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.

The azygos vein is lateral to the esophagus on the right side, running inferiorly to join the superior vein cava at the fourth interspace. At the point where the azygos vein turns medially, it may receive some branches, which may be divided if necessary (Fig. 26-6).

Fig. 26-6 Retromediastinal structures in the right thoracic cavity. The azygos vein is running in a cephalocaudal direction. The right sympathetic chain contributes to the formation of the greater splanchnic nerve.

The sympathetic chain runs vertically and lies on the top of the heads of the ribs, at the anterior edge of the foramina. From the intercostal nerves the chain receives the rami communicantes. A section of a few of these will be of no functional consequence as long as the major chain is preserved. From the inferior thoracic ganglia larger trunks are derived; these constitute the splanchnic nerves and should be spared.

LATERAL EXTRA-CAVITARY APPROACH

The lateral extra-cavitary approach (LECA) is an extension of costotransversectomy. The more extensive rib resection provides the more ventral and wider operative view across the midline.⁷

LECA is indicated for the removal of extradural mass lesions anterior and lateral to the spinal cord or cauda equina, followed by anterior vertebral fusion.

It can be applied for the management of thoracic disc herniation, upper lumbar disc herniation, trauma, tumors, and inflammatory diseases involving up to three and sometimes four vertebral levels.^1,8 It may not be applicable above the T4 level because of the scapula and below L4 level because of the iliac crest.

POSITIONING AND INCISION

A midline vertical incision (three levels above and three levels below) is made with a gently curved lateral portion (12–14 cm). This incision offers access to both the posterior midline and anterior vertebral body through the lateral approach (Fig. 26-7).⁹

Fig. 26-7 L-shaped skin incision on the thoracic and lumbar spine.

MUSCLE DISSECTION

Skin and subcutaneous tissue are incised and reflected to the extended incision side. Then the thoracodorsal fascia is dissected from the midline and incised along the horizontal skin incision line. The thoracolumbar fascia appears silver to white. When it is dissected and retracted, the lateral branch of the dorsal ramus of the spinal nerve is seen to run over the surface of the muscle layer (Fig. 26-8).¹⁰

Fig. 26-8 Exposure of thoracolumbar fascia and erector spinae muscle. The lateral branch of the dorsal ramus of the spinal nerve is tied with string.

Trapezius muscle or latissimus dorsi muscle is divided with the attached fascia depending on the level of the lesion. The entire skin, subcutaneous tissue, muscle, and fascia flap are then elevated and retracted laterally. A plane is defined at the lateral aspect of the erector spinae group, and these muscles are elevated as a layer off the ribs and are retracted medially (Fig. 26-9).

Fig. 26-9 The erector spinae muscle group is elevated and retracted medially to expose the posterolateral surface of the spinal canal.

RIB RESECTION

All muscles and attached ligaments are cleaned from the ribs, and the rib is transected 7–10 cm lateral to the costovertebral junction. The endothoracic fascia and pleura are separated using blunt dissection. On identification of the neurovascular bundle, the intercostal nerve is separated from the vessels. The transverse processes and associated intertransversarii muscles are removed.

The superior costotransverse ligaments, radiate ligament, posterior costotransverse ligament are incised with a scalpel. After the costovertebral joint is opened, the rib head is elevated out of the field. It is important to remove the rib and transverse process at the articulation to ensure full exposure (Fig. 26-10).

Fig. 26-10 The exposure of the intercostal bundle (vein, artery, and nerve) under the lower margin of the rib. The indicator is passed under the rib and over the intercostal bundle.

IDENTIFICATION OF NEURAL FORAMEN

Each intercostal nerve is then traced into its respective foramen (Fig. 26-11). A ligature is placed around the nerve, which is cut 3.0 cm distal to the dorsal root ganglia, and the nerve is retracted to the dorsal side. The retracted nerve roots cause spinal cord retraction, which enables the surgeon to view the vertebral body across the midline.

