WO1997025006A1

WO1997025006A1 - Patellofemoral joint prosthesis

Info

Publication number: WO1997025006A1
Application number: PCT/GB1997/000032
Authority: WO
Inventors: Andrew Arthur Amis
Original assignee: Imperial College Of Science, Technology & Medicine
Priority date: 1996-01-03
Filing date: 1997-01-03
Publication date: 1997-07-17
Also published as: GB9600036D0; AU1387697A

Abstract

A patellofemoral joint prosthesis comprising a femoral component adapted for location in the region of the femoral trochlear groove comprising an articular bearing surface, and a patellar component adapted for location within the patella and having a co-operating articular bearing surface, said femoral component comprising a segment of resilient material whose bearing surface geometry is at least partly defined by a pair of generally part convex surfaces located either side of an articular valley, said patellar component also comprising resilient material with a bearing surface formed to articulate from flexion to extension of the joint prosthesis with the bearing surface of said femoral component, the patellar bearing surface including a pair of spaced part concave depressions whose geometry permits articulation with said part convex surfaces of the femoral component, and wherein an upstanding ridge of the patellar component permits articulation along at least part of said valley from flexion to extension, the geometry of the bearing surface of said patellar component being such as to permit sliding engagement with and disengagement from the femoral articular surface.

Description

PATELLOFEMORAL JOINT PROSTHESIS

This invention is concerned with an artificial replacement for the patellofemoral joint herein referred to as the "pfj", that forms the bearing surface between the patella 1 and the end of the femur 2 in the knee joint. When the knee is flexed, see fig 6, the patella slides over the femoral bearing surface, in what is usually referred-to as the trochlear groove, and it is damage to the articular cartilage surface in the natural knee joint (i.e. arthritis) that causes the pain that leads to patients needing a joint replacement. This is often a generalised degeneration around the knee, which can require the well-known 'total knee replacement' operation. In younger patients, it is sometimes found that the pfj has been damaged in isolation, and a total replacement would then be too destructive, if a localised implant could replace the localised damage in the pfj.

The pfj prosthesis consists of two parts, one to replace the bearing surface of the patella 3, the other to replace the bearing surface of the femoral trochlear groove 4.

According to this invention there is provided a patellofemoral joint prosthesis comprising a femoral component adapted for location in the region of the femoral trochlear groove comprising an articular bearing surface, and a patellar component adapted for location within the patella and having a co-operating articular bearing surface, said femoral component comprising a segment of resilient material whose bearing surface geometry is at least partly defined by a pair of generally part convex surfaces located either side of an articular valley, said patellar component also comprising resilient material with a bearing surface formed to articulate from flexion to extension of the joint prosthesis with the bearing surface of said femoral component, the patellar bearing surface including a pair of spaced part concave depressions whose geometry permits articulation with said part convex surfaces of the femoral component, and wherein an upstanding ridge of the patellar component permits articulation along at least part of said valley from flexion to extension, the geometry of the bearing surface of said patellar component being such as to permit sliding engagement with and disengagement from the femoral articular surface.

In use, and by virtue of the geometry of the articular bearing surfaces of both components, the patellar articulation is preferably trimmed away distally, allowing a smooth sliding engagement into and disengagement out of articulation.

It is most preferred that the lowermost surfaces of the part concave depressions pass directly to surfaces which are generally flat in a proximal-distal direction extending away distally therefrom. Moreover other geometry could be utilised which allows the smooth sliding of the two components into articular engagement and disengagement.

Preferably the extent of the segment at the lateral surface is greater than the extent at the medial surface.

The segment may have an upper surface edge which extends from the medial side and upwardly towards the lateral side of the femoral component.

The segment can have lowermost ,-jt out portions formed to avoid, in use, contact between the segment and eniscal or other bearing surfaces on the tibia in the general region of the femoral trochlear groove.

The segment geometry can be defined by a section revolved about a common axis of rotation based on a pair of part circles each centred on that axis of revolution and linked by a blending radius.

