GB2181354A

GB2181354A - Improvements relating to orthopaedic implants

Info

Publication number: GB2181354A
Application number: GB08623617A
Authority: GB
Inventors: Michael Antony Tuke; Michael Alexander Reyk Freeman
Original assignee: FINSBURY
Current assignee: FINSBURY
Priority date: 1985-10-08
Filing date: 1986-10-01
Publication date: 1987-04-23
Also published as: GB8623617D0; GB2181354B; CH672060A5; GB8524823D0

Abstract

An orthopaedic implant, such as a hip implant, has at least one surface area, integral with the implant and adapted in use to come into contact with bone, having a finely patterned area. This area has a patterned conformation composed of a plurality of raised portions separated from each other by indented portions. The indented portions are of a width and depth to allow bone penetration thereinto.

Description

SPECIFICATION Improvements Relating to Orthopaedic Implants This invention relates to orthopaedic implants, and is especially concerned with implants which make direct contact with adjacent bone, such as hip and knee implants.

The use of implants to replace or strengthen all or part of bones in the human, or animal, body is well known, for example in hip and knee joint replacement operations. In all cases where the implant, usually of metal, contacts the remaining bone it is important that the material of the implant and the bone be compatible and it is highly desirable that the bone should graft to the implant to give stability and strength to the replaced joint and minimize later deterioration of the remaining bone.

Several methods have been proposed for assisting in the integration of the implant with the adjacent bone. A currently favoured method involves the application to a portion of the surface of the implant, such as a hip implant, of a coating of sintered balls of a metal the same as, or compatible with, the implant. The aim of this method is to give surface porosity so as to encourage bone grafting within the pores. However this method is subject to several disadvantages as follows.

Firstly, it is inherently difficult, and consequently expensive, to manufacture such implants with sufficient adhesion of the balls to the implant surface. The method of application involves considerable heating near the melting point of the materials involved which can adversely affect the properties of the material. Thus, of the commonly used materials for implant manufacture, cobalt' chromium alloys show adverse reactions on heat treatment while titanium alloys are prone to fatigue problems and the presence of even minor surface defects can have a considerable effect. It is not possible to control the surface geometry of the ballcoated product and it is thus impossible to provide implants of precise size and of controlled porosity.

There are advantages in providing a precision fit of the implant in the bone with minimal use of cementation techniques: however such a precision fit cannot be obtained with implants of imprecise surface geometry. Further, x-ray examinations have revealed that some of the surface balls can migrate from the implant and become embedded in the bone, which is clearly undesirable. The use of surface balls also increases greatly the total surface of exposed metal adjacent the bone, for example by as much as ten times. Thus the inherent toxicity problems between bone and implant are greatly magnified. Further the area over which the balls are applied is often greaterthan ideal: thus the balls have been applied to a comparatively large area of the implant surface with consequent problems in retrieval of the implant if that becomes necessary.

Alternatives to the use of sintered balls include the external fixing, for example by heat and welding, of mesh pads, for example of titanium, on portions of the implants, or the welding of rigid matrices to the sides of the implants. Such separately applied pads and matrices are easily damaged and subject to most of the problems outlined above. Implants have also been proposed having relatively large area shallow indented portions which can be packed with powdered bone in an effort to promote bone growth as shown for example in our copending European application No.

85302469.3. However these indented portions may not have more than a superficial effect in encouraging grafting of the bone to the implant, especially when treating older patients with diseased bones.

The present invention seeks to provide implants capable of providing in use a comparatively strong and controllable interlock between the implant and the adjacent bone so as to facilitate the use of precision fitting techniques.

According to this invention we provide an orthopaedic implant having at least one surface area, integral with the adjacent portion of the implant and adapted in use to contact bone, having a finely patterned conformation composed of a plurality of raised portions separated from each other by indented portions, the indented portions being of a width and depth to allow bone penetration therein to in use to promote an interference fit between the implant and adjacent bone in the region of the patterned area.

Preferably the indented portions have a depth which is equal to or greater than the spacing between the raised portions. The raised portions may conveniently have a diameter of 200 to 2000 jl, preferably 400 to 1000 p, with a range of 450 to 550 p, e.g. 500 cm being particularly preferred. The spacing between adjacent raised portions is usually 200 to 2000 p, preferably 250 to 1000 p, with a range of 700 to 800 p, e.g.7501l, being preferred.

Preferably the height of the raised portions is from 250 to 1500 p. The patterned area may be formed so that the raised portions stand proud of the surrounding surfaces with the bases of the indented portions at the level of the surrounding surfaces or, the raised portions may be at the level of the surrounding surfaces with the indented portions deeper than the surrounding surfaces. Of course conformations between these two extremes are possible, as is also a combination thereof.

The implant may be provided with one or more patterned surface areas dependent on the sites where it is desired to have a firm linkage between the bone and the implant. However it is advantageous to position these areas at easily accessible locations in case the linking needs to be severed for retrieval of the implant.

Any suitable method may be used to provide the patterned surface areas, two suitable methods being electrodeposition machining (sometimes called "spark erosion") or electrochemical machining. The implant itself may be of any suitable material, usually metal, the currently most commonly used materials being cobalt/chromium alloys or titanium alloys.

After the formation of the patterned surface areas, the outer surface of the implant, including the patterned areas, may be given a fine coating of titanium nitride, preferably by vapour deposition, to improve compatibility of the implant with the body into which it is to be placed. We have found that the titanium nitride coating gives the advantages of increased hardness (especially for titanium or titanium alloy implants) andlor reduction in the ion release rate of the basic implant material (especially for cobalt/chromium alloys).

