Prosthesis

A distal component for an interphalangeal prosthesis suitable for insertion into a proximal end of a distal phalangeal bone includes a stem portion shaped to be received within a surgically prepared bore in the proximal end of the distal bone. A head portion has an under-surface and an upper-surface, the under-surface of which in use will bear on the bone into which the stem is implanted.

FIELD OF THE INVENTION

The present invention relates to a prosthesis. More particularly it relates to a prosthesis for use in replacing the interphalangeal joint or the metacarpal-phalageal joint. For ease of reference the prosthesis of the present invention will be referred to as an “interphalangeal prosthesis” but it will be understood that the term will cover both prostheses for interphalangeal joints and those for the metacarpal-phalangeal joints whether in the hands or feet.

BACKGROUND OF THE INVENTION

The efficient functioning of the finger and/or toe joints is important to the well being and mobility of the human body. An interphalangeal joint is comprised by the distal end portion of a first phalange and the proximal end portion of the adjacent phalange. Similarly, the metacarpal-phalangeal joint is formed by the distal end of the metacarpus and the proximal end of the proximal phalange. In a normal healthy body these joints will be covered with articular cartilage. The presence of this cartilage allows the bones of the joint to move smoothly and freely through their range of motion.

Diseases such as rheumatoid- and osteo-arthritis can cause erosion of the articular cartilage so that the components of the joint rub together causing pain and further erosion. Bone erosion may cause the bones themselves to attempt to compensate for the erosion which may result in the bone being reshaped. This misshapen joint may cause pain and may eventually cease to function altogether.

Operations to replace the interphalangeal joint are known. In these operations prosthetic joints may be introduced. One type of known prosthesis is a one-piece prosthesis comprising a central body and two oppositely projecting stem portions of reduced cross-section. These stem portions are implanted into the machined ends of bones adjacent the joint. One example of a prosthesis of this kind is described in GB1192960. A modification of these types of prostheses having a hinge-type mechanism between the two stem portions is described in WO96/25129. Whilst joints of this kind go some way to addressing the requirement for an acceptable interphalangeal prosthesis, they suffer from various disadvantages including the long-term stiffening of the joint. It is therefore desirable to provide a prosthesis which more accurately reflects the natural joint.

The natural joint comprises a ball shaped articular head on the distal end of the proximal bone which articulates against a generally socket-shaped surface on the head of the proximal end of the distal bone. Thus, a prosthesis which more accurately reflects the natural joint will generally comprise two separate components such that the resultant movement of the joint most accurately replicates the natural movement of the digit into which it is implanted. Examples of two-part prosthesis comprising one part having an articulating head and a second part having a socket shaped surface on which the head of the first part can articulate are described in WO98/19637, WO92/00709, U.S. Pat. No. 4,231,121 and EP0338715. In each of these prosthesis arrangements there is described a proximal component comprising a shaft for implantation in the resected proximal bone and a head which is at least in part convex and a distal component comprising a shaft for implantation in the resected distal bone and a head which is at least in part concave such that in use the convex head of the proximal component is in articulating contact with the complementary concave head of the distal component.

Whilst these arrangements go some way to resolving the problems detailed above, they still suffer from certain disadvantages and drawbacks. In particular, they generally require substantial resection of the bone into which the prosthesis component is to be inserted before the prosthesis can be used. This is not generally desirable. Further, as the resection required to accommodate these devices is flat to accommodate the flat underside of the prosthesis, there is a high risk of damage to, or a loss of, the collateral ligaments.

FIG. 1illustrates a schematic plan view of a distal component of a conventional two-part prosthesis which can be inserted into a suitably machined proximal end of a distal bone forming the joint. The distal component of the prosthesis comprises a shaft1and a head2having a flat bearing component which may be integral with the shaft or may be a separate component which may be removably attachable to the shaft. X represents the minimum amount of material under the articulating surface.

It is therefore desirable to provide a component which provides which reduces the amount of bone which has to be resected to enable the prosthesis to be inserted. However, it will be understood that there should be a depth of material under the articulating surface to provide sufficient support. Preferably it is desirable to provide a prosthesis which when inserted in position enables the collateral ligaments to be retained.

