The anatomy of the anterior minimally invasive approach is essentially the anatomy of the chest contents in the right thorax, where the vast majority of procedures are performed. The thorax is essentially a container that is bordered by the spine posteriorly and the chest wall, which is supported by the ribs and intercostal musculature and lined by the parietal pleura. The lungs fill the contents of the thorax bilaterally and must be deflated during a VATS procedure to gain access to the spine. The spine, covered by the parietal pleura, has peaks representing the intervertebral disks and valleys compatible with the central portions of the vertebrae which have the segmental vessels traversing horizontally at each level as a continuation of the intercostal vessels running along the caudal border of each rib. The rib head attaches at the cephalad onethird of the vertebrae and overlaps the lateral portion of the vertebra by 2 to 3 mm. Deep to the rib head lies the pedicle. The rib head provides a useful landmark for placement of vertebral body screws and for gaining orientation to the spinal canal spanning the posterior border of the vertebrae. The mediastinum at its upper portion contains the trachea, esophagus, and large vessels, and within this envelope, the vagus and phrenic nerves course downward. The sympathetic trunk with its ganglia lies along the rib head or neck longitudinally throughout the thoracic spine and give off the greater and lesser splanchnic nerves providing sympathetic innervation to the abdomen. These are often sacrificed during pleural dissection for exposure of the spine without significant deleterious impact (Fig. 18.1). The majority of procedures are done through the right chest consistent with right thoracic scoliosis presenting most frequently. Occasionally, left-sided approaches can be done but the aorta must be contended with in those cases. In the right chest, the azygous vein coming off the superior vena cava runs along the anterior portion of the vertebral bodies and has tributaries from each segmental vein and from the superior intercostal vein which emanates off of the second, third, and fourth intercostal veins. The azygous anatomy must be respected when performing VATS approaches to avoid bleeding complications.

VATS instrumentation and fusion for scoliosis has fallen out of favor except for one to two institutions worldwide. The technique of screw placement, learned from thoracoscopic instrumentation for fusion a decade ago, is now applicable to the technique of fusionless VBT. The fusion procedure will not be outlined here; rather the techniques for anterior spinal release and instrumentation for tethering will be discussed.

Anterior release is utilized in cases of severe magnitude, rigid deformity, and in cases of marked hypokyphosis or frank lordosis. The role of anterior release has evolved considerably over decades and is much less commonly performed today than in years past. However, the indication remains and when performed thoracoscopically, is associated with very little morbidity especially compared to open thoracotomy approaches.

Anterior release is performed as part of a posterior-instrumented arthrodesis (Fig. 18.2). In the past, the patient was positioned in the left lateral decubitus position, the procedure

performed, and then the patient was repositioned for the posterior procedure. Today, the entire anterior and posterior procedure is performed prone. The back and right chest wall are prepped and draped. Care must be taken in the draping to leave adequate room for a thoracotomy in case of bleeding complication. Single-lung ventilation with either bronchial blocker or double lumen endotracheal tube is not required. CO₂ insufflation can be performed using special 12 mm ports. Typically, two portals into the chest cavity are placed in the mid or posterior axillary line. The first portal is placed one to two levels cephalad to the apex of the scoliosis and the second is placed one to two levels caudad to the apex under direct visualization after the scope has been introduced into the thoracic cavity through the first portal. Direct visualization allows the second port to be placed safely without jeopardizing the diaphragm or other vital structures. The CO₂ insufflator is set at a maximum pressure of 15 mm Hg. Tidal volumes can be dropped slightly to facilitate visualization while minimizing CO₂ pressures, which can affect cardiac output and oxygen saturation if too high. The use of CO₂ permits the procedure to be performed through two portals instead of three, as an endoscopic retractor is not required to hold the diaphragm out of the operative field. The pleura along the spinal column, is incised longitudinally using a harmonic scalpel with hooked spatula attachment that permits easy dissection of the pleura and coagulation of the segmental vessels. The dissection of the pleura is taken to the vertebrae proximal and distal to the intervertebral disks to be released. The dissected pleura is then swept posteriorly to the level of the rib head and anteriorly around to the contralateral side of the spine which is rotated and becomes easily accessible especially using angled scopes of 30 to 45 degrees. The segmental vessels overlying the exposed segments are then coagulated and incised with the harmonic scalpel and are swept posteriorly and anteriorly. The latter move facilitates the mobilization of the azygous vein away from the spine to avoid laceration during the release. After exposure is completed, apical disks that do not correct on bending x-rays are removed. This is achieved by incising the annulus fibrosus and anterior longitudinal ligament with the harmonic scalpel, followed by diskectomy using endoscopic pituitary rongeurs and curettes. Two-thirds or more of the exposed annulus can be removed in this fashion. Care must be taken to avoid the azygous vein when removing the contralateral disk and annulus and this should be done under direct visualization with the scope and camera. It is not essential to achieve complete endplate preparation for purposes of fusion for the majority of cases since the posterior procedure is relied upon for achieving arthrodesis. In revision cases with prior laminectomy, pseudarthrosis, and prior irradiation, an anterior arthrodesis is desirable and this is carried through meticulous preparation of the endplates. The posterior annulus is not routinely removed and adds a level of neurologic risk, potential for epidural bleeding, and increased operative time. The rib head can be removed to enhance the release and internal thoracoplasty can also be performed for a large rib prominence although this author prefers to do that from the posterior vantage.