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Trauma to the Spinal Column
Traumatic injuries of the spinal column reflect the mechanisms that produced them. The patterns of these injuries on imaging studies, therefore, help to predict the underlying mechanisms of trauma and lead, in turn, to more complete description of the pathologic process, more thorough search for associated abnormalities, and better patient prognostication. Differences in the anatomy and mobility of each spinal segment influence the pathologic process suffered.
Cervical spine injuries are typically classified as craniocervical or as subaxial. The craniocervical injuries are usually distractions at the atlanto-occipital and atlantoaxial junctions. These distractions are rare but are identified with increased frequency. The subaxial cervical spine injuries are predominantly flexion and extension injuries. Because the ribs stabilize the upper thoracic spine, few injuries are seen there. Spinal mobility increases further inferiorly, however, so fractures and fracture-dislocations occur more often in the lower thoracic and lumbar spine, especially at the thoracolumbar junction.
Spinal stability indicates that the bony and ligamentous elements of the spinal column will remain in the same relative positions without shifting or separating from each other over time. Spinal instability indicates that, without stabilization, the spinal elements may shift and, by shifting, induce additional neurologic, soft tissue, or osseous injury. A number of radiographic and CT signs suggest the presence of concurrent ligamentous injuries and high likelihood of instability. MRI visualizes many ligaments directly, leading to better diagnosis of ligamentous injuries. MRI shows injuries of spinal ligaments as thinning of the ligaments, separations of the ligaments from the underlying bone, areas of discontinuity within the ligaments, and perifocal edema. MRI also shows concurrent injuries of the spinal cord. Therefore, MRI is particularly useful in patients with suspected unstable injuries, in patients with neurologic deficits, and in patients who are obtunded.
The three-column model is often used to assess and characterize stability of the spinal column. In this model, the spinal column is considered to have three stability elements or “columns.” The anterior column consists of the anterior longitudinal ligament plus the anterior two thirds of the vertebral bodies, discs, and annuli. The middle column consists of the posterior third of the vertebral bodies and discs, the posterior longitudinal ligament, and the posterior portion of the annuli. The posterior column includes all of the osseous and ligamentous structures posterior to the posterior longitudinal ligament. Denis described unstable injuries as either an injury involving all three columns or an injury that involves two contiguous columns, that is, the anterior and middle columns or the middle and posterior columns.
Associated injuries include disc injuries or herniations, vertebral artery injuries, spinal cord injuries, intramedullary and extramedullary hematomas, meningeal tears, and nerve root trauma. Identification of these injuries affects patient management and outcome.
ATLANTO-OCCIPITAL DISSOCIATION INJURIES
Atlanto-occipital dissociations are dislocations or subluxations of the occiput from C1 (the atlas). The direction of dissociation varies with the specific conjunction of vertical distraction with anterior, posterior, or lateral shear force. Atlanto-occipital dissociations can be combined ligamentous and osseous injuries of the craniocervical junction or may be purely ligamentous. These dissociations may also be referred to as dislocations or subluxations.
Epidemiology
Atlanto-occipital dislocation is seen in 8% to 35% of deaths related to motor vehicle accidents. The 30-day mortality of those patients who survive the injury is 35%. The majority of patients present after a motor vehicle accident, but atlanto-occipital dislocation may also result from pedestrian injury. Gregg and associates reviewed 135 survivors of atlanto-occipital dislocations. Of these, 80 were children and 55 were adults. There were 53 females, 79 males, and 3 cases with no gender specified. Bucholtz and colleagues reviewed 112 trauma victims post mortem and identified 9 patients (8%) with atlanto-occipital injuries. All were male. Three victims were children younger than 18 years of age; 4 were between 18 and 24 years of age, and 2 were older.
Clinical Presentation
Atlanto-occipital dislocations are usually fatal. Improved imaging and better initial emergency management, however, have led to increased numbers of survivors. These patients typically present to the emergency department with spinal cord and other neurologic injuries, especially brain stem dysfunction and palsies of the lower cranial nerves V through XII. They may also have significant vascular injuries. Because the radiographic and CT presentation of purely ligamentous injuries can sometimes be subtle, clinical identification of signs and symptoms of brain stem injury can help to suggest this diagnosis.
Pathophysiology
Atlanto-occipital dislocations most commonly result from distraction with either hyperflexion or hyperextension. The major ligaments likely to be injured include the anterior atlanto-occipital membrane, the tectorial membrane, the transverse ligament, and the alar ligaments. The tectorial membrane is believed to counter hyperextension, whereas the dens and foramen magnum counter hyperflexion. These structures must be injured, therefore, by hyperextension and hyperflexion injuries.
