Trauma Surgery: Low Lumbar Injuries

Summary of Key Points

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Low lumbar fractures below L3 do not behave like thoracolumbar injuries.
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Neurologic deficits are not as common as thoracolumbar injuries.
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Sagittal alignment is much more important, and focal kyphosis is not tolerated in low lumbar injuries.
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Surgery should be considered for significant neurologic injury and inability of the fractured segment to maintain sagittal alignment.
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Surgical management should consider limiting fusion levels.

Traumatic fractures to the lumbar spine present with a set of challenges when it comes to diagnosis, management, and long-term outcomes. This is due to its unique anatomy, its biomechanics, and the surrounding neurologic elements. This region transitions from the largely immobile thoracic area to the more mobile lumbar segment to the sacrum, which makes up an important portion of the pelvic ring. This region of the spine can be divided into four distinct biomechanical segments: the thoracolumbar junction, the midlumbar spine, the lumbosacral junction, and the sacrum. The spinal cord terminates in the thoracolumbar spine with nerve roots innervating the lower extremities, bowel, bladder, and genitalia. Thus, appropriate management of fractures in the lumbar spine requires a thorough understanding of the unique fracture patterns and their associated neurologic injuries.

The thoracolumbar junction should be thought of as part of the upper lumbar spine (L1-2). It is a transition point between the largely immobile thoracic spine, stabilized by the rib cage, and the much more mobile lumbar spine. Although the majority of the thoracic spine resists rotational forces due to the rib cage, the thoracolumbar spine has a transitional facet structure without rib support, making it unable to resist this force pattern. As a result, 60% of all spinal fractures occur between T12 and L2 and 90% occur between T11 and L4.

Low lumbar fractures (L3 to L5) are distinct from injuries that occur in the thoracolumbar region. In the low lumbar spine, the body is unable to compensate for any resultant kyphosis, which may result in worsening symptoms. Focal kyphosis is tolerated in the thoracolumbar junction because the sagittal plane deformity can be compensated for by the upper thoracic and lower lumbar spine. Sagittal deformity in the low lumbar spine disrupts the overall sagittal alignment and mechanics of the spine. The facets in the upper lumbar spine are more oblique, and transition to a sagittal orientation at the lumbosacral junction results in more translational mobility. The increase in vertebral body size allows for greater axial load acceptance and has additional stability from the more muscular attachments. Hence, fractures of L3 to L5 account for 4% of all spine fractures.

Initial Assessment

The assessment of trauma patients should follow the algorithm outlined by the Advanced Trauma Life Support protocol. All traumatically injured patients should be treated as if they have a spine injury until it is ruled out. A skilled trauma team quickly and immediately evaluates airway, breathing, circulation, disability, and exposure. Multiperson patient transfers, log rolling, cervical orthosis, and a backboard should be utilized in order to maintain spinal column alignment. It is important to prevent hypoxia and hypotension in order to avert the neural element injury after trauma. Once stabilized, a secondary survey including full physical examination and neurologic examination should be performed. Intoxication, head injury, dementia, or distracting injury to the extremities may hinder a thorough examination. These patients require a high level of clinical suspicion for spinal cord injury, as reassessment should take place frequently in the first 48 hours. It is important to note the potential for neurologic injury from a missed injury, thus it is imperative that polytrauma patients have spine imaging done in the form of a computed tomography (CT) of the entire spine.

Specific injury mechanisms can be associated with spinal fractures. For example, someone who has fallen from a steep height can present with long bone, calcaneus, and pelvis injuries with a high risk for accompanying lumbar and thoracolumbar spine injuries. Bruising of the abdomen in lap belt–only seatbelts can be a sign of flexion distraction injuries along with abdominal and urologic trauma. Lumbar fractures have been associated with abdominal and urologic trauma. Sacral fractures are often associated with injury to the pelvic ring.

Neurologic examination and documentation should be performed in a systemic fashion. The American Spinal Injury Association (ASIA) motor score is based on manual muscle testing. Ten muscles are used to score motor function, five in the upper extremities and five in the lower extremities. Each muscle is scored out of 5 bilaterally, giving a total possible score of 100 for motor function. A score of 5 is normal strength, 4 can be overcome by resistance, 3 is strength against gravity only, 2 is muscle movement but fails against gravity, and 1 is muscle contraction only. Sensation is determined in each dermatome and graded as absent, impaired, or normal, while noting sacral (S4-5) sparing. When there is injury to an isolated nerve root, it should be described by the affected level and appropriate laterality. It is important to establish neurologic status, as progressive neurologic deterioration is an indication for acute intervention. Those who are neurologically intact should be monitored for new neurologic symptoms.

Examination of the back should include the entire spine from the cervical spine to the sacrum. With inline stabilization of the cervical spine, log rolling of trauma patients is imperative to assess for soft tissue integrity, tenderness, crepitus, stepoffs, and gross malalignment of the spine. Low lumbar injuries, especially at L5, may be associated with a degloving injury, the so-called Morel-Lavallee lesion, which may complicate any planned surgery and can be associated with considerable blood loss.

