Variation in the morphology of the extensor pollicis longus and extensor digitorum muscles: A case report

Abstract

This case report aims to highlight the variation in the morphology of the extensor pollicis longus (EPL) and the extensor digitorum (ED) muscles. EPL originates from the posterolateral surface of the ulna, and the ED originates from the lateral epicondyle of the humerus. Both of these muscles are innervated by the radial nerve. EPL’s primary function serves to extend the pollex, while the ED serves to aid in the extension of the metacarpal phalangeal, proximal interphalangeal, and distal interphalangeal joints of digits 2 through 5. During routine anatomical dissections of 48 upper limbs from 24 cadavers (n=24) at Oakland University William Beaumont School of Medicine, one adult female (75‐years‐old) was observed to have an EPL with an accessory tendon attaching on the 2nd metacarpal‐phalangeal joint and an accessory tendon of the ED inserting on the distal phalanx of the pollex unilaterally on the left side. The EPL is known to be one of the most anatomically consistent structures with its tendon, only inserting into the pollex (Kim, Lee, & Baek). This variation of the EPL was described as extensor pollicis et indicis accessorius, which was found in 1.2% of the cadavers (n=431) in Yoashida’s study. Although there are a few isolated patient cases describing the similar ED variation, such as in Hanrahan et al., very few results have been found regarding the prevalence of the accessory ED muscle belly in the general population. It is likely that the ED tendon inserting into the pollex weakly assists with the extension function. Additional variations of the extensor indicis muscle providing an accessory tendon to the 3rd digit and extensor digiti minimi muscle providing an accessory tendon to the 4th digit were noticed bilaterally on the same cadaver. The knowledge of these variations will be vital in proper surgical planning for tendon transfers, grafts, and transplantations.