Alternatively, the segment geometry could be defined by a straight line that would form a cylindrical roller geometry if revolved around the ax s. These are examples only of many possible geometric forms for the segment, which in itself is not critical providing it allows for articulation with a co-operating formation in the patellar component articular surface.

The segment valley can be, for example, U-shaped w th curvature extending in the base of the "U", or V-shaped with curvature extending in the base of the "V", although it is desired to be complementary to the upstanding ridge on the patellar component.

A further example would be a pfj (patellofemoral joint) prosthesis in which the femoral groove geometry is the same as in a total knee joint replacement prosthesis, that also has a tibial component. With the patellar component of the pfj prosthesis adapted to the geometry of the total knee joint prosthesis, this would facilitate later conversion of the pfj replacement to a total knee replacement, when the patellar component need not be removed and replaced.

The part concave surfaces of the patellar component are preferably part spherical surfaces and more preferably complementary to the part spherical femoral articular surfaces located at either side of the trochlear groove.

The patellar component part concave depressions can be so formed as to be complementary to co-operate in articulation with the said part convex surfaces to permit articulation with a substantial part of these surfaces in mutual contact from extension to flexion; moreover in such articulation the upstanding ridge preferably contacts a major part of the said articular valley.

The patellar component part concave depressions are located relatively proximally and extend relatively distally into, for example, a flat, sloping or curved formation comprising extensions of the part concave bearing surfaces either side of the upstanding ridge. Such formation can be of constant cross section.

The patellar component part concave depressions can be spaced either side of said upstanding ridge. The patellar component distal end preferably has a chamfered or rounded/radiused off surface which extends below the plane of the lowermost surface part of the said part concave surfaces, to facilitate gliding engagement as the knee flexes.

One method of making the femoral component, by way of example only, based on use of a bispherical shell (as subsequently described) involves investment casting or forging of a suitable alloy such as a cobalt-chrome-molybdenum alloy into a bispherical shell, followed by grinding and polishing operations on the articular surfaces, as appropriate. The reverse side can be formed in such a manner as to permit subsequent fixation to the bone by the surgeon, or for fixation onto a base which is then fixed in proper location by the surgeon.

One method of making the patellar component, by way of example only, involves machining the end-face of an ultra-high molecular weight polyethylene rod using a circular bladed fly-cutter, that would form each of the part-spherical concavities, the cutter being moved distally at a constant height so as to form the distal part of the articular surfaces that has relatively constant cross-section. After cutting from the rod, the reverse surface could either be prepared for fixation to the patellar bone, or else for fixation to a metallic backing, that is then used for fixing to the bone.

In order that the invention may be illustrated, exemplified and readily carried into effect by one skilled in this art, an embodiment thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings, and in which:

Figure 1 is a cross sectional view of a right-hand knee femoral component in the form of a metallic shell-like segment indicating the geometry defining its articular bearing surface,

Figure 2 is a part-sectional end elevation of a bispherical shell component indicating two cuts at approximately 90° tangential to the bore which can be made thereto to form the femoral component,

Figure 3 is a view of the end face of the right-hand knee femoral component part as applied to the femur in the femoral trochlear groove, formed with cut out portions which avoid contact with meniscal surfaces on the tibia during extension,

Figure 4 is a plan view of the femoral component shown n figures 1 and 3 from above indicating the preference for removing a section,

Figure 5 is a plan view of a left-hand knee patellar component depicting the articular bearing surfaces, this component (in mirror image form) being adapted to articulate with the femoral component shown in Figures 1, 3 and 4,

Figure 6 is a view of the prosthetic implant affixed to the knee joint of a patient, in flexion,

Figure 7 is a view of the same prosthetic implant shown in Figure 6 but in the extension condition,

Figure 8 is a view of an alternative femoral component in cross section, with flatter transverse geometry than that shown in figure 1,

Figure 9 is a view of a still further femoral component in cross section, with flatter transverse geometry than that shown in figure 1,

Figure 10 shows a femoral component that is mounted onto 3 flat cut surfaces and three stages of femoral bone preparation prior to attachment of femoral component, and

Figures 11 and 12 are views of further patellofemoral joint prostheses wherein transverse geometry of the patellar component is not widely congruent with the femoral component.

