GB2246957A

GB2246957A - Artificial hip joint

Info

Publication number: GB2246957A
Application number: GB9116050A
Authority: GB
Inventors: Michael Alexander Reyk Freeman
Original assignee: FINSBURY
Current assignee: FINSBURY
Priority date: 1990-07-24
Filing date: 1991-07-24
Publication date: 1992-02-19
Anticipated expiration: 2011-07-24
Also published as: GB2246957B; GB9016232D0; GB9116050D0

Abstract

A peg or an array of pegs is embedded in a patient's acetabulum with a bearing surface for a ball head of a femoral implant. In the case of an array of pegs the bearing surfaces of the pegs define an envelope whose shape matches that of the femoral ball of the femoral implant. The peg may have a concave bearing surface. <IMAGE>

Description

ARTIFICIAL HIP JOINT This invention relates to an artificial hip joint.

The human hip joint is constituted by the upper portion of the upper leg bone (the femur) which terminates in an offset bony neck surmounted by a ball-headed portion which rotates within a socket (the acetabulum) in the pelvis. Diseases such as rheumatoid-and osteo-arthritis can cause erosion of the cartilage lining the acetabulum so that the ball of the femur and the pelvis rub together causing pain and further erosion. Bone erosion causes the bone cells to attempt to compensate and to reshape, thus giving a mis-shapen joint which may well cease to function altogether.

The replacement of the hip joint by an artificial implant is now an extremely common orthopaedic surgical procedure. Although in the case of a fracture of the femur the orthopaedic surgeon may decide to implant only a femoral implant, it is often necessary or desirable to fit the patient with an artificial acetabulum, particularly when the patient suffers from rheumatoid- or osteo-arthritis.

A variety of designs of artificial acetabulum have been proposed. Typically these comprise a cup-shaped member made of a plastics material, such as high density polyethylene, or a metal cup-shaped member with a lining of plastics material. Such forms of artificial acetabulum may be secured in place during the surgical operation by means of a cement. Alternatively they may be designed to be a press fit in the patient's own acetabulum. Other designs have external screw threads and are forced into place in the pelvis by surgeon using a rotational movement. Still a further method of securing an artificial acetabulum in place uses screws.

In course of time all cup shaped artificial acetabula sink into the natural bone due to the loading imposed upon the hip joint as a result of the patient walking or standing. Some times the amount of sinkage is only about 0.5 mm but it can be much more. Since it is the superior bone of the hip into which the sinkage occurs, the result is that the opening of the cup changes its angle relative to the vertical. In particular the cup turns outwardly, i.e. it rotates and moves its centre laterally.

Any such lateral movement, or indeed any loosening of the artificial acetabulum, accelerates failure of the joint by causing further loosening of the acetabulum. In addition debris formation and tissue reaction can hasten deterioration of the proper action of the hip joint.

When a surgeon decides to implant only a femoral prosthesis, then the ball head of the femoral prosthesis bears in the patient's natural acetabulum. In this case the size of the ball head corresponds to the size of the bony ball head that it is replacing. Typically the ball head of such a femoral implant has a diameter of between about 45 mm and about 65 mm. On the other hand, if the surgeon decides to implant an artificial acetabulum, then a smaller ball head is normally used on the femoral prosthesis since the external diameter of the artificial acetabulum normally approximates that of the natural acetabulum in the bone and hence reduces the space available for the ball head on the femoral implant, this space being further reduced by any plastics liner for the artificial acetabulum.Typically the ball head of a femoral prosthesis, when an artificial acetabulum is also implanted, is in the range of from about 22 mm to about 35 mm.

US-A-3840904 discloses various forms of acetabular cup prosthesis. Figure 13 thereof discloses use of a screw to secure the acetabular cup prosthesis in the patient's hip bone. According to column 3, line 46: "A cluster of screws could be used, also".

EP-A-0149425 discloses an acetabular prosthesis 1 with an offset peg 3. The prosthesis is coated with hydroxyapatite.

EP-A-0237751 discloses an artificial acetabulum made of titanium or a titanium alloy with an external screw thread 6 and a series of circular rings 7 in a polar region thereof.

US-A-4044403 discloses in Figure 7 an acetabular prosthesis comprising a shield or casing member 68 having an elongated stem 70 with a plurality of serrated projections 72 designed to secure the casing 68 to a corresponding anatomical structure.

