WO1995033425A1

WO1995033425A1 - Prosthetic finger joint

Info

Publication number: WO1995033425A1
Application number: PCT/GB1995/001341
Authority: WO
Inventors: David John Beevers
Original assignee: University Of Leeds
Priority date: 1994-06-08
Filing date: 1995-06-08
Publication date: 1995-12-14
Also published as: GB9411418D0

Abstract

A finger joint prosthesis for implantation between a first bone and a second bone, the prosthesis comprising a first component capable of being operatively connected to said first bone, and a second component capable of being operatively connected to said second bone, each component comprising an anchor part for connection to said first bone and a bearing part for pivotal engagement with the bearing part of the other component, the anchor and bearing parts of the first component being distinct elements for connection together so that, when the implanted joint is in use, there is no relative movement between said elements. The joint is usually a metacarpophalangeal joint and the anchor part of the first component is, preferably, a sleeve fitted into the metacarpal bone to secure the replacement metacarpal head. The two-part first bone component allows replacement of worn heads without disturbing the sleeve anchor.

Description

PROSTHETIC FINGER JOINT

This invention relates to prosthetic finger joints and more particularly, though not exclusively, is concerned with prostheses for the metacarpophalangeal (MCP) joint.

MCP joints have been previously proposed, and some used, which fall into three main categories, namely, hinge, flexible and surface prostheses.

The first generation of hinged prostheses were simple uni-axis designs, fixed to the metacarpal and proximal phalanx intramedullary canals. They were manufactured entirely from metal and only allowed flexion and extension movements. The movement of the MCP joint is in practice seldom only one of flexion and extension but normally incorporates both abduction and adduction.

The first generation hinged prostheses were followed by single component flexible prostheses, the majority of which were allowed to slide freely within the intramedullary canals. They permitted flexion, extension, abduction and adduction.

The second generation of hinged prostheses used multi-axes hinges and had snap-fit assemblies, generally using cement for fixation to the bones. Either a metal or ceramic component acted against the polymer bearing surface.

Surface prostheses involve separate metacarpal and proximal phalanx components which come into contact to form a bearing surface.

The most popular MCP joint is the Swanson silicone rubber prosthesis, a joint of the flexible type, which is a one component prosthesis with two stems introduced into the bone cavities, without any fixation. It restores the joint from a cosmetic point of view, but not its function to any appreciable extent. In the course of time, natural ligament material may be completely damaged by disease and the absence of ligaments renders a joint unstable. However there is a general reluctance by orthopaedic surgeons to provide early surgical intervention for rheumatoid arthritis (RA) patients. The patients are considered to be able to cope even if their hands are grossly deformed. At the time of surgery their hands may be so badly affected with arthritis that only a salvage operation can be achieved to restore cosmesis and slight function. Hence the popularity of the flexible prosthesis and in particular the Swanson joint.

If the hands are operated on at an early stage of the arthritis, an MCP prosthesis that more accurately restores the surfaces of the joint and which can take normal joint forces without fracture could restore the biomechanics of the joint and allow normal function - a concept that has worked well in the knee. However when designing an MCP joint replacement, it is important to recognise some essential differences between it and a knee, or a hip, arthroplasty. During its use an MCP joint experiences the high loads during gripping and pinching actions where the sliding movement of the articulating surfaces is minimal or non-existent.

Another important aspect to consider when developing an MCP prosthesis is its fixation to the bone. The disadvantage of using cement is that bone cement particles can cause damage to the bearing surfaces of the prosthesis, causing increased wear rates. These particles may be from debris left behind during surgery between the bearing surfaces or from wear particles of cement generated due to a loose prosthesis. Cement also lengthens the operation time since time is needed to prepare and apply the cement.

For the use of cement, there is a requirement for good bone stock. Only a thin amount of cement is necessary and the temperature increase at the bone-cement interface due to the exothermic polymerization reaction of the cement is minimal. However, if cement is used in an aged or rheumatoid hand, where the cortical bone is thin and the cancellous bone is normally spongy and therefore needs removing, a large amount of cement is necessary to fill the intramedullary canal. With this large mass of cement the temperature would exceed 56°C and thermal necrosis of the bone will occur. As the cortical bones of the rheumatoid hand are thin, all the bone will be destroyed. Cement therefore should not be used in rheumatoid hands for stem fixation.

