Mechanics of Metacarpophalangeal Joint Extension.

PURPOSE: It is a common belief that extension of the metacarpophalangeal (MCP) joint of the finger is achieved via the sagittal bands acting as a sling or lasso to attach the extensor tendon to the base of the proximal phalanx. The aim of this study was to test the hypotheses that (1) division of the sagittal bands reduces extension force or torque of the MCP joint, and (2) division of the extensor tendon distal to the sagittal band will not affect the extension force or torque of the MCP joint. METHODS: Ten cadaver limbs were secured to a jig to allow for testing of the extension force of the MCP joints of the index, middle, and ring fingers. A 1-kg load was applied to the forearm extensor digitorum communis tendon and the extension force was measured with the MCP joint positioned at 0° (neutral extension) and again at 45° flexion. These measurements were repeated after the sagittal bands were divided in 15 specimens; in the other 15 specimens, the extensor tendon was divided just distal to the sagittal bands. RESULTS: After sagittal band division, extension force was similar in the 2 groups (0.11 N reduction after division with the MCP joints in neutral and 0.14 N in 45° flexion). There was significantly less extension force after division of the extensor tendon in both joint positions (0.95 N reduction after division in neutral extension and 0.66 N in 45° flexion). CONCLUSIONS: The sagittal bands do not primarily extend the MCP as a sling or lasso. The extensor tendon continuation to the extensor hood and middle phalanx is the major extension motor. The MCP joint is extended by the torque generated by the extensor tendon passing the joint carrying a force and possessing an extension moment arm. CLINICAL RELEVANCE: This principle should be correctly understood in the literature to ensure that clinical decisions related to injury and/or repair of the extensor tendon and sagittal bands are based on a sound understanding of their mechanics.