The anatomy of Wassel type IV-D thumb duplication.

We describe Wassel type IV-D thumb duplication anatomy after surgery on 11 affected children (12 hands, seven boys (eight hands) and four girls). We studied the structure and course of the flexor pollicis longus tendon and its action at the joint. Four patients had secondary deformity associated with an absent A2 pulley and a tendon that clung to the radial side of a small thumb. In patients with primary deformity, the flexor tendon sheath became membranous in the A2 area and attached to neighbouring sites on the opposite side of the proximal phalanx. In the proximal A2 area, the tendon divided - one division attached on the ulnar side of the distal phalanx base; the other, the base of the radial side. There was slight ulnar angulation of the distal phalanx on the radial portion of the duplication and slight ulnar angulation on the radial portion. LEVEL OF EVIDENCE: V.