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Folia Morphol (Warsz). 2018 Oct 9. doi: 10.5603/FM.a2018.0091. [Epub ahead of print]

Accessory head of the flexor pollicis longus muscle: anatomical study and clinical significance.

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Universidad Industrial de Santander, Carrera 32 N. 29-31, 630001 Bucaramanga, Colombia.



The accessory head of the flexor pollicis longus (AHFPL) has an oblique trajectory from medial to lateral aspect of the forearm below the flexor digitorum superficialis muscle and then joins the flexor pollicis longus muscle. When the anterior interosseous nerve (AIN) courses underneath the muscle belly of the AHFPL an entrapment neuropathy may occur, known as anterior interosseous nerve syndrome (AINS).


This descriptive cross-sectional study evaluated 106 fresh upper extremities. When the AHFPL was present, its fascicle was traced up to evaluate the origin site. The morphometric variables were measured using a digital micrometer (Mitutoyo, Japan). The relationship between the AHFLP and the AIN was evaluated.


The AHFPL was found in 34 (32.1%) of the 106 forearms. The AHFPL arose from the flexor digitorum superficialis muscle in 16 forearms (47.1%), the medial epicondyle of the humerus in ten forearms (29.4%) and the coronoid process of ulna in eight forearms (23.5%). The average total length of the AHFPL was 94.11 ± 10.33 mm. The AIN was located lateral to the AHFPL in three forearms (8.8%), posterolateral in seven forearms (20.6%) and posterior in 24 forearms (70.6%).


This study performed in a South American population sample revealed a prevalence of the AHFPL in a lower range compared to previous studies in North Americans and Asians. The AIN coursed more frequently underneath the muscle belly of AHFPL. This finding has clinical significance in the onset of the AINS and the subsequent surgical procedure for the AIN decompression.


anatomic variation; anterior interosseous nerve; flexor pollicis longus; nerve compression syndromes; orthopedic procedures

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