BILATERAL APPEARANCE OF THE ABDUCTOR DIGITI MINIMI ACCESSORIUS ORIGINATING FROM THE PALMARIS LONGUS MUSCLES: A CASE STUDY

Background. The palmaris longus (PL) is a slender muscle which arises from the common flexor tendon on the medial epicondyle of the humerus, runs along the medial side of flexor carpi radialis, and attaches distally to the palmar aponeurosis. It consists of a short muscle belly proximally and a long tendon distally. Functionally, the PL aids the flexor carpi radialis and flexor carpi ulnaris in wrist flexion. The PL is known to be one of the most variable muscles in the body. Previous reports include cases of both unilateral and bilateral: reversal, double appearance, absence, variable size and position of muscle belly, and hypertrophy. Therefore, the clinician must consider the possibility of abnormal PL when conducting differential diagnosis of distal forearm pathology. Purpose (a): The purposes of this case study were to 1) examine the bilateral variations of the PL in a male cadaver at the University of North Texas Health Science Center (UNTHSC), and 2) determine the potential clinical significance of this variation. Methods (b): During a routine dissection of a 59-year old male cadaver in year 2005, an unusual PL muscle was discovered in both right and left upper extremities. The dissection was performed by physical therapy students at UNTHSC. Results (c): The PL muscle originated at the medial epicondyle of the humerus and split into two connected tendon bundles at the upper one-fourth of the anterior forearm. The tendon bundles then passed down into the lower one-third of the anterior forearm and separated completely, forming a proper PL tendon and a variant muscle mass –the belly of the ADMA muscle. The PL tendon passed through, as normally seen, to fuse with the palmar aponeurosis, while the ADMA continued distally and medially, passing underneath the palmaris brevis muscle but immediately anterior to Guyon's canal. The ADMA joined, but did not fuse with, the intrinsic abductor digiti minimi muscle, to insert into the medial base of the proximal phalanx of the little finger. Conclusions (d): This case presents a novel bilateral appearance of the ADMA originating from the PL. This is relevant to surgeons who perform operations on or using the PL tendon, and to clinicians diagnosing and treating afflictions of the distal forearm, especially with regard to issues concerning Guyon's canal.