Extraspinal Anatomy and Surgical Approaches to the Thoracic Spine

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Chapter 35 Extraspinal Anatomy and Surgical Approaches to the Thoracic Spine

Brendan Killory, M. Yashar, S. Kalani, Timothy Uschold, Nicholas Theodore

The thoracic spine contains more vertebrae than any other segment of the spinal column. With its 12 vertebrae, the thoracic spine is responsible for the load bearing and flexibility that has allowed Homo sapiens to stand erect. Given its critical role in the biomechanics of movement and its large contribution to the spinal column (almost a third of the total vertebrae), it is not surprising that the thoracic segment is also a frequent site of pathology (Table 35-1). Trauma, primary and metastatic tumors of the column, infections, vascular malformations, congenital disorders, and deformity all affect the thoracic column, making the ability to operate in this region an essential skill set for the competent neurosurgeon.

TABLE 35-1 Indications for Surgery

Indication	Type
Trauma	Vertebral body fracture causing spinal cord compression
Infection	Tuberculosis of the vertebral body
Deformity	Scoliosis, kyphosis
Degeneration	Any type
Tumor	Primary and metastatic

Anatomy

The thoracic vertebrae arise from a mesodermal origin. There are three primary centers of ossification in the cartilaginous template of the vertebra, the centrum and the two neural arches.^1,² These initial three centers of primary ossification mature into five secondary centers at the tips of the transverse processes, the spinous processes, and the annular epiphysial discs.^1,² Development of the spinal column proceeds postnatally and continues into adolescence, whereby the lordotic and kyphotic curves necessary for weight-bearing are established and completed.

During early development, the intricate connection between the ribs and the thoracic spine begins to contour the posture of humans. The ribs articulate with the vertebral bodies via the costovertebral joints, the transverse processes via the costotransverse joints, and the pedicle of the vertebrae. As is the rule in the spinal column, the size of a vertebra increases from the cervical to the lumbar regions. Therefore the size of the thoracic vertebrae is intermediate compared with their adjacent vertebrae (Fig. 35-1).³ From T1 to T12, the length of the transverse processes decreases. The spinous processes of the thoracic vertebrae are not uniform. At the midthoracic levels, the spinous processes are long and oriented inferiorly compared with their more horizontal orientation at the lower thoracic levels. From T1 to T4, the spinal canal is heart shaped and gradually transitions to a more circular shape from T4 to T8. An imaging study of the thoracic spine frequently shows a vascular groove caused by the impression of the descending aorta. Relative to their cervical counterparts, the thoracic laminae are thicker and deeper, albeit their width is considerably decreased.⁴ The thoracic pedicles are short, and their height and radius increase from T1 to T12.^3,⁵

FIGURE 35-1 Schematic representation of the thoracic vertebrae.

Throughout the thoracic spine, the angle between the pedicle and midsagittal plane changes dramatically depending on the level.⁵ This observation has important clinical implications, such as for the placement of pedicle screws for fixation.⁶ At T1 the angle between the pedicle and the midsagittal plane is wide, but by T12 the pedicles are parallel to the midsagittal plane. The thoracic pedicles are shorter and thinner than their lumbar counterparts, making them more susceptible to perforation during screw placement.

The relationship between the transverse process and the pedicle is variable in the thoracic spine; this variability makes the use of intraoperative fluoroscopy a necessity for thoracic cases.⁷ At the upper thoracic levels, the transverse process is located rostral to the pedicle; at the lower levels, the transverse process is caudal to the pedicle with the crossover occurring at T6-7.⁷

From T1 to T10 the facets are oriented coronally. The orientation becomes oblique between T10 and T12.⁸ The coronal orientation is important for flexion-extension movements in the lower thoracic spine. The thickness and width of the laminae overlying the facets increase as they progress from rostral to caudal in the thoracic spine. Throughout the thoracic levels, the short and broad laminae in the upper and middle thoracic spine prevent hyperextension.³ The multitude of ligamentous connections, most notably the anterior longitudinal ligament, provides additional stability by increasing the tensile strength of the column. The articular surface of the superior facets is on the ventral aspect, whereas the articular surface of the inferior facets is dorsal. The articular surfaces of the facet joints are flat and slope in an oblique coronal plane, in the same plane as the lamina.

