Plain Radiography
Radiologic evaluation is used to confirm the diagnosis and estimate the degree of spinal instability. Initial evaluation includes open-mouth anterior-posterior and flexion-extension lateral radiographs. Os odontoideum appears as a round or oval ossicle with a smooth uniform cortex separated from the base of the axis by a wide gap (see the image below). The ossicle border does not directly match up with the axis body. The gap separating the os and the axis proper should lie above the level of the superior articular facets. [55, 56, 57]
A coronal reconstruction of an orthotopic os odontoideum. Note the wide gap between the rounded ossicle and the base of the axis.
Orthotopic os odontoideum (see the image above) may appear free and in a relatively anatomic position, where it may be difficult to differentiate from an unfused neurocentral synchondrosis, odontoid hypoplasia, or odontoid fracture nonunion. In children younger than 5 years, the neurocentral synchondrosis often has not fused. Dynamic lateral radiographs of those with an unfused synchondrosis do not demonstrate motion, whereas radiographs of individuals with an os odontoideum may demonstrate motion.
A dystopic ossicle may be fixed to the clivus or to the anterior ring of the atlas. The remaining axis is hypoplastic as well. With a dystopic os odontoideum (see the image below), the radiographic diagnosis is clear.
A sagittal reconstruction of a CT scan demonstrating a dystopic os odontoideum. Note that the ossicle appears fused to the clivus (anterior portion of the foramen magnum). Also note the smooth corticated border of the ossicle.
A dens (odontoid process) fracture nonunion (see the image below) typically exhibits a narrow gap between the axis base and the dens. The normal shape and size of the dens are preserved on the open-mouth view.
Anterior-posterior tomogram view of a type II dens fracture. The fracture line is narrow and lower on the waist of the dens, unlike the fracture line of an os odontoideum. No cortication is noted along the fracture line.
With an os odontoideum, hypertrophy of the anterior arch of the atlas may be seen. This hypertrophy is believed to represent osseous reaction to chronic atlantoaxial instability and is unlikely with an acute dens fracture.
Flexion-extension lateral radiographs typically show motion in symptomatic patients. This abnormal motion is typically seen in the anterior-posterior plane. Some patients are unstable in all directions. In one series, the average translatory motion was 1 cm.
Important prognostic indices are as follows:
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Anterior atlantoaxial translation
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Posterior atlantodens interval (PADI)
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Instability index - The change in the space available for the cord in flexion versus extension
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Sagittal plane angulation - The difference in the atlantoaxial angle between flexion and extension
In some forms of atlantoaxial subluxation, the anterior atlantodens interval (AADI) is used to measure instability. However, with os odontoideum, the os fragment often moves with the atlas. Therefore, the AADI does not reflect the abnormal motion of the segment. Direct measurement of the motion of C1 on the body of C2 is more useful.
Anterior atlantoaxial translation represents the space between a line projected superiorly from the anterior border of the body of the axis and a line projected inferiorly from the posterior border of the anterior arch of the atlas. More than 3 mm of separation is pathologic.
CT and MRI
Critical evaluation of the bony anatomy of the upper cervical spine is often difficult with plain radiographs alone. A fine-cut (1-mm) sagittally reconstructed computed tomography (CT) scan allows a more detailed depiction of the atlantoaxial articulation.
Previously, the distance between the posterior border of the dens and the anterior border of the posterior ring of the atlas on plain radiographs was termed the space available for the cord (SAC). Currently, this distance is more frequently referred to as the PADI. These terms are occasionally used synonymously.
With magnetic resonance imaging (MRI) or CT myelography, however, the actual space for the cord can be readily measured, and the SAC can be used to refer to the actual anterior-posterior canal dimension. Practically, SAC refers to the PADI minus additional compression from soft tissue. This measurement should be carried from the superior posterior corner of C2 to the posterior ring of C1. Soft-tissue structures such as synovial cysts may further diminish the SAC in some cases of os odontoideum. A PADI of less than 13 mm is associated with neurologic decline.
In one study, plain radiographic measurements of translation and PADI did not accurately reflect clinical status. MRI measurement of cord compression was more predictive of symptomatology. Hadley’s literature review was unable to establish a linear relationship between PADI and neurologic status. [8]
MRI also may delineate pathologic changes within the cord. On T2-weighted MRI (see the image below) sequences, an increased signal in the substance of the cord may reflect edema or myelomalacia. [58, 59, 60] An altered T1 signal typically portends a more grim prognosis neurologically, in that it may reflect hemorrhage or necrosis within the cord.
T2-weighted parasagittal MRI image of a patient with os odontoideum and mild compression of the upper cervical spine. This patient presented with transient quadriparesis.
A number of dynamic imaging modalities have been recommended as means to more completely understand the degree and nature of abnormal motion in patients with os odontoideum. For example, cineradiographs and dynamic (flexion-extension) MRI have been recommended because of their ability to define the relationship of the os to surrounding bony elements. [59, 60, 61] As of this writing, these modalities are not widely used. CT angiography (CTA) and magnetic resonance angiography (MRA) are more commonly helpful in understanding aberrant vertebral artery anatomy or persistence of the first segmental artery. [62, 63]
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The axis has 5 primary and 2 secondary ossification centers. C0, C1, and C2 sclerotomes contribute to various portions of the dens. The principal portion of the dens body arises from the original center of C1.
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A coronal reconstruction of an orthotopic os odontoideum. Note the wide gap between the rounded ossicle and the base of the axis.
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A sagittal reconstruction of a CT scan demonstrating a dystopic os odontoideum. Note that the ossicle appears fused to the clivus (anterior portion of the foramen magnum). Also note the smooth corticated border of the ossicle.
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Anterior-posterior tomogram view of a type II dens fracture. The fracture line is narrow and lower on the waist of the dens, unlike the fracture line of an os odontoideum. No cortication is noted along the fracture line.
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Flexion lateral radiograph of a dystopic displaced os odontoideum. Note the limited posterior interval between the dens and the posterior C1 ring (PADI). This patient had reported electric shocks radiating down the body (ie, Lhermitte sign), as well as progressive weakness and ataxia.
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Extension lateral radiograph of a dystopic displaced os odontoideum. This deformity did not reduce with extension.
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T2-weighted parasagittal MRI image of a patient with os odontoideum and mild compression of the upper cervical spine. This patient presented with transient quadriparesis.
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Lateral radiograph of a dystopic displaced os odontoideum 6 months after posterior wiring. After the wiring was performed, the patient had a solid arthrodesis with no motion on flexion and extension. Her neurologic symptoms resolved despite the failure to obtain a reduction. Had she continued to have severe symptoms, anterior odontoidectomy could have been considered.
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Lateral radiograph demonstrating fixation of a reduced os odontoideum with Magerl screws in a patient with an incomplete posterior arch of C1.
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Lateral radiograph demonstrating fixation of a reduced os odontoideum with the Harms technique.
