WO1995025484A1

WO1995025484A1 - Prosthesis assembly

Info

Publication number: WO1995025484A1
Application number: PCT/GB1995/000514
Authority: WO
Inventors: Michael Anthoney Tuke
Original assignee: Corin Medical Limited
Priority date: 1994-03-24
Filing date: 1995-03-10
Publication date: 1995-09-28
Also published as: GB2301538B; GB9405883D0; GB9619222D0; AU1856695A; GB2301538A

Abstract

A prosthesis assembly for a knee comprising a femoral component (10) having two condylar surfaces (12) and an intercondylar notch (14), a meniscal component (20) having two condylar surfaces (22) and an upwardly extending intercondylar spine (24), and a tibial component (30) for supporting the meniscal component (20), wherein interengaging means (28a, 28b, 36a, 36b) are provided between the tibial and meniscal components (30, 20) such that limited rotational, anterior and posterior movement of the meniscal component (20) with respect to the tibial component (30) is allowed. The assembly retains the advantages of a posterior stabilised knee whilst more closely resembling the articulation of a natural healthy knee.

Description

PROSTHESIS ASSEMBLY

The invention relates to a prosthesis assembly, particularly to a prosthesis assembly for a knee.

Posterior stabilised knees are known in which the tibial component carries an intercondylar tibial spine which abuts against a cam in the intercondylar notch on the femoral component. The interaction of the spine and notch means that the femoral component is prevented from moving anteriorly with respect to the tibial component when the forces applied act to make it do so, for example, when descending stairs. However, although posterior stablised knees are effective in preventing this movement and thus increasing the stability of the knee, they do not make adequate allowance for other natural movements of the femur with respect to the tibia. In a normal healthy knee joint, for example, the femur will rotate relative to the tibia during movement between flexed and unflexed positions. Also, when the knee joint is flexed, for example when climbing stairs, the tibia moves posteriorly with respect to the femur. None of the known posterior stablised knee joints allows for relative rotation or anterior/posterior movement between the tibia and the femur. Indeed, the aim of the intercondylar notch and spine is to positively prevent anterior movement of the femur with respect to the tibia, although limited movement is in fact desirable.

It is an objection of the present invention to provides a prosthesis assembly for a knee which is posteriorly stablised and which also articulates in a manner more closely resembling that of a natural healthy knee joint.

The invention provides a prosthesis assembly as set out in claim 1. Advantageous features are out in the subsidiary claims.

The prosthesis assembly of the invention retains all of the advantages of a known posterior stablised knee but also allows the femur of the recipient to rotate and to move anteriorly and posteriorly to a limited extent with respect to the tibia. This results in the movement of the prosthesis assembly closely resembling that of a normal, healthy knee joint but with the added stability of posterior stablisation.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, wherein:

Figure la is a rear view of the femoral component of a prosthesis assembly according to the invention;

Figure lb is a side view of the component of Figure la;

Figure 2a is a side view of the meniscal component of an assembly according to the invention; Figure 2b is a front view of the component of Figure 2a;

Figure 2c is' a plan view of the component of Figures 2a and 2b;

Figure 3a is a side view of the tibial component of an assembly according to the invention;

Figure 3b is a front view of the component of Figure 3a;

Figure 3c is a plan view of the component of Figures 3a and 3b; and

Figure 4 is a side view showing the interaction of the components shown in Figures la to 3c.

Figures la and lb of the drawings show the femoral component 10 of a prosthesis assembly according to the invention. For the sake of clarity, details of any features which allow fixing of the prosthesis to a femur have been omitted, but can comprise appropriate pegs or stems. The femoral component may also be fixed by cementing.

The femoral component 10 essentially comprises two σondylar surfaces 12 and an intercondylar notch 14. The condylar surfaces 12 are shaped in a known manner to allow optimum load transfer without excessive pressures occurring. The remainder of the component has features which do not form part of the present invention. The notch 14 is shaped as illustrated to allow the prosthesis to function as a posterior stabilised knee. Figures 2a, 2b and 2c illustrate the meniscal component 20 of the prosthesis. The meniscal component 20 essentially comprises two condylar surfaces 22 and an intercondylar spine 24 which extends upwardly between the condylar surfaces 22. The profile of the spine 24 is shaped as illustrated in Figure 2a so as to allow the meniscal component and the femoral component to interact in the manner of a posterior stablised knee.

