GB2178320A

GB2178320A - Hip implant

Info

Publication number: GB2178320A
Application number: GB08519079A
Authority: GB
Inventors: Michael Alexander Reyk Freeman; George Cremore
Original assignee: FINSBURY
Current assignee: FINSBURY
Priority date: 1985-07-29
Filing date: 1985-07-29
Publication date: 1987-02-11
Also published as: GB2178320B; GB8519079D0

Abstract

A hip implant has a substantially linear stem of circular cross section from which medially extends a wedge- shaped portion surmounted by a ball head. That end of the stem which will be lowermost in use has a surface of lower coefficient of friction than the surface of that end of the stem which will be uppermost in use.

Description

SPECIFICATION Hip implant This invention relates to artificial bone implants and is particularly concerned with implants for replacing hip joints.

The efficient functioning of the hip joints is extremely important to the well being and mobility of the human body. Each hip joint is constituted by the upper portion of the upper leg bone (femur) which terminates in an offset bony neck surmounted by a ball-headed portion which rotates within a socket (acetabulum) in the hip bone.

Diseases such as rheumatoid- and osteo-arthritis can cause erosion of the cartilage lining the acetabulum so that the ball of the femur and hip bone rub together causing pain and further erosion.

Bone erosion causes the bones themselves to attempt to compensate and reshape, thus giving a misshapen joint which may well cease to function altogether.

The replacement of the hip joint by an artificial implant is widely practised but the implants conventionally used can suffer from a number of disadvantages.

Thus, conventional hip implants are usually inserted by resecting the neck of the femur and reaming a comparatively large cavity down the femur to receive a bow-shaped implant surmounted by a ball which is then cemented in place using, for example, an acrylic filler material. The implant is bow-shaped to correspond to the angle which the intact femur ball head makes with the downwardly extending stem of the femur. Examples of such hip implants are shown in GB Patent Specifications 1409053 and 1409054.

The bottom portion of the implant, while tapering, is conventionally of non-circular flattened crosssection so as to resist rotational forces within the reamed cavity. Such non-circular cross sections are subject to uneven loading so that, for example, for a substantially rectangular cross-section, the corner position would bear increased load causing cracking of the surrounding cement.

Furthermore various surface finishings for the implant have been proposed in an attempt to increase the adhesion between the implant and the surrounding bone and/or sheath of cement. Such surface finishings may comprise the provision of grooves as shown in US Patent No. 4430761 or the use of porous regions on the surface of the implant as shown in US Patent No. 3605123. Such implants, if correctly cemented can be comparatively efficient but, if not correctly cemented, or after a long period of use, the cement may work loose, thus allowing movement of the implant and causing bone erosion.

Bone erosion can lead to tissue reactions which themselves can lead to further bone destruction. As a result, the whole joint can get out of alignment and/or damaged beyond repair.

Furthermore the adhesion between the cement and the implant, makes removal and replacement of the implant extremely difficult, as it is necessary to sever the bonding before the implant can be withdrawn.

Ourcopending European Patent Application No.

85302469.3 describes an improved hip implant which has a substantially linear stem portion and, at least in the region of the lower end, a substantially circular cross section. A wedge-shaped portion extends medially from the stem at or near the upper end and is surmounded by a ball head. It is possible to machine an accurate cavityfor such an implant so that one can dispense with cement and give an implant which can be readily withdrawn.

Furthermore the shape of the implant allows one to retain a large portion of the long neck of the femur and to ensure that both compression and torsional loads on the implant are well distributed.

Nevertheless, there are many occasions when it is desirable to incorporate cement around the hip implant. Thus, for example, in some patients, particularly those suffering from rheumatoid arthritis, the femur has become spongy and does not have sufficient strength in itself to bear the necessary load. The insertion of a sheath of cement can therefore act as a strengthening agent by bonding with the adjacent bone to provide a mantle of bone plus cement on the outside and cement alone on the inside to receive the implant.

When using an implant with a stem of circular cross section such a mantle may grip the implant and make its subsequent removal and replacement difficult.

The present invention seeks to overcome this problem.

According to this invention we provide a hip implant comprising a stem portion, with a substantially linear axis and a substantially circular cross section substantially throughout its length and a wedge-shaped portion surmounted by a ball extending medially from the stem portion at or near that end of the stem which will be uppermost in use, that end of the stem which will be lowermost in use having been provided with a surface with a lower coefficient of friction than the surface of the stem at or near that end of the stem which will be uppermost in use. The surface with a lower coefficient of friction may be provided by any suitable means such as by giving the material of the insert a highly polished surface and/or by providing a non stick coating such as polytetrafluoroethylene (ptfe) on the material of the insert.

Preferably that end of the stem which will be uppermost in use has a roughened surface; for example it may have been textured, coarse blasted or rendered porous.

One form of the invention will now be described with reference to the accompanying drawings, wherein Figure 1 is a side view of an implant according to the invention; Figure 2 is a section on line X-X of Figure 1; Figure 3 is a partial section on line Y-Y of Figure 1.

