ADI greater than 5mm | Down Syndrome Association | Transverse Ligament Key | Fusion if Symptomatic
INSTABILITY PATTERN
Critical Must-Knows
- ADI greater than 5mm in children indicates transverse ligament incompetence
- SAC less than 14mm indicates cord compression risk
- Down syndrome screening controversial but required before sports/surgery
- Neurological symptoms mandate surgical stabilization
- C1-C2 posterior fusion is gold standard treatment
Clinical Pearls
- "Transverse ligament is PRIMARY restraint to anterior translation of C1
- "Alar and apical ligaments are SECONDARY restraints
- "Down syndrome has ligamentous laxity + os odontoideum association
- "Neurological deterioration can be sudden and catastrophic
Critical Atlantoaxial Instability Exam Points
ADI Measurement
Atlantodental interval (ADI) measured from anterior arch of C1 to front of dens. Normal is less than 3mm adult, less than 5mm child. Greater than 5mm indicates transverse ligament failure and instability.
Space Available for Cord
SAC (posterior ADI) is distance from posterior dens to anterior C1 posterior arch. SAC less than 14mm indicates cord at risk. Steel's rule of thirds: 1/3 dens, 1/3 cord, 1/3 space.
Down Syndrome Association
10-30% of Down syndrome patients have atlantoaxial instability on imaging. Most are asymptomatic. Screen with lateral flexion-extension views before anaesthesia or contact sports participation.
Surgical Indications
Absolute indications: Neurological symptoms (myelopathy, weakness). Relative: ADI greater than 10mm, SAC less than 14mm, progressive instability. C1-C2 posterior fusion is standard treatment.
Quick Decision Guide - Atlantoaxial Instability
| ADI/SAC | Neurological Status | Management |
|---|---|---|
| ADI 3-5mm | Asymptomatic | Activity restriction, annual screening X-rays |
| ADI 5-10mm | Asymptomatic | Contact sport restriction, 6-monthly monitoring |
| ADI greater than 10mm | Asymptomatic | Consider prophylactic C1-C2 fusion |
| SAC less than 14mm | Any symptoms | Urgent MRI, surgical stabilization |
| Any ADI elevation | Myelopathy/weakness | Emergency C1-C2 posterior fusion |
AAI - AAAI - Anatomy at Risk
| A | Atlantodental interval Measure ADI on lateral X-ray |
| A | Alar and Apical ligaments Secondary stabilizers |
| I | Integrity of transverse ligament Primary stabilizer - prevents anterior translation |
| A | Atlantodental interval Measure ADI on lateral X-ray |
| A | Alar and Apical ligaments Secondary stabilizers |
| I | Integrity of transverse ligament Primary stabilizer - prevents anterior translation |
Hook:AAI reminds you of the key structures - the transverse ligament is the most important
DOWNS - RDOWNS - Risk Factors
| D | Down syndrome 10-30% have AAI on imaging |
| O | Os odontoideum Associated developmental anomaly |
| W | Weakness/myelopathy Indication for surgery |
| N | Neck pain with neurological signs Red flag symptoms |
| S | Space Available for Cord measurement SAC less than 14mm is critical |
| D | Down syndrome 10-30% have AAI on imaging | N | Neck pain with neurological signs Red flag symptoms |
| O | Os odontoideum Associated developmental anomaly | S | Space Available for Cord measurement SAC less than 14mm is critical |
| W | Weakness/myelopathy Indication for surgery |
Hook:DOWNS syndrome patients need careful cervical spine screening
5-14-3 R5-14-3 Rule
| 5 | ADI greater than 5mm Abnormal in children |
| 14 | SAC less than 14mm Cord at risk |
| 3 | ADI greater than 3mm Abnormal in adults |
| 5 | ADI greater than 5mm Abnormal in children |
| 14 | SAC less than 14mm Cord at risk |
| 3 | ADI greater than 3mm Abnormal in adults |
Hook:These numbers define when atlantoaxial instability becomes clinically significant
STEELSTEEL's Rule of Thirds
| 1/3 | Odontoid process Anterior third of canal |
| 1/3 | Spinal cord Middle third of canal |
| 1/3 | Space (safety zone) Posterior third - buffer zone |
| 1/3 | Odontoid process Anterior third of canal |
| 1/3 | Spinal cord Middle third of canal |
| 1/3 | Space (safety zone) Posterior third - buffer zone |
Hook:This explains why some subluxation is tolerated before cord compression
Overview and Epidemiology
Atlantoaxial instability (AAI) refers to excessive movement at the C1-C2 articulation, most commonly anterior subluxation of C1 on C2. This can lead to spinal cord compression and potentially catastrophic neurological injury.
