GB2386558A

GB2386558A - Two-part replacement patella prosthesis

Info

Publication number: GB2386558A
Application number: GB0206514A
Authority: GB
Inventors: Sheo B Tibrewal
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-03-20
Filing date: 2002-03-20
Publication date: 2003-09-24
Anticipated expiration: 2022-03-20
Also published as: GB0206514D0; GB2386558B

Abstract

A two-part replacement patella prosthesis comprises a patellar component and a femoral component. The patellar component has an oval articulating face 1 which is divided by a medial ridge 2, slightly off-centre of the oval face 1. The medial ridge 2 separates a slightly convex medial facet 3 and a slightly concave lateral facet 4 of the face 1. The femoral component (6) caps the natural articulating surfaces of the femur (7) and the femoral component (6) is provided with a medial facet (8) and a lateral facet (9). In this case, the medial facet (8) is slightly concave and the lateral facet (9) is slightly convex so as to complement the oppositely curved facets 3, 4 of the patellar component. In another aspect, a surgical apparatus for the preparation of a knee joint for patella replacement prosthesis comprising a clamp consisting of a pair of limbs (31, 32, figure 2), a first limb (31) having a cradle portion (33) for seating a patella (P) and a second limb (32) incorporating a foot (34) for securing a seated patella (P) in position for cutting, the two limbs (31, 32) being connected and held at a substantially parallel separation by means of one or more clamping screws (35) located distal from the cradle portion (33) and foot (34), and a resection guide (36) consisting of a reference pointer (37) and an adjustment screw (38) for adjusting the position of the resection guide (36) relative to the seated patella (P).

Description

APPARATUS FOR PATELLA REPLACEMENT

This invention relates to surgical apparatus for use in the repair and replacement of damaged tissues in the knee of a human or animal body. In particular, the invention provides a patella replacement prosthesis and a method and apparatus for the surgical implantation thereof.

Studies have shown that the most frequent reasons for re-operation after knee replacement surgery are complications related to the patella component, including; loosening, subluxation, dislocation, fracture, material wear, soft tissue impingement and disruption. Various attempts have been made to improve or extend the life of the patella in such replacements, many of which are based on " two part " patella designs.

These prior art designs are generally focussed on the rotational and medio-

lateral movements of the patella and are configured to provide contact at a single point or line. It is postulated that this may result in wear due to the inconsistent engagement of the patella with the femoral component. The inventors have identified other possible failings of these prior art arrangements as follows:

1) Patella thickness is not accurately restored during replacement; 2) There is no reproducibility of planar resection of the patella so that patella alignment cannot be accurately restored; 3) Appropriate congruous articulation in the patella facet of the femoral component is not provided; 4) There is no quantification of the degree of medio-lateral movements of the patellae. Complications related to patella or the extensor mechanism of the knee are the most frequent reasons for reoperation after an otherwise successful knee replacement.

Various procedures are adopted by surgeons and some choose not to resurface the

patella in order to avoid these problems, others selectively resurface the patella.

Problems related to patella replacement include loosening, subluxation, dislocation, fracture, polyethylene wear, soft tissue impingement, extensor mechanism disruption and avascular necrosis. Recent follow-up studies have shown that resurfacing or replacement of a patella results in a lower incidence of such problems but the incidence is still unacceptably high.

Particular problems incurred with patella replacement include; 1) The relationship between articular surfaces change with flexion 2) Translational movements occur in medial, lateral, sagittal and coronal directions 3) Stress distribution is dependent on the joint geometry and muscle strength 4) Normal function of the extensor mechanism depends on multiple factors such as; a) stable articular geometry b) correct interplay of muscles and passive soft tissue restraints that act on the patella for it to track normally.

The inventors have concluded that the following factors are of particular importance in patella replacement; a) the highest point (or vertical ridge) of the patella and the patella thickness should be restored for it to distribute load evenly across the patella femoral joint. b) the patellar component should track congruously in the trochlear groove of the femoral component to restore natural tracking of the knee. (This should enable restoration of normal femoral function of the knee extensor mechanism.

c) a congruous articular surface at each arc of movement between joint components is desirable.

