Locking of the metacarpophalangeal joint of the thumb.

We clinically examined the hands of twenty-three patients and experimented with amputation specimens to clarify the characteristic features and pathomechanism of locking of the metacarpophalangeal joint of the thumb. In all of the patients the locking had resulted from forced hyperextension of the thumb. Every injured thumb showed a mild hyperextension deformity of the metacarpophalangeal joint, with the joint having become fixed. Manual reduction was successful in seven patients, but the other sixteen required open reduction, during which we found that the proximal palmar ligament was ruptured transversely with its distal part riding over the volar prominence of the radial condyle of the metacarpal together with the accessory collateral ligament. These dislocated parts of the ligament formed a constricting bundle on the distal joint surface of the condyle that prevented closed reduction. The locking was released by cutting the constricted ligament bundle. Our description of this mechanism is supported by the experimental evidence that we obtained from examining the amputation specimens.