Limitation of thumb flexion due to an unusual insertion of the flexor pollicis longus

A case is described in which an anomalous insertion of the flexor pollicis longus tendon into the region of the carpal tunnel caused limited active flexion of the thumb in a child. This is the first reported case of this anomaly in which an apparent familial relationship has been noted, and the first reported case in the Caucasian race. Two similar cases in Japanese have been described. Abnormal insertion of a portion of the flexor pollicis longus must be included in the differential diagnosis of inadequate thumb flexion.