Combinatorial roles of extrinsic and intrinsic muscles in extension strength of the distal interphalangeal joint.

In this study, we observed a combinatorial relationship between intrinsic and extrinsic muscles to extend the distal phalanx via the terminal tendon of the finger extensor. Eleven fresh-frozen human cadaver digit rays were used in these experiments (four index, four middle, three ring digits). All fingers had full joint motion without degenerative diseases. Fingers were individually mounted in a custom built jig fixed by a 1-mm Kirschner wire that was driven into the rotational center of the distal interphalangeal (DIP) and proximal interphalangeal joints. Loads were applied to the extensor digit and dorsal interosseous tendons without flexor tendons, via sutures attached over low-friction pulleys. Extension forces that crossed the DIP joint were measured by a force transducer coupled with a materials-testing machine. We observed a steep inclination of the extensor forces produced by the intrinsic muscles, and there were no significant differences in comparisons among loads (200, 400, 600, and 800 g). The inclination slope increased with an increase of load (p < 0.001). Additionally, the inclinations of the non-linear phases among all finger types were not statistically different. Thus, the contribution between the extrinsic and intrinsic muscles in providing extensor forces to the distal phalanx was similar.