Razi Journal of Medical Sciences

  Introduction: Congenital clasped thumb refers to spectrum of thumb anomalies with diversity of very mild cases with deficiency in extensor mechanism to severe abnormality in soft tissue and thenar muscles. Hypoplasia and absence of extensor pollicis brevis are usual and the patient is unable to extend the metacarpophalangeal joint of thumb.

  Case Report: The present patient is a 12-year-old left-handed boy who has been referred due to bilateral deformity in his thumbs. General physical examination was normal except in his thumbs. The first web space was narrow bilaterally. In spite of full passive range of motion in the joints of thumbs, he was unable to extend the metacarpophalangeal joint of thumbs but he could extend the interphalangeal and carpometacarpal joints. During surgical treatment it became clear that the cause of inability to extend the metacarpophalangeal joint of thumb was not hypoplasia or aplasia of extensor pollicis brevis but abnormal insertion of this tendon. Both extensor pollicis brevis and longus tendons were inserted to the distal phalanx. Reinsertion of extensor pollicis brevis to the base of proximal phalanx was done in the right hand.

  Conclusion: This report presents a new pathophysiology for congenital clasped thumb for the first time. According to these findings, in addition to aplasia or hypoplasia of extensor pollicis brevis tendon, abnormal insertion of extensor pollicis brevis to the distal phalanx can be an underlying causative factor in congenital clasped thumb.