Pathology
Atlanto-occipital injuries can be associated with skull fractures, most often involving the occipital condyles and including avulsion fractures of the alar ligaments.
Because this injury involves the craniocervical junction, injury can damage the cranial nerves, especially lower cranial nerves V through XII. The injury may involve the nucleus of the cranial nerve or its peripheral portion, such as with injury of the hypoglossal canal (cranial nerve XII) or the jugular foramen (cranial nerves IX-XI).
Imaging
CT
The craniocervical region is often difficult to display on plain radiographs because the anatomy is complex and many significant structures overlap. Patient condition may preclude optimal positioning for the radiographs. Obtunded patients cannot cooperate with the examination. Spasm of the cervical musculature often elevates the shoulders to overlap the neck. When seen, prevertebral soft tissue swelling suggests adjacent spinal injury.
CT is easier to perform, displays the bony anatomy and fractures better, and, with multiplanar reformatting, can be used to image any portion of the spine at the precise angle needed. There may be prevertebral soft tissue swelling and separation of the occipital condyles from the lateral masses of C1. Possible ligamentous injuries may be inferred by identifying increased lengths of two intervals: the basion-dental interval and the basion-axial interval ( Figs. 11-1 to 11-3 ). The basion-dental interval is the measurement from the basion to the superior tip of the dens. The basion-axial interval is the perpendicular measurement from the basion to a line parallel to the posterior cortex of the axis. Normally, these intervals should both be less than 12 mm. The Power ratio (the distance from basion to the posterior arch of C1 divided by the distance from the opisthion to the anterior arch of C1) has also been employed but can be more difficult to determine on plain films.
MRI
MRI depicts the tectorial membrane, alar ligaments, and anterior atlanto-occipital membrane as well as other ligamentous and soft tissue structures of the craniocervical region ( Fig. 11-4 ). Abnormal separation between the occiput and C1 and increased signal within the joint capsules on T2-weighted (T2W) and short tau inversion recovery (STIR) MR sequences also suggest atlanto-occipital injuries. MRI may also show hemorrhage and edema in the soft tissues, subarachnoid and epidural hemorrhages at the craniocervical junction, and contusion, edema, and hemorrhage of the spinal cord.
Special Procedures
The walls of the vertebral arteries can be injured by stretching due to atlanto-occipital distraction, with subsequent dissection, thrombosis/obstruction, and pseudoaneurysm formation. CT angiography (CTA), MR angiography (MRA), or conventional angiography may be employed to detect and characterize these vascular injuries.
ATLANTOAXIAL DISSOCIATION INJURIES
Atlantoaxial dissociation injuries are dislocations or subluxations of C1 on C2 due to injuries of the ligaments and osseous structures of the atlantoaxial (C1-C2) articulations. Because these injuries involve the craniocervical junction, consequences of injury and instability can be severe. These injuries are also referred to as distractions or subluxations.
Epidemiology
Post-traumatic atlantoaxial distraction injuries most frequently result from motor vehicle accidents, falls, and sports. Overall, post-traumatic atlantoaxial distractions and subluxations are not common.
Clinical Presentation
These patients typically present with neck pain in the setting of trauma. With advanced degrees of subluxation, patients may also suffer neurologic injuries due to spinal cord damage or vertebral artery injury. Concurrent head trauma may cause additional intracranial injury.
Pathophysiology
The mechanism of injury typically involves distraction with extension or flexion. The transverse ligament normally prevents anterior displacement of the atlas, so the transverse ligament is often seen to be injured in these cases. The alar ligaments help maintain stability but may not be able to support the C1-C2 articulation when the transverse ligament is injured. The anterior longitudinal ligament, posterior longitudinal ligament, tectorial membrane, and other cruciate ligaments also provide support for the C1-C2 articulation. Disruption of these ligaments can lead to subluxation or dislocation. Type II fractures of the odontoid process may also lead to subluxation or dislocation of C1 on C2. Underlying inflammatory arthritides, infections, or neoplasms weaken these ligaments, so they may become disrupted by relatively mild injuries.
White and associates describe five major types of atlantoaxial injury: bilateral anterior displacement, bilateral posterior displacement, unilateral anterior displacement, unilateral posterior displacement, and unilateral combined anterior and posterior C1-C2 subluxations and dislocations.
Pathology
If the mechanism of trauma includes an anteroposterior force or lateral force, fractures of the odontoid may be seen in association with atlantoaxial subluxation or dislocation.
Imaging
CT
Prevertebral soft tissue swelling usually signifies prevertebral edema and/or hemorrhage, suggesting possible subtle atlantoaxial subluxation. Other signs of atlantoaxial subluxation on axial and reformatted CT images include displacement of the C1 lateral masses with respect to the C2 lateral masses, widening between the facets of C1 and C2, and widening of the anterior atlantodental interval. The normal anterior atlantodental interval measures up to 3 mm in adults and 5 mm in children, so measurements greater than these indicate possible subluxation.
