Spine Stability

A discussion of spine fractures necessitates a definition of spine stability since the goal of any clinically applicable characterization of fracture pattern and subsequent treatment paradigm relies on the concept of restoring the spine to its preinjury functional capacity. White and Panjabi define spinal stability as the ability of the spine to maintain physiologic loads without pain, deformity, or neurologic deficit.⁹ Conversely, Benzel describes instability as the inability to limit excessive or abnormal spinal displacement.¹⁰ These terms are somewhat nebulous, and we agree with the concept that spinal stability should be considered as part of a dynamic process that takes into account multiple parameters, such as actual segment loading, activity level, and chronicity. A detailed discussion of spinal stability is outside the scope of this chapter, but the spine surgeon must have a general concept of acute and chronic instability. With trauma, the initial decision of whether to operate depends in part on whether the fracture is acutely unstable. As described later in this chapter, multiple classification systems based on radiographic and clinical examination help to determine if the injury is acutely unstable. However, the potential of chronic instability should not be ignored. Recognizing injury patterns that lead to chronic instability and that pose a risk of posttraumatic deformity is essential to the long-term care of trauma patients.

Mechanism of Injuries

In general, the mechanism of injury to the lumbosacral spine is blunt, although penetrating trauma does occur, especially in a military or combat situation. For penetrating injuries, the key component to the extent and pattern of injury is the amount of kinetic energy (KE) released at the time of impact. The formula KE = 1/2 mv², where m is mass and v is velocity, explains that the higher the velocity of the projectile, the more kinetic energy it disperses on impact, and, hence, the greater the resulting damage. Most spine injury classifications and subsequent treatment paradigms are based on blunt trauma, with penetrating trauma patterns imposed within that system. Like any injury, the mechanism should be considered when determining a treatment plan, with the understanding that penetrating injuries of the spine are different from their blunt counterparts; this difference is primarily determined by the kinetic energy imparted by the projectile (Fig. 67-1).

FIGURE 67-1 Fragmentation injury to the abdomen resulting in pelvic and sacral disruption.

(Courtesy of Michael Rosner.)

Prehospital Management

All patients involved in major trauma should be treated as having a potential spinal injury until proven otherwise. This treatment includes multiperson patient transfers, log rolling, and the use of a cervical orthosis and a backboard. Every attempt should be made to maintain the spinal column in as near neutral alignment as possible. Initial management in the field should follow an advanced life support protocol. A stable airway, adequate ventilation, and hemodynamic stability should be maintained. Preventing hypoxia and hypotension is critical to preserving neural element function after trauma. Once a patient is stabilized, a neurologic examination should be performed, keeping in mind that concomitant head injury, intoxication, or injury to the extremities may make a thorough examination difficult or impossible.¹¹ A “normal” neurologic examination in the field does not rule out a spinal injury.

Initial Hospital Management

Because the thoracolumbar spine is relatively resistant to injury, the amount of force required to result in a fracture makes these traumas high risk for associated neurologic and retroperitoneal injury. It has been reported that 4.4% of all patients arriving at a level I trauma center have a fracture in the thoracolumbar spine and approximately 19% to 50% have a neurologic deficit.^12–15 Even with modern spinal immobilization techniques, the concern still exists for an unstable fracture that could result in a new neurologic injury or neurologic deterioration. In a study by Reid et al., fractures of the thoracolumbar spine that were diagnosed in a delayed fashion had a higher incidence of a new neurologic deficit than those diagnosed at the time of admission (10.5% vs. 1.4%).¹⁶ Unfortunately, delay in the diagnosis of thoracolumbar injuries is not uncommon. In a retrospective review, Dai et al. found that 28 of 147 patients with acute thoracolumbar injuries had a thoracolumbar fracture that was not diagnosed at the time of admission. Although some diagnoses were delayed secondary to resuscitation efforts or acute surgical intervention, 37% of these patients (7/19) with fractures did not undergo initial radiographic evaluation because of a lack of clinical suspicion and another 26% (5/19) had radiographs but their fractures were either misclassified or not diagnosed at all.¹⁷ Because of the potential for neurologic injury from a missed injury, some authors have suggested that the thoracolumbar spine should be imaged in all patients with polytrauma.^16,18