Fig. 26-11 The exposure of the neural foramen following the intercostal nerve. The trace of the nerve can identify the neural foramen and epidural fat.

The parietal pleura is dissected off the vertebral bodies using a Cobb elevator. If one divides the rami communicantes connecting the nerve root and sympathetic ganglion, one can expose the vertebral body easily to the ventral tip. The sympathetic chain is contained within a fascial compartment formed by fusion of the mediastinal and prevertebral fascia over the costovertebral articulation. Displacing the sympathetic chain anterolaterally via subperiosteal dissection reveals the anterolateral surface of the vertebral body, pedicle, and foramen. The segmental arteries are dissected off the vertebral bodies and divided between clips. The foraminal margins above and below the lesion are defined with a blunt nerve hook. Care is taken not to dissect into the spinal canal. After identification of the foramen, the pedicle is removed using a combination of rongeurs and thin foot-plated punches. The table is then rotated 15–20 degrees away from the surgeon to maximize visualization of the spinal canal.

Removal of the pedicle provides the lateral view of spinal cord. To facilitate the exposure of the dorsal cord, the ipsilateral facet complex and lamina can be removed.

When the epidural space is opened, epidural venous plexus bleeding is severe.

CORPECTOMY OR DISCECTOMY

The annuli adjacent to the vertebral bodies to be removed are incised with a number 15 blade, and a punch is used to create a seat for a drill bit (Figs. 26-12 and 26-13). Using a brace and bit, the disc material and endplates are drilled out about three-fourths of the way across the vertebral body, thus ensuring that the surgeon is across the spinal canal. The posterior intervertebral disc space, about 1.0 cm ventral to the canal, is the portion to be drilled out. The intervening vertebral bone is removed using a rongeur or a high-speed drill to go deep through the vertebra. At this point, there should be at least 1 to 2 cm of bone left anteriorly and dorsal shelf posteriorly. Careful dissection of the dura-bone interface can be helpful to break up adhesions and define spicules, which may be stuck to the sac. The backward-angled curette is used to remove the posterior cortex from the ventral dura. The posterior cortex can be removed in a single piece by working primarily at the junctions of the intervertebral discs, and posterior cortex removal can be extended across the spinal canal (Fig. 26-14).

Fig. 26-12 During the retraction of the dorsal rami of the spinal nerve, the discectomy is performed. The string is tied on the spinal nerve to be retracted.

Fig. 26-13 After partial removal of disc, the intervening vertebral body is resected. The distal end of the dorsal root ganglion (DRG) is cut.

Fig. 26-14 After the vertebral body is removed, the ventral dura is decompressed.

TRANSCOSTOVERTEBRAL APPROACH

Posterior surgical approaches to the thoracic spine are the transpedicular, transfacetal, costotransversectomy, and LECA. The transpedicular and transfacetal approaches are included in the transcostovertebral approach.¹¹

Midline incision and usual posterior exposure are performed. The transverse process of the involved level is resected en bloc to uncover the costotransverse junction and to provide access to the costovertebral joint (Fig. 26-15).⁴

Fig. 26-15 Transverse process resection with pedicle. The facet joint is drilled away with the superior portion of the pedicle and whole portion of transverse process. The underlying rib head is cut away partially.

The lateral portion of the facet joint and superior half of the pedicle are removed with a drill. The thoracic pedicle can be identified by following the superior facet. The pedicle is the landmark for the inferior margin of the disc. After removing the facet and pedicle, one can reach the costovertebral joint (Fig. 26-16).

Fig. 26-16 After the facet and pedicle are removed, the costovertebral joint is seen. The dotted line represents disc space.

This joint is composed of the lateral end of the disc, rib head, and lateral aspects of the pedicle. From the center of the joint, the drilling is continued outward circumferentially to include immediately adjacent structures such as the posterior cortex of the rib head and lateral endplates above and below the annulus (Fig. 26-17).

