INTRODUCTION

Several surgical approaches have been developed to remove thoracic vertebral lesions from the anterior to posterolateral direction. The choice of surgical approach is decided depending on the disease entity, level of the lesion, laterality, multiplicity, presence of instability, and the necessity of reconstruction. The posterior approach provides less extensive exposure of the vertebral body than the anterior approach, but the involved anatomy is familiar to the spinal surgeons and it can be applied to patients with anesthetic risk.¹

Advantages of the posterior approach over the anterior approach are as follows^2,3:

With increasing bone removal, from the transpedicular approach to the costotransversectomy, and finally to the lateral extra-cavitary/lateral parascapular approach, the operative trajectory becomes more lateral to better visualize the affected vertebrae.⁴ In this chapter, overall review of the posterior or posterolateral approach to thoracic spine will be presented with the pertinent anatomical assessment.

MUSCULAR ANATOMY

The most superficial muscle of the dorsal spine is the trapezius muscle (Fig. 26-1). The trapezius originates along the external occipital protuberance and each spinous process from C1 to T12. The insertion of the trapezius is the lateral third of the clavicle, the acromion and the scapular spine. This muscle provides the stabilization and abduction of the shoulder. Immediately deep to the trapezius muscle on the upper thoracic level lie the rhomboid major, rhomboid minor, and levator scapulae muscles (see Fig. 26-1). The rhomboid muscles originate from the spinous processes of the cervical and thoracic spine and insert to the ventral edge of the scapula.⁵ The levator scapulae muscle connects the scapula to the upper cervical vertebrae.

Fig. 26-1 Superficial layer muscles: trapezius, rhomboids, levator scapulae muscles, and latissimus dorsi muscle.

The serratus posterior superior muscle is another muscle that fixes the cervicothoracic junction area spinous processes to the lateral part of rib cage (Fig. 26-2). In the exposure of the cervicothoracic junction, the spinous process insertions of these muscles are taken down as a single group for lateral retraction.⁶ As these muscles are taken down, the scapula is released from its attachments to the spinous processes and rotates anterolaterally out of the operation field.

Fig. 26-2 Deep layer muscles: erector spinae muscle and transversospinalis muscle.

On the lower portion of the back, the latissimus dorsi muscle spans over the body. It originates from the spinous processes of the six lower thoracic vertebrae, lumbar and sacral vertebrae, and ilium, inserting onto the humerus (see Fig. 26-1).

In the deeper part of the back, two large groups of muscles are located: the erector spinae muscles (sacrospinalis muscle) and the transversospinalis muscles.

The erector spinae muscles are a group of muscles running from the sacrum and iliac crest to the ribs or transverse process of the vertebrae. They have three separated groups: iliocostalis (lateral), longissimus (middle), spinalis (medial) (see Fig. 26-2). The iliocostalis muscle is inserted into the angles of the ribs and into the cervical transverse processes from C4 through C6. The longissimus thoracis muscles are inserted into the thoracic transverse processes and nearby parts of the ribs between T2 and T12. The spinalis muscle is largely aponeurotic and extends from the upper lumbar to the lower cervical spinous processes.

The transversospinalis group of muscle passes obliquely cephalad from the transverse processes to the spinous processes immediately deep to the erector spinae muscle. These muscles fall into three layers. The most superficial layer, the semispinalis muscle, arises from the tips of the transverse process and inserts to the tips of the spinous processes. The semispinalis capitis passes from the upper six thoracic transverse processes and lower four cervical articular processes to the occipital bone between the superior and inferior nuchal lines. The semispinalis cervicis muscle also starts from the upper thoracic and lower cervical transverse processes and attaches to spinous processes of C2 through C5.

The semispinalis thoracis muscle runs from the transverse processes of the lower six thoracic vertebrae onto the spinous processes of the upper thoracic and last two cervical vertebrae. The intermediate layer, the multifidus, arises from the sacrum, posterior sacroiliac ligament, accessory processes of the lumbar spine, and the articular processes of the thoracic spine and inserts to the spinous processes of the vertebrae up to C2 vertebra. The deepest muscles of this group, the rotators, are small muscles that bridge from the transverse processes to the lamina of the vertebra directly above.

POSTERIOR THORACIC CAGE

The head of a rib articulates with the adjacent parts of its own vertebral body, the vertebra above, and the intervertebral disc between them (Fig. 26-3).

Fig. 26-3 Thoracic bony structures, view from the inside of the thoracic cavity. The rib head contacts the intervertebral disc on the superior portion of the pedicle. At the lower thoracic levels, the rib head contacts on the mid-portion of the pedicle.

The exception of this general rule is the first, 11th, and 12th ribs, which articulate only with their own vertebral body. On the vertebral body from the second to 10th level, each rib head has two synovial joints with the vertebral body and intervening radiate ligament enforcing the joint. These are two independent joint surfaces, which are separated by the posterolateral position of the intervertebral disc. The inferior articular surface, which is numbered the same way as the rib, has a height that is slightly larger than the pedicle, and its posterior limit corresponds to the point of insertion of the pedicle. Its height represents about one-third of the height of the vertebral body. In contrast, the superior facet represents only half the height of the inferior facet.

The third synovial joint is a costotransverse joint that is strengthened by superior and lateral costotransverse ligaments (Fig. 26-4). The superior costotransverse ligament joins the neck of the rib to the transverse process immediately above.

Fig. 26-4 Rib head contact with the vertebral body via two joints. Costotransverse and costovertebral joints. The costotransverse joint is wrapped with lateral costotransverse ligament, and the costovertebral joint is surrounded with radiate ligament.

The ribs also are attached to one another through the intercostal musculature, which originates medially on each superior rib and inserts laterally on its immediately inferior rib. This strip of muscles contains the intercostal nerve, artery, and vein. Most often, the intercostal vein is most cephalad with the intercostal artery close to it but caudad (Fig. 26-5).

Fig. 26-5 Retromediastinal structures in left thoracic cavity. The aorta is seen to run along the anterolateral surface of the thoracic vertebral body. The sympathetic chain is laid over the rib heads.

The intercostal nerve is commonly found separate from these structures and is located most caudad of the three. Immediately ventral to the intercostal bundle and intercostal muscles lie the pleura.

POSTERIOR MEDIASTINAL SPACE AND NEUROVASCULAR STRUCTURE

There are some neurovascular structures related to the spine in the posterior mediastinal space. Between T4 and T7 the aorta has a close relationship with the left lateral surface of the vertebral bodies. It then moves medially to occupy a more anterior position, and at the level of the diaphragm, the aorta is strictly prevertebral. The segmental arteries arise from the posterior surface of the thoracic aorta and run horizontally, following the concavity of the vertebral body. At the level of the foramen, they bifurcate into a radiculomedullary and an intercostal branch. The principal medullary artery, the artery of Adamkiewicz, is located on the left side in about 60% of the cases and originates mostly between T9 and T11.

In the upper thoracic region, the first two intercostal spaces are supplied by branches of the costocervical trunk through the highest intercostal artery. Because the aorta is displaced downward and to the left, the upper four intercostal arteries ascend to reach intercostal spaces three through six. They stretch obliquely across each vertebral body from caudad to cephalad in direct apposition to the periosteum of the vertebral body and are located deep to the azygos of the hemiazygos vein, thoracic duct, and sympathetic trunk. The superior hemiazygos vein occupies, on the left side, a position lateral to the aorta, receiving collateral branches down to the sixth or seventh interspace.