Applied Surgical Anatomy of the Arm ‌

Overview

The critical neurovascular structures in surgery of the arm do not stay neatly in one operative field, but cross from compartment to compartment as they course down the arm. Therefore, it is easiest to view the anatomy of the arm as consisting of two major muscle compartments, flexor and extensor, that share responsibility for three major nerves and arteries (Fig. 2-39).

Muscle Compartments

1. The anterior flexor compartment contains three muscles: The coracobrachialis, the biceps brachii, and the brachialis. Two are flexors of the elbow; all are supplied by the musculocutaneous nerve.

2. The posterior extensor compartment consists of one muscle, the triceps brachii, which is supplied by the radial nerve. In the distal two-thirds of the arm, the muscle compartments are separated by lateral and medial intermuscular septa.

Nerves

1. The radial nerve, which is the key surgical landmark in the arm, is the continuation of the posterior cord of the brachial plexus. It begins behind the axillary artery at the shoulder, runs along the posterior wall of the axilla (on the subscapularis, latissimus dorsi, and teres major muscles), and then passes through the triangular space between the long head of the triceps muscle and the shaft of the humerus beneath the teres major muscle. In the arm, the nerve lies in the spiral groove on the posterior aspect of the humerus between the lateral and medial (deep) heads of the triceps muscle. After crossing the back of the humerus and giving off branches to the lateral head and the lateral part of the medial head of the triceps, the radial nerve pierces the lateral intermuscular septum, entering the anterior compartment. At this point, the nerve may be vulnerable to distal locking bolts inserted from the lateral side of the arm. The nerve lies between the brachioradialis and brachialis muscles as it crosses the elbow joint. There, it supplies the brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, and anconeus muscles (see Figs. 2-51 and 2-52). Although a radial nerve palsy is not uncommon following fractures of the humeral shaft, the vast majority of these are due to a neurapraxia. Exploration of the nerve is, therefore, not mandatory if a nerve palsy is present following fracture.17 The presence of a nerve palsy following reduction in a patient without an initial neurological lesion is a good indication for exploration as the nerve may have become trapped between the bony fragments during reduction.

    

    

   Figure 2-39 Schematic diagram of the upper arm. The compartments of the arm are shown. The muscles are removed partially to show the course of the radial, ulnar, and median nerves as they run down the arm. The relationships of the nerves to the compartments and septa are seen.

    

2. The median nerve remains in the anterior compartment, anteromedial to the humerus. It runs with the brachial artery, lateral to it in the upper arm and medial to it in the cubital fossa.

3. The ulnar nerve lies behind the brachial artery in the anterior

compartment of the upper half of the arm. It pierces the medial intermuscular septum about two-thirds of the way down the arm to enter the posterior compartment, where it lies with the triceps muscle. It then travels on the back of the medial epicondyle of the humerus, where it is almost subcutaneous in location. Similar to the median nerve, it has no branches in the arm (see Figs. 2-43, 2-49, and 2-51).

Arteries

The vascular organization of the arm is relatively simple; each nerve takes one artery with it.

1. The brachial artery runs with the median nerve down the medial border of the arm under the biceps brachii muscle and onto the brachialis muscle. The artery can be palpated along its entire length, because the deep fascia of the arm is the only medial covering. The artery lies medial to the humerus in the upper two-thirds of the arm. At the elbow, it curves laterally to lie over the anterior surface of the bone, where it may be damaged in supracondylar fractures of the humerus (Figs. 2-40 and 2-41).

    

    

    

   Figure 2-40 Superficial layer of muscles of the arm. Note the course of the brachial artery and the median and ulnar nerves. The brachial artery starts medial to the median nerve. In the distal part of the arm, it moves lateral to the median nerve before entering the cubital fossa.

    

    

   Figure 2-41 The anterior fibers of the deltoid have been removed. The pectoralis major and minor have been resected at their insertions. Note the relationship of the nerves to the teres major, subscapularis, and latissimus dorsi, as well as the point where the musculocutaneous nerve enters the coracobrachialis muscle. Distally, note the position of the brachial artery and median nerve at the tendinous insertion of the biceps.

    

2. The profunda brachii artery runs with the radial nerve, supplying the triceps brachii muscle (see Figs. 2-47 and 2-48).

3. The ulnar collateral artery runs with the ulnar nerve. The three arteries anastomose freely with one another around the elbow joint.

Landmarks and Incision

Incisions

A longitudinal incision on the anterior aspect of the arm closely parallels the relaxed skin tension line (lines of cleavage of the skin). More proximally, however, the same incision crosses perpendicular to the relaxed skin tension line. The cosmetic appearance of anterior scars, therefore, is variable and dependent on their location.

A longitudinal incision on the posterior aspect of the humerus crosses the relaxed skin tension lines at almost 90 degrees. Scars made by posterior incisions are likely to be broad.

Superficial Surgical Dissection

Anterior Approach to the Humerus

Proximally, the internervous plane lies between the deltoid muscle (which is supplied by the axillary nerve) and the pectoralis major muscle (which is supplied by the lateral and medial pectoral nerves; see Anterior Approach in Chapter 1). Distally, the approach involves the muscles of the flexor compartment of the arm (Figs. 2-42 through 2-44; see Figs. 2-40 and 2-

41).

The coracobrachialis is a largely vestigial muscle arising from the coracoid process (see Applied Surgical Anatomy of the Anterior Approach in Chapter 1).

The biceps brachii is a powerful flexor of the elbow and supinator of the forearm (see Applied Surgical Anatomy of the Anterior Approach in Chapter 1).

The brachialis is the main elbow flexor, the workhorse of the upper arm. The biceps only really comes into play when extra strength or speed of flexion is required.

