Accessory head of flexor pollicis longus muscle in fetuses and adult cadavers and its relation to anterior interosseous nerve.

This study was conducted to examine the accessory head of flexor pollicis longus muscle (ahFPL) and its relation with the anterior interosseous nerve (AIN) in human fetuses and adult cadavers. Ninety fetus forearms and 52 adult cadaver forearms were dissected to evaluate the incidence, morphology, and innervation of the ahFPL. The ahFPL was observed in 29/90 (32%) of the fetus forearms and 20/52 (39%) of the adult cadaver forearms. The overall side incidence was 34.5% (49/142) among total forearms examined. On the other hand, the population incidence of ahFPL was 42% (19/45) in fetuses while it was 50% (12/24) in adult cadavers. So, the overall incidence in humans was 44.9% (31/69) in the population studied. Compression of the AIN in the forearm by the ahFPL is known as one of the causes of the anterior interosseous nerve syndrome (AINS). So, the relation of ahFPL with the AIN was evaluated, and in light of previous classifications a modified new classification is proposed. The most common relation detected in this study was Type IVa (71.4%) (AIN and its branches coursed posterior to the ahFPL). While Type I was not observed in this study, the incidences of Type II, Type III, and Type IVb (all AIN branches 'without AIN itself' coursed posterior to the ahFPL) were 2%, 14.3%, and 12.3%, respectively. The Types I, IVa, and IVb are thought to be associated with complete or incomplete types of AINS and Type III with incomplete type of AINS only.