WO2005067822A1

WO2005067822A1 - A flexible finger implant

Info

Publication number: WO2005067822A1
Application number: PCT/EP2005/000989
Authority: WO
Inventors: Gareth Thomas; John Knowles Stanley; Ian Alexander Trail
Original assignee: Gareth Thomas; John Knowles Stanley; Ian Alexander Trail
Priority date: 2004-01-17
Filing date: 2005-01-17
Publication date: 2005-07-28
Also published as: GB0401049D0; GB2409979A

Abstract

The invention relates to a flexible finger implant (1), particularly for the metacarpal phalangeal (MCP) joints. The implant is a single piece moulded construction, consisting of two stems (2, 4), which fit in the bone canals of the metacarpal and phalangeal bones respectively. The stems are connected by a hinge mechanism. It has been established that during flexion the natural MCP joint of the hand, rotates around 5° from the lateral to the medial side. The invention has an asymmetrical hinge (6), being thicker on the lateral side than on the medial side (of the finger). This causes the implant to rotate around 5° to the medial side during flexion.

Description

A Flexible Finαer Implant

Background

The present invention relates to a flexible finger implant, particularly for the metacaφal phalangeal (MCP) joints.

The human hand comprises a number of bones, cartilage and tendons. Fingers of the hand comprise three distinct groups of bones. The first group of phalangeal bones articulates with a group of bones called the metacaφel bones. This group is also known as the proximal phalanges. The joint formed between these bones is known as the metacaφal-phalangeal joint. Articulated to the proximal phalanges are the middle row of phalanges. The third group is referred to as the distal phalanges.

As the natural finger joints degrade, for example as a result of arthritis, it is common to replace natural joint with an artificial implant or prosthesis. These implants or prostheses usually are of unitary moulded construction (eg of silicone) consisting of two stems which fit into canals formed in bone canals of the metacaφal and phalangeal bones respectively.

Prior Art

An example of an artificial implant, which has been particularly successful, comprises a one piece body of flexible, inert material which includes a pair of outwardly extending portions, known as stem portions. These stem portions are shaped and dimensioned to be received by intermedullary canals of bones. The stems are connected by a thickened portion which defines a hinge.

One example of such an artificial implant is the moulded joint that is described in US Patent US 3 875 594 (Dow Coming Coφ). The joint comprises a unitary moulded construction with an enlarged central portion and oppositely projecting stem portions. The stem portions articulate about a depression formed in the volar side of the central portion of the joint so as to accommodate articulating movement of the said stem portions.

Although successful, the prosthesis described in the aforementioned US Patent suffered from the fact that the natural metacaφal phalangeal (MCP) joints of the hand, typically rotate around 5° (towards the medial side) during flexion of the phalanges. The result of this flexion has had the effect of imposing strain on muscular tissues and the metacaφal- phalangeal joints. This tendency has led to the tendons becoming misaligned with the axis of the joints, particularly in the third and fourth phalanges. This misalignment resulted eventually in the tendons slipping off the joints and lead to the condition known as "ulna drift".

The condition of ulna drift has generally not been improved by existing implants, because they do not create a metacaφal phalangeal (MCP) joint which allows significant rotation. Prosthetic finger joints have therefore not alleviated the stresses experienced by the implant. Likewise existing joints have not reduced strains applied to the soft tissues of the hand.

Another example of a finger joint prosthesis is described in US Patent US 6454 808 (M- E-System Inc). The joint comprises a ball and socket type arrangement that has an opening and a spherical inner side. It is stated that the joint prosthesis matches motional characteristics of a finger in which it is implanted. The device is complex and expensive to fabricate. Moreover there has been relatively little uptake of the joint by surgeons.

The invention seeks to provide an implant for a metacaφal phalangeal joint which allows rotation (supination) of around 5° to the medial side during flexion.

Another object of the invention is to provide an implant for a metacaφal phalangeal joint which is reliable in operation. A further object of the invention is to provide an implant for a joint which permits a degree of rotation to the medial side during flexion ideally the amount of rotation is around 5°.

An additional object of the invention is to reduce the re-occurrence of ulna drift post- operatively.

Summary of the Invention

According to the present invention there is provided a flexible implant of unitary construction comprising: a) a first stem to fit a first bone canal, b) a second stem substantially in line before flexion with the first stem to fit a second bone canal, c) a hinge formed between an end of the first stem and an end of the second stem, said hinge having a varying thickness in a direction substantially normal to the line of said first and second stem, whereby, in use of said implant, said hinge allows rotation to the medial side during relative flexion of the first and second bones.

This movement produces a more natural arc of motion during flexion, thus leading to less stress on the implant and soft tissues.

According to a second aspect of the invention there is provided a flexible finger implant of unitary construction comprising: a) a first stem to fit the bone canal of the metacaφal bone, b) a second stem substantially in line before flexion with the first stem to fit the bone canal of the phalangeal bone, c) a hinge formed between an end of the first stem and an end of the second stem, said hinge having a declining thickness in a direction substantially normal to the line of said first and second stem, in use of said implant, said hinge allows rotation to the medial side during flexion of a finger.

An advantage of the varying thickness is that it twists (rotates) about its longitudinal axis as a consequence of the same bending moment being applied to different amounts of material. The result is therefore that a rotational compensation of the natural twisting in the phalangeal joints is achieved. By suitable dimensioning of the thickness and variation of the amount of material different amounts of supination of the finger joint is achieved.

