Occipitocervical / Craniocervical Junction Instability

• Traumatic AOD - a rare, highly unstable distraction injury from motor-vehicle and pedestrian trauma; historically often fatal at the scene, though better pre-hospital care has improved survival and detection. Children and young adults are over-represented (more horizontal occipital condyles, a relatively large head).
• Atraumatic instability - rheumatoid arthritis (the classic), Down syndrome, and congenital os odontoideum / skeletal dysplasias produce slowly progressive atlanto-axial or occipito-atlantal instability, sometimes with myelopathy.
• Why it is examinable: AOD is lethal and easily missed, the no-traction rule is a hard safety point, and the at-risk-group screening (RA, Down) is a recurring clinical-governance theme.

The craniocervical junction (CCJ) — the occiput, atlas (C1) and axis (C2) — provides nearly half of cervical flexion-extension and rotation, yet its bony articulations are shallow and incongruent, so stability depends on ligaments:

• the transverse ligament (part of the cruciate ligament) holds the odontoid (dens) against the anterior arch of the atlas — the key restraint to anterior atlanto-axial translation;
• the alar ligaments run from the dens to the occipital condyles, restraining rotation and distraction;
• the tectorial membrane (the cephalad continuation of the posterior longitudinal ligament) and the apical and atlanto-occipital membranes/ligaments span the dens/atlas to the occiput.

Disruption of these — by trauma or disease — produces craniocervical instability.

AOD is frequently missed, so look for it actively on the craniocervical CT using objective measurements:

• C1-condyle interval (CCI) — the gap between the occipital condyle and the C1 lateral mass; an increased CCI is the most sensitive sign of AOD (in adults and children). Know the number, and know that it differs by age — see the box below.
• Harris lines — the basion-dental interval (BDI) (basion to the tip of the dens) and the basion-axial interval (BAI) (basion to the posterior axial line); each should normally be under about 12 mm.
• Powers ratio (basion–posterior arch / opisthion–anterior arch) — over ~1 suggests anterior dislocation (less reliable for distraction/posterior injuries).

MRI assesses the ligaments, cord signal, oedema and epidural haematoma. Always evaluate the whole cervical spine and the neurology.

Unstable craniocervical injury/instability is treated with posterior occipitocervical stabilisation and fusion — commonly occiput to C2 (C0–C2) — using an occipital plate and C1/C2 (± occipital) screw-rod constructs with bone graft. Isolated atlanto-axial instability (transverse-ligament rupture, os odontoideum) may be treated by C1–C2 fusion, whereas occipito-atlantal disruption requires inclusion of the occiput. In rheumatoid disease, decompression and fusion address instability/cranial settling and myelopathy. Milder, stable unilateral ligamentous injuries may be managed non-operatively. Recognising the injury early, before deterioration, is the key to survival and outcome.

• CCJ stability is ligamentous — transverse (odontoid–atlas), alar (rotation/distraction), tectorial membrane; the bone alone is insufficient.
• AOD is highly lethal and easily missed — children predisposed; diagnose on CT (C1-condyle interval most sensitive; Harris BDI/BAI under ~12 mm; Powers ratio).
• NEVER apply traction to a distraction injury — rigid immobilisation instead.
• Atraumatic at-risk groups: rheumatoid arthritis (pannus, cranial settling), Down syndrome (laxity/os odontoideum), congenital os odontoideum/dysplasias — screen them.
• Unstable → posterior occipitocervical fusion (C0–C2); C1–C2 for isolated atlanto-axial instability.
• Milder stable unilateral injuries can be non-operative — CCJ injury is a spectrum.