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.