Preferably the implant is of silicone. An advantage of silicone is that it is a well-tried and tested material that is resilient and biocompatible. The material is also relatively straightforward to mould or cast and this leads to a reliable method of fabrication of the prosthesis.

The implant may be of other bio-compatible elastomeric materials.

The surface of the implant may be coated with dimples or other texturing which may assist in the insertion of the joint into the bone canal(s).

Ideally a recess is provided in the volar or dorsal side of the joint. An advantage of the recess is that it permits flexion of the finger.

Preferably the end of the first stem of the joint and the end of the second stem of the joint each includes a flange, and said hinge joins the two flanges.

The thickening of the hinge may be linear or non-linear. If it is non-linear the thickening of the hinge may be curved or undulating.

The thickness of the hinge portion may vary in more than one plane. An advantage of this is that by modifying the precise amount of thickness the prosthesis joint can be tailored to suit a particular person, a particular sized finger or to compensate a particular degree (or likelihood) of ulna drift.

Preferably the stems are tapered, and preferably the stems are of a substantially rectangular based pyramid shape.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings in which:

Brief Description of the Figures

Figures 1A and 1B show diagrammatical perspective views of an implant prior to flexion;

Figures 2A and 2B show plan and side views of the implant of Figure 1 prior to flexion; and

Figures 3A and 3B show plan and side views of the implant of Figure 1 during flexion.

Detailed Description of Preferred Embodiments

Referring to the Figures there is shown a flexible finger implant 1 of unitary construction from preferably from silicone.

Implant 1 has a first stem 2 to fit a bone canal of the metacaφal bone. The first stem 2 tapers from a first wide end 2a to a relatively narrow end 2b. The first stem 2 is generally of a rectangular based pyramid shape. A flange 3 is provided adjacent the relatively wider end 2a.

Implant 1 has a second stem 4 substantially in line before flexion with the first stem to fit the bone canal of the phalangeal bone. Second stem 4 tapers from a first wide end 4a to a narrow end 4b and is generally square based pyramid shape. A flange 5 is provided adjacent wide end 4a. Second stem 4 may be shorter than first stem 2.

A hinge 6 is formed between flange 3 at the end of the first stem and flange 5 at the end of the second stem. Hinge 6 has a linearly declining thickness in a direction substantially normal to the line of first stem 2 and second stem 4, as one end 6a of hinge 6 is thicker than end 6b. As shown in Figure 3b during flexion the top edges of flanges 3 and 5 come together. Because of the declining thickness of hinge 6 and as shown in Figure 3A, the top edges of flanges 3 and 5 adjacent hinge end 6b come closer together than the top edges of flanges 3 and 5 adjacent hinge end 6a. This is designed to cause rotation of around 5° as shown by the arrow of Figure 3A.

When fitted between the metacaφal and phalangeal bones, the hinge 6 thus allows rotation to the medial side during flexion of a finger.

The invention may take a form different to that specifically described above. For example the implant shown in the drawings is shown with hard edges, but in practice a preferred embodiment of the implant may have more rounded edges. In particular the flanges and/or the hinges may have rounded edges. The stems may have a more rounded, anatomical shape.

It will be appreciated that variation to the aforementioned embodiments may be made without departing from the scope of the invention. Thus, for example, the implant may be suitably modified for use with, in or by other joints in the human or animal body.

Further modifications will be apparent to those skilled in the art.

Claims

1. A flexible implant of unitary construction comprising: a first stem to fit a first bone canal, a second stem substantially in line before flexion with the first stem to fit a second bone canal, a hinge formed between an end of the first stem and an end of the second stem, said hinge having a declining thickness in a direction substantially normal to the line of said first and second stem, in use of said implant, said hinge allows rotation to the medial side during relative flexion of the first and second bones.

2. A flexible finger implant of unitary construction comprising: a first stem to fit the bone canal of the metacaφal bone, a second stem substantially in line before flexion with the first stem to fit the bone canal of the phalangeal bone, a hinge formed between an end of the first stem and an end of the second stem, said hinge having a declining thickness in a direction substantially normal to the line of said first and second stem, in use of said implant, said hinge allows rotation to the medial side during flexion of a finger.

3. A flexible implant according to claim 2, wherein th Ae implant is formed from a bio- compatible elastomeric material.

4. A flexible implant according to claim 3, wherein the bio-compatible elastomeric material is silicone.

5. A flexible implant according to any of claims 2 to 4 wherein the end of the first stem and the end of the second stem each includes a flange, and said hinge joins the two flanges.

6. A flexible implant according to any preceding claim wherein the stems are tapered, and preferably the stems are of rectangular based pyramid shape.

7. A flexible implant according to any preceding claim wherein said hinge allows rotation of around 5 ° to the medial side during flexion of a finger.

8. A flexible implant according to any preceding claim wherein edges of the joint are rounded.

9. A flexible implant according to any preceding claim wherein a recess is provided in the volar side of the joint.

10. A flexible implant according to any preceding claim wherein the end of the first stem and the end of the second stem each includes a flange, and said hinge joins the two flanges.

11. A flexible implant according to any preceding claim wherein said thickening of the hinge is non-linear the thickening of the hinge may be curved or undulating.

12. A flexible implant according to any of claims 1 to 9 wherein said thickening of the hinge is linear.

13. A flexible implant according to any preceding claim, the surface of the implant may be coated with dimples or texturing.

14. A flexible finger implant substantially as hereinbefore described with reference to and as shown in the accompanying drawings.