The lower surface 26 of the meniscal component 20 is planar and smooth for reasons which will become clear below. Located in the lower surface 26 are an anterior recess 28a and a posterior recess 28b. The anterior and posterior recesses 28a, 28b are arranged at the anterior and posterior ends respectively of the longitudinal axis of the meniscal component 20 and are located between the condylar surfaces 22 and below the intercondylar spine 24. The recesses 28a, 28b are both open towards the lower surface 26 and the anterior recess 28a is open towards the front of the component 20 whereas the posterior recess 28b is open towards the rear of the component 20. Each recess 28a, 28b is substantially D-shaped in plan view but the mouth of the anterior recess 28a is wider than the mouth of the posterior recess 28b. The function of the anterior and posterior recesses 28a, 28b will be explained below.

Figures 3a, 3b and 3c show the tibial component 30 of an assembly according to the invention. The tibial component 30 essentially comprises a plateau 32 and a fixing element 34. The fixing element 34 does not form part of this invention and could be replaced by other suitable fixing means as are well known in the art. The plateau 32 is planar and smooth and carries, at the anterior and posterior ends respectively of the longitudinal axis of the component 30, two pegs or bollards 36a, 36b. The height of the bollards 36a, 36b above the plateau 32 is slightly less than the depth of the recesses 28a, 28b of the meniscal component 20 and the diameter of the bollards 36a, 36b is considerably less than the width of either of the recesses 28a, 28b.

The interaction of the three components 10, 20, 30 is illustrated in Figure 4 which shows the prosthesis assembly in a partially flexed configuration. The meniscal component 20 is located between the femoral and tibial components 10, 30 and rests on the plateau 32 of the tibial component 30. The femoral component 10 rests on the meniscal component 20 with the condylar surfaces 12, 22 and the intercondylar notch 14 and spine 24 in contact to allow the complete assembly to act in the manner of a posterior stabilised knee and with the condylar surfaces 12, 22 fully congruent. However, instead of the meniscal component 20 being immovably held with respect to the tibial component 30, the meniscal component 20 is located so that the lower surface 26 of the meniscal component 20 merely rests on the upper surface of the plateau 32 and, within certain limits, can glide across the plateau 32. The bollards 36a, 36b of the tibial component 30 extend into the anterior and posterior recesses 28a, 28b.

The limits of movement of the meniscal component 20 are set by the dimensions and spacing of the bollards 36a, 36b and the recesses 28a, 28b. Anterior movement of the meniscal component 20 with respect to the tibial component 30 is limited by the anterior bollard 36a coming into abutting contact with the posterior face of the anterior recess 28a; similarly, posterior movement of the meniscal component 20 with respect to the tibial component 30 is limited by the posterior bollard 36a coming into abutting contact with the anterior face of the posterior recess 28b. The extent of the anterior/posterior movement of the meniscal component 20 with respect to the tibial component 30 is thus dictated by the difference between the spacing of the bollards 36a, 36b and the spacing of the recesses 28a, 28b.

The fact that the lateral dimensions of each recess 28a, 28b are larger than the diameter of the bollards 36a, 36b means that a certain amount of rotation of the meniscal component 20 can occur with respect to the tibial component 30, the rotation taking place about an axis substantially perpendicular to the upper surface of the plateau 32 and the lower surface 26 of the meniscal component 20. This anterior, posterior and rotational movement of the meniscal component 20 with respect to the tibial component 30 allows a posterior stablised knee to articulate in a manner more closely resembling that of a natural healthy knee than has hitherto been possible. Also, loosening forces which would otherwise be transmitted to the tibia are minimised.

