Prevalence of accessory head of flexor pollicis longus muscle and its relation to anterior interosseous nerve in Thai population.

A detailed description of the accessory head of flexor pollicis longus muscle (AHFPL) in the Thai population has not been reported. Because it is one of the causes of anterior interosseous nerve syndrome (AINS), a study was carried out on 120 Thai cadavers (70 embalmed, 50 fresh; 78 male, 42 female) to elucidate the prevalence of AHFPL, its morphology and relationship with the anterior interosseous nerve (AIN). The prevalence of AHFPL was 62.1% (149/240) with 74.5% (111/149) of its origin on medial epicondyle, 23.5% (35/149) on coronoid process and 2% (3/149) on flexor digitorum superficialis muscle. One hundred percent of its insertion was on the ulnar border of flexor pollicis longus tendon, and it was 98% (146/149) fusiform-shaped and 2% (3/149) slender shaped, with a diameter between 0.8-16.0 mm (average 6.7 mm), averaging 6.5 mm on the right and 4.2 mm on the left. The right was significantly statistically larger than the left (P < 0.05). The average distance from the mid-point of the distal wrist crease to the insertion point of AHFPL was 12.8 cm. Four patterns of relationship with AIN were noted including: 1) I AIN passed anterior to AHFPL, 13.4% (20/149); 2) AIN passed lateral to AHFPL, 65.8% (98/149); 3) AIN passed posterior to AHFPL, 8.1% (12/149); and 4) AIN passed both lateral and posterior to AHFPL, 12.8% (19/149). We believe that the latter two patterns (3 and 4) with AIN passing posteriorly would be more likely to be associated with AINS due to anatomic considerations.