DE-A-2911754 teaches a hemispherical acetabular prosthesis with a series of ribs on its outer surface.

Various forms of artificial acetabulum with a hemispherical outer surface and projections therefrom have been marketed by Waldemar Link GmbH & Co., of Barkhausenweg 10, D-2000 Hamburg 63, Federal Republic of Germany, under the Trade Mark "LINK SPII LUBINUS TOTAL HIP SYSTEM". An advertisement thereof was published, for example, as an insert to "The Journal of Bone and Joint Surgery", British Volume 68-B (1986), Number 3 (May) following page ii.

Another proposal with external fins permitting cementless immediate mechanical skeletal fixation by spherical thread design is that sold as "The S-Rom Acetabular System" by Joint Medical Products Corporation"; an advertisement thereof appeared in "The Journal of Bone and Joint Surgery", Volume 68-A, April 1986 at page 91.

A hip cup for a hip joint acetabular prosthesis with a liner is disclosed in US-A-4715859.

US-A-4666450 teaches another design of acetabular cup assembly with a socket insert that has a central fastening tab which locks into a support shell.

In EP-A-0289192 there is disclosed an artificial acetabulum which has an outer cup 2 and an inner liner 10.

The outer cup 2 has a hollow locating peg 6 projecting from its base. A series of screw holes 13 are provided each for reception of a bone fixation screw.

The present invention seeks to provide an improved form of artificial hip joint. In particular it seeks to provide a form of artificial hip joint in which the problems caused by sinkage are minimised or obviated. It further seeks to provide a novel form of artificial acetabulum.

According to the present invention there is provided a kit of parts for forming an artificial acetabular surface comprising a plurality of pegs of selected length for implantation in an acetabulum of a patient, each peg having an insertion end and at the opposite end thereto a bearing means adapted to define, upon implantation in an array in the natural acetabulum of the patient together with the corresponding bearing means of the other implanted pegs of the array, a bearing envelope for a ball head of a femoral implant. Preferably at least one peg of the kit has a different length from that of at least one other peg of the kit.

It will be appreciated by those skilled in the art that the invention provides, in place of the conventional acetabular cup prosthesis, a plurality of pegs providing a series of discrete, individual bearing surfaces which are typically implanted in an array in the acetabulum of a patient.

The invention further provides a hip replacement joint comprising (a) a femoral implant comprising a stem for insertion in a prepared cavity in a femur of a patient and a ball head, and (b) an acetabular implant means comprising a plurality of pegs of selected length for implantation in an acetabulum of the patient, each peg having an insertion end and at the opposite end thereto a bearing means adapted to define, upon implantation in an array in the natural acetabulum of the patient together with the corresponding bearing means of the other implanted pegs of the array, a bearing envelope for the ball head of the femoral implant.

In such a hip replacement joint preferably at least one of said pegs has a different length from the length of at least one other said peg.

The number of pegs in the array may vary from 3 to about 30. Typically each peg comprises a stem and an enlarged, relatively thin head portion. In such a peg the head is preferably from about 0.5 mm up to about 4 mm in thickness, and is preferably less than about 2 mm in thickness. The length of the pegs may be chosen in dependence upon the available thickness of the pelvic bone at the site of insertion of the peg measured in the intended direction of insertion. Typically the pegs each have a stem ranging in length from about 5 mm to about 30 mm, e.g. from about 5 mm to about 20 mm. The pegs may be fixed in place in the patient's pelvic bone by an interference fit, by means of cement, or by screwing into the pelvic bone. The pegs may be coated with hydroxyapatite which may be applied over a plain or textured surface.The pegs may be made of a plastics material, such as high density polyethylene; alternatively they may be made of a metal which may be selected in dependence on the material of the ball head of the femoral implant. It is preferred that maximum dimension of the head portion of each peg, normally its diameter, ranges from about 5 mm up to about 30 mm.

It is further envisaged that it may suffice in certain circumstances to implant only a single peg into the acetabulum of a patient. In this case the surgeon would implant the peg so that the axis of the peg is as near vertical as possible in order to support the weight of the patient in optimum fashion. Hence in another aspect of the invention there is provided a bearing peg for implantation in an acetabulum of a patient, said bearing peg having an insertion end and at the opposite end thereto a bearing means adapted to provide a bearing surface for a ball head of a femoral implant.Such a bearing peg can be used in conjunction with a femoral implant to provide a hip replacement joint comprising (a) a femoral implant comprising a stem for insertion in a prepared cavity in a femur of a patient and a ball head, and (b) an acetabular implant means comprising a bearing peg for implantation in an acetabulum of the patient, said bearing peg having an insertion end and at the opposite end thereto a bearing surface for the ball head of the femoral implant.

