REVIEW Clinical Prevalence of Palmaris Longus Agenesis: A Systematic Review and Meta-Analysis KAISSAR YAMMINE* Foot and Hand Clinic, The Orthopedic Department and the Center for Evidence-Based Orthopedic Research, Emirates Hospital, Jumeirah Beach Road, Dubai, UAE We report a systematic review and a proportion meta-analysis of prevalence studies evaluating the prevalence of palmaris longus agenesis (PLA) in the lit- erature. The overall PLA rate was defined to be the primary outcome. Second- ary outcomes were rates of PLA in relation to ethnicity, laterality, side, gender, age, and hand dominance. We identified 26 articles which met the inclusion cri- teria. Meta-analyses showed an overall PLA pooled rate of 20.25%, higher than the commonly reported overall rate of 15%. Our results also showed signifi- cantly lower pooled rates in Africans (11.3%) and East Asians (4.5%) when compared to Arab Middle Eastern population (41.7%). A subgroup analysis of the African group showed a pooled rate of 2.71%, the lowest, in the East and South East African population. The pooled rate was 26.3% among Caucasians, 26.16% among South and Southeast Asians and 34.13% among Turkish. In discordance with the literature, PLA was statistically more predominant on the right side. No significant differences in PLA rates were found for laterality, gen- der, the combination of gender and side or the combination of gender and lat- erality. The lowest rate of PLA found in East and South East African populations might be indicative of the subsequent phylogenetic degeneration of the palma- ris longus muscle in modern humans after the “Out of Africa” migration. Clin. Anat. 00:000–000, 2013. V C 2013 Wiley Periodicals, Inc. Key words: palmaris longus; evidence-based; anatomy INTRODUCTION Anatomy and Clinical Relevance Palmaris longus (PL) is a superficial flexor muscle of the forearm that lies anteriorly between the flexor carpi ulnaris and the flexor carpi radialis muscles. It is a spindle fusiform shaped muscle and takes origin from the medial epicondyle of the distal humerus within the common flexor tendon, and from adjacent intermuscular septa and deep ante-brachial fascia (Standring, 2005; Moore and Dalley, 2010). Its tendon attaches superficially to the flexor retinaculum and it fans out into the palmar aponeurosis of the hand. In humans, PL acts as a weak accessory flexor of the wrist. It also opposes strong shearing forces on the skin of the palm during gripping, aiding in cupping of the hand (Snell, 2007). In most cases, the distal tendon has a lateral slip that attaches to the superfi- cial surface of the abductor pollicis brevis that may contribute to thumb abduction (Gangata et al., 2010). PL is often described as one of the most variable muscles in the human body (Schneider, 1999; Kapoor et al., 2008). It is known that PL is only present in mammals and it is more developed in those who use their forelimb for locomotion (Vanderhooft, 1996). The PL muscle is classified as a phylogenetically retro- gressive muscle (Koo and Roberts, 1997) because it has a short belly with a long tendon; it is always pres- ent in the orangutan but is variably absent in other *Correspondence to: Kaissar Yammine, Head of the Foot and Hand Clinic, The Orthopedic Department and the Center for Evidence-Based Orthopedic Research, Emirates Hospital, Jumeirah Beach Road, P.O. Box 73663, Dubai, UAE. E-mail: kayseryam@yahoo.com Received 30 April 2013; Revised 25 May 2013; Accepted 4 June 2013 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/ca.22289 V V C 2013 Wiley Periodicals, Inc. Clinical Anatomy 00:00–00 (2013)