axis. A component with this configuration can be generated by revolving such a straight line around an axis substantially parallel to the medial-lateral axis of the knee. More than one such axis may be used to generate respective portions of the component. Different radii can be used to generate different portions of the bearing surface. Figures 3 and 4 are viewed of another femoral component which is tapered on its the bearing surface. The bearing surface is generally rounded in a convex sense at its medial and lateral extremities, the groove 9 being provided in the region between these extremities. The configuration of the component shown in Figures 3 and 4 is generated by revolving a curved line, especially an arc of a circle. Figure 5 shows the femoral component 1 of a knee joint prostheses engaging a patellar component 41. The femoral component engages the bearing surface 43 of the patellar component, in contact with the bearing surface 5 of the femoral component. A peg 43 on the patellar component is received in the groove 9 in the femoral component. Figures 6a and 6b are sections through a femoral component at two locations along the groove 9. Figure 6a is a sectional elevation taken at the lower end of the groove, in a location occupied by patella 41 when the knee joint is flexed. Figure 6b is a sectional elevation through the component at a location towards the upper end of the groove, which the patella occupies when the knee joint is straightened. Referring to Figure 6a, the width of the groove 9 in the femoral component 1 is approximately equal to the width of the peg 45 on the patella 41. The depth of the groove 9 is greater than that of the peg 45. Referring to Figure 6b, the width of the groove 9 is greater than that of the peg 45 on the patella 41. Indeed, the width is greater than the width of the patella, measured from one side of the peg to the opposite edge of the patella. Consequently, when the knee joint is straightened and the patella is pulled laterally, the patella moves to one side of the groove. As the knee joint is subsequently flexed, the patella moves along the groove towards the narrowed portion thereof, so that the patella comes into engagement with the other edge of the groove, towards the orientation shown in Figure 6a. Figures 7a and 7b are elevational views of a femoral component 1 in which a groove 9 formed in the bearing surface 5 is defined medially and laterally by awls 51,53 which extend into the space between the condylar portions 55,57 on each side of the groove to an extent that is greater than the thickness of the condylar portions adjacent to the groove. Figures 8a and 8b are sections through the component shown in Figure 7a and 7b, on the lines I-I and II-II in Figure 7b. A patella component 59 is shown. The groove 9 in the femoral component shown in Figures 7 and 8 is wider at its upper end towards the upper anterior edge of the component than towards its other end.

Claims: C AIMS : 1. A knee joint prosthesis which comprises: (a) a tibial component having a bearing surface which defines a recess which is tapered inwardly towards its base when viewed in cross-section; (b) a meniscal component having (i) a tibial bearing surface that is appropriately conical to engage the tibial component bearing surface, and (ii) a femoral bearing surface for engaging a femoral component, which defines a recess which is tapered inwardly towards its base when viewed along the anterior-posterior axis, at least towards its medial and lateral extremities; (c) a femoral component that is appropriately tapered on its bearing surface to engage the relevant bearing surface of the meniscal component. 2. A knee joint prosthesis as claimed in claim 1, in which the portions of the femoral bearing surface of the meniscal component which define the tapered portion of the recess are substantially straight when viewed in cross-section. 3. A knee joint prosthesis as claimed in claim 1, in which the portions of the femoral bearing surface of the meniscal component which define the tapered portion of the recess are generally rounded in a concave sense. 4. A knee joint prosthesis as claimed in any one of claims 1 to 3, in which the thickness of the meniscal component, measured between the tibial and meniscal bearing surfaces, is substantially constant at least over a portion of the component. 5. A knee joint prosthesis as claimed in any one of claims 1 to 4, in which the femoral component has a groove in its bearing surface and the meniscal component has a peg protruding from its femoral bearing surface which can be received in the groove in the femoral component bearing surface. 6. A knee joint prosthesis as claimed in any one of claims 1 to 5, in which the femoral component bearing surface is a surface of revolution. 7. A knee joint prosthesis as claimed in claim 6, in which the surface of revolution is defined by revolving a line about one or more axes. 8. A femoral component as claimed in claim 7, in which the radius about which the line is rotated changes along the femoral component bearing surface. 