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

Abstract

Background: 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). Materials and methods: 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 micrometre (Mitutoyo, Japan). The relationship between the AHFLP and the AIN was evaluated. Results: The AHFPL was found in 34 (32.1%) of the 106 forearms. The AHFPL arose from the flexor digitorum superficialis muscle in 16 (47.1%) forearms, the medial epicondyle of the humerus in 10 (29.4%) forearms and the coronoid process of ulna in 8 (23.5%) forearms. The average total length of the AHFPL was 94.11 ± ± 10.33 mm. The AIN was located lateral to the AHFPL in 3 (8.8%) forearms, posterolateral in 7 (20.6%) forearms and posterior in 24 (70.6%) forearms. Conclusions: 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.

Two muscular variations in the elbow associated with the anterior interosseous nerve

“…The presence of this uhPT type, i.e., originating with a single band from the biceps brachii, may shift the contraction force from the biceps brachii over the elbow joint, thus helping the biceps brachii flex the elbow. The ahFPL may originate from the medial epicondyle of the humerus, the coronoid process of the ulna or the flexor digitorum superficialis muscle [ 2 , 9 , 10 , 16 , 29 ]. In the present case report, the ahFPL was found to originate from the medial epincondyle, under the flexor digitorum superficialis.…”

“…The incidence of the ahFPL is quite high and ranges from 25 to 73.6%. [ 2 , 9 , 10 , 12 , 16 , 29 ]. The ahFPL arises during the development of the common flexor mass.…”

“…A similar situation can occur at the site of the median nerve ran between the two pronator teres heads, as well as the site where the median nerve crosses the palmaris longus tendon [ 20 , 21 ]. The accessory head of the flexor pollicis longus (ahFPL) may lead to entrapment neuropathy of the anterior interosseous nerve (AIN) [ 1 , 2 , 6 , 9 , 13 , 15 , 19 , 25 ].…”

“…Earlier studies on the morphological variability of the pronator teres (PT) focused mainly on the incidence of the ulnar head [ 7 , 11 , 14 , 18 , 21 , 28 ] or its relationship to the median nerve [ 7 , 11 , 14 , 18 , 21 , 28 ]. In contrast, variations in the flexor pollicis longus mainly concerned its morphological variability, and studies focused on the incidence of composite accessory heads and place of origin [ 1 , 2 , 6 , 9 , 13 , 15 , 19 , 25 ].…”

Coexistence of two accessory flexor pollicis longus heads or coexistence of two-headed flexor pollicis longus with an unrecognized anatomical structure?

“…Another pathology can result from an additional FPL head: compression of the interosseous nerve, which is located underneath this head. This can lead to an entrapment neuropathy known as anterior interosseous nerve syndrome (AIS) [ 3 ].…”