SELECTIVE BLOCK OF THE BRACHIALIS MOTOR POINT - AN ANATOMIC INVESTIGATION OF MUSCULOCUTANEOUS NERVE BRANCHING

Background and Objectives. Injections of neurolytic agents designed to block the musculocutaneous nerve often eliminate all elbow flexion mo vements, 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 brach ialis is the largest contributor to elbow flexion. This factor, togeth er with this muscle's lack of a role in supination, makes it the targe t of choice for controlling flexion spasticity. There are few descript ions of brachialis motor point blocks, and they fail to provide satisf actory instructions for the procedure. The goal of this study was to d etermine the brachialis motor point site and to quantitatively describ e its location. Methods. In this prospective, randomized study of 26 c adaver arms, the innervation site of the brachialis muscle from the mu sculocutaneous nerve was measured. Measurements were taken from the la teral epicondyle and were compared with the distance to the biceps mot or point. These lengths were normalized across subjects by dividing by the arm length (from lateral epicondyle to the acromion). Results. Th e brachialis was found to be innervated at approximately one third of the distance from the elbow to the acromion. This site is significantl y 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. Conclusions. An injection one third of the distance fro m the lateral epicondyle to the acromion along the medial aspect is re commended to provide best access to the brachialis motor point. By inj ecting from the medial aspect, one avoids the humerus (encountered in a lateral approach) and the need to pass through the biceps brachii (a s in an anterior approach).