Epidemiology:
- Down syndrome: radiographic AAI in roughly 6.8-27% (varies with age); fewer than 1-2% become symptomatic [FSEM 2020; Pueschel & Scola 1987]
- Rheumatoid arthritis: a leading adult cause; atlantoaxial subluxation prevalence relates to disease activity, and modern disease-modifying therapy has reduced its incidence [Veldman 2022]
- Os odontoideum: frequently associated with atlantoaxial instability
- Trauma: transverse ligament rupture, often with Jefferson (C1 burst) fractures
- Other conditions: Morquio syndrome, Grisel syndrome, Klippel-Feil
Causes by age group:
| Population | Common Causes |
|---|---|
| Pediatric | Down syndrome, os odontoideum, Morquio syndrome |
| Adult | Rheumatoid arthritis, trauma, ankylosing spondylitis |
| Any age | Infection (Grisel syndrome), tumor, congenital anomalies |
Down Syndrome Screening
Screening recommendations are controversial. The American Academy of Pediatrics no longer recommends routine screening X-rays. However, many institutions still require lateral flexion-extension X-rays before anaesthesia, contact sports, or surgery (especially intubation).
Anatomy and Pathophysiology
C1-C2 articulation: The atlantoaxial complex is unique in the cervical spine - it has:
- No intervertebral disc between C1 and C2
- Specialized ligamentous stability rather than bony congruence
- Allows 50% of cervical rotation (approximately 45 degrees each side)
Key stabilizing ligaments:
| Ligament | Function | Clinical Significance |
|---|---|---|
| Transverse ligament | PRIMARY restraint to C1 anterior translation | ADI greater than 5mm = failure |
| Alar ligaments | Limit rotation and lateral bending | Secondary restraint |
| Apical ligament | Connects dens tip to foramen magnum | Secondary restraint |
| Tectorial membrane | Continuation of PLL | Provides additional stability |
| Cruciform ligament | Transverse + vertical bands | Holds dens against C1 |
Transverse Ligament Critical
The transverse ligament is the PRIMARY restraint to anterior translation of C1 on C2. Rupture or laxity of this ligament (from trauma, RA, or congenital conditions) is the main cause of atlantoaxial instability and increases ADI.
Biomechanics of instability:
In flexion:
- Normal: Transverse ligament prevents anterior C1 translation
- AAI: C1 slides forward on C2, narrowing SAC
- Severe: Cord compression between posterior C1 arch and dens
In extension:
- May allow some reduction of subluxation
- Less dangerous position for cord
Steel's Rule of Thirds:
- Anterior 1/3: Odontoid process
- Middle 1/3: Spinal cord
- Posterior 1/3: Safety space (allows some subluxation before cord compression)
Catastrophic Risk
Sudden neurological deterioration can occur with AAI during intubation, trauma, or sports. This includes quadriplegia or sudden death. All patients with known AAI require careful cervical spine precautions during any procedure requiring neck manipulation.
Classification Systems
Fielding-Hawkins Classification - Rotatory Atlantoaxial Subluxation
| Type | Description | ADI | Treatment |
|---|---|---|---|
| I | Rotatory fixation, no anterior displacement | Normal | Collar, reduction |
| II | Rotatory fixation with anterior displacement 3-5mm | 3-5mm | Halter traction, surgery if fails |
| III | Rotatory fixation with anterior displacement greater than 5mm | Greater than 5mm | Traction, likely fusion |
| IV | Rotatory fixation with posterior displacement | Variable | Rare, usually traumatic |
Type II and III
Types II and III indicate transverse ligament incompetence. Type I may be treated conservatively with collar, but types II-IV often require surgical fusion if reduction cannot be maintained.