Approaches

The choice of the approach to the thoracic spine largely depends on the location of the pathology that the surgeon is treating (Tables 35-2 and 35-3; Fig. 35-2). Traditionally, the operative technique of choice for treatment of pathology involving the thoracic spine has been laminectomy. Although this approach is useful for reaching dorsal pathology, it can cause great damage if used to treat ventral lesions.^9,¹⁰ With the recent advent of novel microsurgical techniques and hardware for ventral stabilization, ventral approaches have become commonplace for the treatment of ventral pathology.

TABLE 35-2 Approaches to the Thoracic Spine as a Function of Level of Pathology

Approach	Vertebral Level
Standard ventral cervical	T1-4
Transsternal	T1-4
Transthoracic	T3-11
Transthoracic, transdiaphragmatic, retroperitoneal	T11-L1

TABLE 35-3 Surgical Approaches to the Thoracic Spine

FIGURE 35-2 The choice of approach to the thoracic region depends on the location of the lesion that the surgeon hopes to treat. For posteriorly located lesions, the approach is posterior. Ventral lesions can be approached via a thoracostomy or by using one of the dorsolateral approaches. The approach to each thoracic level is complicated by the intricate anatomy of the mediastinum and abdomen, and the surgeon must be cautious to avoid complications.

The dorsal approaches include laminectomy, the lateral gutter approach, and the transpedicular approach. These approaches usually involve the removal of facet and pedicles with a high-speed drill.

Approaches to ventral lesions of the thoracic spine include dorsal, dorsolateral, lateral, and ventrolateral approaches. Each approach provides a unique visualization of the ventral thoracic spine, and the utility of each approach depends on the location of the pathology. The simplest ventral approach for lesions from T4 to T11 is via a thoracotomy incision with the patient in the lateral decubitus position. Dorsolateral approaches including costotransversectomy, lateral extracavitary, and parascapular approaches are performed via an incision in the back and require mobilization of the rib and transverse process to gain access to the vertebral body.

Dorsal Approaches

Dorsal approaches can be used to gain access to the entire spinal column. Although the approach is relatively straightforward, the adjacent muscles are likely to be damaged. With the advent of minimally invasive spinal approaches, many surgeons now use muscle-splitting approaches to minimize such damage to the paraspinal muscles. The indications for the use of this approach include correction of spinal stenosis, correction of disc herniation, correction of deformity and thoracolumbar factures caused by trauma, resection of thoracic level tumors, and the treatment of infections.

The patient is positioned on the operating table in the prone position. A padded headrest should be used to minimize pressure on the patient’s face. The patient’s abdomen must be free and hanging to avoid compression and congestion of the venous networks, which can be a significant cause of bleeding during this approach. Fluoroscopy should be used to identify the spinal level requiring treatment, thus minimizing the occurrence of surgery performed at the wrong level. The landmarks for dorsal approaches are the spinous processes, iliac crest, and posterior superior iliac spine.

The skin is incised in the midline above the spinous process, and the subcutaneous tissue is dissected (Fig. 35-3A). The thoracolumbar fascia is incised with electrocautery. The paraspinal muscles are detached subperiosteally from the spinous process and lamina (Fig. 35-3B). A sponge may be used to perform the blunt dissection of the paraspinal muscles laterally. The sponge may also be used to control bleeding. During the blunt dissection it is important to ensure that the joint capsule of the facet is not damaged.

FIGURE 35-3 A, The posterior thoracic approach. B, The dissection shows the spinous process and paraspinal muscles. Dissection of the paraspinal muscles exposes key bony portions of the vertebra, which can be removed during decompression or used as a platform for instrumentation.

