Ulnar tendinous interconnection in type IIIA thumb hypoplasia--a pollex adductus.

We present an anatomic variation of type IIIA thumb hypoplasia in which a tendinous interconnection between the flexor pollicis longus and the extensor pollicis longus occurred on the ulnar aspect of the thumb. Interconnection between extrinsic flexor and extensor systems is well known in type IIIA thumb hypoplasia and has been reported on the radial side (pollex abductus). In our patient the ulnar location of the interconnection remained unrecognized until recurrence of the deformity in adolescence.