SUMMARY OF THE INVENTION

The present invention relates in one aspect to a distal component for an interphalangeal prosthesis suitable for insertion into a proximal end of a distal phalangeal bone, which component generally comprising:

a stem portion shaped to be received within a surgically prepared bore in the proximal end of the distal bone, said stem portion generally tapering inwardly from a proximal end to a distal end and having a central longitudinal axis extending from the distal end to the proximal end; and

a head portion having an under-surface and an upper-surface, the under-surface of which in use will bear on the bone into which the stem is implanted, said head portion extending from the stem portion and being ramped away therefrom at an angle of greater than 90° to the longitudinal axis of the shaft; said upper-surface comprising an articulating surface shaped to enable a cooperating surface on a distal end of a proximal bone forming said joint to articulate there-against.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated inFIG. 2the prosthetic component of the present invention comprises a stem portion4and a head portion5. The head portion5has an upper surface6which in use provides an articulating surface. The undersurface7will, in use, bear on the bone into which the stem is implanted. The head component6extending from the stem portion4and is ramped away therefrom at an angle θ which is greater than 90° to the longitudinal axis A of the shaft. As inFIG. 1, X represents the minimum amount of material required under the articulating surface. It will be noted by comparison with the arrangement illustrated inFIG. 1that a significant amount of bone may be retained in using the present invention having the generally conical profile.

In the preferred arrangement, the head portion is formed of biocompatible plastics material such as ultra high molecular weight polyethylene and the stem portion from a biocompatible metal such as cobalt chrome.

In use the upper surface6of the head portion5provides an articulating surface.FIG. 3illustrates the articulation of the prosthetic component8which may be used in the distal end of the proximal bone forming the joint. Generally both ends of bone forming the joint will require resecting and the insertion of a prosthetic component. However, it will be understood that where the distal end of the proximal bone is healthy and does not require the insertion of a prosthesis, the articulating surface6of the prosthetic component of the present invention will in use articulate against natural bone.

As illustrated inFIGS. 4 and 5the overall configuration of the component when viewed from above can be regarded as being generally D-shaped.

The profile of the prosthetic component of the present invention is illustrated by line9on the component ofFIG. 6. The profile is illustrated by line9on the joint illustrated inFIG. 6. This shows the function of the profile of the proximal resection of the middle phalanx10in retention of the collateral ligaments12. It can readily be seen that the conical form of the profile maximizes material thickness in the middle phalangeal component whilst sparing the ligaments. This retention of the ligaments will assist in the correct articulation and functioning of the joint on the distal end of the proximal phalanx13.

The head portion5includes an upper section and a separate lower section. The upper section of the head portion5and the stem portion4are preferably provided as two components as illustrated inFIG. 7. Here the stem portion includes a stalk14extending from and surrounded by the head supporting surface15forming at least a part of the lower section of the head that is integrally formed with the stem. The stalk includes a barb16extending around its circumference. As illustrated inFIG. 8, the stalk is inserted into a bore17provided in the underside of the upper section of the head portion. The upper section of the head portion5is preferably a snap-fit onto the stalk14.

The prosthetic component of the present invention located in the interphalangeal joint between the proximal phalanx18and the middle phalanx19of the middle finger is illustrated inFIG. 9. The finger also includes a distal phalanx20. The articulating surface of the prosthesis is acting against the articulating surface of a proximal phalangeal component8.

In order to obtain the conical bore in the proximal end of the bone a conical mill21such as that illustrated inFIG. 9may be used. The conical profile achieved by the mill21is illustrated by line22.

With the arrangement in the present invention, a depth of material is provided to give support for the articulation surface whilst minimizing the amount of bone which has to be resected to enable the prosthesis to be inserted.

As indicated, the special shaping of the component of the present invention minimizes the amount of natural bone which has to be resected. It is generally desirable to maintain as much of the natural bone as possible. However, there is a particular advantage in retaining natural bone in the areas where the ligaments attach. The conical shape of the stem portion of the prosthesis means that a conical resection can be used which will generally allow the connection point for collateral ligaments to be retained. For example, where the prosthesis is for use at the proximal end of the middle phalanx, the resection required to enable the prosthesis of the present invention to be inserted, enables the collateral ligaments which are attached close to the proximal end of the middle phalanx to be retained. Thus it will be understood that a benefit of the present arrangement is that the attachment points of the collateral ligaments can be maintained and there is no risk of the prosthesis interfering with the action of the flexor tendons and the collateral ligaments.

