The extensor medii proprius muscle

The extensor indicis is a deep muscle of the posterior group of the forearm, which originates from the interosse­us membrane and the distal part of the body of the ulna. Its tendon passes under the fourth compartment of the extensor retinaculum and projects into the distal aponeurosis of the index finger. In the literature, many studies have discussed the clinical significance of the variations of this muscle in the region of plastic reconstruction of the hand and upper limb surgery. The present case report presents an additional muscle, which was observed in the posterior forearm region dur­ing a routine anatomical dissection of the left upper limb of a 69-year-old female cadaver. The muscle originated from the distal part of the extensor indicis muscle and its bundles ran parallel in distal direction. The final ten­don passed through the fourth extensor compartment along with the extensor indicis and the extensor digitorum and inserted into the dorsal aspect of the capsule of the metacarpophalangeal joint of the middle finger. The dis­section revealed that this muscle was innervated by the radial nerve. Numerous variations of the extensor indicis muscle have been described - absence, extremely distal origin, the presence of two heads or complete duplication of the muscle, the existence of extensor indicis brevis muscle, etc. The reported incidence of the extensor medii proprius muscle in cadaveric dissections ranges from 0% to 12%. Knowledge of the described muscle is clinically important, because it may be injured by inappropriate dissection or misdiagnosed as tumor-mimicking lesion. Furthermore, the tendon of this additional muscle may be used as a flap in reconstructive hand surgery.