Intrinsic stability of an unconstrained metacarpophalangeal joint implant

Objective. To compare the intrinsic stability of an unconstrained resurfacing metacarpophalangeal arthroplasty to that of a normal human cadaveric joint. Design. Cadaveric joints and metacarpophalangeal prostheses were studied in a mechanical testing machine at different angles and axial loads to determine the stability ratio in eight directions of movement. Background. An unconstrained resurfacing arthroplasty was designed to replicate the normal anatomy with the exception of the proximal component having a greater arc of curvature on its dorsal aspect. Methods. Eight fresh-frozen cadaveric joints and five different sizes of the image metacarpophalangeal prosthesis were studied at 0°, 45° and 90° angles of flexion and at eight different directions of motion with three different axial loads (0, 20, 40 N). A 6-component load cell measured the force needed to sublux the joint. The stability ratio was the measured outcome and is defined as ratio of the force of subluxation to the axial force. Results and conclusions. The unconstrained resurfacing arthroplasty has more intrinsic stability than the cadaveric metacarpophalangeal joint in all eight directions tested. Relevance A major complication of metacarpophalangeal implants is ulnopalmar subluxation. The image implant is designed to decrease the risk of ulnopalmar subluxation by having a greater arc of curvature on the dorsal aspect of the proximal component. This study shows that the designed implant has greater stability due to the geometry of the implant compared to that of the anatomical joint.