The surgical importance of the brachialis lies in its nerve supply. The lateral part of the muscle is supplied by the radial nerve, and the medial part is supplied by the musculocutaneous nerve. Thus, the muscle can be split longitudinally without either side being denervated. Because the musculocutaneous nerve is the major nerve supply to the brachialis, even cutting the radial nerve supply to the muscle seems to have little clinical effect. That is why the plane between the brachialis and the adjacent lateral muscle, the brachioradialis, is useful in surgery.

Posterior Approach to the Humerus

The posterior approach involves splitting the triceps brachii muscle (Figs. 2-45 through 2-49).

Figure 2-42 The biceps muscle has been removed at its proximal origins—its conjoined tendon and long head. A portion of the coracobrachialis has been removed to reveal the musculocutaneous nerve running on the brachialis muscle, supplying it. The median nerve and ulnar nerve course through the arm without supplying its muscles.

Figure 2-43 The central portion of the brachialis and the extensor carpi radialis longus have been resected to reveal the distal humerus and the course of the radial

nerve as it pierces the lateral intermuscular septum to enter the anterior compartment. The radial nerve continues distally into the elbow before entering the supinator muscle. Medially, the relationships of the median nerve, brachial artery, and ulnar nerve are revealed. The median nerve is anterior to the brachial artery. The ulnar nerve, situated posteriorly, penetrates the medial intermuscular septum to enter the posterior compartment of the arm. The partially resected flexor–pronator group reveals the deeper structures at the level of the elbow.

Figure 2-44 The origins and insertions of the muscles of the arm. Brachialis. Origin. Lower two-thirds of anterior surface of humerus. Insertion. Coronoid process and tuberosity of ulna. Action. Flexor of forearm. Nerve supply. Musculocutaneous and radial nerves.

Figure 2-45 The anatomy of the posterior aspect of the arm. Note the cleavage plane between the long and lateral heads of the triceps.

Figure 2-46 The most posterior portion of the deltoid muscle has been removed to reveal the origin of the lateral head of the triceps. Triceps Brachii. Origin. Long head from infraglenoid tuberosity of scapula. Lateral head from posterior and

lateral aspect of humerus. Medial (deep) head from lower posterior surface of humerus. Insertion. Upper posterior surface of olecranon. Action. Extensor of forearm. Weak adductor of shoulder. Nerve supply. Radial nerve.

Figure 2-47 The central portion of the lateral head of the triceps has been removed to reveal the courses of the radial nerve and profunda brachii artery in the spiral groove. The fibers of the lateral head of the triceps arise from the lateral lip of the spiral groove. The medial head arises from the medial side of the spiral groove. the nerve lies between these two muscle origins. Detail of the relationship among the radial nerve, the axillary artery, and the profunda brachii artery (inset). The axillary artery becomes the brachial artery on the anterior surface of the humerus. There it gives off a branch, the profunda brachii artery, which continues posteriorly with the radial nerve through the triangular interval and the spiral groove.

Figure 2-48 Resection of the proximal half of the triceps. The radial nerve and profunda brachii artery run in the spiral groove between the origins of the lateral and deep heads of the triceps. The nerve and vessel penetrate the lateral intermuscular septum before entering the anterior compartment of the arm. The ulnar nerve pierces the medial intermuscular septum to gain entrance to the posterior compartment of the arm.

Figure 2-49 The entire triceps muscle has been removed, uncovering the entire posterior surface of the humerus. The medial and lateral intermuscular septa and the nerves that penetrate them are seen.

Figure 2-50 The lateral aspect of the humerus, with the overlying superficial cutaneous nerves.

Figure 2-51 The posterior aspect of the humerus and elbow joint and the course of the ulnar nerve. The lateral intermuscular septum runs beneath the brachioradialis. The main continuation of the radial nerve is the posterior interosseous nerve, which pierces the supinator muscle through the arcade of Frohse.

Figure 2-52 The lateral intermuscular septum and the course of the radial nerve as it passes from the spiral groove through the intermuscular septum to emerge in the forearm from between the brachialis and the brachioradialis. The muscles covering the posterolateral aspect of the joint have been removed to reveal the joint capsule.

Figure 2-53 The muscles have been removed completely, showing the origins of the musculature of the posterior humerus.

The long head of the triceps brachii receives its radial nerve supply high up in the axilla, close to its origin; the lateral head receives its supply lower, at the upper level of the spiral groove. The two heads can be split up to the level of the spiral groove without compromising the nerve supply of either (see Fig. 2-53; see Figs. 2-45 through 2-49).

The medial (deep) head has a dual nerve supply. The medial half receives fibers from the ulnar nerve. These fibers originate from the radial nerve and run alongside the ulnar nerve, so closely bound to it that they once were thought of as branches of the ulnar nerve. They actually are radial fibers that are “hitchhiking” in the ulnar nerve substance.18

The lateral half of the medial head receives its nerve supply from the main trunk of the radial nerve as it crosses the back of the humerus in the spiral groove. Because of its dual nerve supply, the medial head may be split longitudinally to expose the posterior surface of the humerus.

Special Anatomic Points

In some patients, the coracobrachialis muscle has an additional head that attaches to the ligament of Struthers.19 This ligament connects a supracondylar spur of bone to the medial epicondyle of the humerus. It may trap the median nerve between itself and the underlying bone. Entrapment produces symptoms similar to those of carpal tunnel syndrome.20 Compression of the median nerve at this level can be differentiated from compression within the carpal tunnel because the flexor muscles of the forearm, as well as the palmar cutaneous branches of the median nerve, are affected. All these branches come off below the ligament and above the carpal tunnel.

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