Existing prostheses and surgical apparatus have been found not to permit reproducible restoration of patellar thickness or patellar alignment between a prosthesis and the natural patella.

It is an object of the present invention to provide a patella component which goes some way to overcoming the identified deficiencies of the prior art components.

It is a further object of the invention to provide an apparatus to assist the surgeon in accurately positioning a patella component for optimal performance.

Patella Prosthesis In a first aspect, the invention provides a two part patella replacement prosthesis for implantation in a human or animal knee joint comprising a patella component and a femoral component, the patella component having articulating surfaces congruous with the patellar articulating surfaces of the femoral component, the articulating surfaces extending over a defined two dimensional area and not a point or line of contact.

The articulating surfaces preferably are curved surfaces extending over a substantial part of an articulating face of the patellar component. Optionally the articulating surfaces comprise a medial and a lateral facet separated by a medial ridge The lateral facet is desirably slightly concave and the medial facet desirable slightly convex. The articulating facets are desirably positioned off-centre of the articulating face towards the upper pole of the patella component The patella component desirably has a substantially oval cross section in the plane of the articulating face. Preferably, the component comprises a medialised ridge in the vertical plane (when the implant is in position and the limb in the standing position) and a lateral and medial articulating surface positioned either side of the medialised ridge. Each of the medial and lateral articulating surfaces desirably have a substantially equal radius of curvature in the two planes orthogonal to the vertical plane. The femoral component is provided with an articulating surface shaped to

complement the contours of the patella femoral articulating surface. For instance, where the medial facet of the patella component is slightly convex, the complementary facet on the femoral component would be slightly concave and of a similar radius of curvature to the medial facet of the femoral component. Optionally, the femoral component may also be provided with a high posterior arc radius on the posterior condyle this may provide improved knee flexion in the joint after surgery. The characteristics of the femoral component are otherwise as known in the prior art.

The patella component is desirably comprised of a big-compatible polymeric material such as polyethylene (typically a high density polyethylene). Other suitable materials conventionally used in orthopacdic and other medical implants will no doubt occur to the skilled addressee and do not depart from the scope of the invention as claimed herein.

Surgical Apparatus In another aspect, the invention provides a surgical apparatus for the preparation of a knee joint for a patella replacement prosthesis implantation comprising; a clamp, consisting of a pair of limbs, a first limb having a cradle portion for seating a patella and a second limb incorporating a foot for securing a seated patella in position in the cradle for cutting, the two limbs are connected and held at a substantially parallel separation by means of one or more clamping screws located distal from the cradle portion and foot and a resection guide consisting of a reference pointer and an adjustment screw for adjusting the position of the resection guide relative to a seated patella. A particular advantage of the apparatus is that, by means of the placement of the guide pin perpendicular to the surface of the patella and the resection guide, it enables the surgeon to perform an accurately measured 90 degree planar cut which is reproducible. Preferably, the foot is provided with an aperture for receiving a guide wire or

pin for locating the highest point of the patella and/or determining the dimensions of the patella. The guide wire or pin may, optionally, be calibrated for ease of taking measurements. Optionally or in the alternative, the resection guide may include a calibrated adjustment screw for pre-selecting a size of patella (or portion of patella) to be resected. The resection guide is optionally slotted through a slot provided in one of the two limbs and may be repositionable along the length of the limbs to accommodate different sized patellae or preferences of a surgeon performing a procedure using the apparatus. The resection guide may be provided separately from the clamp for use therewith. In a preferred implantation method, the patella is first averted and marginal osteophytes are excised to contour the patella to its anatomical shape. The highest point of the vertical medial ridge of the patella is identified and marked, this provides a reference point for measuring the maximum thickness of the patella. In accordance with another aspect of the invention, a calibrated guide pin or wire is introduced into the patella substantially orthogonally to the marked surface and through the centre of the mark, to the opposing surface of the patella. The patella thickness may then be read directly from the scale provided on the pin. The foot of a clamp in accordance with the surgical apparatus of the invention (and which includes an aperture for receiving the guide pin or wire) is positioned around the pin. The cradle portion of the first limb of the clamping apparatus is positioned adjacent the surface of the patella opposing the marked surface. The two limbs are then clamped into position by means of the clamping screw.

