10 The Cervical Spine

Although the cervical spine is injured in only 2% to 3% of blunt trauma accidents,1 the potential for instability and critical neurologic injury makes prompt identification and management of cervical spine injuries important. Patients with suspected cervical spine injury should be evaluated initially with conventional radiographs (AP, lateral, and open-mouth odontoid views). CT imaging offers greater osseous detail than does conventional radiography and may reveal fractures or details that are not detected with radiography. CT is especially helpful in assessing fractures of the occipital condyles and cervicothoracic junction, where osseous overlap on conventional radiographs makes fracture detection difficult. MRI provides soft-tissue visualization superior to that of conventional radiography or CT and is useful for the assessment of spinal cord injury, ligamentous injury, degree of spinal stenosis, and additional fracture evaluation. Occult fractures not visible on conventional radiographs or CT images may be detected by the presence of vertebral body edema on MR images. Although MRI is extremely sensitive in identifying cervical spine fractures, their characteristics and the exact appearance of the osseous components can be challenging; CT may be a better choice for assessing such details. In addition, MRI is useful for the evaluation of obtunded patients or those with cervical spine injury, neurologic deficits, or an unreliable physical examination.2–7

MRI is indicated specifically when neurologic deficit, vascular injury, or soft-tissue injury is suspected in the setting of trauma. It is also useful in assessing posttraumatic sequelae.⁸ Imaging spinal gunshot injuries is controversial. Theoretically, a ferrous gunshot fragment may become mobile, but most bullets are nonferrous, and therefore such patients can usually be imaged without consequences. Unfortunately, the exact composition of a gunshot fragment is seldom known, and therefore MRI remains controversial and dependent on the clinical need.^9,10

It should be noted that there are obstacles to obtaining MRI studies in the trauma setting, especially with regard to cervical spine trauma, because patients may have clinically significant neurologic deficits. These obstacles include the following:

• Lack of availability of MRI capabilities on an urgent basis

• MR-incompatibility of some ventilators, traction devices, and other equipment

• Lack of clinical access to patients during the imaging study

MRI protocols vary by institution, but commonly used sequences in trauma evaluation include the following¹¹:

• Sagittal T1-weighted images to assess the alignment of the cervical spine, vertebral body integrity, fractures, and spinal cord caliber

• Sagittal T2-weighted images to assess for the presence of cord edema, compression, and spondylotic changes

• Sagittal STIR images to assess for the presence of para-spinal ligamentous injury and bone marrow edema

• Axial T1-weighted and T2-weighted images to assess for the presence of posterior element fractures, to evaluate for spinal stenosis, to better define disc pathology, and to confirm the precise location of abnormalities detected on sagittal images

• Sagittal T2-weighted gradient-echo images (in some institutions) to assess for the presence of acute spinal cord hemorrhage and disc herniation (high signal in the disc even with severe osseous degeneration, which enables the distinction between bone fragments and a disc herniation)

Source: Takhtani D, Melhelm ER. MR imaging in cervical spine trauma. Magn Reson Imaging Clin N Am 2001;8:615–634. Modified with permission.

Regardless of the specific institutional MRI protocol, a systematic approach (see Chapter 3) for the evaluation of cervical spine MRI should be used to avoid missing pathologic conditions (see Table 10.1 for important cervical spine structures to evaluate). In addition, it is essential that the interpretation of the MRI findings be performed in conjunction with that of the other available imaging modalities, including conventional radiographs (with flexion and extension views if clinically indicated) and CT (see Chapter 17).

Classification of Cervical Spine Trauma

Cervical spine injuries can be classified based on the mechanism of injury. Although six categories have been described (vertical compression, compressive flexion, distractive flexion, lateral flexion, compressive extension, and distractive extension¹²) (Fig. 10.1), the classification scheme is simplified here into three broad categories:

• Hyperflexion

• Hyperextension

• Axial loading

In many instances, the mechanism of injury can be difficult to determine from an analysis of the clinical situation (in the absence of imaging findings), and therefore clinicians may choose to broadly classify cervical spine injuries as follows:

• Secondary to blunt trauma

• Secondary to penetrating trauma

In addition, cervical spine injuries can be subdivided based on the region of injury within the occipitocervical spine:

More recently, the subaxial cervical spine injury classification system has been described as an approach that recognizes the importance of fracture morphology, neurologic injury, and integrity of the discoligamentous complex.13 A systematic evaluation of these three components can be used to guide the treatment of patients with cervical spine fractures.

