Assessing ulnar instability of the metacarpophalangeal joint of the thumb.

Seven hundred and fifty normal thumbs were examined to determine the normal range of radioulnar mobility of the metacarpophalangeal joint with the joint in full extension, 15 degrees of flexion, and full flexion. Full flexion was the position of greatest stability. Twenty-five adult unembalmed cadaver thumbs were examined to determine the optimal position for testing the metacarpophalangeal joint for ulnar collateral ligament stability. When radial stress was applied to the metacarpophalangeal joint of 20 specimens after cutting the adductor aponeurosis and capsule, minimal ulnar instability was found. However, further sectioning of the ulnar collateral ligament revealed marked ulnar instability most significantly when the thumbs were examined in full metacarpophalangeal flexion. Finally, division of the adductor aponeurosis, dorsal capsule, ulnar collateral ligament, accessory collateral ligament, and volar plate resulted in complete instability of the metacarpophalangeal joint in all positions tested.