An anatomical study of extensor expansion of thumb

Abstract

Introduction The presence of extensor expansion of the thumb has been reported in the literature and its mode of formation and contribution from the palmar muscles has been studied earlier. Yet, some authors deny its presence in the thumb. Objective This article evaluates the presence of the extensor expansion of thumb (EET) and its formation and contributions from palmar muscles. Methods Dissection was carried on the dorsum of the hand on 80 free upper limbs. The dorsum of thumb was carefully cleaned to look for the presence of extensor expansion. When present, its mode of formation and the contributing muscles were looked into. The mode of insertion of the extensor tendons onto the phalanges was also observed. Results In all specimens, EET had tendons of extensor pollicis longus (EPL) and extensor pollicis brevis (EPB) along its central axis. It received expansion from abductor pollicis brevis (APB; 100%) and flexor pollicis brevis (FPB; 41.2%) on the lateral side, and adductor pollicis (AP; 100%) and first palmar interosseous (PI; 50%) on the medial side. Attachment of EPL to distal phalanx (DP) was seen in 45%, and to both phalanges in 55%. EPB insertion onto proximal phalanx alone, DP alone, and both phalanges was in 46.2, 25, and 23.75%, respectively. Conclusion Extensor expansion was noted in all limbs studied. It is formed by the extensor tendons of the thumb with expansions from APB and AP on its margins. Contribution from the first PI and FPB was found to be variable. Differences in insertion of EPL and EPB to the phalanges were also noted.