Clinical Presentation and Assessment
History:
- 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
Physical examination:
Clinical Findings
| Finding | Significance | Next Step |
|---|---|---|
| Torticollis (cock-robin position) | Rotatory subluxation | CT to confirm, attempt reduction |
| Upper motor neuron signs (hyperreflexia, Babinski) | Myelopathy | Urgent MRI, surgical consultation |
| Weakness in all four limbs | Cord compression | Emergency stabilization |
| Neck pain with limited rotation | Possible AAI | Flexion-extension X-rays |
| Gait disturbance, wide-based gait | Myelopathy | MRI, consider surgery |
| Normal neurological exam with radiographic AAI | Asymptomatic AAI | Activity modification, monitor |
Red flags requiring urgent workup:
- Progressive weakness or numbness
- Gait deterioration
- Bowel or bladder dysfunction
- Respiratory compromise (high cord compression)
- Worsening headache with neck movement
Myelopathy Signs
Look for myelopathy signs: hyperreflexia, clonus, positive Babinski, Hoffmann sign, gait disturbance, hand clumsiness. These indicate cord compression and mandate urgent imaging and surgical consideration.
Differential diagnosis:
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.
Differential Diagnosis of Suspected Atlantoaxial Instability
| Diagnosis | Discriminating Features | Key Investigation |
|---|---|---|
| Atlantoaxial rotatory subluxation (Grisel/Fielding-Hawkins) | Acute painful torticollis (cock-robin), often post-URTI or ENT surgery, chin rotated away from tilt | Dynamic CT showing fixed C1-C2 rotation |
| Os odontoideum | Mobile ossicle separate from C2 body, may be incidental or unstable on flexion-extension | Flexion-extension radiographs + CT (orthotopic vs dystopic) |
| Odontoid (type II) fracture | Trauma history, acute pain, cortical break of dens base | CT cervical spine |
| Muscular/idiopathic torticollis | Painless or mild, full passive correction, no neurology | Normal dynamic radiographs |
| Retropharyngeal abscess / discitis-osteomyelitis | Fever, raised inflammatory markers, prevertebral soft-tissue swelling | MRI with contrast, bloods |
| Posterior fossa / cervical cord tumour | Progressive myelopathy without instability, night pain | MRI brain and whole spine |
| Rheumatoid / inflammatory cervical disease | Adult, polyarthritis, pannus, may have basilar invagination | MRI (pannus, cord) + flexion-extension radiographs |
Investigations
Imaging protocol:
1. Plain radiographs:
- Lateral flexion-extension views - KEY for assessing instability
- Measure ADI in flexion (should be less than 3mm adult, less than 5mm child)
- Measure SAC (should be greater than 14mm)
- Open-mouth (odontoid) view - assess dens, lateral masses
- Standard AP and lateral views
2. CT scan:
- Best for bony anatomy
- Identifies os odontoideum, fractures, bony anomalies
- CT with 3D reconstruction for surgical planning
- Dynamic CT (flexion-extension) in select cases
3. MRI:
- Essential if neurological symptoms
- Shows cord compression, signal change (myelomalacia)
- Identifies transverse ligament integrity
- Shows pannus (in RA)
- Rule out other cord pathology
Imaging Findings
| Modality | Normal | Abnormal |
|---|---|---|
| ADI (lateral X-ray) | Less than 3mm adult, less than 5mm child | Greater than 3mm adult, greater than 5mm child |
| SAC (lateral X-ray) | Greater than 14mm | Less than 14mm (cord at risk) |
| Powers ratio | Less than 1.0 | Greater than 1.0 suggests anterior subluxation |
| MRI cord signal | Normal | T2 hyperintensity = myelomalacia |
Special measurements:
| Measurement | Calculation | Interpretation |
|---|---|---|
| ADI | Anterior C1 arch to anterior dens | Greater than 5mm child = unstable |
| SAC | Posterior dens to anterior C1 posterior arch | Less than 14mm = cord at risk |
| Powers ratio | BC/OA (basion to C1, opisthion to A arch) | Greater than 1.0 = anterior subluxation |
MRI Mandatory
MRI is mandatory before surgery and whenever there are neurological symptoms. It shows cord compression, myelomalacia, and transverse ligament status. T2 hyperintensity in the cord indicates established injury.