Hyperflexion Injuries

Flexion-compression injuries range from the minor anterior compression of the anterosuperior end plate (Fig. 10.2) to a severe teardrop or quadrangular fracture. These injuries are associated with retrolisthesis, kyphosis, and circumferential soft-tissue disruption. The radiographic evaluation of flexion-compression injuries includes inspection for the following:

• Anterior and middle column compromise

• Vertebral body-height loss

• Translation

• Angulation

• Posterior element competence

Although conventional radiographs and CT scans can evaluate fracture pattern, alignment, angulation, and translation, MRI provides additional diagnostic value and can assist with the determination of treatment options for such patients because it facilitates the assessment of spinal cord compression and posterior element compromise.

Flexion-distraction forces can lead to facet subluxations, dislocations, or fracture-dislocations. These injuries represent a spectrum of osteoligamentous pathology, ranging from the purely ligamentous dislocation to fracture of the facet and lateral mass. MRI helps assess the compromise of posterior musculature, interspinous ligaments, ligamentum flavum, and facet capsules that is often seen with flex ion-distraction injuries.¹⁴ The role of MRI in the treatment algorithm of patients who present with bilateral cervical facet dislocations (Fig. 10.3) without neurologic compromise is the subject of substantial debate in the literature and among spine surgeons.^14–16 The treatment options include MRI before attempting closed reduction or surgical intervention; closed reduction with traction while monitoring the patient’s neurologic examination; and surgical intervention via anterior, posterior, or combined approaches.^14–16 One of the purposes of obtaining an MRI study before the reduction of bilateral facet dislocations is to rule out the possibility of an extruded disc fragment that may displace into the spinal canal during a closed reduction (Fig. 10.4).

Most flexion injuries are well visualized on MRI, and MRI is particularly effective for the assessment of the following11:

• Alignment

• Fractures

• Ligamentous injury

• Cord abnormalities

• Acute disc herniations

• The cause of anterior subluxation, either chronic degenerative changes or hyperflexion sprain

Facet joint injuries may be seen on parasagittal or axial images, which show increased signal on T2-weighted images secondary to edema from facet capsule tears.^11,17–19 Injury to posterior ligaments may be seen as areas of hyperintensity on T2-weighted images, especially fat-suppressed T2-weighted or STIR images (Fig. 10.5).

Hyperextension Injuries

Cervical spine extension injury results in the posterior translation or rotation of a vertebral body in the sagittal plane.^6,11,20 Hyperextension injuries often are produced by rear-impact motor-vehicle collisions or direct facial trauma. In cervical spine hyperextension injuries, potential findings include the following^{6,11,17,19,20}:

• Horizontal intervertebral disc rupture (Fig. 10.6)

More severe and potentially unstable hyperextension injuries may be associated with the following⁶:

• Prevertebral hematoma

• Widening of the disc space

• Posterior ligament complex edema

• Herniated disc

Elderly patients with spondylosis and kyphosis of the cervical spine may suffer spinal cord injury without fracture or ligamentous injury because of posterior infolding of the ligamentum flavum upon a spinal canal already narrowed by posterior vertebral osteophytes.⁶

Whiplash injuries often have no associated osseous injury on standard radiographs or CT images, and flexion-extension radiographs may be nondiagnostic because of poor excursion secondary to pain. However, MRI is of limited value for the assessment of whiplash; several studies have failed to show positive MRI findings in the absence of neurologic symptoms.^18,21 In contrast, patients with a fused cervical spine secondary to ankylosing spondylitis or diffuse idiopathic skeletal hyperostosis may benefit from an MRI examination to assess for acute fracture, instability, or neurologic compromise. In such patients, the fused cervical spine acts like a long-bone fracture, and even minimally displaced fractures may be unstable (Fig. 10.7).²²

