A Variant of Extensor Medii Proprius: A Case Report




Liu, Hao (Howe)
Rosales, Armando
Kirchhoff, Claire A.


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Purpose: The extensor medii proprius (EMP), a documented variation of human hand extensor muscles, originates from the distal 1/3 of the ulna and inserts on the dorsal aponeurosis (extensor expansion) of the 3rd manual digit. This case report describes an anomaly in which the EMP originates from the lunate, an origination reported in the medical literature. Methods: During routine gross anatomy dissection of the left hand of a 67-year old female who died of “stroke,” an unusual EMP muscle was observed and documented. Results: The flat, fleshy muscle originated from the lunate bone, narrowed into a flat tendon near the 3rd metacarpophalangeal joint, and continued distally to insert on the extensor expansion of the 3rd digit. A branch from the posterior interosseous nerve was traced to the EMP. We propose that this previously unreported variation be termed extensor medii proprius brevis (EMPB). Conclusions: Several anomalies found here have not been previously reported in the literature. First, the EMP originated at the lunate, which means the muscle does not cross the radiocarpal joint as described in previous reports. Second, the co-existence of the EMP with the extensor indicis or related muscles was not observed in this case. Third, the EMP tendon traveled deep to the intertendinous connection between the 2nd and 3rd tendon slips of the extensor digitorum muscle. Finally, this is the first time that the EMP was found to be innervated by a branch from the posterior interosseous nerve. In functional terms, the EMP/EMPB may act as an accessory MCP extensor. Due to its small size, however, the impact of EMP/EMPB on MCP extension may be negligible. Clinicians should still be aware of this variation, since swelling or tenderness of the muscle may lead to misdiagnoses of ganglion cysts or adipose tumors around this area of the dorsal hand.