Management
Non-operative management:
Indicated for:
- Asymptomatic radiographic AAI with ADI less than 10mm
- SAC greater than 14mm
- No neurological symptoms
- Stable on dynamic imaging
Management includes:
- Activity modification (avoid contact sports, high-risk activities)
- Cervical collar for acute symptoms (soft or rigid)
- Regular clinical and radiographic surveillance (annual flexion-extension X-rays)
- Patient and family education about warning signs
Activity restrictions:
- No contact sports (football, rugby, wrestling)
- No diving
- No activities with high cervical hyperflexion risk
- Trampolines contraindicated
Monitoring Required
Conservative management requires regular follow-up. Instruct patients and families about neurological warning signs. Any new symptoms mandate immediate evaluation and imaging.
Surgical Technique
Posterior approach to C1-C2:
Patient positioning:
- Prone on Jackson table or Mayfield
- Head secured in Mayfield pins (or halo if preoperative traction)
- Neck in slight flexion for access, neutral for fusion
- Neuromonitoring: SSEPs and MEPs
Exposure:
- Midline incision from occiput to C3
- Subperiosteal dissection exposing C1 posterior arch and C2 lamina/spinous process
- Identify C1 lateral masses (limit lateral dissection to avoid vertebral artery)
- Identify C2 pars interarticularis and pedicles
- Expose surfaces for fusion (C1-C2 facet joints)
Vertebral Artery Risk
The vertebral artery runs in the foramen transversarium and is at risk during lateral C1 exposure. Stay within 15mm lateral to midline at C1. Use fluoroscopy to confirm screw trajectories.
Complications
Complications of C1-C2 Fusion
| Complication | Incidence | Prevention/Management |
|---|---|---|
| Vertebral artery injury | 0.2-4% | Careful screw trajectory, preoperative CT angiography if anomaly suspected |
| Nonunion/pseudarthrosis | 5-10% | Adequate decortication, bone graft, rigid fixation, postoperative immobilization |
| Wound infection | 2-5% | Prophylactic antibiotics, meticulous sterile technique |
| Screw malposition | Variable | Intraoperative imaging, navigation if available |
| Neurological deterioration | Rare if done properly | Careful reduction, neuromonitoring, avoid overdistraction |
| Hardware failure | 1-3% | Adequate screw purchase, proper rod contouring |
| Subaxial hypermobility | Long-term | May accelerate adjacent segment degeneration - rare issue in children |
| Occipital neuralgia | 5-10% | Avoid C2 nerve root injury, consider nerve sectioning if severe |
Vertebral artery injury:
- Most serious complication
- Can cause stroke, death
- Prevention: Preoperative CT/CTA if anatomy abnormal, careful screw placement
- Management: Control bleeding (bone wax, hemostatic agents), consider endovascular if ongoing
Nonunion:
- More common with wire techniques than screw fixation
- Check with CT at 3-6 months
- May require revision with more rigid fixation
C2 Nerve Root
The C2 nerve root exits beneath C1-C2 facet. It may be sacrificed for better visualization or screw placement. This causes occipital numbness (greater occipital nerve) which is usually well tolerated.
