Occasional head of flexor pollicis longus muscle: a study of its morphology and clinical significance.

A cadaveric dissection study of 54 upper extremities to determine the incidence of occurrence, morphology and relations of the occasional head of the flexor pollicis longus muscle is presented. The occasional head of the flexor pollicis longus muscle was found to be present more frequently (66.66%) than absent. It mainly arose from the medical epicondyle of the humerus (55.55%) and the medial border of the coronoid process of the ulna (16.66%). It was found to be in close association with the median nerve (anteriorly) and the anterior interosseous nerve (posteriorly). The clinical implications of these findings are discussed viz. entrapment neuropathies of the median and anterior interosseous nerves, cicatricial contraction of the occasional head leading to flexion deformity of the thumb and the likely necessity to lengthen/release the occasional head in spastic paralysis of the flexor pollicis longus muscle.