The invention is not intended to be limited to the exact features of the embodiment described above. As has previously been mentioned, the femoral and tibial components can be provided with any suitable fixing means for fixing to the femur and tibia of the recipient. Many alternative fixing means are known in the art and need not be further described here. It is also possible to provide different configurations of bollard (or other projection) and recess to achieve an equivalent freedom of movement. For example, a single bollard l'ocated substantially centrally of the plateau extending into a corresponding recess located in the lower surface of the meniscal component would allow limited anterior and posterior movement of the meniscal component with respect to the tibial component depending upon the relative dimensions of the bollard and recess. Rotational movement would be unhindered if the bollard and recess were circular in shape, but different shapings, eg. elliptical, rectangular, would provide some restriction of rotational movement. Furthermore, the bollards or other projections may be provided on the meniscal component and the recesses may be formed in the plateau of the tibial component.

The femoral and tibial components 10, 30 of the assembly are preferably made of metal, for example a cobalt chrome alloy, with a highly polished surface. The meniscal component 20 is preferably made of a heavy-duty low-friction plastics material, preferably a polyethylene material. At least one of the femoral and tibial components may have a textured surface on a face designed to contact natural bone, which encourages homogenous bone ingrowth. Either or both of the femoral and tibial components can, if desired, carry a coating of Hydroxyapatite on surfaces designed to contact natural bone.

As will be appreciated, the prosthesis assembly described above acts in the manner of a posterior stablised knee. Preferably, the spine and notch comprise complementary arcuate surfaces having substantially identical radii of curvature. The spine may have a concave arcuate surface and the notch has a convex arcuate surface and, ideally, the radius of curvature of the arcuate surfaces is between 7mm and 15mm, advantageously substantially 11mm. It is also advantageous if a substantial area of contact between the spine and the notch is achieved, in use, when the knee is flexed to an angle of at least 20^*, preferably substantially 70^*. The spine and notch are preferably maintained in abutting contact during flexing of the knee. More preferably, there will be abutting contact between the spine and the notch at all times. This ensures full control over the contact area of the spine and both condylar surfaces thus allowing the contact area between the condylar surfaces to remain fully operational and load bearing. The minimum surface area of contact between the tibial and femoral components is preferably

2 between 170 and 200mm . Ideally the minimum surface

2 area of contact is 188mm . This arrangement means that, when the forces acting on the components are such that the femoral component is pressed anteriorly with respect to the tibial component, the anterior force is spread over a substantial area thus reducing the pressure transmitted between the spine and the notch.

Also, if the load bearing area of the condylar surfaces is reduced, this is compensated by the area of contact between the spine and the notch. The components are therefore less prone to wear and have an extended life.

Furthermore, the invention provides a posterior stabilised knee which does not compromise area contact between the femur and the tibia.

Other modifications and variations within the scope of the invention will be apparent to a reader skilled in the art.

Claims

CLAI MS

1. A prosthesis assembly for a knee comprising a femoral component having two condylar surfaces and an intercondylar notch, a meniscal component having two condylar surfaces and an upwardly extending intercondylar spine, and a tibial component for supporting the meniscal component, wherein interengaging means are provided between the tibial and meniscal components such that limited rotational, anterior and posterior movement of the meniscal component with respect to the tibial component is allowed, characterised in that the intercondylar notch and spine are shaped such that, when the prosthesis assembly is articulated in use, there is a substantial area of contact between the spine and the notch and a substantial area of contact is maintained between the condylar surfaces at all angles of articulation.

2. A prosthesis assembly as claimed in claim 1, wherein the interengaging means comprise at least one projection locatable in at least one enlarged recess.

3. A prosthesis as claimed in claim 2, wherein the at least one projection is provided on the tibial component and the at least one recess is provided in the meniscal component.

4. A prosthesis assembly as claimed in claim 2 or 3, wherein two projections and two recesses are provided.

5. A prosthesis assembly as claimed in any one of the preceding claims, wherein the femoral and tibial components are made of metal.

6. A prosthesis assembly as claimed in claim 5, wherein the metal is a cobalt chrome alloy.

7. A prosthesis assembly as claimed in in any one of the preceding claims, wherein the meniscal component is made from a heavy-duty, low-friction plastics material.

8. A prosthesis assembly substantially as hereinbefore described with reference to and as shown in the accompanying drawings.