Atlantoaxial Rotatory Subluxation

Atlantoaxial rotatory subluxation (AARS) is an abnormal, locked rotation of the atlas (C1) on the axis (C2); when fixed and persistent it is often called atlantoaxial rotatory fixation (AARF). The patient presents with a painful, fixed torticollis, the cock-robin posture, which unlike ordinary muscular torticollis cannot be voluntarily corrected.

Why it matters. This is the classic "dangerous torticollis" that must not be missed. Recognise it, image it correctly with dynamic CT, and treat it early, before the rotation becomes permanently fixed.

Who. Children are the predominant group. A 15-year paediatric series of 55 children with AARS/F had a mean age of about 8.5 years with a roughly even sex distribution [Spinnato 2020]. Adults can develop AARF, almost always after trauma: a systematic review of published adult cases found a mean age of about 36 years, with motor-vehicle accidents and falls the leading mechanisms [Zafarshamspour 2024].

Aetiology. Three main settings account for the cases:

The joint. The C1-C2 complex provides roughly 50% of all cervical rotation, about 45 degrees to each side, pivoting around the dens. There is no intervertebral disc, so stability depends entirely on bone shape and ligaments: the joint is shallow and ligamentously dependent. The transverse ligament is the primary restraint, holding the dens against the anterior arch of C1 and preventing anterior translation; the alar ligaments check rotation and the capsular ligaments of the facet joints stabilise the lateral masses.

View larger

View larger

The normal rhythm. Normative CT motion studies in children showed a highly reproducible pattern as the head turns: C1 first moves alone (the single-motion phase), then C1 and C2 move together with C1 faster (the double-motion phase), and beyond about 65 degrees they move in unison with a fixed maximal separation [Pang 2010]. AARF is a loss of this normal, predictable C1-on-C2 rhythm: the two vertebrae stay locked together regardless of how the head is turned.

The mechanical lesion. The atlas rotates on the axis and then fails to return to neutral. Proposed mechanisms include the lateral mass capsule and synovium becoming caught or swollen, the facet articulation effectively over-riding or jamming, and muscle splinting that maintains the abnormal position. Once locked, the joint behaves like a fixed deformity rather than a transient spasm.

View larger

Why inflammation locks it (Grisel syndrome). Infection or inflammation in the pharynx or retropharyngeal space, or after ENT surgery (post-adenoidectomy or tonsillectomy is a classic trigger), spreads to the upper cervical region. The classic explanation is drainage through the pharyngovertebral (periodontoidal) venous plexus, causing local hyperaemia, ligamentous laxity and decalcification around the C1-C2 joint, which lets the atlas slip and lock. Abnormally lax soft tissues and a diagnosis that is often delayed are the reasons this group carries the worst prognosis: it is the subgroup most likely to need prolonged treatment or fusion.

Why children. Compared with adults, children have more horizontal (less locked) facet joints, greater ligamentous laxity, a relatively large head and incompletely ossified bony stabilisers. These features allow a larger physiological range of C1-C2 rotation and make the joint easier to lock, so relatively minor force or inflammation can lock C1 on C2.

Two systems are in use and they grade different things. Fielding-Hawkins, the traditional anatomy-based system, grades the degree of anterior atlantodental shift; Pang grades how "stuck" the C1-C2 joint is on 3-position dynamic CT rather than the static shift, and adds a chronicity grading that runs alongside as a prognostic axis independent of type.

View larger

History. Acute onset of a painful, twisted neck, often in a child. A trigger may be identified: minor trauma, or a recent upper respiratory or ENT infection or ENT surgery (Grisel). The head is held in a characteristic position and the patient resists attempts to straighten it.

The cock-robin deformity. The head is tilted toward one side and the chin rotated to the opposite side, with slight flexion; the posture resembles a robin cocking its head to listen. The sternocleidomastoid that looks contracted is typically on the side the chin points to. It is splinting, not the primary cause, which is the key difference from congenital muscular torticollis, where the tight SCM is on the side opposite the chin.

Examination. The most useful bedside question is whether the patient can rotate the head past the midline toward the side the chin points to. In AARS this movement is blocked and painful because the joint is mechanically fixed; in muscular torticollis it is usually possible, if uncomfortable.

Differential Diagnosis of Paediatric Torticollis

AARS is only one cause of a child holding the head twisted. The examinable skill is a structured differential that separates the benign, the time-critical and the dangerous. Each of these causes is worked up in its own right in torticollis in children, and two of the structural mimics have dedicated accounts: Klippel-Feil syndrome and pseudosubluxation of the cervical spine, the normal paediatric variant that is misread as displacement.

Plain radiographs screen. On the open-mouth (odontoid) view the lateral mass that has rotated forward looks wider and closer to the dens, while the contralateral mass looks narrower and further away: the asymmetric "wink" appearance. Asymmetry of the dens-lateral mass spaces raises suspicion. The lateral view may show the assimilated posterior arch of C1 and an increased atlantodental interval if there is anterior shift. Plain films are suggestive, not confirmatory: positioning the twisted neck reproducibly is difficult, and a normal film does not exclude AARS.

CT confirms. CT is the diagnostic test of choice. A static CT demonstrates the rotational malalignment of C1 on C2, but a single scan can show rotation that is actually within physiological range if the head happened to be turned. The 3-position dynamic protocol (neutral, then maximal active rotation to each side) settles the question: in a normal child the C1-C2 relationship changes predictably, whereas in AARF the atlas and axis stay locked together and the pathological rotation persists. The same study both confirms AARF and grades it, because Pang's normative and prospective studies established the protocol as the means to diagnose and grade AARF by plotting the C1-C2 motion curve against a normal template [Pang 2010; Pang and Li 2005].

