Radial instability of the metacarpophalangeal joint of the thumb. A biomechanical investigation.

Human cadaver thumbs were tested to evaluate stability of the radial side of the MP joint. The contributions of the dorsal capsule, radial collateral ligament, accessory collateral ligament, and volar plate were examined with the joint in 0 degrees and 30 degrees of flexion. At 0 degrees flexion, the average joint angulation increased 4 degrees following isolated radial collateral ligament transection and 6 degrees following isolated accessory collateral ligament transection. Release of both the accessory and radial collateral ligaments produced marked instability with joint angulation of at least 46 degrees. The accessory collateral ligament helped to stabilize the extended MP joint. There were no significant contributions to stability from the dorsal capsule and volar plate when the collateral ligaments were intact. In the laboratory setting, radial-side instability of the MP joint of the thumb requires transection of both the proper and accessory radial collateral ligament.