Finally, MRI can assess intervertebral disc injury and subtle fractures caused by any of the above-mentioned mechanisms.^{11,17–19,23} Intervertebral disc injury may range from tear (s) of the outer annulus fibrosis (seen as increased T2-weighted signal in the outer annular fibers) to frank intervertebral disc herniation. The identification of an annular tear on MRI does not indicate acute traumatic injury and can be seen in asymptomatic individuals.^24,25 Intervertebral disc separation from the adjacent vertebral body may be seen as a horizontal hyperintense T2-weighted signal.^11,17,19 Subtle fractures, such as vertebral end-plate fractures, may be best visualized with MRI because it can detect osseous edema and hemorrhage not seen on conventional radiographs or CT images.^11,17,19

Axial load injuries are caused by the axial transmission of force through the skull, through the occipital condyles, and into the spine. This force transmission can cause a Jefferson burst fracture or burst fractures of the subaxial cervical spine. MRI is useful for the assessment of C1 compression fractures and associated pathologies such as lateral mass displacement on coronal images, atlantodental interval increase on sagittal images, and transverse ligament disruption on axial images.11 For burst fractures, MRI is useful for diagnosing associated spinal cord injury caused by an acute herniated disc or retropulsion of osseous fragments (Fig. 10.8). Because a purely axial force subjects the posterior capsuloligamentous structures to compression only, these posterior structures should remain intact.^11,20 However, there often is some degree of spine flexion during the traumatic event that may cause injury to the posterior spinal elements, which can be detected by MRI.²⁰ It is important to carefully scrutinize the fat-suppressed T2-weighted and other images for evidence of injury to the posterior ligamentous and osseous structures because such injury will lead to consideration of posterior fusion in addition to the anterior decompression and fusion that is often performed for patients with cervical burst fractures.

Occipitocervical Junction Injuries

Although injury to the occipitocervical junction occurs in a small percentage of blunt trauma victims (0.8% in one study²⁶), recognition of such injuries is crucial because of their devastating effects.^27–30 A detailed discussion of occipitocervical craniotomy and the various measurement techniques for evaluation of occipitocervical pathology is beyond the scope of this chapter, but presented here is an overview of the major types of occipitocervical traumatic findings as seen on MRI. It is important to keep in mind that MRI studies of the occipitocervical junction should be reviewed in conjunction with conventional radiographic and CT imaging.

Atlantooccipital Dissociation

Atlantooccipital dissociation is any separation of the atlantooccipital articulation. The skull may displace anteriorly, posteriorly, or superiorly, and may be complete (dislocation) or partial (subluxation). Atlantooccipital dissociation can be a devastating injury.^27–30 The primary injury is to the ligaments that provide structural support to the cervicocranial junction. In addition, even without frank dislocation, the occiput–C1 junction may be injured, as indicated by postmortem studies.^27,28 Although this injury may be fatal, improvement in resuscitative and medical treatment has increased survival rates. CT imaging may be used to assess associated fractures or relationships among the basion, dens, occipital condyles, and atlas in conjunction with atlanto-occipital dissociation, whereas MRI is better at detecting injury to the cervicocranial ligaments (e.g., transverse, apical, cruciate, atlantooccipital membrane and capsular ligaments, tectorial membrane), brainstem, or spinal cord.^11,19,31

Axial load to the occipitocervical junction at the atlas may result in a burst fracture of the atlas. The injury is visualized on open-mouth odontoid radiographs or coronal CT images.32,33 As indicated by the cadaver study of Spence et al.,34 combined overhang of the lateral masses of C1 over C2 of ≥6.9 mm is associated with transverse ligament rupture and indicates a relatively unstable Jefferson burst fracture. The axial T2-weighted images can be critically evaluated to rule in or rule out injury to the transverse ligament. These images should be carefully scrutinized for increased T2-weighted signal in or along the course of the transverse ligament. The ligament should also be evaluated for regions of discontinuity.