Postoperative Care and Rehabilitation
Immediate postoperative:
- ICU or close monitoring initially
- Wound check, drain management
- Neurological checks hourly then 4-hourly
- Hard collar or halo (depending on fixation stability)
- DVT prophylaxis
- Remove drain (typically day 1-2)
- Upright X-rays in collar
- Begin mobilization
- Soft diet initially (pharyngeal swelling possible)
- Discharge planning
- Wear collar full-time (rigid collar)
- Wound review at 2 weeks
- No lifting, bending, or neck rotation
- Light activities of daily living
- School/work return (sedentary) at 4-6 weeks
- Flexion-extension X-rays at 6-8 weeks
- CT for fusion assessment at 3 months
- Begin weaning collar if fusion progressing
- Gradual activity increase
- CT confirmation of solid fusion
- Discontinue collar when fused
- Return to most activities
- Contact sports typically not recommended long-term
Collar use:
- Rigid collar (Miami J or Philadelphia) for 6-12 weeks
- Halo vest if very unstable or poor bone quality
- Soft collar for comfort transition
Activity restrictions:
- No contact sports permanently (fused motion segment)
- No high-risk activities for 6-12 months
- Full activities (non-contact) when fusion confirmed
Understanding these activity limitations is important for patient counseling.
Outcomes and Prognosis
Fusion rates:
- Screw fixation (Harms, Magerl): 95-100%
- Wire techniques (Gallie, Brooks): 85-90%
Neurological outcomes:
- Patients with myelopathy: 60-80% improve, 10-20% stable, 5-10% deteriorate
- Prophylactic surgery in asymptomatic: Prevents deterioration in vast majority
- Established myelomalacia on MRI: Less likely to fully recover
Functional outcomes:
| Preoperative Status | Expected Outcome |
|---|---|
| Asymptomatic AAI (prophylactic surgery) | Excellent, maintain function |
| Mild symptoms (neck pain, minimal neuro) | Good, most improve |
| Myelopathy (ambulatory) | Fair to good, majority improve |
| Severe myelopathy (non-ambulatory) | Guarded, stabilize but limited recovery |
Loss of motion:
- C1-C2 provides 50% of cervical rotation
- Fusion results in 50% rotation loss
- Well compensated in most patients
- May cause some adjacent segment stress long-term
Early Surgery Better
Earlier surgery = better neurological outcomes. Patients with mild myelopathy have high likelihood of improvement. Severe, established cord damage (myelomalacia on MRI) has limited recovery potential - emphasizes need for timely intervention.
Evidence Base
- Described individual polyaxial screw fixation of the C1 lateral mass and C2 pedicle connected by rods in 37 patients. The construct permits fluoroscopically controlled reduction after screw insertion and avoids the fixed-alignment and structural-graft requirements of transarticular screws. Solid fusion was reported in all patients with no neural or vascular injury.
- Examined 404 individuals with Down syndrome. Atlantoaxial instability was present in 59 (14.6%): 53 (13.1%) asymptomatic and 6 (1.5%) symptomatic requiring surgery. Atlanto-dens interval measurements were significantly greater in flexion than in neutral or extension, confirming the need for dynamic views.
- In ten cadaveric specimens with sectioned alar, transverse and capsular ligaments, transarticular (Magerl) screw fixation, Brooks and Halifax constructs all controlled motion significantly better than the Gallie wire construct in flexion-extension, axial rotation and lateral bending. The Magerl transarticular screw tended to allow the least rotation.
- Pooled 137 Down syndrome patients across 51 studies. Screw-and-rod constructs achieved significantly greater bony union and lower rates of revision, loss of reduction or pseudarthrosis, halo use and early neurological decline than wiring alone (all p less than 0.05). Wire-and-rod constructs also fused better than wiring alone.
- Across 8 studies (393 patients), bony fusion after C1-C2 screw fixation ranged 96.7-100%. Sacrificing the C2 nerve root reduced blood loss (mean -195 mL) and operative time (mean -57 min) and increased occipital numbness, but did not change the rate of occipital neuralgia.
- The 2011 AAP guideline withdrew routine asymptomatic cervical-spine radiographic screening for all children with Down syndrome, citing poor predictive value, and moved to symptom-based assessment for myelopathy, with imaging reserved for symptomatic children or before high-risk procedures.
