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.

Classification of Injuries

Several classification systems have been developed to describe fractures of the thoracic and lumbar spine. In 1983, both Denis and McAfee et al. independently published three-column models that have become widely accepted (Fig. 67-2).^2,40 These models are very similar but do have some fundamental differences (Table 67-1).⁴¹ Each model divides the vertebra into three columns: anterior column (anterior longitudinal ligament, ventral half of the vertebral body, and ventral half of the anulus fibrosus), middle column (posterior longitudinal ligament, dorsal half of the vertebral body, and dorsal half of the anulus fibrosus), and posterior column (supraspinous and infraspinous ligaments, ligamentum flavum, articular processes, joint capsules, spinous processes, and laminae). In Denis’s model, instability requires injury to at least two columns with an emphasis on preservation of the middle column for the maintenance of stability. Fractures are divided into four groups: wedge compression (Fig. 67-3), burst fracture (Fig. 67-4), lap belt–type injury (Fig. 67-5), and fracture-dislocation (Fig. 67-6). Instability varies in magnitude: first degree (mechanical), second degree (neurologic), and third degree (mechanical and neurologic). McAfee’s system places more emphasis on preservation of the posterior column for the maintenance of stability and defines six fracture patterns: wedge compression, Chance fracture, flexion-distraction injury, stable burst, unstable burst, and translational injury.

FIGURE 67-2 Three columns of the spine: anterior column (AC), middle column (MC), and posterior column (PC). ALL, anterior longitudinal ligament; PLL, posterior longitudinal ligament; SSL, supraspinous ligament.

(From Haber TR, Felmly WT, O’Brien M: Thoracic and lumbar fractures: diagnosis and management. In Bridwell KH, DeWald RL, editors: Textbook of spinal surgery, vol 2, Philadelphia, 1991, JB Lippincott, pp 857–910.)

FIGURE 67-3 Four types of compression fractures, as described by Denis: A, Compression fracture detaching anterior vertebral body from posterior vertebral body. B, Failure of superior end plate. C, Failure of inferior end plate. D, Failure of superior and inferior end plates.

(From Denis F: The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine [Phila Pa 1976] 8:817–831, 1983.)

FIGURE 67-4 Classification of burst fractures (by Denis). A, Fracture of both end plates. B, Fracture of the superior end plate. C, Fracture of the inferior end plate. D, Burst rotation. E, Burst lateral flexion.

(From Denis F: The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine [Phila Pa 1976] 8:817–831, 1983.)

FIGURE 67-5 A, One-level lap belt–type injury through the bone (Chance fracture). B, One-level lap belt–type injury through the ligament. C and D, Variants of two-level lap belt–type injuries: two-level lap belt–type injury through the bone (C); two-level lap belt–type injury through the ligaments (D).

(From Denis F: The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine [Phila Pa 1976] 8:817–831, 1983.)

FIGURE 67-6 Fracture-dislocation types (according to Denis). Flexion-rotation type through the bone (A) and the disc (B). Note the difference in rotation between both spinal segments. C, Shear (posteroanterior) subtype; the segment above is sheared off forward on the top of the segment below. D, This shearing may leave a floating dorsal arch. A dural tear and complete paraplegia are very common with this type of fracture. E, Shear (anteroposterior subtype). F, Flexion-distraction type resembles a lap belt–type fracture, but, in addition, there is stripping of the anterior longitudinal ligament during subluxation or dislocation.

(From Denis F: The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine [Phila Pa 1976] 8:817–831, 1983.)

In 1994, Magerl et al. proposed the Arbeitsgemeinschaft für Osteosynthesefragen (AO) system, based on the review of 1445 consecutively treated thoracolumbar injuries. There are three main types of fractures (types A, B, C), with a progression in the severity of injury from type A to type C. Each category contains subclassifications. Type A injuries are compressive forces acting on the vertebral body. Type B injuries are characterized by a distraction mechanism with disruption of the anterior and posterior elements. Type C lesions are rotational injuries involving the anterior and posterior elements.⁴² Although more accurately descriptive than the three-column classification, the AO system is often viewed as cumbersome to apply in a clinical setting.

In 2005, the Spine Trauma Study Group devised a thoracolumbar injury classification and severity score scale.⁴³ This system relies on three characteristics to describe the injury: injury morphology, the integrity of the posterior ligamentous complex, and neurologic status (Table 67-2) Points are assigned to the various subcategories. The total score among the three categories determines the injury severity score. Scores of 5 or higher suggest the need for operative treatment due to the unstable nature of the injury, and scores of 3 or lower suggest nonoperative management. A score of 4 may be treated either way. If multiple fractures are present, the injury with the greatest score dictates the treatment.