The stem portion and the head portion may be formed as a single piece. In this arrangement, each size of component may be supplied in a number of forms, each matching the range of proximal components with which in size they may be combined. In a preferred arrangement, they may be formed in separate pieces.

In an alternative preferred embodiment, the stem portion and at least a lower section of the head portion may be formed as a single piece such that the stem portion and the section of the head portion which forms the under-surface which in use will bear on the bone into which the stem is implanted is a single piece. Thus this arrangement provides a single element which provides a tray that in use supports a separate upper section of the head portion which provides the upper-surface which in use is the cooperating surface on the distal end of the proximal bone.

In the embodiment in which separate components are used, the components may be interconnected by any suitable means. In one arrangement, a stalk may extend from a supporting cone congruent with said stem portion. In view of the shaping of the stem portion discussed in detail below, when the stalk is present, it will form a perfoliate arrangement. Where the stalk is present, a bore may be provided in the underside of the head portion into which the stalk may be inserted to lock the head portion in position. In a particularly preferred arrangement, the head portion may be a snap-fit on the stalk portion which may include a barb to assist in interlocking the stalk portion in the bore. The barb may extend around the outer circumference of a portion or the whole of the stalk or may simply be one or more flanges extending from the stalk.

This arrangement of the present invention offers a further advantage in that it provides some initial flexibility in the rotational position of the head portion. This flexibility in use enables the surgeon to correct any misalignment in the location of the stem portion of the component in the machined bone.

A further advantage of having separate components is that the prosthesis can be regarded as modular such that the surgeon can select the appropriate stem and appropriate articulating surface for a particular patient. In particular, this modularity allows the surgeon to select independently the most appropriate sized stem, based upon the individual bones which maintaining optimal conformity of bearing surfaces.

Thus it will be understood that in the arrangement in which the upper-surface component is a snap-fit on a stalk, the various possible combinations of implant sizes is matched by a range of snap-on components from which in use the surgeon can select the appropriate ones. From the foregoing, it will be understood that the prosthesis component of the present invention can be provided in any suitable size. Generally, the overall size can be provided in the same five sizes conventionally used, namely the nominal sizes7,8,9,10and11. It will be understood that these sizes describe the maximum width across the upper-surface of the head which is in the dorsal/palmer plane.

The prosthesis of the present invention may be formed of any suitable material. Where the stem portion and the head portion are formed of separate pieces, they may be formed of the same or different materials. In one arrangement, the head portion may be formed from a biocompatible plastics material. Suitable biocompatible plastics materials include ultra high molecular weight polyethylene. In the same or a different arrangement, the stem portion may be formed from a biocompatible metal or polymeric material. Suitable biocompatible metal materials include cobalt-chrome, titanium, titanium alloys, titanium coated cobalt-chrome, and stainless steel.

The surface of the stem portion may have a polished surface. Alternatively, the stem portion may have a roughened surface. In one arrangement, the stem may be ribbed.

The articulation surface of the head portion may be of any suitable shape but is preferably shaped to mimic that of the natural bone. In one arrangement, it will be shaped to accept the bicondular features on the distal end of the proximal bone. However, concave configurations may also be used.

The stem portion is preferably elliptical in cross-section. Where the head portion of the prosthesis is separate from the stem component, the upper surface of the stem component preferably provides a tray on which in use the head portion sits. The tray is preferably higher on one side than on the other. The higher side is preferably the palmer side. Thus the tray can be envisaged as being of a ruff configuration around the stalk, where present. In one embodiment the outer profile of the tray can be considered as being generally D-shaped (albeit with some curvature on both sides) with the most curved side being located in use on the dorsal side of the joint.

The notional line from the end of the stem to the edge of the upper-surface may be of any suitable configuration. It may be a linear slope or it may be curved either in a convex or a concave manner.

The component of the present invention may be provided in a kit additionally including a proximal component for an interphalangeal prosthesis suitable for insertion into a distal end of the proximal phalangeal bone.