M USCLES OF U PPER A RM AND E LBOW (Continued)

(distally), radial nerve (proximally), and radial recurrent arteries. The median nerve, ulnar nerve, and brachial artery run along the medial aspect of the upper arm, while the radial nerve runs laterally. The musculocuta-neous nerve travels more in the midline, between the biceps brachii and brachialis muscles. Key origin and insertion points of the muscles of the arm are pictured on Plate 2-7.

Coracobrachialis Muscle

A short, bandlike muscle of the upper arm, the coraco-brachialis, arises from the tip of the coracoid process

Latissimus dorsi muscle Biceps brachii tendons (cut)

obliquely over the brachial artery and median nerve to blend with the antebrachial fascia over the flexor group of the forearm (see Plate 2-8). The pull of the bicipital aponeurosis is largely exerted on the ulna.

Brachialis Muscle

This muscle arises from the lower half of the anterior surface of the humerus and the two intermuscular septa and lies deep to the biceps. Its upper extent has two pointed processes positioned on either side of the inser-tion of the deltoid muscle (see Plate 2-8). The muscular fibers converge to a thick tendon, which adheres to the capsule of the elbow joint and inserts on the tuberosity of the ulna and on the anterior surface of its coronoid process. Its major attachment is to the coronoid process about 2 mm distal from the articular margin. This muscle bulges beyond the biceps brachii muscle on

either side, and anterior to its medial border lie the brachial vessels and the median nerve. The medial half of this muscle is supplied by the musculocutaneous nerve, whereas the lateral portion is supplied by the radial nerve. The muscle’s main action is to flex the elbow. The brachialis has the largest cross-sectional area of any of the muscles that flex the elbow, but it has a poor mechanical advantage owing to its close proxim-ity to the axis of rotation. This natural internervous plane within the muscle allows it to be split during a routine anterolateral surgical approach to the humerus to come down on the anterior surface of the humerus, typically for fixation of a humeral shaft fracture.

Triceps Brachii Muscle

This large muscle with three heads occupies the entire dorsum of the arm (see Plate 2-9). Each head of the

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musculocutaneous nerve supplies both heads of the biceps brachii, whose function is unique in that it acts across both the shoulder and elbow joints. At the shoul-der, the muscle assists in forward flexion, joint stabiliza-tion (long head), and adducstabiliza-tion (short head). At the elbow, its main actions are to flex the elbow and supi-nate the forearm. The biceps acts as an elbow flexor particularly when the forearm is supinated and is a strong supinator with the elbow at least partially flexed and with the forearm in a more pronated position.

The bicipital aponeurosis, or lacertus fibrosis, formed from the more anterior and medial tendon fibers of the muscle, arises at the bend of the elbow and passes

Deltoid muscle (cut and reflected) Superior lateral brachial cutaneous nerve

of triceps Extensor carpi radialis brevis muscle

Capsule of glenohumeral joint Teres major muscle and tendon

Medial epicondyle of humerus and medial head of triceps brachii muscle Medial head of triceps brachii muscle

Lateral head of triceps brachii muscle (cut) Long head of triceps brachii muscle

radial nerve and the profunda brachii artery. The lateral head takes origin from the posterior surface and lateral border of the humerus above and lateral to the radial groove and from the lateral intermuscular septum.

Crossing the groove and concealing the radial nerve and deep brachial vessels, its fibers join in the common tendon insertion on the olecranon. The lateral head is the lateral border of both the quadrangular space and triangular interval. The medial head arises from the humerus entirely medial and below the radial groove from as high as the insertion of the teres major muscle to as low as the olecranon fossa of the humerus (see Plate 2-9). It also takes origin from the entire length of the medial intermuscular septum and from the lateral septum below the radial nerve groove. The medial head is deep to the other heads and is hidden by them. The

tendon of the muscle appears as a flat band covering its distal two fifths. It inserts on the posterior part of the olecranon and into the deep fascia of the forearm on either side of it.

All three heads of the triceps brachii are innervated by the radial nerve and have a primary action of elbow extension. The long and lateral heads are innervated by branches of the radial nerve that arise proximal to the radial groove, whereas the branch to the medial head originates distal to the radial groove and also innervates the anconeus. A radial nerve injury at the midshaft of the humerus, therefore, usually does not disrupt func-tion of the more proximally innervated long and lateral heads of the triceps. The humeral shaft can be surgically approached posteriorly by splitting the triceps muscle to come down onto the posterior cortex or by elevating

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triceps (long, lateral, medial) originates distal to the other, with a progressively larger area of origin. The long head arises by a strong tendon from the infragle-noid tubercle of the scapula. Its belly descends between the teres major and teres minor muscles and joins the lateral and medial heads of the triceps in a common insertion on the olecranon. The long head is a defining border of the triangular space (lateral border), which contains the circumflex scapular artery; the quadrangu-lar space (medial border), which contains the axilquadrangu-lary nerve and the posterior humeral circumflex artery; and triangular interval (medial border), which contains the

Pectoralis major muscle and tendon Cephalic vein Posterior antebrachial cutaneous nerve (from radial nerve)

Radial collateral artery Extensor carpi radialis longus muscle

Posterior antebrachial cutaneous nerve

Profunda brachii (deep brachial) artery Ulnar nerve

Triceps brachii muscle and tendon Lateral intermuscular septum Triceps brachii muscle

Biceps brachii muscle

Triceps brachii muscle

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fascia and extends across the elbow and the superior radioulnar joints. It is innervated by the terminal branch of the radial nerve that also innervates the medial head of the triceps. The function of this muscle has been the subject of some debate and includes assisting in elbow extension and stabilizing the elbow joint. Joint stabiliza-tion may be its primary role. The anconeus serves as a key landmark in the lateral or Kocher approach to the elbow, in which the interval between the extensor carpi ulnaris anteriorly and the anconeus posteriorly is uti-lized to approach the lateral or posterolateral aspect of the elbow.

Muscle Actions

The principal movements produced by the muscles of the arm are flexion and extension of the forearm at the elbow. The brachialis and biceps brachii muscles are the

principal flexors. In this action, the brachialis muscle is always active; the biceps brachii muscle becomes active against resistance and is most effective when flexion of the forearm is combined with supination. It is a powerful supinator of the forearm and one of the primary muscles for producing this movement. Exten-sion of the forearm is produced by the triceps brachii muscle and assisted by the anconeus muscle. The medial head of the triceps brachii muscle is usually active, and the lateral and long heads are recruited for extra powers.

Certain heads of these muscles are active at the shoulder joint. The long head of the biceps brachii muscle flexes the arm at the shoulder, and its tendon aids in stabilization of the joint. The long head of the triceps brachii muscle assists in extension and adduction of the arm.

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the muscle along its lateral border and reflecting all three heads of the muscle medially. This approach may afford more exposure than an anterolateral approach in more distal fractures of the humeral shaft.

Anconeus Muscle

This is a small, triangular muscle that arises from a broad site on the posterior aspect of the lateral epicon-dyle of the humerus (see Plate 2-9). Its fibers diverge from this origin and insert into the side of the olecranon and the adjacent one fourth of the posterior surface of the ulna. The muscle is deep to the dorsal antebrachial

CROSS SECTIONAL ANATOMYOF ELBOW

Pronator teres muscle

Brachioradialis muscle

Extensor carpi radialis longus muscle and extensor carpi radialis brevis muscle

Radial head Posterior bundle of ulnar collateral ligament

Joint capsule

Brachialis muscle and tendon

Trochlea Common flexor tendon

Bicipital aponeurosis (lacertus fibrosus)