Once the patella is securely clamped between the limbs, the resection guide is positioned such that the reference pointer is along-side the patella. The desired position of the reference pointer may be obtained by adjustment of the adjustment screw with reference to calibrations thereon and/or calibration provided on the guide pin or wire.

Once the reference pointer is in position, a surgeon uses a suitably configured blade to cut the patella along a guide line provided by the reference pointer. Implantation of a patella component can then be carried out in accordance with known practices.

Where the patella component includes a peg for securing to the remaining portion of the patella, a locating hole can be drilled at the highest point on the ridge.

A drill bit can conveniently be inserted through the aperture in the foot of the second limb of the clamp after removal of the guide pin.

The apparatus and method described above can be utilised to insert various types of patella component known in the prior art. It enables a surgeon to remove an

amount of bone from the patella which is substantially equal in size to the patella component of the prosthesis. This ensures that the tissues of the treated knee joint are correctly tensioned ensuring that, post operatively, quadriceps loading and patella loading are maintained as before the operation. The patella and patella component may be positioned accurately in the anterior /posterior and mediolateral planes to reproduce natural tracking with re-aligned tibio-femoral articulations.

The apparatus may also be used in the centralising/alignment of the femoral component relative to the patella highest central point. This may be achieved by advancing the guide wire or pin in an ante-grade fashion in the femoral trochlea with the knee in full extension after resection of the patella. As for the patella, the resection guide can be aligned with reference to the guide pin or wire thereby enabling more accurate and reproducible cuts to be made.

For the purposes of exemplification, some embodiments of the invention will now be further described with reference to the following figures in which: Figure 1 shows an embodiment of a patella replacement prosthesis in accordance with the present invention: Figure 2 shows a surgical apparatus for the preparation of a knee joint for a patella replacement prosthesis implantation in accordance with the invention.

Figure 3 shows a femoral component of an embodiment of a patella replacement

prosthesis in accordance with the present invention, the articulating surface being marked. As can be seen in Figure 1, a two part patella replacement prosthesis in accordance with the invention comprises a patella component (as shown in a) and b)) and a femoral component (as shown in c)). Figure la) shows the patella component from a perspective of the articulating face. Figure lb) shows a front face of the patellar component which, when implanted would sit in the place of the natural patella. Figure lo) shows a congruous femoral component for the prosthesis.

As shown in Figure la) and b), the patella component has a generally oval articulating face, 1. The face is divided by a medial ridge 2 slightly off centre of the oval face 1. The medial ridge 2 separates a medial 3 and lateral 4 facet of the articulating face 1 of the component. The medial facet 3 has a slightly convex radial curvature relative to the plane of the articulating face 1, and the lateral facet 4 has a slightly concave radial curvature. A slightly curved and generally triangularly shaped front face 5 extends forwardly of the articulating face, representative of a natural patella. As can be seen in Figure 1 c), the femoral component 6 essentially caps the natural articulating surfaces of the femur 7. As with the patella component, the femoral component 6 is provided with a medial 8 and a lateral 9 articulating facet. In this case, the medial facet 8 is slightly concave and the lateral facet 9 is slightly convex, so as to complement the oppositely curved facets 3,4 of the patella component.

Figure 2 shows a surgical apparatus in accordance with the invention. As can be seen, the apparatus comprises a clamp, consisting of a pair of limbs 31, 32. First limb 31 has a cradle portion 33 for seating a patella P. Second limb 32 has a foot 34 for securing a seated patella P in position in the cradle 33 for cutting. The two limbs 31,32 are connected and held at a substantially parallel separation by means of a clamping screw 35 located distal from the cradle portion 33 and foot 34. A resection

guide 36 consisting of a reference pointer 37 and an adjustment screw 38 for adjusting the position of the resection guide 36 relative to the seated patella P is also provided.

