Extraspinal Anatomy and Surgical Approaches to the Thoracic Spine

Summary of Key Points

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Surgical success depends primarily on the surgeon’s proficiency and experience, knowledge of the thoracic spine anatomy, operative techniques, spinal biomechanics, and adequate indications for surgery.
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Key and vital structures are encountered when accessing the thoracic spine operatively, especially via ventral approaches.
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Dorsal approaches involve skin and occasionally extensive muscular dissection; however, the dorsal extraspinal anatomy lacks major vascular and neurologic structures.
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The choice of the approach to the thoracic spine largely depends on the location of the pathology that the surgeon is treating, the surrounding anatomy, and whether there is a history of previous surgeries.
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Novel microsurgical techniques and hardware for ventral stabilization, together with advanced techniques, have allowed surgeons to access thoracic spine pathologies ventrally with acceptable morbidity and mortality rates.
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Each approach provides a unique visualization of the ventral thoracic spine, and the utility of each approach depends on the location of the pathology.
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Dorsolateral approaches include costotransversectomy, lateral extracavitary, and parascapular methods.

The thoracic spine contains more vertebrae than any other segment of the spinal column. With its 12 vertebrae, the thoracic spine contributes to 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 (a third of the total vertebrae), it is not surprising that the thoracic segment is also a frequent site of pathology ( Table 47-1 ). Trauma, primary and metastatic tumors of the spinal column, infections, vascular malformations, congenital and degenerative disorders, and deformity all affect the thoracic column, making the ability to operate in this region an essential skill set for the competent neurosurgeon. Surgical success depends primarily on the surgeon’s proficiency and experience, thoracic spine anatomic knowledge, operative techniques, spinal biomechanics, and adequate indications for surgery.

TABLE 47-1

Indications for Surgery

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

Thoracic Spine Anatomy

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 and 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. 47-1 ). 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. They serve as attachment sites for the supraspinatus and intraspinous ligaments and paravertebral muscles. 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, albeit their width is considerably decreased. The thoracic pedicles are short, and their height and radius increase from T1 to T12. The pedicles appear kidney shaped with medial convexity. They are composed primarily of cancellous bone surrounded by cortical bone and are thicker medially than laterally, which is why screw perforation occurs more frequently laterally. The nerve root is numbered according to the pedicle located immediately superior to it.

Throughout the thoracic spine, the angle between the pedicle and midsagittal plane changes dramatically depending on the level. This observation has important clinical implications, such as for the placement of pedicle screws for fixation. 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. In the midthoracic spine (T7-T9), the transverse process is oriented more inferiorly relative to the pedicle. As you move superiorly and inferiorly from this segment, the center of the pedicle becomes more aligned with the center of the transverse process.

From T1 to T10, the facets are oriented coronally. The orientation becomes oblique between T10 and T12. 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. The multitude of ligamentous connections, most notably the anterior longitudinal ligament, provides additional stability by increasing the tensile strength of the column with a relatively higher proportion of collagen. The posterior longitudinal ligament limits hyperextension and has approximately half the tensile strength of the anterior longitudinal ligament. The articular surface of the superior facets is on the dorsal aspect, whereas the articular surface of the inferior facets is ventral. The articular surfaces of the facet joints are flat and slope in an oblique coronal plane, in the same plane as the lamina. Their orientation allows lateral bending, flexion, and axial rotation. Extensive damage of mechanoreceptors and proprioceptors located at the capsule of the facet joints may lead to trunk motion dysfunction and dyskinesia.

Extraspinal Anatomy

Key and vital structures are encountered when accessing the thoracic spine operatively, especially via ventral approaches. Ample knowledge of the extraspinal anatomy is paramount for the spine surgeon. Dorsal approaches involve skin and, occasionally, extensive muscular dissection; however, dorsal extraspinal anatomy lacks major vascular and neurologic structures. Paravertebral muscles are innervated by posterior primary rami and their branches, with the exception of those innervated by the cranial nerves and brachial plexus. As a general rule, in the upper thoracic spine, the lateral branches supply muscles and the medial branches are musculocutaneous; the opposite occurs in the lower thoracic spine, and the midthoracic spine has transitional innervation. A similar overlap is evident in the vascular supply. This overlap becomes relevant because iatrogenic muscular ischemia may lead to muscular fibrosis due to excessive pressure or vascular injury. Wide exposures may also cause muscular denervation leading to atrophy.

