Metacarpal-phalangeal (MCP) joint prosthesis

Claim 1

A metacarpal-phalangeal MCP joint prosthesis for a human or animal body, comprising first (20) and second (40) components each made of bio-compatible material and adapted for secural to respective bones (13,14) of the body in the vicinity of a natural MCP joint (16) after surgical excision of the natural MCP joint (16), the components (20,40) of the prosthesis (12) being unattached to each other and, in use, being held together to form the prosthetic joint by the natural ligaments, tendons and muscles of the body in the vicinity of the MCP natural joint (16), said first component (20) comprising a stem (21) sized for reception by a metacarpal bone cavity and a bulbous head (22) presenting a substantially-spheroidally curved convex articulation surface (23) which in the sagittal plane has a radius of curvature which changes within the arc length of the surface and in that plane the length of said arc being at least 90°, said bulbous head (22) further comprising a single lateral protrusion (27) extending in the coronal direction and being sized for differentially controlling tension of the collateral ligaments when the prosthetic joint is in the flexed condition, and said second component (40) comprising a stem (41) sized for reception by a phalangeal bone cavity and a head (42) presenting a concave articulation surface (43), said concave surface (43) forming part of a substantially hollow spheroid and being dimensioned to slide over said convex surface (23) substantially along said 90° arc length in the sagittal plane to provide flexing movement of the prosthetic joint with the instantaneous center of rotation of the joint changing during said flexing movement due to said changes of radius of curvature so as to change the distance between the respective bones (13,14) and thereby allow natural collateral ligament function around the prosthetic joint to aid flexor muscle effect when the joint is extended and extensor muscle effect when the joint is flexed, said concave articulation surface (43) having a curvature which provides only ring-like peripheral contact with the substantially-spheroidally curved convex articulation surface (23) when the joint is in its extended condition and only central contact when the joint is in its flexed condition.