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A variant extensor indicis muscle and the branching pattern of the deep radial nerve could explain hand functionality and clinical symptoms in the living patient.

The purpose of this study is to document the topographic anatomy of an extensor indicis (EI) muscle with a double tendon and the associated distribution of the deep branch of the radial nerve (DBRN). Both EI tendons were positioned deep to the tendons of the extensor digitorum as they traversed the dorsal osseofibrous tunnel. They then joined the medial slips of the extensor expansion of the second and third digits. In all other dissected forearms, a tendon of the EI muscle joined the medial slip of the extensor expansion to the index finger. The DBRN provided short branches to the superficial extensor muscles, long branches to the abductor pollicis longus and extensor pollicis brevis muscles, and terminated as the posterior interosseous nerve. Descending deep to the extensor pollicis longus muscle, the posterior interosseous nerve sent branches to the extensor pollicis brevis and EI muscles. Understanding of the topographic anatomy of an EI with a double tendon, and the associated distribution of the DBRN, may contribute to accurate diagnosis and treatment of hand lesions.