Chapter Preview
|
|
|
|
The evaluation of a patient with a suspected spinal abnormality always begins with a thorough history and physical examination. The next most important tool in the spine surgeon’s armamentarium is the ability to evaluate imaging studies accurately. Imaging begins with conventional radiographs and often progresses to advanced planar imaging studies such as computed tomography (CT) and magnetic resonance imaging (MRI) (see Chapter 10 ). In the context of correlating clinical findings, the ability to order and interpret radiographic studies appropriately leads to more accurate diagnosis and treatment. This chapter focuses on the individual radiographic views that aid the clinician in the evaluation of the cervical spine. A discussion of the indications for CT-based evaluation of the cervical spine is also included.
Conventional Radiographic Evaluation
Conventional radiographs are commonly obtained to (1) diagnose (e.g., fracture from trauma), (2) localize the level or levels of abnormality, (3) observe and follow the progression of disease (e.g., tumor, infection, or degenerative or inflammatory conditions such as rheumatoid arthritis or diffuse idiopathic skeletal hyperostosis), (4) observe and follow the progression of deformity (e.g., kyphosis, scoliosis), (5) plan the levels and extent of surgery preoperatively, and (6) follow-up operative procedures.
An understanding of the information that can be obtained from individual radiographic views is necessary to ensure that appropriate studies are initially ordered. Next, the clinician must develop a systematic approach to radiographic studies. This approach should begin by ensuring that the image is of the correct patient and that it adequately visualizes the anatomic structures to be evaluated and allows for assessment of spinal alignment.
Lateral View
The lateral cervical spine radiograph provides most of the information for the evaluation of patients with cervical spine disorders or suspected cervical spine abnormalities.
For a cervical spine lateral radiograph to be considered adequate, the clinician must be able to visualize the area from the occiput to the superior end plate of T1 ( Fig. 9-1 ). The overall spinal alignment should be noted in terms of lordosis, straightening, or kyphosis. Normal vertebral bodies are symmetric and rectangular. The margins of the vertebral body should be visually traced to rule out fracture or an osteolytic process, such as tumor. Disk space height should also be evaluated; a loss of disk space height may indicate degenerative disk disease or chronic infectious conditions. A loss of disk space height with nonbridging, nonmarginal osteophytes or syndesmophytes is a classic finding in patients with degenerative spinal disease.
Harris and associates described five lines for the evaluation of the cervical spine on the lateral radiograph: (1) the anterior vertebral body line, (2) the posterior vertebral body line, (3) the spinolaminar line, (4) the spinous process line, and (5) the soft tissue line. These lines should be evaluated carefully in every patient. Disruption of one of these lines, even if subtle, should prompt the examiner to scrutinize that area further for abnormality. For example, spondylolisthesis, or displacement of one vertebral body over another, as classified by Wiltse and colleagues and graded by Meyerding, results in disruption of these radiographic lines and indicates abnormality with the potential for instability. These findings should be interpreted according to the clinical situation. For example, after acute trauma to the cervical spine, such a finding may warrant immediate immobilization or surgical stabilization ( Fig. 9-2 ). In contrast, in the setting of chronic degenerative or rheumatologic disease, this finding would prompt further clinical or radiographic evaluation, such as flexion and extension views, as described later.
In addition to osseous structures, soft tissue shadows can be appreciated on lateral radiographs. In particular, the shadow anterior to the vertebral bodies representing the retropharyngeal soft tissues should be evaluated. According to some clinicians, the shadow should be less than 5 mm at the C3 level ( Fig. 9-3 ), and it should be less than 22 mm at the C6 level. However, other clinicians have found this measurement to be unreliable. A larger soft tissue shadow may be the result of edema related to a fracture, an infection in a patient with a retropharyngeal abscess, or a retropharyngeal hematoma in a patient who recently underwent an anterior cervical spinal procedure.
Occipitocervical Junction
The occipitocervical junction can be a particularly challenging region to evaluate on conventional radiographs because of the overlap of anatomic landmarks. Radiographic lines and parameters have been described to aid in evaluating the relationship of the base of the occiput with C1 and C2 for disassociation, basilar invagination, and cranial settling. The Harris “rule of twelves” is one such relationship with which the spine surgeon should be familiar, especially in the setting of major occipitocervical trauma. The dens-basion interval, measured as the distance from the basion to the tip of the odontoid process, should be less than 12 mm. Similarly, the basion-axial interval, the distance from a vertical line drawn along the posterior aspect of the dens (termed the posterior axial line) to the basion, should be less than 12 mm. A distance of more than 12 mm for either interval indicates atlanto-occipital dissociation. Additional radiographic lines and parameters can aid in the evaluation of the occipitocervical junction ( Fig. 9-4 and Table 9-1 ). The reliable use of these lines and parameters largely depends on the ability to visualize their corresponding landmarks. CT and MRI have aided substantially in the accurate evaluation of these parameters in this region and have widely replaced conventional radiographs for definitive evaluation.
