The Influence of Thumb Metacarpophalangeal Joint Rotation on the Evaluation of Ulnar Collateral Ligament Injuries: A Biomechanical Study in a Cadaver Model

PurposeTo determine whether variation in thumb metacarpophalangeal (MCP) joint pronosupination influences perceived ulnar collateral ligament (UCL) stability during clinical stress testing.MethodsTwelve fresh-frozen specimens underwent sequential evaluation for the following conditions: ligament intact (LI), proper UCL deficient (-pUCL), and proper and accessory UCL deficient (*UCL). Valgus stress testing was completed in both 0° and 30° MCP joint flexion for thumb pronation, neutral, and supination.ResultsCompared with neutral MCP joint rotation, supination decreased and pronation increased stability such that established treatment guidelines could be incorrectly applied. During evaluation in supination and 0° flexion, 9/12 -pUCL had greater than 35° laxity and, similarly, the mean laxity of -pUCL was similar to the *UCL group in neutral rotation and 0° flexion, incorrectly suggesting a complete ligament tear. In comparison, mean laxity of the *UCL in pronation and 0° flexion was not different than -pUCL in neutral rotation and 0° flexion, emphasizing the stabilizing effect of pronation.ConclusionsThumb MCP joint pronosupination significantly influenced the evaluation of joint stability, where pronation improved valgus stability in contrast to supination that tended to increase joint instability, In pronation and 0° flexion, a complete UCL injury could be misdiagnosed as a partial injury. In supination and 30° flexion, an intact UCL could be misdiagnosed as a partial UCL injury. In supination and 0°, a partial UCL injury could be misdiagnosed as a complete UCL injury.Clinical relevanceAccurate evaluation of thumb UCL stability is critical for guiding treatment. Variations in thumb MCP joint rotation during stress testing may influence clinical interpretation and, therefore, we recommend standardization of testing with the thumb MCP joint in neutral rotation.