21 Anterior Approaches to the Spine for Neuromuscular Spinal Deformity

10.1055/b-0038-162482

21 Anterior Approaches to the Spine for Neuromuscular Spinal Deformity

Peter O. Newton

Abstract

Anterior exposure to the spine for the purpose of performing an anterior release has become less common as the posterior methods have become more powerful. The goals of spinal fusion in the patients with neuromuscular scoliosis cannot be forgotten, and in selected cases an anterior release (with or without instrumentation), particularly in the thoracolumbar curve pattern, is an excellent alternative to achieve pelvic obliquity correction and sitting balance. An anteroposterior approach may be safer than a posterior three-column osteotomy for the most severe cases. An approach surgeon should be utilized if the spinal deformity surgeon is not experienced with the anterior anatomy.

21.1 Introduction

The spinal deformities of patients with neuromuscular disorders may develop at a young age and are often treated when the curves are large and rigid. Traditionally, the two main goals for an anterior procedure are the desire to prevent crankshaft, which is relatively common in these patients, and the attainment of curve flexibility. Despite the increasing popularity of posterior-only approaches for scoliosis, there remain appropriate indications for the addition of an anterior procedure in some patients with neuromuscular scoliosis.

The surgical exposure of the anterior aspect of the spine may be made by open or endoscopic methods, and the choice is determined by the intended procedure as well as the experience of the surgeon with each method. Neither of the approaches is extensile, although the standard thoracotomy can be extended into the lumbar region by dividing the diaphragm as in the thoracoabdominal approach. The thoracoscopic method is an attractive alternative to an open thoracotomy when the region of interest is between T4 and L1, allowing a less invasive alternative for performing diskectomy/release.

Additionally, there is a group of patients with specific curve features that may benefit from the addition of anterior instrumentation, generally prior to performing a longer posterior instrumented fusion procedure. The most likely candidates are those with severe/rigid thoracolumbar curves associated with marked pelvic obliquity.

The risk and benefits of adding an anterior procedure must be carefully considered in this vulnerable population. The association of medical comorbidities with many of the neuromuscular conditions should always cause one to pause when contemplating the addition of a second surgical approach. The goals of spinal deformity surgery are more likely based on functional outcomes associated with pulmonary function and comfortable sitting. Anterior procedures should be added only when the gains in achieving these goals are thought to outweigh the risks of the more invasive anterior approach. Having said that, in the myelomeningocele population, an isolated anterior approach with instrumentation may, in fact, have less risk of complications, particularly with regard to postoperative infection. As in all surgical decision making, balancing the risks and benefits is a crucial aspect of achieving success.

21.2 Open Approaches

The spinal deformity surgeon may or may not elect to utilize the skills of a general surgeon colleague for these approaches depending on his or her training and experience. In either event, an understanding of the nonskeletal anatomy (heart, lungs, great vessels, kidney, ureter, liver, bowel, etc.) is of obvious importance. With appropriate training and experience, these vital structures can be safely protected during anterior spinal surgery while enabling access to the anterior vertebral column. ¹ ^, ²

21.2.1 Standard Thoracotomy

Anterior surgical exposure of the thoracic spine is largely achieved via a lateral thoracotomy. This approach utilizes the convex side of the chest in scoliosis cases, but in some cases of severe kyphoscoliosis a concave approach may be appropriate (especially if decompression is required). Typically five or six vertebrae can be reached because the spine is exposed between two ribs that can be spread approximately 10 to 20 cm depending on the size of the patient. The level of thoracotomy must be selected proximally so that the segments of interest can be accessed. In most cases, the thoracotomy should enter via one rib above or the rib at the level of the most proximal vertebral level to be exposed.

The lateral decubitus position with an axillary roll to protect the down side arm/brachial plexus is preferred. Prominences of the greater trochanter and fibular head on the “down” leg should be padded appropriately and the “up” leg scissored and padded separately. The patient should be secured on the table with tape and or bolsters to maintain the position. The arms are positioned in 90 degrees of flexion at the shoulders and elbows. The chest wall should be prepped and draped widely from the axilla to the symphysis and beyond the midline anteriorly and posteriorly.

The skin incision generally follows a rib that may be harvested and utilized for autogenous bone graft. The subcutaneous fat and muscles of the latissimus dorsi and serratus anterior are divided in line with the incision exposing the underlying rib. Entry into the chest may be between ribs or through the rib bed itself. Subperiosteal dissection along the rib and cutting the rib at the proximal and distal extents are standard when the rib is desired for bone graft. The alternative is to divide the intercostal muscles between two ribs. The ribs are spread for access to the spine by a Finochietto chest retractor of appropriate size.

The lungs and great vessels are the relevant nonspinal anatomy within the chest. Pleural adhesions may make retraction of the lung difficult and should be expected in patients with a history of prior chest surgery or infection (fairly common in the neuromuscular population). ³ Dividing any pleural adhesions may be required to retract the lung and pack it with moist lap sponges. Selective lung ventilation is preferred by some to ease the requirement of lung retraction, but this adds complexity to the intubation (see the “Thoracoscopic Approach” section on single lung ventilation). The pleura overlying the spine is a layer that can generally be divided longitudinally in a manner that will allow closure at the end of the procedure. In the right chest, the segmental vessels are divided between the azygous vein (running longitudinally along the anterior aspect of the spine and the rib heads [posteriorly]). In the left chest, the aorta overlies more of the vertebra with shorter segmental vessels and a less developed venous hemiazygos system than in the right chest. A plane is easily developed between the vertebral column and the great vessels, thoracic duct, and esophagus once the segmental vessels are divided. Some prefer to maintain these segmental vessels; however, the circumferential exposure of the spine is limited when doing so. Utilizing the soft-tissue plane between the anterior longitudinal ligament and the great vessels allows for a safe retraction of the great vessels (Fig. 21‑1).

Fig. 21.1 The 12th rib cartilage tip is being split longitudinally with a scalpel. This provides a path to the retroperitoneal space. Blunt dissection deep into the cartilage and especially posteriorly opens the “potential” space posterior and lateral to the peritoneum.

Once the spinal procedure has been addressed, the thoracotomy is closed after a chest tube is placed. In many cases, reapproximation of the pleura over the spine may be possible. This has the advantage of maintaining the position of any morselized bone graft and potentially minimizing pleural adhesions. The chest wall is closed in layers after reapproximating the ribs.

A retropleural approach to the thoracic spine is also possible. This approach avoids exposure of the lungs; however, the exposure of the spine is relatively limited compared to the transpleural thoracotomy. This is more often applicable to a tumor, an infection, or trauma, although a limited diskectomy may be approached this way.

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