Bilateral subluxation of the base of the thumb secondary to an unusual abductor pollicis longus insertion: a case report.

Bilateral subluxation of the trapeziometacarpal joint was related to abnormal insertion of the abductor pollicis longus (APL) tendon and an atrophic extensor pollicis brevis tendon. The APL tendon had four slips, all of which inserted into the fascia of the abductor pollicis brevis muscle distal and palmar to the trapeziometacarpal joint. Active pinch of the thumb resulted in subluxation of the trapeziometacarpal joint. To prevent reciprocal distal deformity, the proximal thumb was held in dynamic balance by attaching two slips of the APL tendon to the radiodorsal base of the first metacarpal. One tendon slip supplemented the tendon of the extensor pollicis brevis muscle. The lax capsule of the trapeziometacarpal joint was reinforced with the remaining tendon slip. The patient retains excellent bilateral function without subjective weakness after surgery.