There is a requirement for an MCP joint which can approach the stability and versatility of the normal joint. This requirement is perhaps best met by a surface joint which will normally be implanted at a relatively early stage in the deterioration of the natural joint. As a result the ligaments and muscles that surround the joint will still be functional at this stage and will be able to provide joint stability.

Existing surface prostheses have not been designed to follow the shape and dimensions of the natural joint. For example the Kessler prosthesis is a single component silicone rubber prosthesis for the replacement of the metacarpal head and has a Dacron-coated intramedullary stem. This prosthesis does not resemble the spherical geometry of the metacarpal head and the proximal phalanx base therefore cannot fit against the prosthesis correctly and provide the accurate bio-mechanical bearing surface and motion that is required. Furthermore the use of Dacron results in fracture of the silastic stems and the silastic material is not strong enough to endure the forces that act in the normal joint.

The Sibly-Unsworth surface prosthesis is manufactured from cross-linked polyethylene which is used for both the metacarpal and proximal phalanx components. However since this prosthesis will be implanted in a much younger population than that for which the flexible prosthesis is appropriate, its lifespan must be correspondingly much greater. Although the forces that act on the joint during flexion are only small, this will cause sufficient wear debris over the years to be of concern. There is a need for a simple finger joint prosthesis which will provide the stability and versatility of the normal joint and not be subject to the disadvantages of existing joints which have been discussed above.

According to the present invention there is provided a finger joint prosthesis for implantation between a first bone and a second bone, the prosthesis comprising a first component capable of being operatively connected to said first bone, and a second component capable of being operatively connected to said second bone, each component comprising an anchor part for connection to said first bone and a bearing part for pivotal engagement with the bearing part of the other component, the anchor and bearing parts of the first component being distinct elements for connection together, so that, when the implanted joint is in use, there is no relative movement between said elements.

It is preferred that the first and second components are not attached together in any way so that, in use, there are no rotational and pull out forces between the components which, if they were connected, would be transferred to the fixating systems, for instance, cement fixation, causing loosening.

A finger joint prosthesis of the present invention is of a "modular" design and, in its simplest form, comprises three entities, namely, a two entity first component and a single entity second component. Typically the three entities will be a metacarpal stem for implantation in the metacarpal shaft, a metacarpal head and a proximal phalanx base component. The metacarpal medullary stem can be implanted with cement and the metacarpal head attached to the stem after the cement has set and the joint has been cleaned. This ensures that no cement debris is left between the bearing surfaces.

The modular approach to the metacarpal component allows the selection of different sizes of metacarpal stems and metacarpal heads to suit the particular joint requirements of the patient. The surgeon may implant the best combination of metacarpal stem and metacarpal head to match the anatomy of the metacarpal bone. Furthermore, if the metacarpal head needs to be replaced due to wear or fracture, the stem may be left cemented within the bone and the metacarpal head replaced, thereby avoiding the problems of having to fix a new stem component to the bone.

A further advantage of a joint in accordance with the present invention is that, if the surrounding muscle and ligamentous structure becomes weakened by arthritis and cannot support the joint prosthesis as originally implanted, the metacarpal head can be replaced with a more stable hinge mechanism without the requirement to remove the metacarpal stem.

Preferably the metacarpal stem is in the form of a sleeve and the metacarpal head is provided with an integral stem for location within the sleeve. The arrangement may be such that the stem of the metacarpal head is a push fit within the metacarpal sleeve. In an alternative arrangement the metacarpal head and metacarpal sleeve may have some form of locking interconnection, for instance, a "snap-fit" arrangement.

Preferably the two metacarpal parts interconnect together in such a way as to prevent relative rotation therebetween. More preferably the metacarpal sleeve is provided with a flat on its distal end which locates in the metacarpal head component.

It is also preferred that the metacarpal sleeve is provided with a flat on its palmar surface for location in the dorsal surface of the metacarpal head, the effect of which is again to prevent relative rotation between these two parts.

