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Purpose A 2-part biomechanical study was constructed to test the hypothesis that coronal morphology of the thumb metacarpophalangeal joint impacts the assessment of instability in the context of radial collateral ligament (RCL) injury. Methods Fourteen cadaveric thumbs were disarticulated at the carpometacarpal joint. Four observers measured the radius of curvature of the metacarpal (MC) heads. In a custom jig, a micrometer was used to measure the RCL length as each thumb was put through a flexion and/or extension arc under a 200 g ulnar deviation load. Strain was calculated at maximal hyperextension, 0°, 15°, 30°, 45°, and maximal flexion. Radial instability was measured with a goniometer under 45 N stress. The RCL was then divided and measurements were repeated. Analysis of variance and Pearson correlation metrics were used. Results The RCL strain notably increased from 0° to 30° and 45° of flexion. With an intact RCL, the radial deviation was 15° at 0° of flexion, 18° at 15°, 17° at 30°, 16° at 45°, and 14° at maximal flexion. With a divided RCL, instability was greatest at 30° of flexion with 31° of deviation. The mean radius of curvature of the MC head was 19 ± 4 mm. Radial instability was inversely correlated with the radius of curvature to a considerable degree only in divided RCL specimens, and only at 0° and 15° of flexion. Conclusions The RCL contributes most to the radial stability of the joint at flexion positions greater than 30°. The results suggest that flatter MC heads contribute to stability when the RCL is ruptured and the joint is tested at 0° to 15° of metacarpophalangeal flexion. Clinical relevance The thumb MC joint should be examined for RCL instability in at least 30° of flexion.

Fractures of the digits and the metacarpals are common injuries in athletes. The majority of these can be treated nonoperatively with careful attention to alignment, rotation, and shortening. Those requiring operative fixation are usually amenable to either K-wire, interfragmentary screw, or plate fixation. Whichever treatment pathway is chosen, the goal should be to provide adequate stability for healing while attempting to maintain mobility of the joints and digits. Dedicated rehabilitation is essential to optimizing functional outcomes, patient satisfaction, and return to sport.