TABLE 67-2 Thoracolumbar Injury Classification and Severity Score Scale

A score <3 is generally considered a stable fracture, and a score ≥5 is considered an unstable fracture and typically requires surgical stabilization.

Treatment of Lumbar Spine Fractures

When initiating treatment for lumbosacral fractures, certain objectives should be kept in mind: (1) Decompress neurologic elements in order to maintain or improve neurologic function. (2) Stabilize the spine. (3) Prevent posttraumatic deformity. With these goals in mind, a determination should be made about whether the patient would benefit from surgical intervention or nonsurgical treatment. It is not always appropriate to discuss nonsurgical management as “conservative treatment” since these patients often face a multitude of risk factors during the healing process.

Nonsurgical treatment, usually consisting of a period of bedrest followed by bracing, in patients who are neurologically intact has a long track record, even in the face of a fracture that by the classification schemes is dubbed “unstable.”⁴⁴ Depending on the type of fracture, the period of bedrest may be as little as 2 weeks or as long as 4 to 6 weeks, depending on the surgeon’s comfort with the fracture pattern. In the acute setting, the use of a specialty bed, specifically a rotating bed, has been found to maintain stability of the spine and decrease pressure on the skin to avoid decubitus ulcers. Although this modality inevitably increases the length of hospitalization, it avoids the risk and costs of surgery. This course of treatment must be accompanied by aggressive pulmonary toilet and mechanical prophylaxis and chemoprophylaxis to prevent deep vein thrombosis.

Indications for surgical treatment include, but are not limited to, incomplete or progressive neurologic injury in the setting of neural element compression, an unstable fracture, or a multitrauma situation in which fixation of the fracture would greatly aid in the treatment of other injuries. If surgery is undertaken, it is important to attempt to reconstruct and stabilize the spine in a normal anatomic alignment to promote healing and prevent deformity. Great thought should go into a case in which decompression alone is performed in the setting of an unstable fracture, unless a period of postoperative immobilization is included. When instrumenting the spine, a general practice is to expose and fuse the least number of segments that will accomplish the goals of surgery. Although considerable debate surrounds the need for a long or short construct, each operation should be tailored for the individual pattern of injury.

Although the literature has established very few standards for treating these patients, a few thoughts are worth discussing. In general, stopping a construct at a transition from a mobile to an immobile section of the spine may result in a junctional kyphosis. In the lumbosacral spine, this means that when a multilevel lumbar construct is undertaken, crossing the thoracic junction should be considered if the construct would end at L1. Long thoracolumbar constructs or sacral fractures that result in lumbar-sacral disassociation often require fixation to the pelvis in the form of iliac screws (Fig. 67-7). Whenever the treatment is this extensive, care should be given to restore normal anatomic alignment so that appropriate sagittal balance is maintained.

FIGURE 67-7 Lumbosacral dislocation from a gunshot wound to the abdomen treated with an L4 to pelvis posterior construct.

(Courtesy of Michael Rosner.)

The next question with surgical intervention becomes a matter of timing: early or delayed. In a retrospective review, Schlegel et al. found that patients treated after 72 hours were at 4.3 times higher risk of being admitted to the ICU, 2.8 times higher risk of being on a ventilator, 12.2 times higher risk for development of pulmonary complications, 4.8 times more likely to develop bed sores, and 3.2 times more likely to develop urinary tract infections than those who had surgery within 48 hours of injury.⁴⁵ A concern about performing early surgery is an increased risk of complications. To examine this problem, McLain and Benson reported on 75 patients treated with spinal instrumentation for fractures and found that surgery within 24 hours was as safe as treatment for those provided care between 24 and 72 hours after injury.⁴⁶ When specifically looking at the thoracolumbar spine, Chipman et al. found that early surgery (<72 hours) in patients with higher injury severity scores (≥15) was associated with shorter periods in the ICU, shorter hospital stays, fewer ventilator days, and fewer complications.⁴⁷ A study by McHenry et al. reviewing 1032 spine fractures echoes these results. They found that surgery performed within 2 days of injury, one of the only variables of the injured patient that the surgeon can control, may decrease the risk of developing respiratory failure.⁴⁸ However, not all the studies on early surgery show an improved outcome. Kerwin et al. compared 361 patients that underwent surgical fixation of their spinal fracture within 48 hours with patients who underwent surgery after 48 hours. The only differences found between the two groups was a statistically significant higher mortality rate in the early-surgery group coupled with a shorter length of hospitalization, which the authors suggest may be related to the higher early mortality.⁴⁹ In general, each case should be evaluated individually. Although early surgery may be beneficial in a neurologically incomplete injury, the overall status of the patient must be taken into account in order to maximize postoperative function.