The bearing means on each peg may comprise a flat end surface or a convex end surface; alternatively it may comprise a concave surface, the radius of which is matched to the radius of the ball head of a femoral implant.

In an alternative embodiment the bearing means of each peg comprises a rolling ball mounted in the end of the peg.

Since the peg or each of the pegs utilised in an artificial hip joint according to the invention has a head portion which is relatively thin, typically of the order of from about 0.5 mm up to about 2 mm, it intrudes relatively little following implantation into the acetabulum of the patient. This means that a femoral implant with a much larger ball head than usual can be used than can be used with a conventional acetabular hip prosthesis. Hence it becomes feasible to use a femoral implant with a fixed or removable ball head having a diameter of, for example, from about 50 mm to about 58 mm, in place of the much smaller ball head (e.g. about 22 mm to about 35 mm) that is conventionally used in conjunction with a typical acetabular hip prosthesis.

In order that the invention may be clearly understood and readily carried into effect a preferred embodiment thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings wherein: Figure 1 is a vertical section through part of a conventional artificial hip joint; Figure 2 is a corresponding vertical section through an artificial hip joint according to the present invention, incorporating the novel acetabular bearing means; Figure 3 is a lateral view of the acetabular socket of the patient of Figure 2 during implantation of an artificial hip joint of the invention; Figures 4 to 7 are each side views of pegs suitable for use in the present invention; and Figure 8 is an end view of the peg of Figure 7.

Referring to the drawings, Figure 1 is an anteriorposterior view of a conventional hip implant. This comprises an acetabulum 1 secured in place in the pelvic bone 2 of a patient by means of a layer of cement 3. A femoral implant 4 has a ball head 5 of diameter 32 mm. In the course of time the acetabulum tends to sink into the patients bone, as a result of loading of the hip joint as the patient stands or walks. The sinkage occurs approximately in the direction indicated by the arrow 6, i.e. proximally. It also tends to rotate in the direction indicated by the arrow 7, i.e. laterally. As a result of such sinkage, which may amount to about 0.5 mm or more, there is a danger of the acetabulum becoming loose. In addition debris from the bone can find its way between the joint surfaces, leading to damage to the joint and encouraging the formation of tissue in unwanted places.

Figure 2 is a similar anterior-posterior view of an artificial hip joint according to the present invention.

This includes a conventional femoral implant 10 having a stem 11, a trunnion 12, and a ball head 13 which approximates in diameter to the diameter of the natural acetabulum of the patient, e.g. about 50 mm. Ball head 13 can be an integral part of the femoral implant or removable therefrom in known manner. Typically ball head 13 ranges in diameter from about 46 mm to about 63 mm, preferably from about 50 mm to about 58 mm.

Instead of having an artificial acetabulum comprising a cup shaped member, as in the conventional implant of Figure 1, the acetabular bearing surface against which ball head 13 rotates is formed by a plurality of pegs or studs 14 implanted in the pelvic bone 15 of the patient.

As can be seen from Figure 3, the surgeon has chosen to use 21 pegs 14 in this instance. The pegs 14 vary in length, depending upon the available depth of bone at the site of implantation in the direction of insertion of the respective peg. Each peg 14 has a head which projects by a small amount, typically by about 1 mm, from the bony acetabulum of the patient. Hence a small clearance is left between the external surface of the ball head 13 and the pelvic bone 15. Since the intrusion of the pegs 14 into the patient's natural acetabular cup is relatively insignificant, the surgeon can implant a femoral implant with a much bigger ball head 13 than the ball head 5 that is typically used with a conventional acetabular cup prosthesis such as acetabulum 1. Hence ball head 13 can range, for example, from about 50 mm to about 58 mm in diameter, whereas ball head 5 is typically only from about 22 mm to about 35 mm in diameter.

Figures 4 to 8 illustrate various designs of peg.

