Surgical Decompression and Stabilization Techniques in Thoracic Trauma

Overview

More than 1 million acute spinal injuries occur in the United States every year, of which approximately 50,000 result in vertebral fractures, and less than 10,000 result in spinal cord injury. Trauma involving the thoracic spine can be quite morbid, given that this segment of the spine has the narrowest canal-to-cord ratio and has the poorest prognosis for neurologic recovery compared with other spinal segments. However, thoracic fractures are less common than fractures of the cervical and lumbar spine, and one study shows that only 16% of spine fractures occur between T1 and T10.

Numerous classification systems have been and continue to be developed. For the purposes of this chapter, understanding the different types of thoracic spine fractures requires understanding the Denis three-column classification system. According to this system, the spine has three columns: anterior, middle, and posterior. The anterior column consists of the anterior half of the disk space and vertebral body, including the anterior longitudinal ligament. The middle column consists of the posterior half of the disk space and vertebral body, including the posterior longitudinal ligament. The posterior column consists of the dorsal bony structures, including the pedicles, facets, pars interarticularis, laminae, and spinous processes. The ligaments that make up the posterior column include the supraspinous and interspinous ligaments and the ligamenta flava. Spinal instability occurs when two of the columns are violated.

In general, five types of fractures can affect the thoracic vertebrae: burst fractures, fracture-dislocation, flexion-distraction, Chance fractures, and compression fractures. Burst fractures are defined as a three-column axial compression injury, and they generally follow the 50 : 20 : 50 rule: greater than 50% loss of vertebral body height and/or greater than 20 degrees of segmental kyphosis and/or less than 50% retained anteroposterior spinal canal diameter as a result of retropulsion of bone. Fracture-dislocation involves all three columns with translation and/or rotation of the spine, a classic example of a shear-type injury. Flexion-distraction fractures occur when the anterior and middle columns are compressed and the posterior column is distracted. Chance fractures, also known as seatbelt fractures, are a flexion-distraction injury in which a transverse fracture extends from the anterior column through the posterior column. Compression fractures, the most common fractures of the thoracic spine, are isolated fractures of the anterior column. With compression fractures, there is no loss of posterior vertebral body height, nor is there evidence of subluxation; these are generally caused by axial loading. All of these fractures require surgical stabilization except for compression fractures, which are generally managed conservatively via orthosis.

In this chapter, we review the pertinent anatomy of the thoracic spine, list the indications and contraindications for surgery, describe techniques to decompress and stabilize the thoracic spine from both posterior and anterolateral approaches, and discuss potential complications for the more common surgical techniques.

Anatomy Review

The thoracic spine is a unique structure that consists of 12 vertebrae, compared with 7 in the cervical and 5 in the lumbar spine. Its alignment is kyphotic, as opposed to the lordotic alignment of cervical and lumbar spines; this is secondary to the anterior portion of the vertebral bodies being 2 to 3 mm shorter than the posterior portion.

As opposed to the consistent orientation of the cervical and lumbar facets, the orientation of the thoracic facets changes from rostral to caudal. In the upper levels of the thoracic spine, the facets are coronally oriented; this changes to a sagittal orientation in the lower levels. As a result, facets of the upper thoracic levels have little resistance to rotational forces but do resist anterior translational forces. The converse is true in the lower thoracic spine, where resistance to rotational forces is greater than to translational ones. These factors are important in terms of mechanism of injury.

Motion in the thoracic spine is further restricted by its articulations with the rib cage. The stability provided by the rib cage requires greater forces to injure the thoracic spine compared with the other segments. In fact, an intact rib cage increases the force needed to compress the spinal column by four times. Adjacency of the rostral and caudal thoracic spine to the flexible cervicothoracic and thoracolumbar junctions lends them protection from injury, whereas the rigidity of the midthoracic spine increases its predilection for fracture. Aside from T12, the most commonly fractured thoracic vertebral bodies are T6–T8.

Familiarity with the anatomy of the thoracic pedicle is essential for proper placement of instrumentation. The thoracic pedicle is oriented in a posterolateral to anteromedial direction by approximately 10 degrees along most of the thoracic spine, although at T12 a slight anterior and lateral angulation of the pedicle is apparent. Techniques for pedicle screw entry will be discussed elsewhere.

Indications and Contraindications

Indications

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Spinal biomechanical instability
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Neurologic dysfunction from spinal cord and/or nerve compression
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Progressive deformity

Contraindications

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Poor pulmonary function or other medical illnesses that would preclude surgery
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Limited life expectancy

Operative Technique

Posterior

In cases of predominantly dorsal compression, or when ventral compression is minimal, decompression of the thoracic spinal cord can be adequately accomplished via thoracic laminectomy. For this procedure, the patient is positioned prone with the arms supported upright on rests. Diligent care must be taken for proper positioning and padding of the upper extremities to avoid positional nerve palsies. Fluoroscopy is used to localize the correct spinal level. Correct localization is imperative, because there may be an anomalous number of thoracic vertebrae, ribs, and so on. A midline incision is planned, and a subperiosteal dissection is performed to expose the lamina. To perform the laminectomy, some surgeons prefer to create troughs in the lateral laminae with rongeurs or a high-speed air drill. The lamina is then removed en bloc. Another technique is to use the drill to remove bone down to the level of the ligamentum flavum, first centrally and then out laterally. Local bone can be saved for autologous grafting if fusion is planned. The ligamentum flavum is then resected to expose the dural sac, completing the laminectomy ( Fig. 32-1 ).

When the amount of ventral compression on the spinal cord is significant, and simple laminectomy for decompression is inadequate, the surgeon can accomplish further decompression via transpedicular decompression . This procedure was first described in 1978, and it has been used for excision of herniated thoracic disks. By removing the pedicle, the surgeon gains access to the anterolateral portion of the thecal sac and, hence, the dorsal aspect of the vertebral body. The positioning and localization for this procedure are the same as described above for the thoracic laminectomy. Subperiosteal dissection is also the same, with the exception that the exposure must be out lateral to the transverse process to adequately visualize the facet and pedicle. Pediculectomy can be performed with a high-speed drill by starting centrally and “hollowing out” the pedicle so that the “eggshell” outer cortex remains. This cortex can then be resected with rongeurs ( Fig. 32-2, A and B ). With the lateral aspect of the spinal canal now exposed, an angled instrument—such as a 45- to 90-degree down-biting curette, Woodson dental instrument, or the like—can be used to tamp down any bony elements that impinge on the spinal cord ventrally (see Fig. 32-2, C ) and to remove disk and/or fractured bone fragments away from the thecal sac (see Fig. 32-2, D ). This procedure can be performed after adequate decompressive laminectomy of that spinal level to further decompress the spinal canal.

Another posterolateral approach to the ventral spine can be accomplished via costotransversectomy ( Fig. 32-3, A ), first described in 1894 to treat a patient with Pott disease and epidural abscesses. The advantages of this approach are avoidance of the morbidity associated with ventral approaches (discussed below) and the ability to decompress the spinal canal and implant instrumentation through the same exposure. The disadvantage of this approach is that, although it provides better visualization of the anterior canal than a transpedicular decompression, it is more limited than a lateral extracavitary approach (described below).