The provision of one or both of the above mentioned means for preventing relative rotation between the parts of the metacarpal component results in the rotational, ulnar and subluxation forces being transmitted between these two metacarpal parts during use of the joint. It is preferred that the interconnection between parts of the metacarpal component is such that the centre of rotation of the spherical head is offset from the axis of the intramedullary stem. As a result the configuration of an actual joint can be closely followed with the intramedullary stem located centrally within the medullary canal and the spherical head being positioned relative to the metacarpal shaft as is the case with the natural joint.

Preferably the intramedullary stem of the metacarpal part is tapered so as to narrow in the proximal direction. The allows the stem to give the closest possible fit within the medullary canal.

Preferably the stem of the proximal phalanx base is also tapered, again to give the closest possible fit within the proximal phalanx bone.

Preferably the intramedullary stem of the metacarpal component is of rectangular cross-section, more preferably of square cross-section.

A finger joint prosthesis in accordance with the present invention allows joints to be produced with a large range of motion. An abduction-adduction arc of 40 degrees together with 30 degrees of extension and 90 degrees of flexion is possible with such a joint.

An embodiment of the present invention will now be described, by way of example only, and with reference to the accompanying drawing which shows, in exploded form, a metacarpophalangeal joint.

Referring to the accompanying drawing, a metacarpophalangeal joint is of the surface type and the components are arranged in the drawing in exploded form with indications of the four directions relative to the joint shown by arrows, these directions being dorsal, distal, palmar and proximal. The joint comprises two components, a proximal phalanx base 1 and a metacarpal component 3. The latter is formed of two parts, a metacarpal head 5 and a metacarpal sleeve 7.

The proximal phalanx base 1 is made of any suitable material, for instance, high density polyethylene and includes a stem portion 9 which tapers in the distal direction, which stem portion is attached integrally to shallow cup portion 11 which provides a part spherical bearing surface 13 shown as a dotted line in the drawing.

Metacarpal head 5 is also made of any suitable material, for instance, stainless steel or a chrome cobalt alloy. It includes a head portion 15 provided with a part spherical bearing surface 17 for engagement with corresponding bearing surface 13 of proximal phalanx base 1. Head portion 15 includes a substantially part spherical distal part and, extending in a proximal direction therefrom from the palmar extremity of part 15, a part spherical extension 19 provided with a flat dorsal surface 21.

Also extending in a proximal direction from head part 15 of metacarpal head 5 is a thin rod-like stem 23 of circular cross-section. Stem 23 extends proximally from the body of head portion 15 which itself is provided with a substantially rectangular recess 25. As indicated in the drawing, stem 23 extends proximally from a position which is dorsally spaced from a parallel axis of rotation 27 of the part spherical head part 15. Accordingly stem 23 is offset from axis 27, so that, when the joint is implanted, the part spherical head part 15 is positioned relative to the intramedullary canal as is the case with the natural joint.

Metacarpal sleeve 7 is made of any suitable material, for instance, high density polyethylene. It is essentially a stem 31 which tapers in a proximal direction and is of substantially square cross-section. At the distal end of stem 31 is an integral collar or flange 33 which viewed distally is substantially rounded at its dorsal comers but has rectangular palmar corners, so provided a substantial palmar flat 35. Extending distally from palmar flat 35 is a projection 37 providing distal flat 39.

Extending from distal flat 39 proximally into stem 31 for about two thirds of its length is blind bore 41. Bore 41 is of circular cross-section and its diameter is such that stem 23 is a push fit within this bore. The dimensions of rectangular projection 37 and flange 33 are such that metacarpal head 5 may be push fitted into sleeve 7 with projection 37 fitting within recess 25 and distal flat 39 abutting against dorsal surface 21 of the part spherical extension 19. The location of rectangular projection 37 within rectangular recess 25, and the abutment of palmar flat 35 against dorsal surface 21 of part spherical extension 19, effectively prevents rotation of metacarpal head 5 relative to sleeve 7.

In order to implant the prosthesis, the natural metacarpal head is removed as necessary and the metacarpal sleeve is cemented within the intramedullary canal. Similarly the proximal phalanx base is located within the proximal phalanx. Finally, the metacarpal head is push fitted into engagement with the metacarpal sleeve.