MRI
MRI displays the C1-C2 supporting ligaments directly, including the transverse ligament, tectorial membrane, anterior atlantodental ligament, and the alar ligaments ( Fig. 11-5 ). In injured patients, MRI shows disruption, avulsion, or thinning of these ligaments plus any concurrent injuries of the anterior or posterior longitudinal ligament. Prevertebral soft tissue hemorrhage and edema may be identified directly. Injury of the facet joints manifests as abnormal separation of the facets and as increased signal on STIR and T2W images within the facet joints from fluid within the joints ( Figs. 11-6 and 11-7 ). MRI also identifies signal abnormalities of the spinal cord and meninges such as cord contusion and intramedullary and extramedullary hemorrhages.
Special Procedures
The walls of the vertebral arteries can be stretched and injured by abnormal motion at the C1-C2 articulation, leading to thrombosis, dissection, or pseudoaneurysm formation. CTA, MRA, or conventional angiography may be employed.
FRACTURES OF THE ATLAS
Osseous injuries of the atlas include fractures of the anterior arch, the posterior arch, and the lateral masses. These C1 fractures may be accompanied by injury to the transverse ligament. Alternate names include Jefferson burst fracture, anterior or posterior arch fractures, and lateral mass fractures.
Epidemiology
Two to 13 percent of cervical spine fractures involve C1.
Clinical Presentation
Modes of injury include motor vehicle accidents, falls, and diving. Neurologic deficits are uncommon with these injuries, because the fragments typically disperse. Neck pain and posterior headaches are the more common symptoms.
Pathophysiology
Because C1 is a ring there will typically be two or more fracture lines through it. Usually, the mechanism of injury involves vertical compression. The pattern of the fractures depends on the position of the head at impact and the degree of rotational force applied ( Figs. 11-8 and 11-9 ). The Jefferson burst fracture typically involves marked axial loading, which is of greater force and in a more vertical direction than seen with other types of C1 fractures. Four fragments usually result. The transverse ligament may be injured.
Imaging
CT
Differentiation of the Jefferson fracture from an isolated fracture may be difficult with plain radiographs. CT demonstrates the fracture pattern more clearly ( Fig. 11-10 ). If the fracture involves the anterior arch, there is usually prevertebral soft tissue swelling due to prevertebral hematoma.
In Jefferson fractures, one should evaluate the integrity of the transverse ligament. One useful indicator involves measurement of the lateral displacement of the C1 lateral masses with respect to the C2 lateral masses. Cadaver studies have shown that a total lateral displacement of both lateral masses greater than 6.9 mm indicates possible disruption of the transverse ligament. Heller and associates report that the proper value in living patients should be taken as more than 8.1 mm to allow for the magnification inherent in obtaining open mouth odontoid radiographs in patients. CT can be used to measure this displacement accurately without concern for magnification. Abnormal widening of the anterior atlantodental interval may also be used to suggest ligamentous injury.
MRI
MRI displays the transverse ligament and any associated injury directly. Type I injury involves disruption of the ligament, whereas type II injury involves an avulsion. The distinction is clinically significant because type I injuries are typically treated with surgery.
DENS FRACTURES
Fractures of C2 may involve the dens or pass inferior to the dens. Nonetheless, three of these injuries are designated dens fractures types I to III. Type I fractures involve only the superior portion of the dens. Type II fractures involve the base of the dens where the dens joins with the vertebral body. Type IIa fractures are comminuted type II fractures. Type III fractures involve the body of C2 inferior to the insertion of the dens itself ( Figs. 11-11 to 11-13 ).
Epidemiology
C2 fractures constitute about 20% of cervical spine fractures. Greene and associates found that dens fractures occurred in 59% of 340 patients with C2 (axis) fractures. Of these, type II fractures were the most common (120 of the 199 dens fractures, 60%). Only 2 patients had type I fractures. Patients older than 50 years of age tended to have type II injuries, whereas younger patients more often had type III injuries. Typical modes of injuries involved motor vehicle accidents, falls, and diving injuries.
Clinical Presentation
Patients present with neck pain after trauma. There may be associated facial injuries. Spinal cord injury may complicate the fracture when there is sufficient displacement.
Pathophysiology
Mechanisms suggested to cause dens fractures include anterior to posterior or posterior to anterior shear/translational injuries as well as hyperextension. An important factor to consider is the stability of the transverse ligament, which can be injured in a similar fashion. Type II fractures have a higher rate of nonunion than other dens fractures.
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