Selective block of the brachialis motor point. An anatomic investigation of musculocutaneous nerve branching.

Injections of neurolytic agents designed to block the musculocutaneous nerve often eliminate all elbow flexion movements, leaving the patient with a flail arm. In such patients, motor point blocks of the biceps brachii or brachialis muscle, or both, may be indicated. By virtue of its relative cross-section area, the brachialis is the largest contributor to elbow flexion. This factor, together with this muscle's lack of a role in supination, makes it the target of choice for controlling flexion spasticity. There are few descriptions of brachialis motor point blocks, and they fail to provide satisfactory instructions for the procedure. The goal of this study was to determine the brachialis motor point site and to quantitatively describe its location.In this prospective, randomized study of 26 cadaver arms, the innervation site of the brachialis muscle from the musculocutaneous nerve was measured. Measurements were taken from the lateral epicondyle and were compared with the distance to the biceps motor point. These lengths were normalized across subjects by dividing by the arm length (from lateral epicondyle to the acromion).The brachialis was found to be innervated at approximately one third of the distance from the elbow to the acromion. This site is significantly different (P < .05) from that of the biceps brachii, which was found to be located at approximately half of the distance from the elbow to the acromion.An injection one third of the distance from the lateral epicondyle to the acromion along the medial aspect is recommended to provide best access to the brachialis motor point. By injecting from the medial aspect, one avoids the humerus (encountered in a lateral approach) and the need to pass through the biceps brachii (as in an anterior approach).