The biomechanical properties of the finger metacarpophalangeal joints to varus and valgus stress.

Purpose: The purpose of this study was to determine the normal biomechanical properties of the passive capsuloligamentous structures about the finger metacarpophalangeal (MCP) joints subjected to dynamic varus/valgus loading and to equate these findings to the clinical situation.

Methods: The finger MCP joints from 9 fresh-frozen cadaver hands were tested in a custom-designed testing apparatus that applied a varus/valgus force in each direction. Testing was performed at 0 degrees, 30 degrees, 60 degrees, and 90 degrees of MCP joint flexion. Load-displacement curves were generated for each specimen. A nonlinear hysteresis curve was apparent on loading and unloading. A region of collateral ligament laxity was identified whereby minimal torque (< 0.5 Nm) caused progressive joint angulation. Subsequently incremental load was required to produce further joint angulation. The slope of this region was used to calculate early and late collateral ligament stiffness.

Results: The index and long fingers showed a significant decrease in the region of collateral ligament laxity between 0 degrees and 90 degrees. The long finger collateral ligament laxity also diminished significantly between 30 degrees and 90 degrees. The collateral ligament laxity did not significantly change in the ring and small digits throughout MCP joint flexion. The early or late phase of collateral ligament stiffness was not affected by the amount of MCP joint flexion across any of the digits, except in late radial collateral ligament stiffness of the long finger between 0 degrees and 60 degrees.

Conclusions: The additional stability and clinical observation of tightening of the MCP in flexion appears related to the decreased laxity of the collateral ligaments and not to alterations in the biomechanical properties of the collateral ligaments.