- The UK Faculty of Sport and Exercise Medicine position statement reports AAI in 6.8-27% of the Down syndrome population (varying with age), with less than 1-2% later developing symptomatic AAI. It notes catastrophic sports-related cord injury is extremely rare and promotes safe physical activity rather than blanket exclusion.
Clinical Decision Scenarios
Use these scenarios to practise clinical reasoning and management decisions
Scenario 1: Down Syndrome Pre-Anaesthesia
"A 6-year-old boy with Down syndrome requires general anaesthesia for dental extractions. The anaesthetist requests cervical spine clearance. How would you assess this patient?"
Scenario 2: Symptomatic AAI Requiring Fusion
"A 12-year-old with Down syndrome presents with progressive gait deterioration and hyperreflexia. X-rays show ADI of 12mm and SAC of 10mm. MRI shows cord compression with T2 hyperintensity. How would you manage this patient?"
Scenario 3: Os Odontoideum with Instability
"A 14-year-old presents with neck pain after a minor fall. CT shows an os odontoideum with ADI of 7mm in flexion. She is neurologically intact. What is your management plan?"
MCQ Practice Points
High-yield MCQ facts:
-
Normal ADI: Less than 3mm adults, less than 5mm children
-
SAC less than 14mm indicates cord at risk - surgical consideration
-
Transverse ligament is the PRIMARY restraint to anterior C1 translation
-
Down syndrome has 10-30% radiographic AAI but only 1-2% symptomatic
-
Steel's Rule of Thirds: 1/3 dens, 1/3 cord, 1/3 safety space
-
Harms technique (C1 lateral mass + C2 pedicle screws) is current gold standard for C1-C2 fusion
-
Os odontoideum - 60% have associated AAI, treat if unstable
-
Myelomalacia on MRI (T2 hyperintensity) indicates established cord injury - limited recovery potential
-
Vertebral artery at risk during C1 lateral mass screw placement - stay within 15mm of midline
-
Fusion rate with screws is 95-100% vs 85-90% with wires
MCQ Q1
Q: What is the normal ADI in children? A: Less than 5mm (vs less than 3mm in adults). The difference is due to greater ligamentous laxity in children.
MCQ Q2
Q: What SAC measurement indicates cord at risk? A: SAC less than 14mm indicates the spinal cord is at risk of compression. SAC less than 10mm is critical.
MCQ Q3
Q: What percentage of Down syndrome patients have symptomatic AAI? A: Only 1-2% have symptomatic AAI despite 10-30% having radiographic instability. This is why routine screening is no longer recommended.
MCQ Q4
Q: What is the primary ligamentous restraint to anterior C1 translation? A: The transverse ligament is the PRIMARY restraint. The alar and apical ligaments are secondary restraints.
MCQ Q5
Q: What percentage of cervical rotation is lost after C1-C2 fusion? A: Approximately 50% of cervical rotation occurs at C1-C2. This loss is generally well tolerated functionally.
MCQ Common Trap
Q: Is routine screening X-ray recommended in Down syndrome? A: NO. The AAP 2022 guidelines no longer recommend routine screening X-rays - it is now symptom-based. However, many institutions still require pre-surgical screening. Know both guidelines and common practice.
Guidelines, Registries & Global Practice
OrthoVellum is a worldwide resource. The principles below are framed for any board worldwide, with regional guidance cited as evidence rather than as the frame.