Figure 4 illustrates a peg 20 of the type disclosed in GB-B-2192543. Peg 20 has a head 21 whose diameter lies in the range of from about 5 mm to about 15 mm and has a length in the range of from about 5 mm to about 30 mm. It is formed with a series of annular lands 22 forming alternate ridges and depressions extending around the peg. Each successive ridge, counting in a direction away from the insertion end 23, has a radially outer surface that extends substantially parallel to the axis of the peg 20 and is spaced, by a comparatively small amount, at a greater distance from the axis of the peg 20 than the radially outer surface of the preceding ridge.

Peg 20 may have a flat head. Alternatively it has a domed surface or a concave surface whose radius matches that of ball head 13.

Figure 5 illustrates an alternative form of peg 30 of generally tapered form formed with a series of circumferential grooves 31. In a similar manner to peg 20, the head 32 of peg 30 can be flat, convex or concave.

Figure 6 illustrates an alternative design of peg 40 in whose head is mounted a captive hard ball.

Figure 7 illustrates a peg 50 with an external screw thread 51. As can be seen from Figure 8 its head 52 is provided with a pair of axial notches 53 by means of which the surgeon may drive the peg 50 home during implantation.

In a modification of the peg 50 of Figures 7 and 8, the screw head 52 is provided with a central hexagonal socket.

Since the acetabulum component of the artificial hip joint of Figures 2 and 3 is separated into several pieces, each such piece can move with the acetabular bone in ordinary flexing of the bone and/or when any other pieces sink into the bone under load. This prevents gross loosening of the whole implant when one area loosens or sinks.

A variety of materials can be used for construction of the components of the artificial hip joint of the invention and a combination of materials of good wear characteristics can be used. For example, cobalt chrome pegs 14 can be used in combination with a cobalt-chrome ball head 13. Alternative materials include ceramics, ultra high molecular weight polyethylene, coated surfaces of titanium nitride, and the like.

As with all forms of artificial hip implant, some wear debris must be expected. This debris would be expected to react locally with soft tissue which will form in the gap between the pegs 14 and produce a fibrous tissue. The soft tissue, and better the fibrous tissue, would allow a load to be taken or shared with the pegs 14 either in any event or with some sinkage of some pegs 14. Thus the ball head 13 will load the pegs 14 and a soft tissue or fibrous tissue matrix and so the artificial hip joint of the invention will be self regulating in load transfer.

It is also envisaged that the pegs 14 can be made of a biodegradable material, that is to say a material which will degrade in the patient's body if present in small particle form, such as debris particles formed through wear, but not when present as a large body, such as a peg 14. In this case the body can degrade wear debris and avoid any problems which would otherwise be caused by the formation thereof.

In some cases the surgeon may decide that the best solution for the patient is to implant a single bearing peg in the natural acetabulum. In this case the surgeon will select a peg with a relatively large head in comparison to the diameter of its shank, for example a head of from about 25 mm to about 35 mm, e.g. about 30 mm, in diameter with a concave surface matching the curvature of the ball head of the femoral implant. The ball head will typically have a diameter of about 50 mm to about 58 mm depending upon the size of the patient's natural acetabulum. The thickness of the head portion will be within the range of from about 0.5 mm to about 2 mm, e.g. about 1 mm. The length of the shank of the peg will typically range from about 10 mm to about 20 mm; the shank may have, for example, any of the designs illustrated in Figures 4 to 8. Thus it can have annular lands like lands 22 of Figure 4 or circumferential grooves like grooves 31 of Figure 5 or an external screw thread like screw thread 51 of Figure 7. Having selected a suitable peg the surgeon will then insert it in the patient's acetabulum so that its axis will be as near vertical as possible when the patient stands up. This would correspond to insertion of a peg with a much larger head than illustrated in Figure 2 between the two right hand pegs 14 shown in Figure 2 with the axis of that single peg lying in a vertical plane.

Implantation of the femoral implant, with a fixed head or a removable head as the surgeon chooses, can be effected in conventional manner.

Claims

1. A kit of parts for forming an artificial acetabular surface comprising a plurality of pegs of selected length for implantation in an acetabulum of a patient, each peg having an insertion end and at the opposite end thereto a bearing means adapted to define, upon implantation in an array in the natural acetabulum of the patient together with the corresponding bearing means of the other implanted pegs of the array, a bearing envelope for a ball of a femoral implant.

2. A kit according to claim 1, in which said plurality of pegs comprises at least one peg having a different length from that of at least one other peg of the kit.

3. A kit according to claim 1 or claim 2, in which said plurality of pegs includes at least one peg with a bearing means comprising a flat end surface.

