Atlantoaxial Instability — Pediatric & RA

Overview & Anatomy

Atlantoaxial instability (AAI) is excessive motion at the C1-C2 (atlas-axis) articulation, causing the potential for neurological injury from spinal cord compression. The atlantoaxial joint is unique — it provides approximately 50% of total cervical rotation and relies predominantly on ligamentous rather than bony stability (the bony geometry provides minimal constraint compared to subaxial cervical levels). Any condition that disrupts the transverse atlantal ligament (TAL), the dens, or the normal bony architecture of the C1-C2 joint can cause instability. The neurological consequences can be catastrophic — cord compression at this level affects breathing, upper and lower limb function.

Anatomy of C1-C2 stability: the atlas (C1) is a ring of bone with no vertebral body; it articulates with the occipital condyles above (atlantooccipital joint) and the axis (C2) below; the dens (odontoid process) projects superiorly from the C2 body and sits within the anterior arch of C1, restrained posteriorly by the transverse atlantal ligament (TAL — the most important stabiliser against anterior atlantoaxial subluxation); the TAL is the primary static stabiliser — its failure allows the dens to sublux posteriorly into the spinal cord; the alar ligaments (paired oblique ligaments from the dens to the occipital condyles) provide secondary restraint; the tectorial membrane (continuation of posterior longitudinal ligament); the cruciate ligament complex (TAL + superior and inferior longitudinal fasciculi)
Atlanto-dens interval (ADI): the space between the anterior arch of C1 and the anterior face of the dens on lateral cervical X-ray in flexion; normal <3 mm in adults; <5 mm in children (paediatric normal is wider due to greater ligament laxity); ADI >3 mm in adults or >5 mm in children = AAI on plain X-ray; the space available for the cord (SAC) = the AP diameter of the spinal canal minus the AP diameter of the cord; SAC <13 mm = neurological compromise risk
Steel`s rule of thirds: the cross-sectional area of the upper cervical canal at the atlantoaxial level is divided into thirds — one-third dens, one-third spinal cord, one-third `spare space` (buffer zone); this buffer zone means that substantial instability can occur before the cord is compressed; however, once the buffer is lost (instability >6–7 mm), any further displacement risks cord injury; this principle also partly explains the high rate of neurological deterioration in AAI that occurs not at rest but during flexion of the neck

Causes of Atlantoaxial Instability

Cause	Mechanism	Key Features
Down syndrome (Trisomy 21)	Ligamentous laxity (collagen abnormality in Down syndrome) allows excessive ADI; hypotonia amplifies instability; TAL is structurally normal but functions suboptimally due to lax capsular tissue	ADI >5 mm on lateral flexion X-ray in ~10–20% of Down syndrome patients; AAI in approximately 10–15% of Down syndrome; most are asymptomatic; NEUROLOGICAL SYMPTOMS are rare (<1–3%) — most cases are radiological abnormality without clinical AAI; the Special Olympics requires cervical flexion-extension X-ray before participation; children with symptomatic AAI (myelopathy signs — hyperreflexia, clonus, ataxia, weakness) or ADI >10 mm require surgical stabilisation (C1-C2 fusion)
Rheumatoid arthritis (RA)	Synovitis of the atlantoaxial synovial joints causes erosion of the TAL, alar ligaments, and dens (pannus formation); also causes subaxial subluxation and cranial settling (vertical translocation of the dens into the foramen magnum); anterior AAI, posterior AAI (dens erosion), and cranial settling all occur	Anterior AAI: most common (65–80% of RA with cervical involvement); ADI >3 mm = AAI; Vertical (cranial) settling: dens migrates superiorly and projects through the foramen magnum — McGregor`s line (tip of dens should be <4.5 mm above McGregor`s line in females and <5.5 mm in males — values above these indicate cranial settling); Subaxial subluxation: multiple level `staircase` pattern; Posterior AAI: dens eroded — posterior subluxation; all RA patients with myelopathy, severe neck pain, or neurological deterioration must have pre-operative cervical assessment before any surgery under general anaesthesia (intubation risk)
Odontoid (dens) fractures	Traumatic disruption of the dens creates an unstable atlantoaxial unit (TAL attachment point on dens is disrupted or the dens is fractured at its base); Anderson-D`Alonzo classification: Type I (apical — rare; stable); Type II (junction of dens and body — most common; UNSTABLE; high non-union rate with conservative treatment — up to 36% in elderly); Type III (through C2 body — often heals well with immobilisation)	Type II odontoid fractures in the elderly are a major cause of preventable mortality; surgical treatment (posterior C1-C2 fusion or anterior odontoid screw fixation) vs collar immobilisation; non-union in elderly with Type II treated conservatively is common → progressive instability; odontoid screw fixation preserves C1-C2 rotation but requires an intact TAL and a fracture pattern allowing lag screw purchase
Congenital anomalies	Os odontoideum (failed fusion of the dens ossification centre — the dens tip separates from its base and becomes a free ossicle); agenesis of the dens; congenital bony anomalies of C1 or C2 (fused C2-C3 — Klippel-Feil syndrome — places extra stress on C1-C2)	Os odontoideum — may be orthotropic (in normal dens position) or dystopic (ectopic); associated with AAI; requires MRI and dynamic X-rays for assessment; associated with increased instability over time; surgical stabilisation recommended for symptomatic or significantly unstable os odontoideum
Rotatory subluxation / fixation (Fielding classification)	C1-C2 rotatory displacement — the atlas rotates relative to the axis and becomes fixed in a rotated position; often follows a URTI or pharyngitis (`Grisel`s syndrome` — inflammation of the C1-C2 synovial joints from adjacent pharyngeal infection → increased joint fluid → rotatory laxity → spontaneous subluxation) or minor trauma	Presents in children as `cock-robin` deformity (head tilted and rotated, chin pointing away from the side of the tilt — the sternomastoid on the side of the tilt is NOT contracted as in torticollis — important distinction); dynamic CT (open-mouth and rotated view) demonstrates fixed rotatory subluxation; Fielding Type I (no ADI widening), Type II (ADI 3–5 mm), Type III (ADI >5 mm), Type IV (posterior displacement); management — early (within 1 week) → halter traction and collar; late (>1 month fixed) → halo traction ± C1-C2 fusion
Ankylosing spondylitis (AS)	Fracture through the fused ankylosed cervical spine creates an unstable segment; the entire fused `bamboo spine` acts as a long lever arm, concentrating forces at the fracture site; small mechanisms can cause highly unstable fractures	Low-energy fractures with high neurological injury risk; often initially missed on plain X-rays; CT or MRI essential; surgical stabilisation required for most cervical AS fractures