The invention will now be described by way of example with reference to the accompanying drawings, wherein: Figure 1 is a side view of a hip implant in accordance with the invention; Figures 2 to 4 illustrate different positions of the raised portions with respect to the implant surface; and Figures 5 and 6 illustrate alternative conformations of raised portions.

As shown in Figure 1, a hip implant 1 comprises a stem portion 2,.which is substantially circular in cross section throughout its length but tapers at its lower end, and a projecting wedge shaped portion 4 of flattened cross section, terminating at its outer end in a neck 5 surmounted by ball head 6. In use the stem portion 2 is intended to fit snugly within an accurately reamed cavity within the femur while the wedge portion 4fits snugly within a machined slot in the upper thickened portion of the femur which remains after the ball of the femur has been removed. The ball 6 of the implant projects from the femur and forms the hip joint with the socket provided by a synthetically lined acetabulum.

It is desirable that at least the lower portion of the stem 2 should be a close fit within the reamed cavity in the femur but that it should not be substantially grafted to the surrounding bone in case there is a need to retrieve the implant. For this purpose, the lower portion of stem 2 below the line 8 may be conveniently provided with a highly polished surface.

On the other hand, it is desirable that the wedge portion 4 be a good interference fit with the surrounding bone and that maximum contact and grafting should occur in this region. To that end, wedge portion 4 is provided with a patterned area 9.

This patterned area has a finely separated series of raised portions 10 spaced by indented portions 12. If desired, an area 11, shown in dotted lines, may also be provided with a similarly patterned area.

The patterning may be provided by any suitable means after manufacture of the implant, such as electrodeposition machining or electrochemical machining and the patterning is integral with the implant itself. The patterning may provide raised portions 10 which rise above the normal surface level 13 as shown in Figure 2, thus leaving indented portions 12 therebetween, the bases of which lie at surface level 13. Alternatively as shown in Figure 3, the tops of raised portions 10 may lie at surface level 13 with the bases of indented portions 12 lying below surface level. Of course, intermediate conformations such as that shown in Figure 4 are possible.

The raised portions shown in Figures 1 to 4 have a cylindrical conformation. Many other conformations are possible. Thus the raised portions may be in the form of domes 14 as shown in Figure 5 or have a thinned stem 15 surrounded by a bulbous portion 16 as shown in Figure 6. The tops of the domes 14 or bulbous portions 16 may be flattened or pointed.

Raised portions of square rather than circular cross section are possible.

It is highly desirable that the height x of the raised portions 10 (i.e. the depth of the indented portions) should be as great or greater than the spacing therebetween. Of course the dimensions of the portions 10 and indented portions 12 will be selected to give maximum strength to the interference fit between the implant and surrounding bone. Thus suitable values for spacing y are from 200 to 2000 p, preferably 250 to 1000 p and more preferably 700 to 800 cm, while the diameterz of portion 10 is suitably 200 to 2000 p, preferably 400 to 1000 p and more preferably 450 to 550 p. Suitable values for the heightx of the raised portions are from 250 to 1500 y.

It will be appreciated that the use of an implant with a patterned surface area as shown allows accurate implantation with an interference fit without the need for cement. Because the patterning is machined into the material of the implant, the geometry of the surface is controllable and there is no danger of particles becoming detached from the implant. The machining can be carried out by methods involving the minimum of heat and so adverse effects on the implant material are avoided.

The raised portions 10, while designed to promote bone contact and grafting, do not increase the surface area of metal-bone contact to any larder extent than necessary. Further, it is possible to control the pore size (i.e. the spacing between raised portions) to give a minimum amount of metal to maximum space for bone penetration, as well as controlling the depth of bone penetration.

It is a simple matter to position the patterned areas accurately at locations which are easy to reach if the grafting has to be severed for retrieval of the implant.

The description above relates to a hip implant. It will be readily appreciated that the invention can be applied to other implants where implant-bone contact occurs, such as knee implants and plates used in other parts of the body such as elbows, shoulders or even teeth.

Claims

1. An orthopaedic implant having at least one surface area, integral with the adjacent portion of the implant and adapted, in use, to contact bone, having a finely patterned conformation composed of a plurality of raised portions separated from each other by indented portions, the indented portions being of a width and depth to allow bone penetration thereinto in use to promote an interference fit between the implant and adjacent bone in the region of the patterned area.

2. An implant according to claim 1 wherein the indented portions have a depth which is equal to or greater than the spacing between the raised portions.

3. An implant according to claim 1 or 2 wherein the raised portions have a diameter of 200 to 2000 p.

4. An implant according to claim 3 wherein the raised portions have a diameter of from 400 to 1000 p.

5. An implant according to claim 4wherein the raised portions have a diameter of from 450 to 550 p.

6. An implant according to any one of the preceding claims wherein the spacing between adjacent raised portions is 200 to 2000 p.

7. An implant according to claim 6 wherein said spacing is 250 to 1000 cm.

8. An implant according to claim 7 wherein said spacing is 700 to 800 p.

9. An implant according to any one of the preceding claims wherein the height of the raised portions is from 250 to 1500 cm.

10. An implant according to any one of the preceding claims wherein the implant is of metal and the at least one patterned surface area has been formed by electrodeposition machining or electrochemical machining.

11. An implant according to any one of the preceding claims wherein at least a portion of the outer surface of the implant, including the at least one patterned area has been given a coating of titanium nitride.

12. An implant according to any one of the preceding claims intended for implantation in a femur and comprising a stem and a flattened wedge portion surmounted by a ball head projecting therefrom.

13. An implant according to claim 12 wherein the flattened wedge portion is provided with a patterned surface area.

14. A hip implant substantially as shown in the accompanying drawing and described herein with reference thereto.