Global epidemiology:
| Population | Burden of AAI | Source |
|---|---|---|
| Down syndrome | Radiographic AAI ~6.8-27% (age-dependent); symptomatic in under 1-2% | FSEM 2020; Pueschel & Scola 1987 |
| Rheumatoid arthritis | Atlantoaxial subluxation prevalence tracks disease activity; falling with modern DMARD/biologic therapy | Veldman 2022 |
| Os odontoideum | Commonly associated with instability; may be congenital or post-traumatic | Grob 1992 (biomechanics of fixation) |
| Down syndrome surgical cohort | Pooled 137 operated patients across 51 studies | Hofler 2019 |
Major guidance, side by side:
| Body / region | Recommendation | Evidence level |
|---|---|---|
| AAP (US, Bull 2011) | No routine asymptomatic radiographic screening in Down syndrome; symptom-based myelopathy surveillance; image symptomatic children or before high-risk procedures | Practice guideline (Level V) |
| FSEM (UK, 2020) | Individualised clinical assessment for sport rather than blanket radiographic exclusion; catastrophic sports injury extremely rare | Position statement (Level V) |
| AO Spine / Magerl-Harms lineage | Rigid screw-rod (Goel-Harms) or transarticular (Magerl) fixation preferred over wiring for unstable C1-C2 | Level III-IV technique evidence |
| General surgical consensus | Operate for myelopathy, progressive instability, ADI greater than 10mm or SAC less than 14mm with concern; reduce before fusing | Level III-IV |
Registry / pooled-evidence note: No dedicated joint registry exists for upper-cervical fusion (unlike hip/knee arthroplasty), so the best comparative evidence is pooled observational data. The Hofler 2019 meta-analysis [PMID 30790735] shows screw-rod constructs outperform wiring for union and complications in Down syndrome, and Badhiwala 2017 [PMID 28098742] reports 96.7-100% fusion with C1-C2 screw constructs.
Global practice variation:
- High-resource settings: navigation/robotics, CT angiography for vertebral artery mapping, and intra-operative neuromonitoring (SSEP/MEP) are routine for screw-rod constructs.
- Limited-resource settings: wiring techniques (Gallie/Brooks) supplemented by halo or prolonged rigid immobilisation remain in use where image guidance and polyaxial implants are unavailable, accepting higher non-union rates.
- Paediatric pathways: managed in tertiary paediatric spine units with transition to adult services at skeletal maturity and lifelong post-fusion activity counselling, regardless of health system.
Atlantoaxial Instability
Clinical summary
Key Numbers
- •ADI greater than 3mm adult, greater than 5mm child = abnormal
- •SAC less than 14mm = cord at risk, less than 10mm = critical
- •Down syndrome: 10-30% radiographic AAI, 1-2% symptomatic
- •C1-C2 provides 50% cervical rotation
- •Fusion rate 95-100% with screws vs 85-90% with wires
- •Vertebral artery safety zone: within 15mm of midline
Critical Anatomy
- •Transverse ligament = PRIMARY restraint to anterior C1 translation
- •Alar and apical ligaments = secondary restraints
- •Steel's Rule of Thirds: 1/3 dens, 1/3 cord, 1/3 safety space
- •Vertebral artery - stay within 15mm of midline at C1
- •C2 pedicle trajectory: medial 20-25 degrees, cephalad 20 degrees
Surgical Indications
- •Neurological symptoms (myelopathy)
- •ADI greater than 10mm (even if asymptomatic)
- •SAC less than 14mm with concern
- •Os odontoideum with instability
- •Progressive instability on serial imaging
Surgical Technique
- •Harms technique = gold standard (C1 lateral mass + C2 pedicle screws)
- •Achieve reduction BEFORE fusion
- •Neuromonitoring essential (SSEPs, MEPs)
- •Autograft bone for pediatric patients
Complications
- •Vertebral artery injury (0.2-4%) - most serious, CT angiography for planning
- •Nonunion (5-10%) - more common with wires than screws
- •C2 nerve root sacrifice - causes occipital numbness, usually tolerable
- •Hardware failure - more common with poor bone quality
- •Adjacent segment degeneration - long-term consideration
Viva Essentials
- •Down syndrome screening: now symptom-based (AAP 2022)
- •MRI mandatory before surgery and if symptoms present
- •Early surgery = better neurological outcomes in symptomatic patients
- •T2 hyperintensity = established myelomalacia = guarded prognosis
- •Os odontoideum: 60% have AAI, treat if unstable (ADI greater than 5mm)
- •Preoperative halo traction for significant subluxation