Morphology of the interphalangeal joint surface and its functional relevance

PURPOSE: To study and to clarify the curvature morphology of the articular surfaces of the proximal interphalangeal (PIP) joint and to relate joint morphology and joint kinematics. METHODS: The radii and centers of curvature of 40 PIP joints were determined by sagittal and transverse intersections of highly precise replicas that were prepared by dental methods. RESULTS: The PIP joint is proved to be a nonconforming joint: the articular surface of the proximal end of the middle phalanx has lesser curvatures than the condyles of the proximal phalanx. In intersections through the apex of the radial and ulnar condyles, the measured differences of the radii between the articular surfaces of the PIP joint were sagittally about 30% and transversely about 49% of the respective radii of the condyles. Incongruity of the joint results in 2 morphologically given axes for extension respective to flexion: (1) an axis given by the articular surfaces of both condyles of the proximal phalanx; and (2) a second axis given by the articular surface of the proximal end of the middle phalanx. Both articulating surfaces have 2 contact points in the transverse plane, one each, central to the apex of radial and ulnar condyles, respectively. In the middle of the joint, in the intercondylar groove, a small joint cavity was present in 37 of 40 joints. CONCLUSIONS: The physiological incongruity of the 2 articular surfaces of the PIP joint was defined quantitatively. This allows the derivation of a theoretical model for PIP joint function that explains the kinematics and mechanical stability of the joint as well as the lubrication and nutrition of the cartilaginous structures.