Anatomy, Positioning, and Surgical Exposure

Upper and lower extremity nerves are considered in turn, with a brief review of anatomy, optimal positioning, and a stepwise operative exposure. More in-depth viewing of positioning for peripheral nerve surgery may be seen on Video 28-1.

The brachial plexus is formed by the ventral primary rami of C5-T1. It provides innervation to the muscles and skin of the upper extremity. Less often, C4 will contribute a branch to C5 (pre-fixed) and T2 will contribute a branch to T1 (post-fixed).¹ The roots of the plexus emerge between the anterior and middle scalene muscles and descend over the first rib, posterior to the clavicle. In the lower part of the posterior cervical triangle, after traversing between the anterior and middle scalene muscles, the C5 and C6 roots unite to form the upper trunk. Before this union, C5 gives off the dorsal scapular nerve to the rhomboids and a branch to the phrenic nerve. The C5 root, along with the C6 and C7 roots, gives off contributions to the long thoracic nerve to the serratus anterior muscle. The C7 root continues on as the middle trunk, and the C8 and T1 roots unite to form the lower trunk. The upper trunk gives off the suprascapular nerve to the supraspinatus and infraspinatus muscles as well as the nerve to the subclavius muscle.

The trunks divide into anterior and posterior divisions. The anterior divisions of the upper and middle trunks form the lateral cord. The anterior division of the lower trunk continues as the medial cord. The posterior divisions of all three trunks unite as the posterior cord. The divisions form at the level of the clavicle, and the cords continue into the infraclavicular space.

The cords form just distal to the clavicle, below the tendinous insertion of the pectoralis minor muscle. They are named according to their spatial relationship to the axillary artery. Each cord ends in two terminal branches. The lateral cord gives off the lateral pectoral nerve and then terminates as the musculocutaneous nerve and the lateral contribution to the median nerve. The medial cord gives off the medial pectoral, medial brachial cutaneous, and medial antebrachial cutaneous nerves and then terminates as the ulnar nerve and the medial contribution to the median nerve. The posterior cord gives off the thoracodorsal and subscapular nerves and then terminates as the axillary and radial nerves.

Although many variations have been reported, true anomalies of this basic anatomy are rare.²

Positioning

For an anterior approach to the brachial plexus, the patient is placed in the supine position with a bolster under the ipsilateral shoulder (Fig. 28-1). The head is placed in a slight amount of extension and is turned 45 degrees to the contralateral side. The arm is abducted on an arm board. Both lower extremities below the knee are sterilely prepared and draped to allow sural nerve harvesting for grafts. If other sources in the upper extremity, such as medial brachial and antebrachial cutaneous nerves, are to be used for grafting, the entire arm should be prepared. The surgeon should have the ability to work from superior to the clavicle and inferior to it. The arm board should be mobile enough to allow the surgeon to abduct or adduct the arm at the shoulder as necessary.

FIGURE 28-1 A patient positioned for a full brachial plexus exploration. The incisions for a supraclavicular and infraclavicular exposure are marked.

In selected cases, a posterior approach to the plexus is indicated. Indications for this approach include a need for very proximal exposure to the neural foramina and the exiting nerve roots. It is also indicated in patients in whom the anterior neck and chest wall have extensive scarring either from radiation therapy, as in breast cancer treatment, or from a previous plexus operation. This approach will clearly expose the roots, trunks, and divisions but exposure for the cords and nerves is inadequate. As there is a greater potential for damage to the cupula pleurae of the lungs and a subsequent pneumothorax, the surgeon should have equipment for a chest tube on standby.

For the posterior approach, the patient is placed in the prone position (Fig. 28-2). Abdominal bolsters are placed to allow the abdomen to hang freely. An additional bolster should be placed under the patient at the level of the clavicle and upper sternum so that the anterior chest wall is lifted slightly off the operating table. The head is turned to the contralateral side. The contralateral arm is protected and tucked next to the patient’s body. The ipsilateral arm is abducted approximately 30 degrees and bent at the elbow. The elbow is then placed on a padded Mayo stand to the patient’s side. The operating table is slightly tilted in a reverse Trendelenburg position. The surgeon works from above the patient’s head as well as from below the patient’s shoulders and side.

FIGURE 28-2 A patient positioned for a posterior subscapular approach to the brachial plexus. This patient had previous radiation treatments to the anterior neck, making a traditional approach to the plexus not feasible.

For further details of this position and approach, the reader is directed to the paper published by Kline and associates ³ or the text by Maniker.⁴

Surgical Exposure

Exposure of the brachial plexus anteriorly may be divided into the supraclavicular and infraclavicular approaches (Fig. 28-3). They can be combined for the full exposure of the plexus or tailored to expose the desired area of the plexus. The supraclavicular portion of the approach exposes the roots, trunks, and proximal portion of the divisions. The infraclavicular portion of the approach exposes the distal portion of the divisions, the cords, and the branches of the brachial plexus.

FIGURE 28-3 The full plexus exposed. This cadaveric exposure has had the clavicle removed for visualization, although it is not usually done in the living. The asterisks indicate the cut ends of the clavicle.

The incision begins 2 to 3 cm above the medial epicondyle over the medial intermuscular septum. Moving inferiorly, it crosses the elbow flexion crease obliquely and just medial to the biceps brachii tendon (Fig. 28-4). The incision continues distally over the bicipital aponeurosis in a gentle curve toward the midline of the forearm. The incision then moves distally down the forearm, following the interval between the flexor digitorum superficialis and brachioradialis. The incision is opened proximally, and the median nerve is located in the interval between the biceps brachii and the medial intermuscular septum. The nerve is traced into the forearm, and the lacertus fibrosus crosses its path obliquely. The nerve next passes deep to the pronator teres muscle. As the nerve is traced, it dives between the two heads of this muscle. The nerve then moves deep to the arch of the flexor digitorum superficialis, and this arch is divided. The median nerve may then be traced to the wrist, where it lies between the flexor carpi radialis and the flexor digitorum superficialis.