Certain injury mechanisms should raise the suspicion of associated spinal fractures. Patients jumping or falling from heights with significant lower-extremity injury, calcaneal injuries in particular, are at high risk for accompanying lumbar and thoracolumbar spine injuries. Patients involved in motor vehicle accidents and wearing lap belts only are at risk for flexion distraction injuries. Lumbar fractures have been associated with abdominal and urologic trauma, particularly in cases of lap belt injuries.¹⁹ Sacral fractures are often associated with injury to the pelvic ring.

The evaluation of the thoracolumbar spine starts with the primary and secondary surveys to include visual inspection and manual palpation for stepoffs or other bony abnormalities. Bruising of the abdomen may be a sign of an underlying lap belt–type injury. An abnormal posture may be indicative of muscle spasm and associated spinal injury. Using full spinal precautions, the patient should be log-rolled and the skin and contour of the neck and back examined. Bruising, stepoff, deviation from the normal spinal curvature, or pain with dorsal midline palpation may be indicative of an underlying bony and ligamentous injury. In the patient with a head injury, verbal response to questions about pain during palpation may not be entirely reliable.

A neurologic examination should be carried out in a systematic and standardized fashion. Results should be communicated using the American Spinal Injury Association (ASIA) grading system (described elsewhere in this book). This facilitates communication between treating specialties. Progressive neurologic deterioration is a widely accepted indication for acute intervention. It is, therefore, imperative that baseline neurologic function be accurately established. In the confused or obtunded patient, facial grimace or withdrawal to painful stimuli can serve as a gross motor and sensory examination. A rectal examination should be performed to assess perianal sensation, rectal tone, and a bulbocavernosus reflex. Radiographic studies of other areas of injury should be evaluated closely for patterns that may indicate a direction of force through the thoracolumbar spine.

The pattern of neurologic injury observed depends on the location of spinal injury. In the majority of patients, the conus medullaris lies directly opposite the L1 vertebral body. The conus medullaris contains the anterior horn cells of the L5 through S5 nerves. Both upper motor neuron (conus injury) and lower motor neuron (cauda equina) injury can occur. The injury pattern runs the spectrum from a complete injury below L1 to incomplete syndromes resulting in partial motor and sensory preservation with sacral dysfunction. The sacral nerve roots are the most sensitive to injury and the least likely to improve after injury. Injury to the sacral spinal cord and nerve roots can result in loss of bowel and bladder function; sexual function is often less severely affected. Injuries from L2-5 can result in isolated root injuries or cauda equina syndrome. Root injuries can appear as monoradiculopathy or polyradiculopathy. Cauda equina syndrome appears as variable sensory, motor, bowel, and/or bladder dysfunction. Sensory and motor loss tend to be asymmetrical. This tendency is in contradistinction to the conus medullaris syndrome, in which deficits are usually more symmetrical.²⁰ Sphincteric dysfunction is common and is often permanent.

Radiographic Evaluation

In compliant patients with unaltered metal status, no distracting injuries, and lack of midline spinal tenderness, the spine can be cleared without obtaining screening plain radiographs.²¹ However, if the patient does not meet any of the preceding criteria, radiographic evaluation is mandatory. One should bear in mind that between 5% and 20% of all spine fractures are multiple, and 5% occur at noncontiguous levels.²² Commonly, radiographic evaluation includes anteroposterior (AP) and lateral views of the cervical, thoracic, lumbar, and sacral spine and an open-mouth odontoid view. Flexion-extension studies are generally avoided in the workup of acute fractures. Spinal alignment should be assessed in both planes. The margins of the vertebral bodies and the spinolaminar line, facets, interspinous and interpedicular distances, and position of the transverse process should be studied. Acute kyphotic angulation or loss of lordosis may be indicative of an acute bony or ligamentous injury. Loss of disc height at the level above a vertebral body fracture is often observed in acute flexion injuries, but it may also be seen in cases of degenerative disc disease. Bare, or “naked,” facets may be indicative of posterior ligamentous injury as a result of a distraction-type injury. Abnormalities of the soft tissues, such as a paraspinal mass or loss of the psoas stripe, can help identify areas of adjacent bony injury.