Atlantoaxial Instability in Rheumatoid Arthritis — Detailed

Prevalence: cervical spine involvement in RA is common — approximately 25–50% of RA patients have radiological evidence of cervical involvement; the atlantoaxial region is the most frequently affected; modern DMARDs (methotrexate, biologics) have dramatically reduced the incidence and severity of RA cervical involvement; cervical RA is becoming less common in newly diagnosed RA patients treated early with biologics
Pre-operative assessment for all RA patients: ALL RA patients undergoing any surgery under general anaesthesia must have pre-operative assessment of the cervical spine (clinical neurological examination + flexion-extension plain X-rays); anaesthetic intubation can cause cord injury if there is occult AAI; ADI >8 mm or SAC <13 mm = high neurological risk → anaesthetist must use awake fibreoptic intubation ± cervical collar; patients with known unstable RA cervical spine should carry a `medical alert` card and inform any medical team
Indications for surgical stabilisation in RA cervical spine: (1) myelopathy (progressive neurological deterioration — loss of hand function, gait ataxia, hyperreflexia, long tract signs); (2) intractable neck pain not responding to non-operative measures; (3) radiological criteria — ADI >8–10 mm, SAC <14 mm, or rapid progressive instability; (4) cranial settling with neurological compromise; surgical options — posterior C1-C2 fusion (Magerl or Brooks-Jenkins techniques); occiput-C2 fusion for cranial settling (occipitocervical fusion); the modern standard is C1-C2 transarticular screw fixation (Magerl) or C1 lateral mass + C2 pedicle screw construct (Harms technique) which provides excellent biomechanical stability
C1-C2 fusion techniques: Brooks-Jenkins — sublaminar wiring with iliac crest bone graft; simple but requires intact C1 and C2 laminae; not ideal for RA (laminae may be fragile); Magerl technique — bilateral transarticular screws through the C2 facets into the C1 lateral masses (biomechanically robust; preserves some rotation at C1-C2); Harms technique — C1 lateral mass screws + C2 pedicle screws connected by rods; the most modern and reliable technique; avoids sublaminar wiring; applicable even when C1 lamina is fragile; allows controlled intraoperative correction of residual subluxation

Exam Pearls

Atlanto-dens interval (ADI): normal <3 mm adults, <5 mm children; ADI >3 mm (adult) or >5 mm (child) = AAI; ADI >10 mm = complete TAL disruption; space available for cord (SAC) <13 mm = neurological risk
Steel`s rule of thirds: upper cervical canal = 1/3 dens + 1/3 cord + 1/3 spare; spare space = buffer against cord injury; when buffer lost → cord at risk with any further displacement
Down syndrome AAI: lax TAL; ADI >5 mm in ~10–20%; neurological symptoms rare (<3%); most are radiological only; symptomatic or ADI >10 mm → C1-C2 fusion; Special Olympics — flexion-extension X-rays required before contact sports
RA cervical involvement: anterior AAI (65–80%); cranial settling (dens migrates into foramen magnum — McGregor`s line); subaxial staircase subluxation; ALL RA patients pre-op must have cervical assessment before GA (intubation risk); modern biologics reducing incidence
McGregor`s line: tip of dens >4.5 mm above line (females) or >5.5 mm (males) = cranial settling (vertical translocation) in RA; requires occiput-C2 fusion
Odontoid fracture classification (Anderson-D`Alonzo): Type I (apical, stable, rare); Type II (dens-body junction, unstable, high non-union — especially elderly); Type III (C2 body, often heals conservatively); Type II in elderly → surgical fixation (C1-C2 fusion or odontoid screw) preferred over collar alone (high non-union rate)
Grisel`s syndrome: rotatory subluxation after URTI/pharyngitis in children; `cock-robin` deformity (head tilted + rotated, chin away from tilt side, normal sternomastoid length — unlike muscular torticollis); Fielding Types I–IV; early → halter traction; late/fixed → halo + fusion
Harms technique: C1 lateral mass + C2 pedicle screws + rods; modern gold standard for C1-C2 fusion; no sublaminar wiring; applicable in fragile RA laminae; most versatile and biomechanically reliable
Pre-op RA cervical assessment: flexion-extension X-rays + neurological exam; ADI >8 mm or SAC <13 mm → awake fibreoptic intubation; communicate cervical instability risk to anaesthetic team; ALL RA patients before GA regardless of symptoms

Atlantoaxial Instability — Pediatric & RA

References

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