Sacral Fractures

Multiple different classification systems for sacral fractures have been proposed. However, the Spine Trauma Study Group advocates for the use of the Denis system and its modifications.^39,51,52 In 1988, Denis et al. published his classification of sacral fractures based on a retrospective review of 236 patients with sacral fractures.²⁸ This system divides the sacrum into three anatomic regions: alar region (zone 1), foraminal region (zone II), and region of the central sacral canal (zone III) (Fig. 67-8).

FIGURE 67-8 A, Classification of sacral fractures into zone I, region of the ala; zone II, region of the sacral foramina; and zone III, region of the central sacral canal. B, Zone II fracture. C, Mechanism of L5 nerve root damage or entrapment in the traumatic “far out” syndrome. The L5 root is caught between the sacral ala and the transverse process of L5 as the fragment migrates superiorly and posteriorly. D, Sagittal view of zone III fractures: normal sacrum (left), sacral burst fracture with a high potential for sacral root compression (middle), and sacral fracture-dislocation with a higher potential for sacral root disruption (right).

(From Denis F, Davis S, Comfort T: Sacral fractures: an important problem: retrospective analysis of 236 cases. Clin Orthop Relat Res 227:67–81, 1988.)

Zone I fractures involve the sacrum lateral to the foraminal line, without injury to the foramina or the sacral canal. These are usually the result of lateral compression of the pelvis with preservation of the sacroiliac ligaments and are composed of two major subtypes: alar fractures and sacrotuberous ligament avulsion. Alar fractures can either be stable if there is minimal displacement or unstable if there is significant displacement. If the vertical shear force on an alar fracture is severe enough for the fracture to become unstable, then the upwardly displaced fragment may stretch the L5 root or entrap it against the L5 transverse process. Displacement of the sacrotuberous ligament represents a severe injury with pelvic instability, typically with some additional anterior pelvic injury. Neurologic deficit was seen in 5.9% of these patients and mainly involved either the L5 root or sciatic nerve. Sacroiliac involvement of zone I fractures should be assessed because this has a deleterious effect on pelvic stability.

Zone II injuries involve one or more of the foramina and may go through the ala but do not violate the central canal. Neurologic injury was seen in 28.4% of these patients, typically involving the L5, S1, or S2 root. Zone II injuries comprise fractures extending through one or more sacral foramina but lateral to the sacral canal. These fractures are often seen in shear-type injuries but may also occur as the result of lateral compression. Vertical shear injuries are the result of significant force transmission to the sacrum and pelvis and usually result in concomitant injury to the sacroiliac joint. As a result, they are unstable. Nerve root injury has been reported in 28% to 54% of zone II injuries.

Zone III fractures include vertical shear injuries, high and low transverse fractures, and traumatic spondylolisthesis of L5 on S1 (highly unstable in flexion). Transverse sacral fractures are commonly seen after falls from great heights. These fractures usually cross the S2-3 level below the level of the sacroiliac joints with ventral and caudal displacement of the spine above the fracture. Zone III fractures primarily involve the central sacral canal, but the fracture line may go through the other two zones. Neurologic deficits were seen in 56.7% of these patients, with 76.1% of the patients with deficits having bowel, bladder, or sexual dysfunction. Zone III fractures have a high incidence of bilateral nerve root injury and cauda equina dysfunction. In addition to the features mentioned, the following are also associated with a risk of instability: high transverse, bilateral sacral, and vertical shear fractures.

Additionally, lumbosacral fracture-dislocations have a high incidence of concomitant transverse sacral fractures and can involve the S1 facet, resulting in instability. The majority of sacral fractures caused by indirect forces are vertical fractures, observed in conjunction with fractures of the pelvis.

Conclusions

Trauma to the lumbosacral spine is fairly common, and a high degree of suspicion should be maintained, especially in patients with an appropriate mechanism of injury, another spine fracture, or neurologic deficit. The stability of these fractures, although not always obvious, should be considered in the development and execution of a treatment strategy. Neurologic deterioration in a patient with a known lumbosacral fracture should prompt immediate evaluation for change in the fracture diagnosis and intervention as warranted.

Acknowledgment

The authors wish to thank Dr. Michael Rosner for images used in this chapter.