Thoracolumbar Spine Fractures

Biomechanics

Forces along the long, rigid kyphotic thoracic spine catalyze an abrupt switch into the shorter, mobile lordotic lumbar spine at the thoracolumbar junction ( Fig. 30.1 ). Biomechanically, this transition zone is susceptible to injury and is the most commonly injured portion of the spine. High-energy trauma (motor vehicle accidents) is the leading cause of injury over this region, followed by falls and sports-related injuries. Owing to the higher energy mechanisms of injury, additional organ systems are often injured in up to 50% of thoracolumbar trauma patients.

The vertebral body is the primary load-bearing structure of the spine, with the intervertebral disk transferring all forces applied to the adjacent vertebral bodies. The annulus fibrosus of the intervertebral disk supports a significant portion of all applied axial and lateral loads and resists tension and shearing. The spinal ligamentous structures are essential in maintaining overall sagittal balance. The posterior longitudinal ligament (PLL) is a relatively weak ligament that provides some restriction to hyperflexion, along with the ligamentum flavum. The thick anterior longitudinal ligament (ALL) functions to resist spinal hyperextension and distraction.

The thoracic spine differs from the remainder of the spinal column because it is supported by and maintains articulations with the ribs. The intact rib cage increases the axial load-resisting capacity of the thoracic spine by a magnitude of four. The rib cage and facet articulations limit rotation, and therefore most thoracic spine fractures occur from a flexion or axial compression force vector. Most of stability in flexion is provided by the costovertebral articulations. A significant factor in the degree and extent of fracture character is the rate of force impact loading.

The thoracolumbar vertebrae are at an increased risk for developing compression fractures after trauma as a consequence of axial loads resulting from the natural kyphotic curvature of the thoracic spine. The kyphotic posture results in the placement of axial forces on the ventral portion of the vertebral body. If the strength of the ventral vertebral body is exceeded, a fracture of the vertebral body occurs, resulting in a vertebral compression fracture (VCF). The traumatic forces may also exceed the strength of the dorsal vertebral body and ligamentous elements, resulting in disruption of the dorsal tension band.

The osseous structures, ligaments, rib cage, and inherent anatomy impart great integrity on the thoracic and lumbar spine. The great kinetic energy needed for a fracture to the spine here is dissipated on impact through the soft tissue and viscous elements contained within and around the thoracic cavity, resulting in a high incidence of concurrent injuries. The incidence of concurrent injuries is reported to be greater than 80%, and these injuries involve the thorax, appendicular skeleton, and abdominal region. These high-energy impacts also affect remote areas from the trauma, such as the cranial vault. Petitjean and associates reported a 65% incidence of head injuries after high-velocity impacts, which resulted in incomplete thoracic spinal cord injury with 12% of these injuries classified as severe (Glasgow Coma Scale [GCS] score less than 8).

Tearing or rupture of the aorta, with associated hemodynamic compromise, has been associated with thoracic vertebral fractures. Pulmonary injuries have been reported in 85% of patients and typically consist of pulmonary contusions. Infrequently, perforation of the esophagus and tracheal injuries have also been associated with thoracic fractures.

The thoracolumbar region is more vulnerable to concurrent injuries than the thoracic region because it is not provided the protection of the thoracic rib cage. Typically these injuries consist of hollow viscous injuries, such as intestinal perforations, mesenteric avulsions, or solid organ injuries.

The most common mechanism of abdominal injuries is distraction or seat-belt injuries. Blunt abdominal aortic dissections are associated with distraction-rotational injuries of the thoracolumbar region. Multiple-level thoracic and lumbar fractures are also associated with a high incidence of abdominal injuries.

Axial load injuries, particularly in patients who have jumped or fallen and landed on their feet, may manifest as both thoracolumbar fractures and calcaneal fractures. Miller and colleagues reported a 48% incidence of concurrent abdominal injuries associated with transverse process fractures. Therefore a physician treating vertebral column injuries must be aware of not only the presence of spinal fractures but also the possibility of concurrent, nonspinal, soft tissue and bony injuries.