EXTENSOR INDICIS BREVIS: A RARE ANATOMICAL VARIANT OF THE UPPER LIMB

Purpose: The purpose of this study was to further investigate the finding of a unique structure, the extensor indicis brevis muscle, found on the dorsum of the hand of a 74 year-old cadaver in the anatomy laboratory at the UNT Health Science Center. Methods: To completely reveal the muscle and its unique variation, first we identified that the normal extensor indicis muscle was missing in the forearm. Next, the tendon of the variant was located and traced to its muscle belly. Once located, skin and fascia were carefully removed on the dorsum of the hand to reveal its origin. Using precise dissection techniques, both the tendons attachment and the origin of the muscle belly were cleaned for identification of attachment sites. Results: The extensor indicis muscle normally originates from the posterior surface of the distal third of the ulna and the interosseus membrane of the forearm. Instead, the identified muscle was found exclusively on the dorsum of the left hand. It had an origin from the posterior surface of the scaphoid bone. This anomaly was found unilaterally. The extensor indicis muscle on the right forearm of this cadaver had a normal origin from the ulna and interosseous membrane. Both the extensor indicis brevis on the dorsum of the left hand and the extensor indicis muscle of the right forearm had normal insertions into the extensor expansion of the 2nd digit. Conclusions: The earliest known case report of this anatomical anomaly was found in a 19- year-old male patient in 1961 (Bingold). Other reports have indicated the extensor indicis brevis is often found bilaterally. However, in this cadaver it was revealed to be unilateral only. Diagnosis of patients with this rare variant has occurred from complaints of pain and swelling in the dorsum of the hand, often leading to its discovery during surgery. However, it is highly likely that some individuals would go throughout their lives without ever knowing they had an anomalous extensor indicis brevis muscle. From the initial description of the extensor indicis brevis, reports indicate the rarity of this anatomical variant to be found in approximately 0.6% to 2.6% of specimens.