View larger

View larger

View larger

MRI is selective. It is indicated if there are neurological signs, to assess the cord and the space available for the cord, and to evaluate soft-tissue or inflammatory change in Grisel syndrome. It also helps exclude a posterior fossa or cord tumour in atypical torticollis.

Measurements. The key measurements and what each tells you:

Both the Fielding-Hawkins type and the measured C1-C2 rotation degree correlated significantly with recovery time in a paediatric series [Spinnato 2020].

The principle. Reduce early, before the joint becomes permanently fixed. Most acute cases respond to conservative measures; chronicity and recurrence drive the need for surgery.

ConservativeSurgical IndicationsSurgical Technique

The first week. Analgesia and muscle relaxants break the splinting cycle, and a soft collar with activity rest suits the very acute, low-grade case. Many early, low-grade (type I) cases reduce spontaneously or with gentle measures within days.

Traction. When the deformity persists or is higher grade, traction reduces the locked joint gradually: halter traction for lower-grade cases, skeletal calipers (Gardner-Wells) for higher-grade or chronic ones, with regular neurological checks throughout.

Holding the reduction. Immobilisation in a collar, SOMI brace or halo, chosen by type and chronicity, maintains alignment while the soft tissues settle.

Grisel syndrome. The same ladder, plus appropriate antibiotics for the underlying infection and attention to any persisting ENT source. Prompt diagnosis can avert surgery.

What the series show. In adult traumatic AARF, conservative management was used in the majority of acute cases but in only a minority of chronic cases, and acutely diagnosed patients were more often treated successfully without surgery [Zafarshamspour 2024]. In children, conservative treatment is effective in most, with delayed recovery concentrated in Grisel cases and higher-grade, larger-rotation deformities [Spinnato 2020; Pang and Li 2005].

View larger

Surgery is reserved for the deformity that cannot be reduced or held, the threatened cord, and the unstable adult injury:

• Irreducible deformity (cannot be reduced by traction)
• Recurrent subluxation after successful reduction (re-slips out of alignment)
• Chronic fixation (especially greater than 3 months), where reduction often fails or does not hold
• Neurological compromise or significant cord-space narrowing
• Ligamentous incompetence / unstable associated fractures (more relevant in adult trauma)

In the adult systematic review, chronically diagnosed patients more often required surgery, and surgery was advised for irreducible dislocations, ligamentous injury, unstable fractures and persistent pain [Zafarshamspour 2024].

Posterior C1-C2 fusion is the standard operation when surgery is required. Aim to reduce first, by pre-operative or intra-operative traction, and fuse in the best achievable alignment.

The Goel-Harms construct (C1 lateral mass screws and C2 pedicle or pars screws connected by rods) is the default: it permits intra-operative reduction and gives high fusion rates. Magerl transarticular screws are an alternative but require intact anatomy and a pre-set alignment, and carry vertebral artery risk. That risk is patient-specific rather than generic: 18% of people have a high-riding transverse foramen prohibiting a transarticular screw on at least one side, 18-23% including borderline anatomy (PMID 8755749; consecutive CT series, n=94). The construct choices are set out in atlantoaxial (C1-C2) fusion, and the wider instability picture in atlantoaxial instability.

Wiring (Gallie/Brooks) is largely historical, with higher non-union and a need for rigid post-operative immobilisation. Autograft bone and intra-operative neuromonitoring are standard.

View larger

Vertebral Artery

The vertebral artery lies in the foramen transversarium and is at risk during C1-C2 instrumentation. Stay close to the midline at C1, plan trajectories on pre-operative CT angiography, and image-guide screw placement.

Chronic fixation. The principal complication of late diagnosis. A long-standing locked joint develops the compensatory occipitocervical changes described above, becomes irreducible, and leaves a permanent cosmetic and functional torticollis, which is the main reason early reduction is stressed.

Recurrence. Re-slippage after reduction is common in higher-grade and chronic cases and predicts a worse outcome. Recurrent AARF patients did markedly worse on every measure, and recurrence was driven by both higher type and greater chronicity [Pang and Li 2005]; recurrent AARF is a leading trigger for moving to fusion.

Loss of C1-C2 rotation. Even after successful treatment, some patients permanently lose normal atlantoaxial rotation. Fusion deliberately sacrifices C1-C2 rotation, about half of cervical rotation, for stability.

Neurological injury. Uncommon but serious with large displacement or during manipulation or surgery. Vertebral artery injury and cord compromise are the feared operative complications.

A viva answer on a child with acute torticollis is best built in this order:

1. Recognise the red flags - acute, painful, post-trauma or post-infection, fixed deformity, no SCM mass
2. Differentiate AARS from muscular torticollis, ocular torticollis, posterior fossa tumour, and infection (osteomyelitis/discitis)
3. Image correctly - plain films to screen, dynamic CT to confirm and grade, MRI if neurology or tumour suspected
4. Classify - Fielding-Hawkins (displacement) and Pang (dynamic, with chronicity)
5. Treat by a ladder - analgesia/collar then traction then bracing then fusion, reducing early
6. Address the cause in Grisel syndrome - antibiotics plus neck treatment

AARS/AARF is a rare condition managed largely on the basis of case series and expert consensus rather than randomised trials or implant registries, so practice is guided by the principles below rather than by a single national guideline.

Points of broad international agreement: