Positioning in Peripheral Nerve Surgery

Published on 14/03/2015 by admin

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Last modified 14/03/2015

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CHAPTER 28 Positioning in Peripheral Nerve Surgery

In performing peripheral nerve surgery, as with any area of surgery, knowledge of the anatomy is of the utmost importance. The surgeon must not only understand the nerve anatomy but must also be able to correlate neural structures with their target muscles and sensory distribution. Knowledge of the vascular and bone anatomy will also be essential to planning the surgery. In much of nerve surgery, the normal anatomy is distorted—from trauma, tumor, or other disease—and the surgeon must have a clear anatomic picture of the normal anatomy before proceeding. A properly planned incision and exposure will allow the correct identification of the vital structures as well as room in which to perform the needed tasks. Although neurosurgeons are comfortable working in the small, confined spaces of the brain and spine, peripheral nerve surgery often allows the luxury of working in a more open, more exposed area. The surgeon should take advantage of this ability and make the exposure generous. Whenever possible, exposure of a nerve should include normal proximal and distal nerve to allow the surgeon to work from the normal to the abnormal.

The specific nerve or nerves to be operated on and the necessity of harvesting grafts will dictate much of the positioning for an operation. If intraoperative electromyographic monitoring of muscles is used, the limb may be draped out of the field. If, however, the observation of muscle contraction will be necessary and no monitoring will be used, either the entire limb must be exposed or clear plastic drapes should be available. In each case, the positioning and exposure must be worked out to suit the individual needs of the particular operation.

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.

Upper Extremity

Brachial Plexus

Anatomy

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.

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.

For further details of this position and approach, the reader is directed to the paper published by Kline and associates3 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.