The implant 1 shown in the drawings is of an acceptable material for introduction to the human body such as cobalt-chromium alloy or titanium alloy, optionally given a corrosion resistant coating of titanium nitride. It may be all of one material or may have a ball head portion of a different material such as a ceramic. The implant 1 comprises a stem 2 which has a substantially linear axis and a substantially circular cross-section as can be seen from Figure 2. The stem 2 tapers towards its lower end 3, the extent of the taper or even absence of any taper depending on individual needs. Spaced from the tapered portion 3, a flattened wedge portion 4 extends medially (i.e. in use, towards the middle of the body) from the stem 2 and is surmounted by a ball head 5.The stem 2 extends beyond the wedge portion 4 to form a projecting end 6 for lateral support and provided with an aperture 7, which, in use, can receive a hook or like means for removing the implant from the bone if this becomes necessary. Wedge portion 4 is provided with indentations 8 which, when the implant is in place, can assist in encouraging bone interlock, e.g. by grafting of the adjacent bone to the implant 1.

Wedge portion 4 may also be gradually reduced in thickness from the proximal to the distal end to assist interference interlock with the bone, especially when the wedge portion has a textured surface.

The provision of a separable ball head may be used to adjust neck length for individual needs.

Thus ball heads incorporating varying neck lengths can be fitted to standard stems with shortened wedge portions.

The lower surface of the implant extending below the line 9 is highly polished while the surface above line 9 can be modified as desired. Thus it may, for example, be plain, textured or coarse blasted, or coated with an inert metal or other material or certain areas of the implant may be coated with material to give a porous surface. The surface above line 9 is rough in comparison with that below line 9.

Instead of being polished, the surface below line 9 can be given a coating of a non stick material such as ptfe. The extent of the polished or coated portion of the stem can vary but it is preferred that the polished or coated portion extend sufficiently far up the stem so that, if it is desired to remove the implant from a patient, it is only necessary to sever any grafting or cement to a comparatively shallow depth which can be readily reached from the top of the implant with a long thin saw blade. In particular the tapered lower region of the implant must be polished as it would be difficult to cleanly sever any grafting in that area to allow implant removal.

In use the femur of a patient has the ball removed while retaining as much as possible of the bony neck of the femur. An aperture for the implant stem and a slotforthe wedge portion are machined in the femur. The aperture is sized to a depth and diameter slightly greater than the dimensions of the implant, while the dimensions of the slot are shown to be a snug fit to encourage bone interlock and to provide prosthesis stability in the neck area. Suitable cement, in fluid form, is injected into the aperture and then the implant inserted which pressurises the cement into any gaps in the surrounding bone. In this way a strengthening mantle of bone plus cement is formed.The polished surface of the lower part of the stem prevents the bonding of the cement to the insert which can then be removed when desired in conjunction with any necessary cutting towards the top of the stem and around the wedge portion.

The implant of the invention is particularly useful for implementation techniques employing cement in conjunction with spacer devices to ensure that the implant is centrally positioned in the cement sheath.

In such techniques a stem-centralising lower spacer tip may be positioned at the bottom of the femur cavity to prevent unwanted cement going down the femur canal and to allow pressurisation into position of the implant and cement. Alternatively or in addition a temporary annular spacer, clipped around the upper part of the insert, may be employed to centralise the insert while liquid cement is injected into the surrounding cavity. The spacer may be removed before the cement has completely set.

It will however be appreciated that the construction of implant described above is equally suitable for use without cement.

Claims

1. A hip implant comprising a stem portion, with a substantially linear axis and a substantially circular cross section substantially throughout its length, and a wedge-shaped portion surmounted by a ball extending medially from the stem portion at or near that end of the stem which will be uppermost in use, that end of the stem which will be lowermost in use having been provided with a surface with a lower coefficient of friction than the surface of the stem at or near that end of the stem which will be uppermost in use.

2. A hip implant according to claim 1, wherein the surface with a lower coefficient of friction is a highly polished surface.

3. A hip implant according to claim 1, wherein the surface with a lower coefficient of friction is provided by a non-stick coating on the material of the implant.

4. A hip implant according to claim 1 wherein the non-stick coating is of polytetrafluoroethylene.

5. A hip implant according to any one of claims 1 to 4 wherein the surface of the stem which will be uppermost in use is roughened.

6. A hip implant according to claim 5, wherein the roughening has been effected by texturing, coarse blasting or rendering porous the material of the implant.

7. A hip implant according to any one of claims 1 to 6, wherein at least a portion of the stem portion tapers towards that end which will be lowermost in use.

8. A hip implant according to claim 7, wherein at least the tapered portion of the stem has the surface with a lower coefficient of friction.

9. A hip implant according to any one ofthe preceding claims wherein the surface with a lower coefficient of friction extends from a circumferential line which in use is below the wedge-shaped portion.

10. A hip implant according to any one of the preceding claims wherein the wedge-shaped portion is provided with a surface which will encourage interlock with adjacent bone.

11. A hip implant according to any one of the preceding claims wherein the ball head is separable 5 from the wedge-portion.

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