Trauma to the Axis

C2 is the most commonly fractured cervical spine level.²⁶ In the elderly, odontoid fractures tend to be posteriorly displaced. Fracture location, displacement, and angulation are important factors that assist with clinical decision-making. Lateral cervical radiographs and CT scans can be used to characterize such fractures and better evaluate the osseous detail. MRI can assist in the evaluation of these fractures by providing assessment of the degree of edema at the fracture site (Fig. 10.9). This information provides insight into the age of the fracture (acute versus subacute versus chronic), which can be used to guide treatment. The sagittal and axial T2-weighted images should also be carefully evaluated to determine the degree of neural compression, from either a displaced fracture or underlying degenerative changes.

Atlantoaxial dissociation may be caused by distraction with superior migration of the atlas away from the axis or by odontoid fractures with anterior or posterior displacement of C1 relative to C2. MRI clearly depicts the associated pathology seen with atlantoaxial dissociation.11 As noted above, MRI is very useful for detecting ligamentous and spinal cord injury. The relationship of the atlas to axis is clearly visible with MRI. Specifically, the integrity of the transverse ligament should be evaluated along with the approximate size of the anterior atlantodens interval. This measurement is more frequently evaluated on flexion and extension lateral cervical spine radiographs (Fig. 10.10).

Vertebral Artery Injury

Vertebral artery injury is associated with blunt cervical trauma, with an incidence as high as 11%.³⁵ This potentially devastating injury may occur with cervical spine fractures extending into the transverse foramen, but it is associated most often with unilateral or bilateral facet dislocations.^19,36 MR angiography may be used to assess vertebral artery patency, especially with such fractures.¹⁹ MR angiography may show areas of vascular stenosis or occlusion. Flowing blood creates a signal void on axial SE images and is seen as a bright signal on gradient-echo images; merging the data from these two modalities can help determine the status of blood flow in the vertebral arteries^19,37 (Fig. 10.11).

The types of vertebral artery injuries are thrombosis, dissection, and transection (rare). MR angiography shows vertebral artery thrombosis as the absence of flow-related enhancement on images in the expected course of the vertebral artery and as an acute thrombus in the foramen transversarium, dissections as a tapering of the vessel, and transections as a focal discontinuity of the vessel. Major clues to vascular injury include changes in vessel caliber, loss of the normal rounded shape, increase in caliber from proximal to distal (except at the carotid bulb), or the presence of an extraluminal thrombus or a slit or dark band through the lumen.

Penetrating Trauma

Penetrating injury to the cervical spine can be caused by projectiles (e.g., bullets) or puncture mechanisms. Missiles can cause spinal cord injury by direct penetration, displacement of bone fragments into the spinal canal compressing the spinal cord, or a blast effect. The osseous architecture of the spine often protects the spinal cord from direct injury from a stabbing mechanism because the lamina and spinous processes can deflect the penetrating object into the para-spinal soft tissues. MRI is useful for assessing the specific location, extent, and type of cord injury from penetrating trauma.

Characterization of Spinal Cord Injury

The severity of spinal cord injury depends on the characteristics of the traumatic event (including the amount, duration, and location of the applied force) and the underlying health of the spinal cord. Spinal cord insult may range from a concussive injury (purely functional and reversible) to complete transection (irreversible). Spinal cord concussive injury often has no MRI evidence of edema (increased T2-weighted signal), or the edema is transient and resolves with time.11 Spinal cord contusion is a more severe injury and may be caused by transient compression or stretching of the spinal cord. Spinal cord compression may show injury characteristics similar to those of spinal contusion, but it can be associated with a specific compressive lesion such as disc herniation (Fig. 10.12) or osseous fragment.

An injured spinal cord segment may have an increase in cord diameter because of swelling, edema, or hemorrhage. MRI characteristics of an injured spinal cord segment are based on the degree of swelling, edema, or hemorrhage, each of which may have a different pattern of signal changes on various pulse sequences.⁷ On T2-weighted images, edema and acute hemorrhage are seen as bright signal, whereas chronic hemorrhage is seen as darker signal. Gradient-echo images show dark areas that are larger than the abnormality on the T2-weighted images. This enlargement, or “blooming,” is the result of the magnetic susceptibility artifact from met-hemoglobin.⁷ The anatomic location, morphology, and length of the spinal cord lesion are important factors in determining the degree of neurologic loss. Initial neurologic deficit and potential for recovery are related directly to the extent of spinal cord damage by hemorrhage or edema.⁷ Evidence of parenchymal hemorrhage on MRI may predict worse functional outcomes or neurologic recovery than is associated with a spinal cord injury with predominantly edematous changes.⁷