The foot 34 has an aperture 39 passing through it for receipt of a guide pin or wire 40. The guide pin may be calibrated by a scale 41. A support 42 is optionally provided extending from the first limb 31 through an aperture in the second limb 32 so as to prevent any rotation of the second limb 32 about the clamping screw 35.

As seen in Figure 3 the femoral component, here viewed looking from the knee when the limb is fully extended. The articulating surface with which the patella component interfaces is shown by shaded area 10.

It is to be understood that the figures are schematic and the shapes of the articulation facets are exaggerated. In practice they would likely be more smoothly integrated with the contours of the components. Other minor variations to the shape and configuration of the embodiments as shown will no doubt occur to the skilled addressee without departing from the scope of the invention as claimed.

Claims

1. A two part patella replacement prosthesis for implantation in a human or animal knee joint comprising; a patella component and a femoral component, the patella component having articulating surfaces congruous with the patellar articulating surfaces of the femoral component, the articulating surfaces extending over a defined two dimensional area and not a point or line of contact. 2. A two part patella replacement prosthesis as claimed in claim 1 wherein the articulating surfaces are curved surfaces extending over a substantial part of an articulating face of the patellar component and femoral component.

3. A two part patella replacement prosthesis as claimed in claim 1 or claim 2 wherein the articulating surfaces of the patella component comprise a medial and a lateral facet separated by a medial ridge.

4. A two part patella replacement prosthesis as claimed in claim 3 wherein the lateral facet is slightly concave and the medial facet is slightly convex.

5. A two part patella replacement prosthesis as claimed in claim 3 or claim 4 wherein the medial and lateral facets are positioned off-centre of the articulating face of the patella component towards the upper pole of the patella component. 6. A two part patella replacement prosthesis as claimed in any of claims 3 to 5 wherein each of the medial and lateral facets have a substantially equal radius of curvature in the two planes orthogonal to the vertical plane of the medial ridge. 7. A two part patella replacement prosthesis as claimed in any preceding claim wherein the patella component has a substantially oval cross section in the plane

of the articulating face.

8. A two part patella replacement prosthesis as claimed in any preceding claim wherein the femoral component is provided with an articulating face shaped to complement the contours of the patella articulating face.

9. A two part patella replacement prosthesis as claimed in any preceding claim the femoral component may also be provided with a high posterior arc radius on the posterior condyle 10. A two part patella replacement prosthesis as claimed in any preceding claim wherein the patella component is comprised of a big-compatible polymeric material. A two part patella replacement prosthesis as claimed in claim 9 wherein the big- compatible polymeric material is a polyethylene.

12. A surgical apparatus for the preparation of a knee joint for a patella replacement prosthesis implantation comprising; a clamp consisting of a pair of limbs, a first limb having a cradle portion for seating a patella and a second limb incorporating a foot for securing a seated patella in position in the cradle for cutting, the two limbs being connected and held at a substantially parallel separation by means of one or more clamping screws located distal from the cradle portion and foot; and a resection guide consisting of a reference pointer and an adjustment screw for adjusting the position of the resection guide relative to a seated patella.

13. A surgical apparatus as claimed in claim 12 wherein the foot is provided with an aperture for receiving a guide wire or pin for locating the highest point of the patella and/or determining the dimensions of the patella.

14. A surgical apparatus as claimed in claim 13 further comprising a guide wire or

pin. 15. A surgical apparatus as claimed in claiml4 wherein the guide wire or pin is provided with a calibrated scale.

16. A surgical apparatus as claimed in any of claims 12 to 15 wherein the resection guide includes a calibrated adjustment screw for pre-selecting a size of patella (or portion of patella) to be resected.

17. A surgical apparatus as claimed in any of claims 12 to 16 wherein the resection guide is slotted through a slot provided in one of the two limbs and is repositionable along the length of the limbs by sliding in the slot.

