The mechanism of acute ulnar instability of the metacarpophalangeal joint of the thumb

Abstract

Acute ulnar instability of the metacarpophalangeal joint of the thumb is divided into two distinct groups, according to the history, clinical, radiological and operative findings. In the first group it is a soft tissue injury, as a result of an unresisted abduction force to the thumb. In the second group, following a resisted abduction force, there is avulsion of a bony fragment from the base of the proximal phalanx, which is the site of common insertion of both the ulnar collateral ligament and the adductor pollicis. This is confirmed by post-mortem studies.