Highly T2-weighted images offer a myelographic effect for the assessment of spinal cord compression. These images should be carefully evaluated in the sagittal and axial planes for regions of effacement of the ventral or dorsal CSF spaces, which indicate spinal stenosis and cord compression. Trauma patients who have underlying degenerative or congenital stenosis are at increased risk for spinal cord injury because of the decrease in the cross-sectional area available for the spinal cord.

Clinical and experimental evidence has shown that surgical decompression of stenotic areas has a beneficial effect on neurologic recovery, which makes prompt identification of stenotic areas and distinguishing these areas from simple contusions important.¹¹ In addition, MRI assessment of cervical spine fractures in obtunded or uncooperative patients may identify disc herniations that may cause spinal cord compression and iatrogenic or progressive neurologic injury during fracture reduction.^20,38,39

White et al.40 defined cervical spine instability as the inability to maintain a normal association between vertebral segments while under a physiologic load. Cervical spine instability may be caused by damage to the osseous and/or ligamentous structures. Conventional radiographs and CT scans often provide the best assessment of osseous injuries. Ligamentous injury contributing to cervical spine instability may be assessed with flexion and extension lateral cervical spine radiographs and by physical examination. MRI can also be used to evaluate for ligamentous injury; the sensitivity for detection of such injuries is greatest within 24 to 72 hours postinjury.5,17,19,41 Important ligaments to assess include the following²⁰:

• Anterior longitudinal ligament

• Posterior longitudinal ligament

• Posterior column ligament complex (supraspinous ligament, interspinous ligament, and ligamentum flavum), which has been recognized as an important restraint to spinal instability (especially kyphosis)

• Transverse ligament

MRI characteristics of ligamentous injury include increased T2-weighted signal (from edema) within the ligamentous and other posterior structures (Fig. 10.13) or loss of ligament continuity (normally a low intensity continuous signal). Ligamentous injury is best assessed on STIR or fat-suppressed T2-weighted images.²⁰ A ligament strain, without complete disruption, may be seen as an elongated or redundant ligament on sagittal MR images. Despite the capability of MRI to detect ligamentous injury, not all MRI-detected ligamentous injuries result in spinal instability or warrant treatment.^20,21 For example, minor motor-vehicle accidents that result in acute whiplash injury of the cervical spine without fracture do not need emergent MRI evaluation for ligamentous injury and may be treated symptomatically only.²¹

With the increasing availability of flexion–extension (kinematic) cervical spine MRI, a dynamic assessment of cervical spinal instability and associated stenosis can be obtained.42 Although such information provides insight into the degree of spinal instability, it tends to be most useful for the evaluation of patients with degenerative disorders of the cervical and lumbar spine (Fig. 10.14).⁴³ Patients who have sustained severe trauma to the cervical spine are likely to be immobilized. After a period of immobilization and after frank instability of the cervical spine has been ruled out with patient-controlled flexion–extension cervical spine radio-graphs, a kinematic cervical spine MRI study can be considered. The information obtained from such kinematic studies can be used to guide surgical treatment and may allow the surgeon to decide among anterior, posterior, or combined surgical approaches.

Degenerative Conditions

Degenerative changes of the cervical spine are common after the fourth decade of life.⁴⁴ Cervical spine degeneration may be asymptomatic or have acute or insidious onset of symptoms; it may result in pain, stiffness, radiculopathy, myelopathy, and even permanent disability. Degenerative pathology may affect multiple areas in the cervical spine, including the following:

• Intervertebral discs

• Facet joints

• Uncovertebral joints of Luschka

• Ligaments

• Paravertebral musculature

Because these elements are biomechanically linked, a single cervical spine level may have multiple degenerative pathologies and cause adjacent-level degenerative changes. Conventional radiographs often are the initial screening studies for evaluating cervical spine degeneration and may guide the selection of more advanced imaging techniques.⁴⁵

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