The prevalence and anatomical characteristics of the accessory head of the flexor pollicis longus muscle: a meta-analysis

Background and Objectives. The accessory head of the flexor pollicis longus muscle (AHFPL), also known as the Gantzer’s muscle, was first described in 1813. The prevalence rates of an AHFPL significantly vary between studies, and no consensus has been reached on the numerous variations reported in its origin, innervation, and relationships to the Anterior Interosseous Nerve (AIN) and the Median Nerve (MN). The aim of our study was to determine the true prevalence of AHFPL and to study its associated anatomical characteristics. Methods. A search of the major electronic databases PubMed, EMBASE, Scopus, ScienceDirect, and Web of Science was performed to identify all articles reporting data on the prevalence of AHPFL in the population. No date or language restriction was set. Additionally, an extensive search of the references of all relevant articles was performed. Data on the prevalence of the AHFPL in upper limbs and its anatomical characteristics and relationships including origin, insertion, innervation, and position was extracted and pooled into a meta-analysis using MetaXL version 2.0. Results. A total of 24 cadaveric studies (n = 2,358 upper limb) were included in the meta-analysis. The pooled prevalence of an AHFPL was 44.2% (95% CI [0.347–0.540]). An AHFPL was found more commonly in men than in women (41.1% vs. 24.1%), and was slightly more prevalent on the right side than on the left side (52.8% vs. 45.2%). The most common origin of the AHFPL was from the medial epicondyle of the humerus with a pooled prevalence of 43.6% (95% CI [0.166–0.521]). In most cases, the AHFPL inserted into the flexor pollicis longus muscle (94.6%, 95% CI [0.731–1.0]) and was innervated by the AIN (97.3%, 95% CI [0.924–0.993]). Conclusion. The AHFPL should be considered as more a part of normal anatomy than an anatomical variant. The variability in its anatomical characteristics, and its potential to cause compression of the AIN and MN, must be taken into account by physicians to avoid iatrogenic injury during decompression procedures and to aid in the diagnosis and treatment of Anterior Interosseous Nerve Syndrome.