18. A surgical apparatus as claimed in any of claims 12 to 17 further comprising a blade for cutting a seated patella along a guideline defined by the reference pointer of the resection guide.

19. A surgical apparatus as claimed in any of claims 12 to 18 further comprising a two part patella replacement prosthesis as claimed in any of claims 1 to 11.

20. A two part patella replacement prosthesis substantially as described herein with reference to the Figure 1.

21. A surgical apparatus substantially as described herein with reference to Figures 1 and 2.

Amendments to the claims have been filed as follows 1. A two part patella replacement prosthesis for implantation in a human or animal knee joint comprising; a patella component and a femoral component, the patella component having articulating surfaces congruous with the patellar articulating surfaces of the femoral component, the articulating surfaces extending over a defined two dimensional area and not a point or line of contact, wherein the articulating surfaces of the patella component comprise a medial and a lateral facet separated by a medial ridge and the lateral facet is slightly concave and the medial facet is slightly convex.

2. A two part patella replacement prosthesis as claimed in claim 1 wherein the articulating surfaces are curved surfaces extending over a substantial part of an articulating face of the patellar component and femoral component.

3. two part patella replacement prosthesis as claimed in claim 1 or 2 wherein the medial and lateral facets are positioned off-centre of the articulating face of the patella component towards the upper pole of the patella component.

4. A two part patella replacement prosthesis as claimed in any of claims 1 to 3 wherein each of the medial and lateral facets have a substantially equal radius of curvature in the two planes orthogonal to the vertical plane of the medial ridge. 5. A two part patella replacement prosthesis as claimed in any preceding claim wherein the patella component has a substantially oval cross section in the plane of the articulating face.

6. A two part patella replacement prosthesis as claimed in any preceding claim wherein the femoral component is provided with an articulating face shaped to complement the contours of the patella articulating face.

7. A two part patella replacement prosthesis as claimed in any preceding claim the femoral component may also be provided with a high posterior arc radius on the posterior condyle 8. A two part patella replacement prosthesis as claimed in any preceding claim wherein the patella component is comprised of a big-compatible polymeric material. 9. A two part patella replacement prosthesis as claimed in claim 8 wherein the bigcompatible polymeric material is a polyethylene.

10. A surgical apparatus for the preparation of a knee joint for a patella replacement prosthesis implantation comprising, a clamp consisting of a pair of limbs, a first limb having a cradle portion for seating a patella and a second limb incorporating a foot for securing a seated patella in position in the cradle for cutting, the two limbs being connected and held at a substantially parallel separation by means of one or more clamping screws located distal from the cradle portion and foot; and a resection guide consisting of a reference pointer and an adjustment screw for adjusting the position of the resection guide relative to a seated patella.

11. A surgical apparatus as claimed in claim 10 wherein the foot is provided with an aperture for receiving a guide wire or pin for locating the highest point of the patella and/or determining the dimensions of the patella.

12. A surgical apparatus as claimed in claim 11 further comprising a guide wire or pin. 13. A surgical apparatus as claimed in claim 12 wherein the guide wire or pin is provided with a calibrated scale.

14. A surgical apparatus as claimed in any of claims 10 to 13 wherein the resection guide includes a calibrated adjustment screw for pre-selecting a size of patella (or portion of patella) to be resected.

15. A surgical apparatus as claimed in any of claims 10 to 14 wherein the resection guide is slotted through a slot provided in one of the two limbs and is repositionable along the length of the limbs by sliding in the slot.

16. A surgical apparatus as claimed in any of claims 10 to 15 further comprising a blade for cutting a seated patella along a guideline defined by the reference pointer of the resection guide.

17. A surgical apparatus as claimed in any of claims 10 to 16 further comprising a two part patella replacement prosthesis as claimed in any of claims 1 to 11.

18. A two part patella replacement prosthesis substantially as described herein with reference to the Figure 1.

19. A surgical apparatus substantially as described herein with reference to Figures 1 and 2.