If this approach is used for spinal fusion, the dorsolateral bed must be prepared for a bone graft. The multifidus muscles must be detached from the lamina, facet joint, and transverse process. While the transverse process is dissected, the periarticular vessels that cross around the facet joint tend to bleed. Closure must involve layer-by-layer correction of the fascia. If necessary, a drain should be inserted and the tissue layers closed using interrupted or running suture.

Ventral Surgical Approaches

Historically, the impetus for the development of spinal approaches and stabilization was the treatment of trauma, tumors, and Pott disease caused by the tuberculosis pandemics of the late 1800s and early 1900s. Hodgson and Stock described the first ventral approach with an acceptable morbidity rate of 2.9% for the debridement of a tuberculosis abscess.¹¹

The ventral approaches can be divided on the basis of the level on the thoracic column that they reach (see Table 35-2). Generally, the higher thoracic levels (T1-3) are readily accessible using a standard ventral cervical approach, occasionally extended with a median sternotomy or sternal window. This approach is excellent for ventrally located disease with minimal paraspinal involvement. The T2-11 vertebrae can be approached through a dorsolateral thoracotomy, usually from the left side to avoid the liver and azygos vein. However, some surgeons prefer the right-sided approach to avoid the aorta. When the approach is used to correct a deformity, the rule is to use the side of the apex or convexity of the curve to allow application of interbody devices. The lower thoracic region and the thoracolumbar junction can be approached via a left thoracotomy combined with dorsal detachment of the diaphragm via a retroperitoneal approach. Again, the preference of the side depends on the surgeon’s comfort and the location of the pathology.

Regardless of the approach used, the patients at the authors’ institution are intubated with a double-lumen endotracheal tube and arterial and venous lines are placed. Baseline somatosensory and motor-evoked potentials are recorded in all major spinal cases.

Cervical Exposure with a Median Sternotomy/Transmanubrial Approach

The high thoracic levels can be accessed via a standard cervical approach (Fig. 35-4A). A cervical approach paralleling the medial border of the sternocleidomastoid muscle is usually appropriate for T1 and T2 lesions. A medial sternotomy or sternal osteotomy is occasionally necessary to extend the surgical field to the level of T3 or T4. Most surgeons prefer a left-sided approach, which lowers the risk of injury to the recurrent laryngeal nerve on this side.¹² A preoperative CT scan can be helpful in determining the relationship of the clavicle and sternum to the spine and for planning purposes.

FIGURE 35-4 A, Cervical exposure via a median sternotomy/transmanubrial approach. The critical anatomy exposed during this approach involves the carotid artery, esophagus, and trachea. B, During this approach, the muscles, clavicle, and sternum can all be mobilized to improve the exposure for reaching the vertebra.

The patient is positioned supine with the midline of the head placed on a donut and the neck extended. A roll can be used between the scapulae to augment the manubrium. The incision is made in a T-shaped or cervicosternal fashion parallel to the border of the sternocleidomastoid muscle. During the dissection it may be necessary to sacrifice veins in the surgical path. The sternocleidomastoid muscle can be detached from its origin to increase the field of view. The sternohyoid and sternothyroid muscles are sectioned above the clavicles and sternal notch. The platysma is divided along its fibers in the direction of the incision. If necessary, the medial third of the clavicles can be sectioned and disarticulated from the manubrium. The inferior thyroid vein is ligated and sectioned. The carotid sheath is identified and retracted laterally. Similarly the trachea and esophagus are mobilized medially to create a plane for dissection. Next, the sternohyoid muscle is mobilized from the medial clavicle and sternum (Fig. 35-4B). Finger dissection is used to create a plane beneath the sternum and into the upper mediastinum. The superior thyroid artery should be identified and ligated.

A median sternotomy is performed down to the manubrium, and the mediastinum is opened with a retractor.¹³ The pleura is opened, and the left innominate vein is divided with ligatures for adequate caudal exposure. A plane is developed beneath the esophagus and above the prevertebral fascia. The longus colli muscles are elevated, and self-retaining retractors are placed. Care must be taken to avoid damage to the recurrent laryngeal nerve. The area of interest is identified using fluoroscopy. At the end of the case, the manubrium is fixed with stainless steel wires and the clavicle with plates and screws.