A full neurologic examination should be completed as soon as the patient’s condition permits it. Complete documentation of motor function in major motor nerve root distributions in both upper and lower extremities should be performed along with documentation of sensation in all major dermatomes including the perianal region. A rectal examination should be performed to assess perianal sensation, rectal tone, and a bulbocavernosus reflex. The bulbocavernosus reflex represents S4 and S5 and is most often spared even in the presence of complete cord injury more cephalad. An absence of this reflex indicates spinal shock is likely present, and the return of the reflex signifies the end of spinal shock. It usually lasts 24 to 48 hours. However, absence of the reflex in low lumbar fractures indicates impairment of the cauda equina.

Neurologic injury is often caused by direct injury to the spinal cord. The conus medullaris lies at L1 and contains the anterior horn cells of the L5 through S5 nerves. Conus and cauda equina injury affects upper motor neurons and lower motor neurons, respectively. Injuries can range from complete injury below L1 to incomplete syndromes resulting in partial motor and sensory preservation with sacral dysfunction. L2-5 injuries can result in isolated root injuries or cauda equina syndrome. Root injuries can present as monoradiculopathy or polyradiculopathy. The sacral nerve roots (S1-5) are the most sensitive and least likely to improve after injury. Injury to this segment can result in loss of bowel and bladder function and, to a lesser extent, sexual function. Cauda equina syndrome presents as variable motor, sensory, bowel, or bladder dysfunction. Compared to cauda equina syndrome, conus medullaris syndrome tends to be associated with symmetric motor and sensory dysfunction. It is also associated with sphincteric dysfunction, which is often permanent.

Classification

Factors Influencing Treatment

As with any other spinal segment, instability and the presence of a neurologic deficit are two factors to consider when treating low lumbar fractures. One unique factor when dealing with these injuries is the ability of the segment to maintain the body in preinjury sagittal alignment. A fractured low lumbar segment may heal without instability but leave the body in positive sagittal alignment, creating significant disability. In comparison, a fracture of the thoracolumbar spine (T10-L2) can tolerate up to 30 to 40 degrees of focal kyphosis without affecting long-term disability. Some authors feel the integrity of the posterior osteoligamentous complex is not as important in the low lumbar spine as in the thoracolumbar spine where it is the primary stabilizing structure. A thorough understanding of spinal stability is important when it comes to evaluation, fracture pattern classification, and treatment. The treatment algorithm relies on the idea of restoring the spine to its preinjury functional capacity. Spinal stability, as described by While and Panjabi, is the ability of the spine to maintain physiologic loads without pain, deformity, or neurologic deficit. Benzel described instability as the inability to limit excessive or abnormal spinal displacement. 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. The initial decision of whether to operate depends in part on whether the fracture is acutely unstable. Multiple classification systems based on radiographic and clinical examination were created to help determine if the injury is acutely unstable. However, chronic instability can occur as well. Identifying fracture patterns that can lead to chronic instability and posttraumatic deformity is important in the long-term care of trauma patients.

Physiologic stresses vary from the thoracolumbar to the lumbosacral segment. Basic fracture biomechanics dictate that structural elements fail at the junction between high stability and low stability, making this area particularly vulnerable to failure under stresses. This is reflected in the incidence of thoracolumbar fractures, which make up 60% of all spine fractures. In contrast, the low lumbar region is a flexible segment with a gradual transition into an immobile pelvis with an incidence of about 4% of all spine fractures. As the low lumbar area transitions into the pelvis and sacrum, the ligamentous attachments provide significant stability, which make isolated injuries to L5 rare. A significant force is required to create a recognizable injury. Often L5 fractures are associated with pelvic ring injuries or involve dissociation of the lumbar spine with the sacrum and pelvis as in traumatic L5 to S1 dislocation.

Radiographic Evaluation

In patients without distracting injuries, unaltered mental status, and lack midline spinal tenderness, the spine can be cleared without obtaining screening plain radiographs. If the patient does not meet any of the preceding criteria, radiographic evaluation is mandatory. During evaluation the clinician must keep in mind that 5% and 20% of all spine fractures are multiple, and 5% occur at noncontiguous levels. Radiographic evaluation includes anteroposterior (AP) and lateral views of the cervical, thoracic, lumbar, and sacral spine and an open-mouth odontoid view. With that being said, radiographs play an increasingly smaller role in the initial evaluation of these injuries, with most patients undergoing CT as an initial screening tool. 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, spinolaminar line, facets, interspinous and interpedicular distances, and the position of the transverse process should be assessed. Acute kyphotic angulation or loss of lordosis may indicate 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 indicate 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. Nork and colleagues identified L5 transverse process fractures and a paradoxic inlet view on AP pelvic radiographs as factors suggestive of sacral fractures. Others have identified the foraminal step-ladder sign, caused by a displaced and overriding transverse fracture, and disruption of the anterior sacral foraminal lines or sacral arcuate line as diagnostic clues. Sacral fracture should be suspected in any patient with a pelvic ring injury associated with a neurologic deficit.

