Impact of Joint Position and Joint Morphology on Assessment of Thumb Metacarpophalangeal Joint Radial Collateral Ligament Integrity

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 degrees, 15 degrees, 30 degrees, 45 degrees, 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 degrees to 30 degrees and 45 degrees of flexion. With an intact RCL, the radial deviation was 15 degrees at 0 degrees of flexion, 18 degrees at 15 degrees, 17 degrees at 30 degrees, 16 degrees at 45 degrees, and 14 degrees at maximal flexion. With a divided RCL, instability was greatest at 30 degrees 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 degrees and 15 degrees of flexion. Conclusions The RCL contributes most to the radial stability of the joint at flexion positions greater than 30 degrees. The results suggest that flatter MC heads contribute to stability when the RCL is ruptured and the joint is tested at 0 degrees to 15 degrees of metacarpophalangeal flexion. Clinical relevance The thumb MC joint should be examined for RCL instability in at least 30 degrees of flexion. Copyright (C) 2015 by the American Society for Surgery of the Hand. All rights reserved.