Atlantoaxial Instability

Fielding-Hawkins Classification - Rotatory Atlantoaxial Subluxation

The anterior displacement is the point of the classification: types II and III indicate transverse ligament incompetence. Type I may be treated conservatively with a collar, but types II-IV often require surgical fusion if reduction cannot be maintained.

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Grading by ADI Measurement

Space Available for Cord (SAC) grading

The ADI grades the ligament; the SAC grades the cord, and it is the crucial measurement because it directly reflects the risk of cord compression.

Dickman Classification - Transverse Atlantal Ligament Injury

A ruptured transverse ligament and an avulsed transverse ligament produce the same widened ADI and the same physiologically incompetent ligament, and they are treated differently, because only one of them has bone to heal.

In the original series of 39 injuries, type I injuries (n=16) were incapable of healing satisfactorily without internal fixation, while type II injuries (n=23) healed in 74% with a rigid cervical orthosis (PMID 8747950; retrospective consecutive series, plain films plus thin-section CT and MRI, level 4).

The 26% that fail. A quarter of type II injuries fail immobilisation, so a type II is not discharged to a collar and forgotten. It is followed with flexion-extension films, and surgery is offered for nonunion with persistent instability after 3 to 4 months.

Os Odontoideum Classification

An os odontoideum is a round or oval ossicle separated from the body of C2. Its origin is debated between a congenital failure of fusion of the ossification centre and an acquired lesion after trauma or a vascular insult.

Treatment follows the instability. An os odontoideum with instability (ADI greater than 5mm) or neurological symptoms requires C1-C2 fusion; a stable, asymptomatic os can be observed with annual flexion-extension radiographs.

Classification by Aetiology

Aetiology guides the treatment approach and the prognosis. Several of these causes are conditions in their own right and are developed in full elsewhere: Down syndrome, atlantoaxial rotatory subluxation, which is where Grisel syndrome belongs, atlantoaxial arthritis for the degenerative C1-C2 joint, and odontoid fractures, the traumatic cause that produces instability through the dens rather than through the ligament.

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History. Ask about the symptoms, then about the reason the patient has them:

• Neck pain (may be occipital headache)
• Torticollis (especially rotatory subluxation)
• Weakness, clumsiness, gait disturbance
• Bowel/bladder dysfunction (late sign)
• History of Down syndrome, RA, or other predisposing condition
• Precipitating trauma or infection

Examination. The examination is a search for myelopathy: hyperreflexia, clonus, a positive Babinski, a Hoffmann sign, gait disturbance and hand clumsiness. These indicate cord compression and mandate urgent imaging and surgical consideration; a normal neurological examination with radiographic AAI is a different problem, managed by activity modification and monitoring.

Red flags that need urgent workup:

• Progressive weakness or numbness
• Gait deterioration
• Bowel or bladder dysfunction
• Respiratory compromise (high cord compression)
• Worsening headache with neck movement

A child or adult presenting with neck pain, torticollis or upper-limb signs has a broad differential. Distinguish true atlantoaxial instability from its mimics before committing to fusion.

Who. Non-operative management is indicated for:

• Asymptomatic radiographic AAI with ADI less than 10mm
• SAC greater than 14mm
• No neurological symptoms
• Stable on dynamic imaging

What it involves. Activity modification, a cervical collar (soft or rigid) for acute symptoms, and regular clinical and radiographic surveillance with annual flexion-extension radiographs. The patient and family are taught the neurological warning signs, because any new symptom mandates immediate evaluation and imaging.

The restrictions are a fixed list:

• No contact sports (football, rugby, wrestling)
• No diving
• No activities with high cervical hyperflexion risk
• Trampolines contraindicated

Absolute

• Neurological symptoms (myelopathy, weakness)
• SAC less than 14mm with symptoms
• Failed conservative management

Relative

• ADI greater than 10mm (even if asymptomatic)
• SAC less than 14mm (asymptomatic)
• Os odontoideum with instability
• Progressive instability on serial imaging
• Patient unable to comply with activity restrictions

The goals of surgery are to decompress the neural elements if they are compressed, reduce the subluxation, achieve a stable bony fusion, and protect the cord from further injury. The end point is solid arthrodesis between C1 and C2.

Reduce before you fuse. Operating on an unreduced C1-C2 fixes the subluxation in place. There are three routes: traction beforehand, reduction on the table, and release when the subluxation is irreducible.

Halo traction is for the reducible subluxation and is gradual: serial radiographs track the reduction, the weight starts at 3-5 lbs in children and is progressively increased, and neurological status is monitored hourly during reduction. Preoperative halo traction achieves gradual, safe reduction before definitive surgery.

Intraoperative reduction uses Gardner-Wells tongs or the Mayfield: manual reduction under fluoroscopy, then the reduction is held while the fusion is performed.

The irreducible subluxation may require an anterior release (odontoidectomy) first and a posterior fusion afterwards, or an occipitocervical fusion if C1-C2 cannot be reduced.

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Positioning. Prone on a Jackson table, the head secured in Mayfield pins (or the halo, if preoperative traction was used), the neck in slight flexion for access and returned to neutral for the fusion. SSEP and MEP neuromonitoring throughout.

Exposure, in order:

1. Midline incision from occiput to C3
2. Subperiosteal dissection exposing C1 posterior arch and C2 lamina/spinous process
3. Identify C1 lateral masses (limit lateral dissection to avoid vertebral artery)
4. Identify C2 pars interarticularis and pedicles
5. Expose surfaces for fusion (C1-C2 facet joints)

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The current gold standard is the Harms technique: C1 lateral mass screws and C2 pars or pedicle screws connected by rods. It is preferred over Magerl for most cases because the screws are placed before the rod, so the reduction can be made intraoperatively; it is safer for the vertebral artery; and it can be done with a deficient C1 arch. Magerl requires intact anatomy and preset alignment.

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C1 lateral mass screw. Enter at the junction of the C1 lateral mass and the inferior margin of the posterior arch, confirming the entry point at the medial border of the lateral mass. Direct the screw 10-15 degrees medial and anteriorly toward the anterior arch. Length is typically 26-32mm, judged on the preoperative CT; the internal carotid artery lies anterior to the lateral mass, so a bicortical tip stops just short of the anterior cortex rather than through it.

C2 pedicle screw. Enter at the superior and medial aspect of the C2 inferior facet and aim 25-30 degrees cephalad and 20-25 degrees medial, toward the anterior cortex of the C2 body. Length 22-28mm.

C2 pars screw. The alternative is a shorter screw contained within the pars interarticularis, entered much as the pedicle screw is but on a shorter and more lateral trajectory. It carries a lower vertebral artery risk and gives less purchase.

Verification. AP and lateral fluoroscopy for every screw, navigation for complex anatomy, and direct palpation of the screw threads. Careful attention to trajectory is what prevents vertebral artery injury and secures good fixation.

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Fusion, in sequence:

1. Decorticate C1-C2 facet joints bilaterally
2. Pack bone graft into decorticated facet joints
3. Posterior structural graft between C1 and C2 lamina (optional but recommended)
4. Use autograft (iliac crest) or allograft supplemented with bone graft substitutes

Graft. Iliac crest autograft is the gold standard for paediatric patients; local bone (lamina, spinous process) and allograft with bone morphogenetic protein (adults) are the alternatives.

Closure and blood loss. Bleeding from the epidural venous plexus is common; a cell saver is useful for larger cases and transfusion may be needed in paediatric patients. After meticulous haemostasis the wound is closed in layers over a drain with a tight fascial closure; a watertight closure prevents CSF leak and wound complications.