In embodiments of the present invention and as shown throughout figures 1 to 7, the femoral component 4 shown in figures 1-4 is formed primarily as a thin shell, preferably metallic, such as of cobalt-chrome alloy. The bearing surface geometry shown in figures 1 and 2 is created by revolving a section 5 around a central axis 6, to form a surface of revolution about that axis. The geometry used consists of two part-circles 7,7', each centred on the axis of revolution 6, linked by a blending radius 8. As the circles are revolved, so part-spherical i.e. convex bearing surfaces are created, linked by a smooth U-shaped valley. The component 4 may be formed from a segment 5 (figure 2) of this revolved surface, cut by two planar cuts 9 tangential to the central bore 6', at approx 90° to each other. A leave a profiled segment 11 of a bispherical shell which ideally corresponds closely to the natural articular anatomy as shown in figure 3. The sideways extent of the bearing shell is determined so that it fits the natural knee, and this means that the lateral spherical surface 12 has a greater extent than the medial surface 13. This reflects the natural balance of bearing forces. On the convex bearing surface, there is a highly polished articular surface, on which the patellar component slides; on the surface underneath (not illustrated) a number of fixation pegs 14 can be provided, perhaps 3 or 4, that pass into the bone or bone cement used by the surgeon after excising damaged articular bone surface to secure it to the femur. The cut out portions 24 are shaped to avoid contact with eniscal surfaces as the knee extends.

The patellar component 15 shown in figure 5 is basically a disc, usually of ultra high molecular weight polyethylene which has the articular bearing surface 16 on one face, and the means for fixing the disc to the patellar bone on the other face (not shown). This fixation means might be a central post, or else a number of pegs 17(figure 7), which could have circular or non-circular sections. Sometimes, a patellar component will be made with a polyethylene bearing shell secured to a metallic backing, which backing includes the fixation means. In the present invention, the disc is preferably circular, but it could be non-circular, such as elliptical. The size of the disc could be chosen so that the edges are surrounded by patellar bone 17A, when the component will be inset, or else it may sit on a flat surface cut right across the patella.

The articular geometry of the patellar articulation 16 is partly congruent with that of the femoral component 5, with two part- spherical concave depressions 18 that mate with the convex part- spherical surfaces 12,13 of the femoral shell. These concavities are linked by an upstanding ridge ISA, that has the same profile as the groove 8 in the femoral component, so the area of contact between the two components is continuous from one concavity to the other.

The congruent bearing configuration described above represents the situation when the knee is flexed as indicated in fig 6. In this situation there is a relatively large contact area 19 in flexion and correspondingly low stresses on the surfaces.

However, as the knee extends, the patella is pulled proximally, up the anterior (frontal) aspect of the femur as depicted in fig 7. This occurs to such an extent that the patella usually moves proximally out of articulation with the femoral groove 20. In order to facilitate the proximal gliding (i.e. during knee extension) in essentially a straight line away from the congruent articulation, the bearing surface of the patellar component has been extended by, for example, removal of material 21 at a constant cross-section, from the lowermost surface of the centre of both concave depressions (i.e. where the concavity is deepest) in a distal direction. This allows it to disengage smoothly from the femoral surface via a 'line' contact 22 without it being balanced on one edge during this transition, a situation that would cause undesirable high localised stresses and accelerate wear of the polyethylene patellar component. In order to help re-engagement, the distal edges 23 of the component can be radiussed off, to prevent protrusion from the bone surface when secured by the surgeon, and minimise the risk of 'catching' or other impediment to smooth gliding action. As with the femoral component, this surface adjustment is preferably asymmetric.

The use of this particular articular geometry for the two components facilitates smooth sliding engagement or disengagement, i.e. taking the concavities 18 in the proximal half of the patellar component, and maintaining a constant 'extruded' section 21 in a distal direction.

