Georges Y. El-Khoury, MD
Professor
Department of Radiology
The University of Iowa
First Published: May 22, 2003
Last Revised: May 22, 2003
Peer Review Status: Internally Peer Reviewed
The thoracic spine is the largest segment of the spine with approximately 10-16% of all thoracolumbar fractures occurring between the T1 and T10 vertebrae. Although the thoracic spine has unique anatomic and biomechanical features, fractures in this region have historically been lumped with fractures of the thoracolumbar junction. Distinguishing features of the thoracic spine are related to the presence of ribs which restrict motion and add stiffness to the spine. Some authors consider the rib cage as part of the upper thoracic spine providing strength and capacity to absorb energy during trauma.
Most thoracic spine injuries occur in flexion and axial loading; this is because there is hardly any rotation in the thoracic spine. Thoracic spine injuries are difficult to detect on chest radiographs and dedicated thoracic spine radiographs should be obtained. These fractures (T1 - T10) do not neatly fit into the commonly used Denis classification which is intended for the thoraco-lumbar junction. Three types of fractures are recognized in the thoracic spine: (1) Wedge compression - which involves the anterior two-thirds of the vertebral body. This fracture is considered to be stable. (2) Sagittal slice - where the basic injury pattern consists of an anterior fracture/dislocation with compression of the vertebral body below. This injury is unstable and frequently associated with neurological damage. (3) Posterior dislocation - this is a high energy unstable injury.
There are two conditions which could mimic vertebral fractures in the thoracic spine and they are physiologic wedging and Scheurmann's. Physiologic wedging typically occurs in the lower thoracic spine between T8 and T12 and it is more pronounced in males. A wedging ratio (anterior vertebral body height divided by posterior vertebral body height) of 0.80 m in males and 0.87 in females at T8 - T10 is considered normal. Scheuermann's disease is due to abnormal growth cartilage with weakening of the vertebral endplates. The vertebral growth is impaired causing anterior wedging which can persist into adulthood. Considerable energy is required to produce a fracture-dislocation, and therefore the incidence of other non-contiguous spinal injuries is fairly high reaching up to 17%. Cord damage is common because the size of the spinal canal is small. Only 12% of patients with fracture-dislocations of the thoracic spine are neurologically intact compared to 41% of patients with lumbar spine and 26% of patients with cervical fracture-dislocations. 62% of patients with thoracic spine fracture-dislocation have complete neurologic deficit, whereas only 2% of patients with lumbar spine and 32% of patients with cervical spine fracture-dislocation have a complete neurologic deficit.
Fractures in the fused thoracic spine constitute a unique subset of injuries. They are seen in patients with ankylosing spondylitis, DISH and advanced degenerative disc disease with bridging osteophytes. The mechanism of injury is typically hyperextension and fracture is always unstable since all three columns are involved. This injury is often misdiagnosed as a neoplasm or infection, especially when a history of acute trauma is lacking. Cord damage is common in this type of fracture.
Disc herniation can occur in the thoracic spine following trauma, but it is rare. When they do occur, they are usually associated with significant neurological damage.
Because of the difficulty in diagnosing thoracic spine fractures on plain radiographs, radiologists have relied on indirect signs to make this diagnosis. These signs include: mediastinal widening, paravertebral hematoma, pleural fluid, rib fractures and costovertebral dislocation, double spinous process sign, and sternal fractures. The first three indirect signs are also seen with traumatic aortic rupture. In aortic rupture, however, the nasogastric tube and trachea are shifted to the right and the left main stem bronchus is depressed >40º from the horizontal.
CT, especially MD- CT, is currently the most effective method for examining the extent of the bony injury in the spine. In conjunction with sagittal reconstruction, CT is very useful in demonstrating retropulsed fragments and spinal canal compromise.
MRI is the imaging modality of choice in patients with a neurological deficit.
The criteria for instability in thoracic spine trauma are not clearly defined; however, most surgeons are inclined to surgically stablize injuries associated with one or more of the following findings: fracture-dislocations; post-traumatic kyphosis >40º; spinal injuries associated with a sternal fracture; concomitant rib fractures and/or costovertebral dislocations.
The thoracolumbar junction is especially vulnerable to injury because its mobility and lack of stabilizing support of the ribs. An important concept which is used for interpretating thoracolumbar injuries is the column theory described by Denis in 1983. This is a widely accepted concept which divides a vertebra into three columns: (1) anterior column which consists of the anterior longitudinal ligament and the anterior part of the vertebral body; (2) the middle column which includes the posterior part of the vertebral body and the posterior longitudinal ligament and (3) the posterior column which includes all the bony and ligamentous structures posterior to the posterior longitudinal ligament. Fractures involving the anterior column are considered stable where as fracture involving the anterior and middle column or all three columns are considered unstable.
Denis described four basic types of thoraco-lumbar fractures: (1) compression fractures, (2) burst fracture; (3) seat belt injury (including Chance fracture); and (4) fracture dislocation. This presentation will discuss imaging of these fracture patterns and comments on their stability.
References
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