4. A kit according to claim 1 or claim 2, in which said plurality of pegs includes at least one peg with a bearing means comprising a concave surface.

5. A kit according to claim 1 or claim 2, in which said plurality of pegs includes at least one peg with a bearing means comprising a rolling ball mounted in the end of the peg.

6. A kit according to any one of claims 1 to 5, in which the number of said pegs is from 3 to about 30.

7. A kit according to any one of claims 1 to 6, in which at least one of said pegs bears a coating of hydroxyapatite.

8. A kit according to any one of claims 1 to 7, in which at least one of said pegs is made from a metal.

9. A kit according to any one of claims 1 to 7, in which at least one of said pegs is made from high density polyethylene.

10. A hip replacement joint comprising (a) a femoral implant comprising a stem for insertion in a prepared cavity in a femur of a patient and a ball head, and (b) an acetabular implant means comprising a plurality of pegs of selected length for implantation in an acetabulum of thepatient, each peg having an insertion end and at the opposite end thereto a bearing means adapted to define, upon implantation in an array in the natural acetabulum of thepatient together with the corresponding bearing means of the other implanted pegs of the array, a bearing envelope for the ball of the femoral implant.

11. A hip replacement joint according to claim 10, in which said plurality of pegs comprises at least one peg having a different length from that of at least one other peg.

12. A hip replacement joint according to claim 10 or claim 11, in which said plurality of pegs includes at least one peg with a bearing means comprising a concave surface.

13. A hip replacement joint according to claim 12, in which the radius of said concave surface is matched to the radius of the ball head of the femoral implant.

14. A hip replacement joint according to claim 10 or claim 11, in which said plurality of pegs includes at least one peg with a bearing means comprising a flat end surface.

15. A hip replacement joint according to claim 10 or claim 11, in which said plurality of pegs includes at least one peg with a bearing means comprising a rolling ball mounted in the end of the peg.

16. A hip replacement joint according to any one of claims 10 to 15, in which the number of said pegs is from 3 to about 30.

17. A hip replacement joint according to any one of claims 10 to 16, in which at least one of said pegs bears a coating of hydroxyapatite.

18. A hip replacement joint according to any one of claims 10 to 17, in which at least one of said pegs is made from a metal selected in dependence on the material of the ball head of the femoral component.

19. A hip replacement joint according to any one of claims 10 to 17, in which at least one of said pegs is made from high density polyethylene.

20. A bearing peg for implantation in an acetabulum of a patient, said bearing peg having an insertion end and at the opposite end thereto a bearing means adapted to provide a bearing surface for a ball head of a femoral implant.

21. A bearing peg according to claim 20, in which said bearing means comprises a concave surface whose radius of curvature is substantially matched to that of the ball head of the femoral implant.

22. A bearing peg according to claim 21, in which the concave surface comprises a spherical surface whose radius of curvature ranges from about 25 mm to about 29 mm.

23. A bearing peg according to any one of claims 20 to 22, in which said bearing means comprises a head portion having a maximum transverse dimension of from about 25 mm to about 35 mm.

24. A bearing peg according to any one of claims 20 to 23, in which the peg has a shank of from about 10 mm to about 20 mm in length.

25. A hip replacement joint comprising (a) a femoral implant comprising a stem for insertion in a prepared cavity in a femur of a patient and a ball head, and (b) an acetabular implant means comprising a bearing peg for implantation in an acetabulum of the patient, said bearing peg having an insertion end and at the opposite end thereto a bearing surface for the ball head of the femoral implant.

26. A hip replacement joint according to claim 25, in which the ball head has a diameter of from about 50 mm to about 58 mm and in which the bearing surface is formed on a head portion whose thickness lies in the range of from about 0.5 up to about 2 mm.

27. A hip replacement joint according to any one of claims 25 to 27, in which the maximum transverse dimension of the head portion ranges from about 25 mm to about 35 mm.

28. A hip replacement joint according to any one of claims 25 to 27, in which the bearing surface is concave with a curvature matched to that of the ball head of the femoral implant.

29. A hip replacement joint constructed and arranged substantially as illustrated in Figures 2 and 3 or in Figures 2 and 3 as modified by Figure 4, by Figure 5, by Figure 6, or by Figures 7 and 8 of the accompanying drawings.

30. A hip replacement joint comprising a bearing peg and a femoral implant constructed and arranged substantially as herein described.