Indications and Examination Objectives
The standard anteroposterior projection of the cervical spine does not allow for a detailed examination of the upper cervical segments due to the heavy superimposition of the cranial bones and mandible. To overcome this obstacle, the open-mouth projection is used.
This imaging study is a crucial part of the evaluation for spinal trauma, as the most mobile segments of the spine (including the cervical spine) are maximally susceptible to injury. The primary objective of the radiograph is to identify or exclude bone fractures, cortical disruption (cortical disruption), and the presence of unstable bone fragments in the region of the odontoid process.
How to distinguish normal anatomy from rotation?
Normally, on the radiograph, the lateral margins of the facet (zygapophyseal) joints of the first and second vertebrae are aligned in strict symmetry, with no contour overlap (overlap). However, in trauma patients, achieving ideal positioning can be extremely challenging.
If the patient's head is slightly rotated to the side, asymmetry in the position of the C2 dens within the atlas (atlas) ring will inevitably appear on the radiograph. To confirm that this is a result of positioning rotation rather than true pathology, a simple radiographic landmark is used:
- A plumb line is drawn strictly between the upper central incisors (teeth 11 and 21).
- If this line does not project precisely through the center of the odontoid process, it confirms that the patient's head was rotated during the imaging.
Specific injuries of the craniocervical region
Open-mouth radiography assists in the comprehensive evaluation of complex craniocervical junction injuries. These include:
- Occipital condyle fractures (occipital condyle fractures). These occur due to axial compression. While generally stable on their own, rotational forces can lead to a dangerous avulsion fracture of the alar ligament (alar ligament), which extends from the C2 dens to the medial surface of the condyle.
- C1 rotatory subluxation (rotatory subluxation). Fixed rotation between C1 and C2 is most commonly seen in children. Characteristically, it typically occurs following a respiratory infection with a completely negative history of trauma.
Method limitations and diagnostic pitfalls
When interpreting radiographs, it is important to keep in mind normal anatomical variants that can mimic trauma. One such variant is the pseudo-Jefferson fracture (pseudo-Jefferson fracture), which is frequently observed on open-mouth radiographs and requires no treatment.
Furthermore, radiography has significant limitations:
- Isolated ligamentous injuries can occur without concomitant bone fractures.
- In severe flexion-extension injuries, particularly in children, SCIWORA syndrome (spinal cord injury without radiographic abnormality) may develop.
For precise assessment of the ligamentous apparatus, determination of stability, and diagnosis of neurological injuries, the "gold standard" is MRI. It is important to remember: performing functional radiographic tests (flexion/extension) in acute trauma is strictly contraindicated due to the risk of causing severe neurological damage.
To check the symmetry of the C2 dens on the radiograph, look for a plumb line from the front teeth: the line between the 11 and 21 incisors should fall directly on the center of the odontoid process.
Class and exam questions
What should be done if C1–C2 joint asymmetry is detected on an open-mouth view?
First, check the plumb line from the central upper incisors. If it is displaced relative to the center of the C2 dens, the asymmetry is caused by the patient's head rotation (a positioning artifact) rather than a true subluxation.
Does the open-mouth view replace MRI in acute trauma?
No. Radiography is good for detecting bony injuries and C2 fractures, but diagnosing isolated ligamentous tears or spinal cord contusion requires an MRI.
Can functional tests be used to check C1–C2 stability?
When acute spinal trauma is suspected, flexion and extension functional tests are strictly contraindicated, as an unstable fragment can damage the spinal cord.
What else to study on this topic
- Anatomy and imaging findings of Harris' ring on lateral cervical radiographs.
- Physiological subluxations and the spinolaminar line: how to avoid overdiagnosis of trauma in children.
- Atlanto-occipital dissociation and methods for assessing craniocervical alignment using the clival line.
- SCIWORA syndrome: pathogenesis, clinical presentation, and imaging algorithm.
- Differential diagnosis of true avulsion fractures and ununited ossification centers.
Master the entire topic in the "Diagnostic Radiology" course
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