Functional anatomy of the deep motor branch of the ulnar nerve.

Our presently inadequate knowledge of the functional anatomy of the deep branch of the ulnar nerve handicaps our management of ulnar nerve lesions. The extensive anatomical variations in the distribution of this nerve preclude adherence to a textbook pattern of innervation. Electrophysiological recordings of single sensory fibers in monkeys along with fascicular stimulation studies provided novel information about the functional organization of the deep branch of the ulnar nerve: (1) there is a wide range of sensory fibers not currently recognized by textbook descriptions; (2) the innervation pattern is far more diffuse and covers greater areas than previously suspected; (3) at the distal forearm level the motor component of the deep ulnar branch is distributed throughout the entire ulnar nerve and is not restricted to specific fascicles; (4) contrary to previous assumptions, the deep branch of the ulnar nerve is mixed, and not purely motor. In fact, it may be mostly sensory. Afferent fibers arise from muscle, joints, deep subcutaneous tissues and even skin. These findings suggest that it is unwise to look at any nerve as purely motor or having a set innervation pattern, and emphasize the pressing need for objective preoperative and/or intraoperative functional assessment in peripheral nerve surgery.