After use the metacarpal head may become worn and a simple operation is required to open up the joint, remove the metacarpal head and replace it by a new head. There is no need to remove the metacarpal sleeve from its cemented imposition within the metacarpal shaft.

If the ligaments and muscles associated with the joint deteriorate, then a further operation may be performed to replace the metacarpal head and proximal phalanx base with a single linked together component, but again without disturbing the metacarpal sleeve.

Claims

1. A finger joint prosthesis for implantation between a first bone and a second bone, the prosthesis comprising a first component capable of being operatively connected to said first bone, and a second component capable of being operatively connected to said second bone, each component comprising an anchor part for connection to said first bone and a bearing part for pivotal engagement with the bearing part of the other component, the anchor and bearing parts of the first component being distinct elements for connection together so that, when the implanted joint is in use, there is no relative movement between said elements.

2. A prosthesis as claimed in claim 1, wherein the first and second components are not attached together in any way.

3. A prosthesis as claimed in claims 1 or 2, wherein the prosthesis is of a "modular" design and comprises three entities, namely, a two entity first component and a single entity second component.

4. A prosthesis as claimed in claim 3, wherein the three entities are a metacarpal stem for implantation in the metacarpal shaft, a metacarpal head and a proximal phalanx base component.

5. A prosthesis as claimed in claim 4, wherein the metacarpal stems and metacarpal heads may be of different sizes to suit the particular joint requirements of the patient.

6. A prosthesis as claimed in claims 4 or 5, wherein the metacarpal stem is in the form of a sleeve and the metacarpal head is provided with an integral stem for location within the sleeve.

7. A prosthesis as claimed in claim 6, wherein the stem of the metacarpal head is a push fit within the metacarpal sleeve.

8. A prosthesis as claimed in claim 6, wherein the metacarpal head and metacarpal sleeve may have some form of locking interconnection.

9. A prosthesis as claimed in claims 4 - 8, wherein the two metacarpal parts interconnect together in such a way as to prevent relative rotation therebetween.

10. A prosthesis as claimed in claim 9, wherein the metacarpal sleeve is provided with a flat on its distal end which locates in the metacarpal head component.

11. A prosthesis as claimed in any of claims 9 or 10, wherein the metacarpal sleeve is provided with a flat on its palmar surface for location in the dorsal surface of the metacarpal head to prevent relative rotation between these two parts.

12. A prosthesis as claimed in any of claims 4 - 11, wherein the interconnection between parts of the metacarpal component is such that the centre of rotation of the spherical head is offset from the axis of the intramedullary stem.

13. A prosthesis as claimed in any of claims 4 - 12, wherein the intramedullary stem of the metacarpal part is tapered so as to narrow in the proximal direction.

14. A prosthesis as claimed in any of claims 4 - 13, wherein the stem of the proximal phalanx base is tapered.

15. A prosthesis as claimed in any of claims 4 - 14, wherein the intramedullary stem of the metacarpal component is of rectangular cross-section.

16. A prosthesis as hereinbefore described with reference to the accompanying drawing.

17. A method of implanting a finger joint prosthesis comprising the steps of:-

(a) removing a natural bone joint head to expose the intramedullary canal of the said bone;

(b) locating a joint anchor component within the said intramedullary canal, and (c) fitting a replacement head into engagement with the said joint anchor component.

18. A method of replacing a prosthesis head comprising the steps of:- (a) removing an existing replacement joint head from an existing anchor component, and

(b) fitting a second replacement head into engagement with the said existing anchor component.

19. A method as claimed in claim 17 or 18, wherein the joint is a metacarpophalangeal joint.

20. A method as claimed in claims 17 - 19, wherein the head is the metacarpal head.

21. A method as claimed in claim 20, wherein a phalanx base is located within the proximal phalanx of the said metacarpal head.

22. A method as claimed in any of claims 18 - 21, wherein the second replacement head is of a different type than the existing replacement head.

23. A method as claimed in any of claims 21 or 22, wherein the head and phalanx base are replaced by a single linked component.

24. A method as claimed in any of claims 17 - 23, wherein the anchor component is a sleeve or stem.

25. A method as hereinbefore described.

26. A method of replacing a prosthesis head as hereinbefore described.