Use of a relatively simple flat cut 9 across the bispherical shell determines the extent of the femoral component. This provides curved edges 24 to the component, that fit very well with the need not to interfere with i.e. impinge upon the other articular structures such as menisci 25 in the knee during extension.

If the pfj prosthesis is adapted to a typical total knee prosthesis system, then the transverse geometry shown in Figure 1 could be relatively flatter (Figure 8), and again this geometry may, for example, be bispherical with a U-shaped valley between the spherical components, or may be inclined sloping surfaces (Figure 9), or even flat, that would cause a cylindrical geometry.

The geometry may also be symmetrical in the medial-lateral direction, as in Figs. 8 and 9.

The fixation to bone may follow bone preparation that requires flat surfaces to be cut (Figure 10), as in total knee replacement systems. The cutting must be guided accurately, and a system to facilitate this is illustrated by way of example. An intramedullary alignment rod s introduced into the femur (Figure 10, A) and a cutting guide block B mounted on it. The cutting guide block s aligned to be tangential to the anterior surface of the femur by means of a probe C. The anterior cut D is then made by introducing a saw blade through a slot E in the guide block. The guide block B and alignment rod A are removed. A distal cutting guide F is located on the anterior cut surface D and on the distal surface of the femur. A bur G is placed through an aperture m the distal cutting guide, the shape of the aperture controlling the area of bone which can be reached by the bur. A collar H controls the depth of the bur in the bone. The shape of the aperture is the same as that of the distal part of the femoral shell of the pfj prosthesis, so that the bone removed is the same as that between cut-outs 24 in Figure 3. The distal guide block s removed and a chamfering guide block I is located onto the anterior and distal cuts. This contains a slot J angled approximately 45°, so that a saw introduced through it will chamfer the anterlor-distal corner of the femur. An alternative embodiment would use a bur through an aperture in a guide block, facilitating preservation of bone and cartilage surfaces to either side. This or another block may also contain guide holes for drilling the fixation peg 14 holes in the bone, n conjunction with a drill with a depth stop.

In a further example, the transverse geometry of the patellar component may not be widely congruent w th the femoral component, to facilitate angular realignments and reduce medial-lateral constraint in the system.

Examples of this are shown in Figures 11, 12. In the first embodiment, the geometry remains partly congruent, and so the patellar concavities 18 match the shape of the femoral convexities 7,7'. The central ridge of the patella 18A has a larger radius than the blending radius 8 of the femoral component, and so ridge 18A does not reach the full depth of the groove between the convex surfaces 7,7'. In further embodiments (Figure 12), the central ridge of the patellar component may be truncated 26, or the articular surfaces may not be congruent in a medial-lateral direction 27. This range of medial-lateral section geometries may all be integrated with the sagittal plane geometry shown in Figures 6 and 7, that allows a smooth gliding engagement of the components as the knee flexes, followed by more congruent articulation in further knee flexion.

Claims

1. A patellofemoral joint prosthesis comprising a femoral component adapted for location in the region of the femoral trochlear groove comprising an articular bearing surface, and a patellar component adapted for location within the patella and having a co¬ operating articular bearing surface, said femoral component comprising a segment of resilient material whose bearing surface geometry is at least partly defined by a pair of generally part convex surfaces located either side of an articular valley, said patellar component also comprising resilient material with a bearing surface formed to articulate from flexion to extension of the joint prosthesis with the bearing surface of said femoral component, the patellar bearing surface including a pair of spaced part concave depressions whose geometry permits articulation with said part convex surfaces of the femoral component, and wherein an upstanding ridge of the patellar component permits articulation along at least part of said valley from flexion to extension, the geometry of the bearing surface of said patellar component being such as to permit sliding engagement with and disengagement from the femoral articular surface.

2. A joint prosthesis according to claim 1 wherein the geometry of the articular bearing surfaces of both components is such that the patellar articulation in use is trimmed away distally permitting a smooth sliding engagement into and disengagement out of articulation.

3. A joint prosthesis according to claim 1 or 2 wherein the lowermost surfaces of the said part concave depressions pass directly to surfaces which are generally flat in a proximal-distal direction extending away distally therefrom.