The remainder of the wound is closed in layers with nonabsorbable sutures and with either suture or clips to the skin. If necessary, a drain should be placed and attention should be paid to pneumothoraces.

Transaxillary Approach

The patient is positioned in the lateral position and tilted 15 degrees dorsally, with the arm abducted and the elbow flexed. An axillary roll is placed under the armpit. A skin incision is made at the third intercostal space between the ventral and dorsal axillary skinfolds. The intercostobrachial nerve is identified and sectioned. The pleural cavity is entered through the second or third intercostal space. The third rib may be removed. Small rib retractors are used to split the ribs aside. The ipsilateral lung is then deflated and retracted. One must use care not to damage the sympathetic chain. The parietal pleura is incised between T1 and T3 to expose the corresponding ventral vertebral bodies.

During closure, a single chest tube is inserted before the pleura is closed with a 3-0 absorbable suture. Three to six interrupted nonabsorbable no. 1 pericostal sutures are used to ensure that the intercostal nerve is not damaged. The remainder of the wound is closed in layers.

Thoracotomy

This approach traverses the thorax to gain access to the spinal column, most notably the vertebrae from T3 to T10.^14,¹⁵ A thoracotomy can be performed from either the right or left side, and the approach largely depends on the location of the pathology. The indications for thoracotomy are correction of deformity, degenerative disease, repair of fractures, resection of tumors, stabilization after trauma, and treatment of infection involving the thoracic column. When the approach is used to correct a deformity, the approach is always from the side of the apex of the curve of the spine. This approach provides excellent visualization of the thoracic vertebrae but is also associated with complications. Among the most avoidable of such complications are procedures performed on the wrong side or the exposure being too high or too low relative to the pathology. These problems can be largely avoided by using fluoroscopy and image-guided systems in the operating room (OR) and the use of standard OR time-outs. Preoperative marking by a radiologist can also be helpful.

Patients are placed in the lateral decubitus position with the arms placed orthogonally, elevated, and flexed at the elbows. The legs are positioned with knees bent and padding placed to prevent the formation of pressure sores. Both the sacrum and symphysis should be well padded and supported.

The incision should be placed as close over the pathology as possible. During this approach, the rib resected usually dictates the highest vertebral level that can be accessed and the best exposure for the vertebra two levels below it. The skin incision starts at the lateral border of the paraspinous muscles and extends to the sternocostal junction of the ribs. After the incision is made through the subcutaneous tissue, the latissimus dorsi and serratus anterior muscles are divided. In the authors’ experience it is best to incise the latissimus dorsi only partially and to lift it with a retractor to minimize the risk of damage to underlying tissues (Fig. 35-5). The serratus anterior muscle should be dissected as far distally as possible, especially in the higher thoracic levels, to minimize damage to the long thoracic nerve.

FIGURE 35-5 A, The approach for a thoracotomy. B, Partial dissection of the latissimus dorsi muscles exposes the underlying tissues during dissection.

Once the ribs are exposed, the periosteum is dissected in the midline and the rib is liberated with blunt dissection and the aid of a rib stripper. The liberated rib is cut with a rib cutter as far ventrally and dorsally as possible to obtain a good exposure. When a rib-preserving thoracotomy is performed, the intercostal muscle is cut in the lower half to preserve the neurovascular bundle lying in the inferior edge of the rib. Immediately deep to the rib lies the parietal pleura, which is mobilized and removed. At this point the anesthesiologist deflates the lung to increase the surgeon’s exposure. The intercostal space can be increased with a retractor, and the lung can be wrapped with a moistened sponge and retracted to further augment the exposure of the vertebral column.

