Anatomy of the extensor mechanism of the thumb in relation to the clinical presentation of extensor pollicis longus rupture

Background Extensor pollicis longus (EPL) tendon rupture is a debilitating condition that presents with impaired interphalangeal joint (IPJ) extension. Standard descriptions of EPL function state that it is the sole thumb IPJ extensor. It has been suggested that the intrinsic muscles of the thumb, acting through a dorsal extensor expansion (DEE), are also capable of producing IPJ extension which can result in a missed diagnosis of EPL rupture. It remains controversial whether the thumb possesses a DEE analogous to the DEE of the fingers. The aim of this study was to determine if a DEE of the thumb exists and to establish if intrinsic hand muscles insert into it to produce IPJ movement. Methods Nine cadaveric hands were dissected, and muscle function was assessed. Results A DEE was observed in all thumbs and its structure closely resembled the DEE of the fingers. The DEEs had clear margins that received contributions from intrinsic muscles of the thumb and EPB. Applying traction to the intrinsic muscle attachment consistently resulted in IPJ extension. Conclusions A DEE is present in the thumb that structurally and functionally resembles that of the fingers. Traction on the EPL tendon produced a compound movement including IPJ and metacarpophalangeal joint extension and supination of the entire first ray around the carpometacarpal joint. This compound movement is, in effect, the antagonistic movement to thumb opposition. This explains the profound loss of function associated with EPL rupture and suggests a more accurate test for EPL function than simply assessing for active IPJ extension. Level of evidence: Level IV, diagnostic study.