Per advanced trauma life support (ATLS) guidelines, all patients sustaining high-energy trauma should have an AP radiograph of the pelvis. However, these images are often inadequate for evaluating the sacrum due to sacral inclination, bowel gas, and, sometimes, the overlying anterior pelvis.²³ Nork et al. identified L5 transverse process fractures and a paradoxic inlet view on AP pelvic radiographs as factors suggestive of sacral fractures.²⁴ Others have identified the foraminal stepladder sign, caused by a displaced and overriding transverse fracture, and disruption of the anterior sacral foraminal lines or sacral arcuate line as diagnostic clues.^25–27 Sacral fracture should be suspected in any patient with a pelvic ring injury associated with a neurologic deficit.^28–30

Computed Tomography

In many institutions, AP and lateral radiographs are the standard screening tools for evaluating thoracolumbar fractures. However, plain radiographs have been criticized for their lack of sensitivity, diagnostic inaccuracies, and for the amount of time required for adequate views. Many have advocated the use of CT as the primary means of evaluating the thoracolumbar spine. Multiple studies have shown that CT scans are more sensitive for detecting fractures in the spine than plain radiographs.^31–34 However, CT carries with it concerns about cost and radiation exposure. In 2004, Brandt et al. found that for 50 patients undergoing radiographic evaluation, the average time for a CT of the chest/abdomen/pelvis was 55 minutes ± 32 minutes with a cost of $654. For thoracic, lumbar, and sacral plain radiographs, the average time was 113 minutes ± 43 minutes. The cost of the radiographic evaluation of the spine in addition to dedicated CT scanning of the spine and viscera was $1487.³⁵ Wintermark et al. found that an average of 4.3 views were needed to adequately evaluate the thoracolumbar spine; 9% of the thoracolumbar films had to be retaken because of insufficient quality. Time needed to perform conventional radiographs was 33 minutes, with 70% (23/33 minutes) devoted to imaging the thoracolumbar spine. This compares with the median time to perform a cervical, thoracic, abdominal, and cranial CT of 40 minutes, including 7 minutes for reformatting and reconstructions of the films.³⁶ There is no level I evidence regarding the use of CT as a standard for diagnosing spine fractures, but a single series of CT scans that can be reformatted specifically to evaluate the spine has proven to be at least equal to if not superior to, plain radiographs in several studies.³⁷

Obtaining appropriate radiographs of sacral injuries is problematic. In one series, 49% of sacral fractures were missed on initial hospital presentation, including 24% with an “unexplained” neurologic deficit that was later explained by the presence of a fracture.³⁸ The sacrum is poorly visualized on standard AP views of the pelvis, so the treating physician must rely on other cues to prompt a more detailed survey of this region. CT with coronal and axial reformations is the most sensitive modality for defining complex pelvic and sacral fractures. For all radiographic measurement parameters, the Spine Trauma Study Group advocates the use of thin-section (1.0–1.5 mm) axial CT scans with coronal and sagittal reconstruction rather than plain radiographs.³⁹

Magnetic Resonance Imaging

MRI can be useful in the study of neurologic deficit to determine the extent of compression on the neural elements. Although current MRI techniques are in many ways more sensitive than CT in detecting areas of acute injury, they do not provide the bony detail provided by conventional CT. Increased signal on T2-weighted imaging may be used to detect fractures that are not well visualized on plain radiographs or CT. Short tau inversion recovery (STIR) sequences are quite sensitive for the detection of ligamentous and intervertebral disc injury not otherwise apparent by plain radiographs or CT.