C1-C2 rotatory subluxation

Biomechanics and imaging of subluxation

The primary function of the C1-C2 joint complex is to enable head rotation relative to the spine. Normally, during head rotation, the articular surfaces move symmetrically. If asymmetry is detected on the initial examination, changing the patient's head position and repeating the scan should completely eliminate this finding.

However, in fixed rotatory subluxation (rotatory subluxation), pathological rotation between the first and second cervical vertebrae remains rigid. It stays unchanged regardless of the direction in which the patient attempts to turn the head. This persistent fixation is the key radiological sign of a true injury.

Diagnostic pitfalls: incorrect positioning

The clinical significance of correctly assessing C1-C2 asymmetry is immense. If the patient has no history of cervical spine trauma, joint asymmetry detected on computed tomography (CT) is most often the result of a routine patient-positioning error rather than a true subluxation. This is a classic example of a diagnostic error (misdiagnosis).

Correct physician tactics:

• The first and most important step when detecting C1-C2 asymmetry without a clear history of trauma is to check the correctness of the patient's positioning on the scanner table.
• If necessary, the examination should be repeated after correcting the head position. Most of the time, the "subluxation" disappears after this.

Differential diagnosis: Jefferson fracture

When evaluating injuries of the craniovertebral junction, it is important to exclude concomitant damage, such as a Jefferson fracture (Jefferson Fracture). This is a fracture of the bony ring of the atlas (C1). Like any other bony ring in the body, C1 typically breaks in more than one place, resulting in disruption of the anterior and posterior semi-rings.

The mechanism of such an injury involves axial force transmitted via the occipital condyles directly to the atlas.

Diagnostics of a Jefferson fracture:

1. Radiography: An open-mouth odontoid view is used (open-mouth peg view). The main diagnostic sign is overhanging of the C1 lateral masses over the edges of the C2 facet joints.
2. CT: The gold standard. Axial slices clearly visualize fractures of the anterior and posterior rings, as well as associated soft tissue injuries.

Important pitfalls: Simple asymmetry of the joint spaces between the odontoid peg and lateral masses on an open-mouth view is not a diagnostic criterion for a Jefferson fracture. Also, incomplete ossification of the posterior arch of C1 (a frequent normal variant) should not be confused with a fracture line.

C2 injuries and ligamentous damage

In addition to rotatory displacements, odontoid fractures (Odontoid Fractures) of the C2 vertebra are frequently encountered.

The classic diagnostic sign of this injury is displacement of the anterior arch of C1 relative to C2. As a rule, displacement is directed posteriorly (posterior displacement). Lateral radiography typically visualizes a fracture of the base of the odontoid process. Important landmark: the anterior spinal line should normally run along the anterior surface of the C1 arch, rather than along the odontoid process itself.

In addition to bony structures, the ligamentous apparatus must be evaluated. On CT in the coronal projection, avulsion of the alar ligament (Avulsion of the Alar Ligament) can be diagnosed. The characteristic presentation is bilateral avulsion of the attachment sites of this ligament from the occipital condyles.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Anatomy and imaging of the alar ligaments
• Classification of odontoid fractures (C2)
• Cervical spine radiography: open-mouth view
• Normal variants of C1 vertebra ossification

Other topics in the "Spine" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.