4. A joint prosthesis according to any preceding claim wherein the extent of the segment at the lateral surface is greater than the extent at the medial surface.

5. A joint prosthesis according to any preceding claim wherein the segment has an upper edge extending from the medial side upwardly towards the lateral side of the femoral component.

6. A joint prosthesis according to any preceding claim wherein the segment has lowermost circular cut out portions formed to avoid in use, contact between the segment and eniscal or other tibial bearing surfaces in the general region of the femoral trochlear groove.

7. A joint prosthesis according to any preceding claim wherein the geometry of the segment is defined by a section revolved about a common axis of rotation based on a pair of part circles each centred on that axis of revolution and linked by a blending radius.

8. A joint prosthesis according to any one of claims 1 to 6 wherein the segment geometry is defined by a straight line that would form a cylindrical roller geometry if revolved around the axis.

9. A joint prosthesis according to any preceding claim wherein the segment valley is U-shaped with curvature extending in the base of the "U", or V-shaped with curvature extending in the base of the "V" provided that the valley is complementary to the upstanding ridge of the patellar component.

10. A joint prosthesis according to any one of claims 1 to 6 wherein the femoral groove geometry of the patellar component corresponds to femoral groove geometry of a total knee joint replacement prosthesis which also has a tibial component.

11. A joint prosthesis according to any preceding claim wherein the part concave surfaces of the patellar component are part spherical surfaces.

12. A joint prosthesis according to claim 11 wherein said part spherical surfaces are complementary to the part spherical femoral articular surfaces located at either side of the trochlear groove.

13. A joint prosthesis according to any preceding claim wherein the patellar component part concave depressions are formed to be complementary to co-operate in articulation with said part convex surfaces to permit articulation with a substantial part of these surfaces in mutual contact from extension to flexion.

14. A joint prosthesis according to claim 13 wherein during such articulation the said upstanding ridge contacts a major part of the said articular valley.

15. A joint prosthesis according to any preceding claim wherein the patellar component part concave depressions are located relatively proximally and extend relatively distally into a formation comprising an extension of the part concave bearing surfaces either side of the upstanding ridge.

16. A joint prosthesis according to claim 15 wherein said formation s flat, sloping or curved.

17. A joint prosthesis according to any preceding claim wherein the patellar component part concave depressions are spaced either side of the upstanding ridge.

18. A joint prosthesis according to any preceding claim wherein the patellar component distal end has a surface extending below the plane of the lowermost surface part of the said part concave surfaces, serving to facilitate gliding engagement during flexion, and which said distal end surface is chamfered or rounded off or radiused off.

19. A method of making a femoral component as defined in claim 1 and adapted for use in a joint prosthesis as claimed in any preceding claim, using investment casting or forging of an alloy into a bispherical shell, followed by grinding and polishing of the articular surfaces, the reverse side being so formed as to permit its subsequent fixation to the femur or for fixation onto a base which base can be subsequently affixed to the femur.

20. A method of making a patellar component as defined in claim 1 and adapted for use in a joint prosthesis as claimed in any preceding claim, by machining the end face of a rod using a circular bladed cutter to form each of the part spherical concavities, the cutter being moved distally at a constant height to form the distal part of the articular surfaces having a relatively constant cross section, the reverse surface being prepared for fixation to the patella or for fixation to a base adapted for attachment to the patella.

21. A system of apparatus adapted for use in securing a joint prosthesis as claimed in any one of claims 1 to 18 to a patient, the system comprising an intramedullary alignment rod, a cutting guide block having a slot suitable for an oscillating saw, a probe, a distal cutting guide having an aperture, a bur and collar for same, and a chamfering guide block with an angled slot or aperture for the bur.

22. A system of apparatus adapted for use in securing a joint prosthesis as claimed in any one of claims 1 to 18 to a patient, the system comprising a bur and a guide block having an aperture in which the bur is mountable, said block including other apertures through which drilling into bone can be effected, otherwise such drilling apertures being absent and the system including a further block containing such drilling apertures.