Martin-Gruber revisited.

A new classification system for Martin-Gruber connections is described. This comprises four types of connection that can theoretically exist and that differ by their source and destination. The world literature has been reexamined and the incidence of Martin-Gruber connections was found to be 17% overall. By the new classification, 60% are type I, sending motor branches from the median to the ulnar nerve to innervate "median" muscles; 35% are type II, sending motor branches from median to ulnar nerves to innervate "ulnar" muscles; 3% are type III, sending motor fibers from the ulnar to the median nerve to innervate "median" muscles; and 1% are type IV, sending motor fibers from the ulnar to the median nerve to innervate "ulnar" muscles.