Morphometric study and occurrence of the palmaris longus muscle in human fetuses

The palmaris longus muscle (PLM) is fusiform, slender and is located on the anterior face of the forearm, between the flexor carpi ulnaris and the flexor carpi radialis muscles, having proximal insertion into the medial epicondyle of the humerus and distally in the palmar aponeurosis. Phylogenetically, the PLM is considered a vestigial muscle in regression because it has a short belly and a long tendon1,2 and its function is considered insignificant in humans,3 although it weakly assists in flexing the wrist and resisting adduction and abduction movements.4 Numerous variations occur in the palmaris longus muscle and may even be absent unilaterally or bilaterally. The absence of the palmaris longus muscle seems to be hereditary, with dominant character, incomplete penetrance and lateral variability.2 When present, the PLM may present variations in the muscular belly and distal tendon. The muscular belly may be central (normal shape), distal (inverted shape), digastric or fully muscular.5 The distal tendon can be divided into two or three parts, that is, bitendinous or tritendinous, being referred to as an “accessory” muscle.6–11 The knowledge of the anatomical variations of the palmaris longus muscle is of great importance in the surgical area, since the PLM tendon meets the necessary length, diameter, availability and can be used in grafts without producing any functional deformity,12 for correction of upper eyelid ptosis, lower eyelid, lips and chins.3 The aim of the present study was to determine the frequency of the palmaris longus muscle, its morphometry and relations between its muscular and tendon parts.