The vertebral pleura, which is frequently covered by the parietal pleura, is lifted from the column and opened to expose the segmental vessels. The segmental vessels may be mobilized and ligated 3 to 4 cm ventral to the head of the rib. At this juncture, it is important to note the contribution of the segmental vessels to the blood supply of the spinal cord. To ensure the safety of the spinal cord and to prevent unwanted ischemic damage, the vessels can be temporarily occluded with an aneurysm clip to test whether ligation affects the blood supply of the spinal cord as indicated by evoked potential monitoring (motor and somatosensory evoked potentials). Once the surgeon is certain that sacrifice of the vessel will not lead to vascular compromise of the spinal cord, he or she can safely ligate the vessel.

Once the segmental vessels are released, the aorta can be mobilized to the right side and the prevertebral area exposed for surgery (Fig. 35-6). A sponge stick can be used to further expose the vertebral bodies. For approaches to T11-L1, monopolar cauterization is used to divide the diaphragm about 1 cm from the costal margin. The retroperitoneal space is then entered.

FIGURE 35-6 A, Anatomy of the mediastinum. B, The release of segmental vessels allows the aorta to shift to the right, exposing the paravertebral area for surgery.

The technique for closure is important because several critical structures, including the lung, azygos vein, and aorta, are in the surgeon’s path. One or two chest tubes are placed, and the operative site is irrigated with antibiotic solution. The chest tubes are set to suction and remain in place until drainage decreases to less than 100 mL/day. Chest tubes are placed to water seal in cases where the dura has opened or cerebrospinal fluid has been encountered. The parietal pleura should be closed whenever possible. At our institution we use thoracic drains. The skin incision for the thoracic drains should be placed one level below the targeted intercostal level. A rib approximator may be used to narrow the cavity between the ribs created by the retractor. The ribs may be reapproximated with a suture. The surgeon must ensure that the neurovascular bundle is excluded. At this point the anesthesiologist can test the patency of the lung by reinflating it. Reinflation of the lung is critical to avoid unwanted atelectasis. The soft tissues are closed sequentially. Other potential sources of complication include the risk of injury to the lung, segmental vessels, azygos vein, and aorta or entry into the intervertebral foramen.

Postoperatively, the patients who require prolonged bedrest or immobilization receive either 5000 U of subcutaneous heparin or 30 mg of subcutaneous enoxaparin sodium twice daily. A thoracolumbosacral orthosis is used in some cases. Serial postoperative radiographs are obtained to assess stability and healing.

Thoracoabdominal Approach

This approach is excellent for the thoracolumbar junction, most notably from T9 to L5.^16,¹⁷ Although this approach is feasible from both the right and left sides, a left-sided approach is preferred because the liver and vena cava are not in the trajectory of the surgeon’s approach. For a left-sided approach, the patient is placed on the right side. The table can be bent above the pelvis to increase the distance between the pelvis and ribcage, adding exposure to this region. Depending on the target level, it is recommended to resect the ninth or tenth rib. After the skin and subcutaneous tissues are incised at the thoracolumbar junction, the muscle is split in the direction of its fibers to open the superficial muscular layer of the rectus anterior, latissimus dorsi, and external oblique muscles.

Starting with a retroperitoneal approach, the external oblique, internal oblique, and transversus muscles are split. The peritoneum is mobilized to the midline and freed with a sponge stick from the diaphragm. The ninth or tenth rib is resected as in a thoracotomy. This rib can be used later as a structural bone graft or morcellized and placed in a cage. The ventral resection is performed as near the cartilage-bone junction of the rib as possible. The costal cartilage is split, and the diaphragm is transected about 2 cm medial to its insertion into the thoracic wall. The transected diaphragm should be mobilized by using holding sutures, which are used during closure to approximate the tissues. The mobilized diaphragm is transected about 2 cm above the medial and lateral arcuate ligaments. The parietal pleura is incised at the thoracic level of the pathology. The psoas muscle should be mobilized dorsally to augment the field of view. The segmental vessels may be ligated at the level of the pathology to minimize bleeding during the approach and operation (Fig. 35-7).

