SPECIFICATIONFinger joint prosthesisThis invention relates to a finger joint prosthesis which is particularly although not exclusively applicable for use in the proximal interphalangeal joint.
Protheses at present in use in the interphalangeal joints usually require complete excision of the joint and they fail to achieve a good range of movement subsequently as fibrous tissue closely envelops the prosthesis and prevents movement.
It is the intention of the present invention to provide a prosthesis which can be inserted into a joint with the minimum of bone resection and retaining the capsular attachments to the greatest extent possible. By this means it is hoped to be able to produce a mobile joint with a good range of movement.
In the past success in hand surgery has only been achieved with simple excision of the joint which is usually referred to as excision arthroplasty, and in these joints replacement of the joint by a spacer made of silicon rubber has produced good results. This is particularly true with the metacarpophalangeal joint. These principles have been applied to the proximal interphalangeal joint of the finger but without success.
According to the present invention a prosthesis for use in an interphalangeal joint of a finger comprises a proximal component which cooperates with a distal component, said proximal component comprising a stem for attachment to the proximal phalange and having a head provided with a curved engagement surface, and said distal component comprising a stem portion for attachment to the distal phalange and also having a head provided with a curved engagement surface adapted to engage in sliding relationship the engagment surface of said proximal component.
Preferably said engagment surfaces are shaped to allow relative angular movement between the components in a single plane and resist relative movement in directions normal to said plane.
Thus, the movement of the joint imitates the natural movement.
In a preferred construction one of the engagment surfaces is substantially concave in cross-section and the other is substantially convex.
The proximal component can have a substantially concave surface and the distal component can be a substantially convex surface but it will be appreciated that the surfaces could be arranged in the opposite manner.
The concave engagement surface on the proximal component may have a substantially Vshaped cross-section as may the engaging convex surface of the distal component, and in order to ensure efficient operation the V shaped crosssection on the distal component is preferably truncated.
The included angle of the V-shaped crosssection in the proximal and distal components may conveniently be substantially 1400 and the engagement surface on the distal component may be arranged to extend through an angle of substantially 1100.
With this arrangement the curved engagement surface on the proximal component can be arranged to extend through an angle of substantially 2100 but not all the engagement surface need be for operative use.
Thus, a mid-portion of the centre of the engagement surface on the proximal component can be part circular and this portion may extend through an angle of substantially 1950.
Preferably the acis of the part circular portion of the bearing surface is offset to one side of the longitudinal axis of the stem.
The proximal component can be provided with an abutment to the stem portion to provide a stop to extension.
The invention can be performed in many ways but one embodiment will now be described by way of example and with reference to the accompanying drawings in which:Figure 1 is an isometric view of the prosthesis according to the invention with the two components spaced apart to show their construction;Figure 2 is a side view of the proximal component shown in Figure 1;Figure 3 is a plan view of the proximal component shown in Figure 1;Figure 4 is an end elevation of the same proximal component;Figure 5 is a cross-sectional view taken onFigure 2;Figure 6 is a side view of the distal component according to the invention drawn to half the scale of the component shown in Figures 2, 3, 4 and 5;Figure 7 is a plan view in part-section of the component shown in Figure 6; and,Figure 8 is an end elevation of the component shown in Figure 6.
As shown in the drawings the prosthesis for use in an interphalangeal joint of the finger comprises a proximal component 1 which cooperates with a distal component 2. The proximal component 1 has a stem portion 3 and a head portion 4. The proximal component is formed from a stainless steel or a chrome cobalt alloy, for example, the metal sold by the Applicants under the Trade Mark VITALLIUM, and the head 4 is provided with an engagement surface 5 which is of concave V-shaped cross-section, as is most clearly shown in Figures 1 and 4. The engagement surface 5 extends through an angle of about 2100. The mid-portion of this concave surface being part-circular about an axis indicated by reference numeral 6, and extending through an angle of approximately 1950. The remaining portion of the engagement surface is curved conveniently as required.
An abutment 9 is provided at one end of the engagement surface, this abutment 9 projecting from the stem portion 3 and acting as a stop to extension.
As will be seen from Figure 2 the axis of the curved surface 5 is offset to one side of the centre line 7 of the stem portion 3 so as to copy the natural anatomical structure, which ensures a tight ligament structure around the joint through the range of motion necessary for stabilizing the prosthesis.
The distal component 2 comprises a substantially conical stem portion 10 from each side of which project flat flanges 1 1 which have tapered sides 13 to match the medullary canal.
The stem tapers to a substantially rectangular cross-section at its free end 1 4.The flanges 1 1 extend to a head 15 on which a curved engagement surface 16 is formed. The engagement surface 16 is of truncated V-shaped cross-section and is substantially part-circular, the radius of the curve being substantially the same as the radius of the curve of the partcircular mid-portion of the surface 5 on the proximal component. The sides of the curve are cut back slightly as indicated at 17 to allow a slight clearance when the components are assembled together, and the centre of curvature 18 is cut back so that it does not engage the centre of curvature 19 of the surface 5.*The engagement merely being between the surfaces 5 and 16 themselves.
The angle of the truncated convex V-shaped surface 16 on the distal component and the Vshaped surface 5 on the proximal component both have an included angle of approximately 1200 to 1400 depending upon the size of the components.
The distal component described above is made from a synthetic plastics material, for example ultra high molecular weight Polyethylene, so as to ensure a good sliding effect with the steel or alloy proximal component but it will be appreciated that the parts could be made from any suitable material provided it is biocompatible.
When the prosthesis is to be used the distal end of the proximal phalange and the proximal end of the distal phalange are partly removed and the bones are prepared to accept the stems 3 and 10 of the components. The components are fastened in place in the usual manner with a suitable cement. In certain instances the component may be inserted without cement. The existing muscles and ligamentous structures act to hold the curved surfaces together. The prosthesis construction allows totally unconstrained movement of the finger in a similar manner to the natural joint, the distal component being able to move from a position in which it is substantially in line with the stem of the proximal component, which produces a straight finger position to a position in which it is approximately 1000 displacement therefrom, that is, the bent finger position, this movement being accommodated by the sliding surfaces.
The capsular attachments with their lining synovium are retained so that the fluid bathing the prosthesis will prevent the ingrowth of fibrous tissue.