For nonoperative treatment, weight-bearing standing radiographs, both AP and lateral, of all fractures should be obtained during the hospital evaluation. This provides a baseline to compare future radiographs against and helps to determine stability. Stable injuries show no change from supine radiographs. Occult, unstable fractures will show changes on radiograph not seen on supping imagining, which will often change the treatment plan.

Computed Tomography

Many institutions use CT as a source of initial evaluation of spinal injuries in the trauma setting. This is in addition to plain radiographs for injury identification. Multidetector CT has supplanted radiographs as the most diagnostically relevant medium for bony injury identification in the lower spine, making them more sensitive for detecting fractures in the spine than plain radiographs. However, CT carries with it concerns about cost and radiation exposure. In 2006, Antevil and colleagues compared diagnostic sensitivity, image timing, costs, and radiation exposure of CT and plain radiographs (PRs) in the initial evaluation of spine trauma. There were 254 patients in the PR group and 319 in the CT group, with similar demographic data, injury severity scores (ISSs), mechanism of injury, and incidence of spine fractures. Sensitivity in the detection of spine fractures was 70% in PR group compared to 100% CT group (p < 0.001). The CT group had a significantly decreased time in radiology compared to the PR group (1 hour versus 1.9 hours; p < 0.001). There was an overall higher cost for CT imaging than PR ($4386 versus $513, p < 0.001) but a similar mean overall spinal imaging cost per patient ($172 versus $164). The radiation exposure was higher with CT versus PR for cervical spine imaging (26 mSv versus 4 mSv), but CT had lower levels of exposure than PR for thoracolumbar imaging (13 mSv versus 26 mSv). Wintermark and colleagues 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. There is no level 1 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 proved to be at least equal to, if not superior to, plain radiographs in several studies. For all radiographic measurement parameters, the Spine Trauma Study Group advocates the use of thin-section (1 to 1.5 mm) axial CT scans with coronal and sagittal reconstruction rather than plain radiographs.

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. 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.

Magnetic Resonance Imaging

Although magnetic resonance imaging (MRI) does not offer the bony detail provided by CT, in many ways it can further aid in decision making. It helps in the evaluation of posterior osteoligamentous disruption. Even though some indirect evidence of ligamentous compromise can be determined based on the amount of vertebral displacement, kyphosis, and height loss with other imaging modalities, MRI is the most sensitive and specific. With plain radiographs, ligamentous compromise could be seen on lateral view as increased interspinous spacing between the lumbar spinous processes. Second, MRI can provide information regarding nerve root, cauda equina compression, and associated disc herniation. Acute injuries not well visualized on radiograph or CT could be seen on MRI as increased signal intensity on T2. With this being said, this is not the most expedient imaging modality to obtain and its judicious use should be dictated by the ability to alter or augment surgical planning. MRI should be advocated for any acute neurologic deficit but is not predictable for evaluating the integrity of posterior ligamentous complex (PLC).

Classification of Injuries

Several classification systems have been developed to describe fractures of the thoracic and lumbar spine. In 1983, both Denis and McAfee and coworkers independently published three-column models that have become widely accepted. Although they are similar in some ways, fundamental differences remain. 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 the Denis 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, burst fracture, lap belt–type injury, and fracture-dislocation. 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.

In 1994, Magerl and colleagues 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 refers to injuries of axial load acting on the vertebral body. Type B injuries are characterized by flexion and extension injuries with disruption of the anterior and posterior elements. Type C lesions are shear and translational injuries involving the anterior and posterior elements. Although more accurately descriptive than the three-column classification, the AO system is often viewed as cumbersome to apply in a clinical setting. Although none of these classification systems combine the morphologic and neurologic components, they are important in guiding management.

The Spine Trauma Study Group proposed the thoracolumbar injury classification and severity score (TLICS) system for levels T10-L2. This system describes the injury morphology, integrity of the posterior ligamentous complex, and neurologic status in order to guide clinicians in choosing the most effective treatment method for thoracolumbar injuries. 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.

Thoracolumbar injuries behave differently when compared to low lumbar injuries (of the L3-5/S1 disc space). They vary greatly in their tolerance for kyphosis. Moore and colleagues found the TLICS system to lack factors that are essential in low lumbar fractures. They include maintenance of lordosis and global sagittal alignment. Compared to thoracolumbar injury, low lumbar integrity of the posterior elements may not be the most important factor when determining operative versus nonoperative treatment. The ability of the anterior and middle columns to support the spine and maintain lordosis is fundamental in dealing with low lumbar injuries. Thus, disruption of sagittal balance may necessitate operative fixation to correct the deformity. Fifteen reviews in the study found L3 injuries to behave like thoracolumbar injuries in terms of management agreement.

The AO thoracolumbar classification was established in 2014 ( Fig. 133-1 ). This classification holds true to the original AO classification of A injuries being compression or flexion mechanisms without distraction; B injuries being distractive forces with failure of the anterior or posterior tension bands; and C injuries being significant torsional, translational, and multivector high energy mechanisms. The new classification has been detailed in earlier chapters and includes modifiers for neurologic injury, posterior osteoligamentous integrity, and primary bone diseases such as diffuse idiopathic skeletal hyperostosis (DISH) and ankylosing spondylitis.