Bilateral extensor medii proprius: A cadaveric case report

Bilateral extensor medii proprius

Abstract

During dissection of a formalin-fixed cadaver in the upper-limb unit of a gross anatomy laboratory, bilateral anomalous muscle bellies and tendons, identified as the extensor medii proprius (EMP), were observed in the posterior compartments of the forearms. The muscle originated from the distal one-third of the ulna and inserted on the extensor hood and proximal interphalangeal joint of the third digit, with an additional tendinous slip connecting to the extensor hood of the third digit and intertendinous connections to the second digit. Both tendons shared a canal with the extensor indicis (EI) and extensor digitorum (ED) tendons. The EMP was distinguished from the EI and ED by its insertion on the third digit and by a separate muscle belly, unlike the ED, which has a common origin and multiple tendons to digits two through five, and the EI, which inserts on the second digit. Although anomalies of the extensor forearm muscles are not uncommon, the literature documenting these variants remains limited. Recognition of these variants is clinically relevant because they can contribute to misdiagnosis when evaluating dorsal hand pain, affect surgical planning, or provide potential graft sources during reconstructive procedures. This report describes the bilateral EMP variant and discusses its prevalence, clinical significance, and possible embryological origin.