FIGURE 35-7 The critical anatomy observed during a thoracoabdominal approach to the thoracic spine. A, The descending aorta passes through the diaphragm at the level of T12. The left crus and right crus attach to vertebral bodies of L2 and L3, respectively. B, The diaphragm and aorta are ghosted to demonstrate the locations of the sympathetic chain, azygos and hemiazygos veins, and segmental arteries.

Closure begins with suturing the parietal pleura. The bilateral stay sutures placed in the diaphragm make its repair simpler. The ribs are reapproximated, and the abdominal wall is closed in three layers.

Complications associated with this procedure include entry into the peritoneal space and damage to the greater splanchnic nerve, ascending lumbar vein, sympathetic trunk, thoracic duct, or the great vessels.

Dorsolateral Surgical Approaches

Costotransversectomy

The patient is typically placed in the prone position; some surgeons also use the lateral decubitus position.^18,¹⁹ A roll may be placed under the scapula to augment the exposure of the chest. The choice of incision depends on the location and extent of the pathology and may include either a longitudinal paraspinal incision or a midline or transverse paraspinal incision. The paraspinal incision is made in a curvilinear fashion almost 2 inches from the midline of the vertebra of choice. Cautery is used to cut the paraspinal muscle and thoracolumbar fascia, and a Cobb elevator is used to separate the muscles at the level of interest (Figs. 35-8A and B).

FIGURE 35-8 A, The approach for a costotransversectomy. B, The skin incision and blunt dissection of the paraspinal muscles show the thoracic vertebrae. C, Using sharp periosteal dissection, the surgeon separates the periosteum from the rib.

The pleura lies immediately deep to the ribs; therefore the surgeon must use caution during the resection. Using sharp periosteal dissection, the surgeon separates the periosteum from the rib (Fig. 35-8C). The costotransverse joint is incised, and the periosteum of the rib is elevated circumferentially. During the dissection, the neurovascular bundle should be identified and care exerted to preserve it. The rib is cut at its angle and disarticulated from the costovertebral joint to enable dissection of the parietal pleura and endothoracic fascia. A self-retaining retractor or a rib spreader can be used to augment the exposure.

At this juncture, the transverse process is brought into the field of view. Further dissection leads to the pedicle and vertebral body. The exiting nerve root can be identified by tracing it to its foramen. After the nerve root is identified and secured, decompression or manipulation can be performed. In dissections ventral to the vertebral body, vital soft tissues are avoided by elevating the prevertebral fascia from the vertebral body and using it to protect structures. A multiple-layer closure is used.

Lateral Extracavitary Approach

This approach is an extension of the costotransversectomy, as described earlier.²⁰ As in a costotransversectomy, the patient is placed in the prone position. This approach can be from the right or left; the most important determinant is the location of the lesion.

Given the nature of the thoracic cord and its location in a vascular watershed zone, it is important to identify the location of the artery of Adamkiewicz via preoperative angiography. Although not always feasible, temporary clip occlusion of a suspected radicular artery with concurrent motor evoked potential monitoring can be attempted. Once this artery is identified, the surgeon should modify the approach to enter from the side that does not place the artery in the entry trajectory. A midline, semilunar, hockey stick, or paramedian skin incision can be used. As explained earlier, a muscle-splitting technique must be used to minimize trauma to the bountiful spinal muscles.

The dissection proceeds deep to the thoracodorsal fascia. The fascia is opened in a paramedian or T-shaped fashion and retracted, revealing the erector spinae muscles, which are detached and retracted medially. Rib dissection proceeds as described earlier; the ribs can be removed and used as bone grafts as needed. The neurovascular bundle is preserved and followed to the neural foramina. The radicular vessels are identified and cauterized. Using a subperiosteal dissection along the lateral aspect of the vertebral bodies, the pedicles are exposed and removed using rongeurs, a Kerrison punch, or a high-speed drill. The wound is closed, as described earlier, in multiple layers.