Dorsolateral, lateral and ventral, or anterior approaches require adequate identification of the pleura and lungs bilaterally and the heart, trachea, and esophagus medially; identification of the aorta, hemiazygos vein, and liver on right approaches and the thoracic duct, vena cava, and azygos vein on left approaches; and identification of the sympathetic trunk, lesser and greater splanchnic nerves, segmental arteries, and artery of Adamkiewicz in approaches close to the vertebral bodies.

The manubrium at the sternal notch correlates with the T2-T3 level, and the sternal angle correlates with T4-T5. The sternohyoid and sternothyroid muscles attach on the dorsal surface of the manubrium, whereas the sternocleidomastoid muscle attaches on the manubrium-clavicular joint. Immediately posterior is the brachiocephalic and subclavian veins and pleural apices covered by the transthoracic fascia. The aortic arch limits the exposure of T3 and T4. The thoracic duct runs dorsal and to the left of the esophagus between the visceral and alar fascia ascending to C7 and lying laterally behind the carotid sheath. It terminates at the junction of the left internal jugular and subclavian veins. The right recurrent laryngeal nerve crosses the retropharyngeal or retromediastinal space anywhere between C7 and T3, whereas the left recurrent laryngeal nerve branches off the vagus nerve, loops around the ligamentum arteriosum, and ascends within the visceral fascia between the esophagus and trachea.

Sufficient exposure of the affected levels and preservation of vital organs is paramount. In specific cases, peripheral nerves and segmental arteries may need to be sacrificed to maximize exposure and perform a corpectomy or vertebrectomy. However, caution must be taken whenever possible to preserve the C8 and T1 nerve roots in order to preserve hand function. Somatosensory and motor-evoked potentials are assessed intraoperatively before ligation and division of segmental arteries by temporary occlusion with an aneurysmal clip of the desired vessel.

Approaches

The choice of the approach to the thoracic spine largely depends on the location of the pathology that the surgeon is treating ( Tables 47-2 and 47-3 ; Fig. 47-2 ), surrounding anatomy, and history of previous surgeries. The left side is usually preferred to avoid the liver and inferior vena cava. The right side is chosen for lesions located at T3 and T4, as the aortic arch limits visualization on the left. The variable course of the thoracic duct must be also considered, as in general it arises at L2 and ascends on the right side, crossing to the left at T5.

TABLE 47-2

Approaches to the Thoracic Spine as a Function of Level of Pathology

Approach	Vertebral Level
Standard ventral cervical	T1-T4
Transsternal	T1-T4
Transthoracic	T3-T11
Transthoracic, transdiaphragmatic, retroperitoneal	T11-L1

TABLE 47-3

Surgical Approaches to the Thoracic Spine

Approach	Incision/Position	Indication	Contraindications	Advantages	Disadvantages
V entral
Ventral	Supine position	Ventral	Dorsal or dorsolateral neural compression	Ventral exposure of dura	Limited to T1-T3
Cervicothoracic	Ventrolateral cervical/median sternotomy			May use instrumentation	Recurrent laryngeal nerve and esophageal injury
Transthoracic	Lateral decubitus position, thoracotomy incision	Ventral compression of spinal cord or roots	Dorsal neural compression	Ventral exposure of dura	Morbidity of thoracotomy
		Ventral release to treat thoracic scoliosis		Excellent for correction of thoracic scoliosis	Staged instrumentation may be necessary
				Control of radicular vessels	Requires mobilization of diaphragm for T10-L1 access
				May use instrumentation
D orsolateral
Costotransversectomy	Prone position	Accessible lateral neural compression without significant ventral component	Polytrauma and medical complications	Lateral and dorsal neural exposure	Extensive muscle dissection
Lateral extracavitary	Hockey stick incision			Dorsal instrumentation can be done simultaneously	Difficult to visualize ventral dura and contralateral pedicle
				Minimal risk of injury to lung and great vessels
D orsal
Laminectomy	Prone position, midline thoracolumbar incision	Dorsal laminar fractures with neural entrapment incision	Ventral neural compression or dorsal epidural hemorrhage with incomplete spinal cord or cauda equina injury	Less surgery Dural tears easy to repair Dorsal instrumentation may be performed	Cord compression May be destabilizing with ventral pathology