9. A femoral component suitable for use in the knee joint prosthesis claimed in any one of claims 1 to 8. 10. A meniscal component suitable for use in the knee joint prosthesis claimed in any one of claims 1 to 8. 11. A knee joint prosthesis which comprises: (a) a femoral component having a bearing surface through which loads are transferred, directly or indirectly, to a tibial component, the femoral component bearing surface having a groove formed in it extending generally in the anterior-posterior plane towards the upper anterior edge of the component; (b) a patellar component having a bearing surface for engaging the bearing surface of the femoral component, the patellar component bearing surface having a peg extending from it which can be received in the groove in the femoral component bearing surface; the groove in the femoral component bearing surface being wider at its upper end than towards the other end of the groove. 12. A knee joint as claimed in claim 11, in which the said groove is flared laterally. 13. A knee joint as claimed in claim 11 or claim 12, in which the said groove is flared medially and laterally. 14. A knee joint as claimed in claim 13, in which the femoral component is appropriately tapered on its " bearing surface to engage a load bearing surface which defines a recess which is tapered inwardly towards its base when viewed along the anterior-posterior axis, at least towards its medial and lateral extremities. 15. A knee joint prosthesis as claimed in claim 14, in which the portions of the bearing surface of the femoral component which define the tapered portion of the component are substantially straight when viewed along the anterior/posterior axis. 16. A knee joint prosthesis as claimed in claim 14, in which the portions of the bearing surface of the femoral component which define the tapered portion of the component are generally rounded in a concave sense when viewed along the anterior/ posterior axis. 17. A knee joint prosthesis as claimed in any one of claims 11 to 16, in which the edges of the groove, where the groove meets the bearing surface of the femoral component, are rounded. 18. A knee joint prosthesis as claimed in any one of claims 11 to 17, in which the corners at the root of the peg are rounded when the component is viewed along the groove in the femoral component. 19. A knee joint prosthesis as claimed in any one of claims 11 to 18, in which the depth of the groove in the femoral component is greater than the depth of the peg on the patellar component, at least towards the lower end of the groove. 20. A knee joint prosthesis as claimed in any one of claims 11 to 19, in which the depth of the groove in the femoral component at the upper end of the groove is approximately equal to the depth of the peg on the patellar component. 21. A knee joint prosthesis as claimed in claim 20, in which the width of the groove in the femoral component at the upper end of the groove is greater than the width of the peg on the patellar component, measured from one side of the peg to the opposite edge of the patellar component. 22. A femoral component of a knee joint prosthesis which has a bearing surface through which loads are transferred, directly or indirectly, to a tibial component, the femoral component bearing surface having a groove formed in it extending generally in the anterior-posterior plane towards the upper anterior edge of the component to define two condylar portions and a space between them, the groove being defined medially and laterally by walls which extend into the space between the condylar portions, to an extent that is greater than the thickness of the condylar portions adjacent to the groove. 23. A femoral component as claimed in claim 22, in which the depth of the walls, measured into the groove, is greater than the thickness of the condylar portions adjacent to the groove, at least in the posterior portion of the component through which load is transmitted to a tibial component when the knee joint is flexed. 24. A femoral component as claimed in claim 22 or claim 23, in which the femoral bearing surface of the meniscal component is substantially conical, at least at its medial and lateral extremities, when viewed along the anterior-posterior axis. 25. A knee joint prosthesis as claimed in claim 22 or claim 23, in which the femoral bearing surface of the meniscal component is generally rounded in a concave sense, at least at its medial and lateral extremities, when viewed along the anterior-posterior axis. 26. A femoral component as claimed in any one of claims 22 to 25, in which the groove in the femoral component bearing surface is wider at its anterosuperior end than towards the other end of the groove. 27. A femoral component as claimed in as claimed in any one of claims 22 to 26, in which the edges of the groove, where the groove meets the bearing surface of the femoral component, are rounded.

Inventors: JOHNSON ROBERT, JOHNSON, ROBERT
Applicants: JOHNSON ROBERT, JOHNSON, ROBERT
Application year: 1995
Main classification: A61F2
IPC codes: A61F2