| Eponym | Parameters | Pathologic Features |
|---|---|---|
| Wackenheim clivus baseline | Tangent drawn along the superior surface of the clivus | Dens should be below the line. |
| Clivus canal angle | Angle formed between Wackenheim line and the posterior vertebral body line | Normal ranges are 180 degrees in extension to 150 degrees in flexion. An angle of <150 degrees is considered abnormal. |
| Chamberlain line | Between the hard palate and the opisthion | Protrusion of the dens >3 mm above this line is considered abnormal. |
| McRae line | Basion to the opisthion | Protrusion of the dens above this line is abnormal. |
| McGregor line | From the hard palate to the most caudal point on the midline occipital curve | Odontoid process rising >4.5 mm above this line is considered abnormal. |
| Ranawat criterion | Distance between the center of the pedicle of C2 and the transverse axis of C1 | Measurement of <15 mm in males and <13 mm in females is abnormal. |
| Welcher basal angle | Tangent to the clivus as it intersects a tangent to the sphenoid bone | The normal range is 125 to 143 degrees. Platybasia exists when the basal angle is >143 degrees. |
Atlantoaxial Region
After the occipitocervical junction, the next region that is evaluated on the lateral cervical spine radiograph is the atlantoaxial junction, which includes the anterior arch of C1, the odontoid process, and the posterior arch of C1. Relationships that should be evaluated in this area include the anterior atlantodens interval and the posterior atlantodens interval. The uses of these intervals have been described primarily in the evaluation of patients with rheumatoid arthritis. The anterior atlantodens interval should measure less than 3 mm in adults and less than 3.5 mm in children less than 10 years old. A posterior atlantodens interval of less than 14 mm is considered a relative indication for surgery in a patient with rheumatoid arthritis.
In the trauma setting, the atlantoaxial region should be evaluated in conjunction with the open-mouth view (see details later) for hangman’s fractures, Jefferson or burst fractures, and odontoid fractures (see Chapter 17 for details on the classification and treatment of these fractures). A more thorough evaluation of these fractures can be performed with CT and MRI.
Subaxial Region
The third region of the cervical spine that is evaluated on conventional radiographs is the subaxial cervical spine (C3 to C7). Along with the five lines described earlier, several other specific parameters should be evaluated in this area. One such parameter is the Pavlov or Torg ratio, which can be evaluated in patients with suspected or known congenital cervical spinal stenosis. A ratio of less than 0.8 (ratio of the anteroposterior [AP] canal diameter to the AP vertebral body diameter) suggests the presence of congenital cervical spinal stenosis. One study described the use of this ratio to limit return to play or participation in contact sports for athletes. The space available for the spinal cord is another similar parameter and can be used to define the degree of stenosis in the subaxial cervical spine. The degree or amount of stenosis is defined as absolute when the space available for the spinal cord is less than 10 mm, and it is defined as relative when the space measures 11 to 13 mm.
The subaxial cervical spine is the primary location where facet subluxations or dislocations may be noted. In the setting of the unilateral facet fracture or subluxation, the degree of spondylolisthesis is typically between 0% and 25%, whereas in bilateral facet dislocation (or “jumped facets”), the degree of subluxation is 50% or greater. An additional radiographic finding in patients with bilateral jumped facets is the sail sign or bow tie sign.
Other fractures with classic radiographic appearances seen in the subaxial cervical spine region include compression fractures, burst fractures, teardrop fractures, quadrangular fractures, and clay-shoveler’s fractures. Denis described the spine as divisible into three columns: anterior, middle, and posterior. Evaluation of the three columns on the lateral radiograph aids in identifying these fractures. For example, a loss of anterior vertebral body height in comparison with maintained posterior vertebral body height signifies an anterior column injury or compression fracture. Involvement of the posterior vertebral body or middle column indicates a two-column injury or burst fracture. Teardrop and quadrangular fractures are associated with interspinous process widening, thereby indicating posterior column disruption or a three-column injury. A clay-shoveler’s fracture results from an avulsion of the spinous process secondary to hyperflexion.
Anteroposterior View
On the AP cervical spine radiograph ( Fig. 9-5 ), the coronal alignment should be evaluated. In the normal cervical spine, the AP radiograph shows the spinous processes in the midline, symmetric uncovertebral joints (joints of Luschka), bilateral lateral masses with undisrupted and undulating lateral cortical margins, vertebral bodies of equal height, and parallel disk spaces. On the AP view, the presence of the first rib serves as a marker to help localize the C7-T1 articulation. In general, the AP cervical spine view does not provide as much information as the lateral view for patients with cervical degenerative and traumatic abnormalities.
