C1-C2 Subluxation

Definition

Loss of the normal relationship between the atlas (C1) and axis (C2). Anterior subluxation of C1 on C2 is the commonest pattern, from incompetence of the transverse ligament or of the odontoid.

• Anterior – transverse ligament or odontoid failure
• Posterior – deficient or absent odontoid
• Lateral – asymmetrical lateral mass erosion
• Rotatory – Fielding and Hawkins types I–IV
• Vertical – cranial settling / basilar invagination

Anatomy

• Three synovial joints: median atlanto-dental and two lateral atlantoaxial. No disc.
• Provides ~50% of cervical rotation; 40–45° each side.
• Stability entirely ligamentous.
• Transverse ligament – primary restraint to anterior translation.
• Alar ligaments – restrain rotation, secondary restraint to translation.
• Tectorial membrane – resists vertical translation and extension.
• Steel’s rule of thirds at C1: 1/3 odontoid, 1/3 cord, 1/3 free space.
• Vertebral artery through C1 and C2 transverse foramina, looping over the C2 pars. High-riding or anomalous artery in ~20%; limits C2 screw placement.

Aetiology

• Inflammatory – rheumatoid arthritis (commonest), JIA, ankylosing spondylitis, psoriatic arthropathy
• Traumatic – transverse ligament rupture, odontoid fracture (type II), combined C1-C2 injury
• Congenital – os odontoideum, odontoid hypoplasia or aplasia
• Syndromic – Down syndrome (up to 15% radiographic instability), Morquio, skeletal dysplasias
• Infective – Grisel’s syndrome; retropharyngeal inflammation causing rotatory subluxation in a child
• Neoplastic – metastatic or primary destruction of C1 or C2

Epidemiology

• Cervical involvement in up to 86% of rheumatoid patients.
• Atlantoaxial subluxation is the earliest and commonest rheumatoid cervical deformity; can appear within 2 years of diagnosis.
• Prevalence falling with DMARD and biologic therapy.

Pathology

• Synovitis of the atlanto-dental and lateral atlantoaxial joints.
• Pannus erodes the transverse ligament and odontoid → anterior translation of C1.
• Retro-odontoid pannus contributes to cord compression independent of translation.
• Lateral mass destruction → odontoid migrates into the foramen magnum (cranial settling). Most dangerous pattern; compression at the cervicomedullary junction.
• Sequence in RA: atlantoaxial subluxation → cranial settling → subaxial subluxation.

Radiographic Measurements

• Anterior atlanto-dental interval (ADI) – abnormal >3 mm adult, >5 mm child. Correlates poorly with neurology.
• Posterior atlanto-dental interval (PADI), space available for cord – <14 mm associated with neurological compromise. Better predictor; common surgical threshold.
• Cervicomedullary angle – <135° suggests cord compromise.
• Cranial settling – Ranawat, Redlund-Johnell, McGregor line.

History

• Occipital and upper cervical pain; cervicogenic headache in greater occipital nerve distribution
• Sensation of the head falling forward on flexion; clunking
• Lhermitte’s phenomenon
• Myelopathy – clumsy hands, deteriorating handwriting, gait disturbance, sphincter change
• Cranial settling – dysphagia, dysarthria, vertigo, drop attacks
• Sudden death reported with acute cord or brainstem compression

Examination

• Upper motor neurone signs; Hoffmann, clonus, hyperreflexia
• Lower cranial nerves if cranial settling suspected
• No provocative flexion testing in known instability

Ranawat Classification of Neurological Status

Class	Description
I	No deficit
II	Subjective weakness, hyperreflexia, dysaesthesia
IIIA	Objective weakness and long tract signs, ambulatory
IIIB	Objective weakness and long tract signs, non-ambulatory

Original publication Ranawat CS, O’Leary P, Pellicci P, Tsairis P, Marchisello P, Dorr L. Cervical spine fusion in rheumatoid arthritis. J Bone Joint Surg Am. 1979;61(7):1003-10.

Investigations

• Lateral flexion / extension radiographs – demonstrates reducibility
• Open-mouth peg view
• CT – bony detail, odontoid morphology, C2 pedicle dimensions, vertebral artery course
• MRI – pannus, cord signal change, PADI, cervicomedullary angle
• Flexion MRI where safe – dynamic compression missed on neutral imaging

Treatment

Non-operative

• Control of underlying disease; analgesia
• Serial imaging for asymptomatic subluxation with preserved PADI
• Collar is symptomatic only; does not prevent progression

Indications for Surgery

• Neurological deficit or progressive myelopathy
• PADI <14 mm
• Cranial settling
• Intractable pain
• Progressive instability on serial films

Operative Options

• Posterior C1-C2 fusion for reducible subluxation without cranial settling
• Wiring (Gallie, Brooks-Jenkins) – requires halo supplementation; largely historical
• Magerl transarticular screws
• Harms–Goel C1 lateral mass and C2 pedicle screw-rod – current standard; rigid fixation, highest fusion rates
• Occipitocervical fusion for cranial settling, deficient C1 or associated occipito-atlantal instability
• Transoral odontoidectomy for irreducible ventral compression persisting after posterior reduction

Outcome

• Neurological recovery correlates with preoperative Ranawat class.
• Ranawat IIIB rarely regain ambulation.
• Assess vertebral artery on CT or CTA before C2 instrumentation.

References

1. Shlobin NA, Dahdaleh NS. Cervical spine manifestations of rheumatoid arthritis: a review. Neurosurg Rev 2021. PubMed 33037539
2. Advances in surgical treatment for atlantoaxial instability focusing on rheumatoid arthritis. Int J Rheum Dis 2023. PubMed 37565304
3. Radiological assessment and surgical management of cervical spine involvement in rheumatoid arthritis. Rheumatol Int 2023. PubMed 36378323
4. Os odontoideum: a comprehensive review. J Craniovertebr Junction Spine 2022. PubMed 36263339
5. Upper cervical trauma. Orthop Clin North Am 2021. PubMed 34538354
6. Surgery for chronic traumatic atlantoaxial dislocation associated with myelopathy. Clin Spine Surg 2017. PubMed 28525491

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.