Originally described by Martin in 1763 and later by Gruber in 1870, this is an anomalous innervation pattern occurring between the median and ulnar nerves in the forearm. Specifically, fibers from the median nerve cross over to the ulnar nerve in the forearm. Martin-Gruber anastomosis is the most common anomalous innervation of the hand, with an incidence of 15% to 28%.