Fig. 26-17 The lateral disc, rib head, and pedicles are removed. This procedure can reveal the lateral surface of the spinal cord.

This maneuver exposes the lateral and anterior aspects of the spinal cord.

COSTOTRANSVERSECTOMY

The costotransversectomy was first used for the drainage of tuberculous paraspinal abscesses in Pott’s disease.^4,12 This approach provides access to the posterior and lateral aspects of the vertebrae. It extends the exposure provided by the pediculectomy by resecting the transverse process, the medial portion of the rib and rib head, the costotransversectomy, and costovertebral ligaments (Fig. 26-18).¹²

Fig. 26-18 Costotransversectomy for ventral lesion removal. The transverse process, the medial portion of the rib and rib head, and the costovertebral ligament are removed.

POSITIONING AND INCISION

The operation may be performed with the patient in the prone or lateral decubitus position. For the upper thoracic region, the arm on the affected side is maximally elevated to move the scapula as far away from the midline as possible.

Three types of skin incision—paramedian, oblique and T shape—can be used depending on the type of lesion.

A straight paramedian incision can be made approximately three finger-breadths beside the spinous process.

An oblique incision following the rib to be resected and extending across the midline is preferred for relatively localized lesions, such as herniated discs or neurofibroma. In more extensive lesions, and particularly in cases where a fusion procedure with instrumentation is needed, a T-shaped incision is better (Fig. 26-19) because it allows the surgeon to resect several ribs and expose the spinal column at multiple levels. The transverse limb is at the level of the vertebra to be exposed.

Fig. 26-19 T-shaped skin and muscle incision. The initial incision is made vertically along the spinous process. Then the additional incision is made as a perpendicular line to extend the surgical field.

MUSCLE DISSECTION

After the skin is incised, the trapezius or latissimus dorsi muscle is divided along the skin incision line to expose the underlying rib. The intrinsic muscles of the back, in accordance with the skin incision, are now first separated from the spinous processes close to the bone. The intrinsic muscles of the back are dissected with a rasp from the vertebral arches and transverse processes above and below the transverse incision. After this, the longissimus muscle is transversely dissected and retracted superiorly and inferiorly (Fig. 26-20). The ribs, laminae, facet joints, and transverse processes are visualized at this point. The proximal 5–6 cm of the selected rib is resected after stripping of the periosteal covering as well as any other soft tissues.

Fig. 26-20 Muscle division and rib resection. The longissimus muscle is incised and retracted superiorly and inferiorly. The proximal 5–6 cm of the selected rib is resected after stripping of the periosteal covering with other soft tissue.

EXPOSURE OF VERTEBRAL BODIES AND SPINAL CORD

Using a peanut sponge stick, the endothoracic fascia beneath the rib periosteum as well as the parietal pleura is cautiously retracted from the anterior side of the vertebrae and the intervertebral discs, sparing the neurovascular bundles.

For easy dissection, the sympathetic chain is identified and released with disconnection of the rami communicantes. The remnants of the intercostal muscles lying between the resected ribs are dissected off the segmental vessels (Fig. 26-21). If necessary, the intercostal vessels may be ligated and transected anterior to the vertebral body, but the segmental nerves should be preserved. After retraction of the parietal pleura from the anterior side of the vertebrae, flexible spatulas may be inserted so that two to three vertebrae are laterally visualized from behind (Fig. 26-22).

Fig. 26-21 Exposure of vertebral body. The intercostal vessels are ligated and transected anterior to the vertebral body. The sympathetic chain is released with disconnection of the rami communicantes.

Fig. 26-22 Exposure of the ventral surface of the vertebral body. The intercostal nerve should be preserved. If it is damaged, the patient may suffer from severe intercostal neuralgia.

The spinal canal is exposed by removing the ipsilateral laminae, facet joint, and pedicle. The lateral aspects of the dural sac and spinal cord are seen at this point. The disc space can be seen and palpated.