Background: Variations in musculature are of interest not only to the anatomist but also to the surgeon, insofar as they can be a source of error in both diagnosis and treatment. Muscular anomalies in the lower arm and in the hand have frequently been described in the literature; some of the lesions they can be confused with are ganglia and other soft tissue tumors.
Case report: In the course of preparation of a small, fine-jointed right upper extremity, an accessory muscle was discovered in the forearm and hand region; it branched off from the extensor carpi radialis brevis (ECRB) muscle. Its tendon crossed over the extensor retinaculum and attached to the dorsal digital expansion of the index finger. Both muscles were innervated by a branch of the deep branch of the radial nerve. The observations made here supplement our knowledge of variations in the antebrachial and dorsal regions of the arm and hand, which should be quite useful in hand surgery.