High Yield Overview

Cervical Instability

Craniocervical and Subaxial Instability

Most common causeTrauma

25-80% have AAIRA

15-20% AAIDown syndrome

70% fatalOC-C1 dislocation

Types of Cervical Instability

OC-C1

PatternOccipitoatlantal dislocation - often fatal

TreatmentOC-C2 fusion

C1-C2 (AAI)

PatternAtlantoaxial instability

TreatmentC1-C2 fusion (Harms)

Subaxial

PatternC3-C7 instability

TreatmentACDF or posterior fusion

Critical Must-Knows

ADI (Atlantodental Interval): Normal is less than 3mm adults, less than 5mm children.
PADI (Posterior ADI): Less than 14mm indicates cord compromise risk.
Powers Ratio: Ratio greater than 1 indicates anterior OC-C1 dislocation.
White-Panjabi Criteria: Score of 5 or greater indicates subaxial instability.
RA patients: Must screen for AAI before anesthesia/intubation.

Examiner's Pearls

"
ADI greater than 3mm in adults is abnormal
"
PADI less than 14mm predicts poor neurological outcome
"
Powers ratio: BC/OA (greater than 1 = anterior dislocation)
"
Always screen RA and Down syndrome patients
"
Harms technique: C1 lateral mass + C2 pedicle screws

Clinical Imaging

Imaging Gallery

Assessing cervical instability. Cervical flexion-extension lateral radiographs in (a) can be difficult to interpret. The pre-dental space is obscured by delayed ossification of the dens and superimpos — Assessing cervical instability. Cervical flexion-extension lateral radiographs in (a) can be difficult to interpret. The pre-dental space is obscured Credit: Solanki GA et al. via J. Inherit. Metab. Dis. via Open-i (NIH) (Open Access (CC BY))

Postoperative lateral cervical spine x-rays showed absence of the osteophytes and normalization of the normal structure.Credit: Constantoyannis C et al. via Cases J via Open-i (NIH) (Open Access (CC BY))

Critical Measurements

ADI Thresholds

Adult: less than 3mm is Normal. Child: less than 5mm is Normal. Reflects transverse ligament integrity.

Cord Risk (PADI)

less than 14mm: Critical threshold for cord compromise. PADI is more prognostic than ADI.

OC-C1 Dislocation

Powers Ratio > 1: Anterior dislocation. BDI > 12mm: Abnormal Basion-Dens Interval.

Subaxial Instability

Translation: greater than 3.5mm. Angulation: greater than 11 degrees. (White-Panjabi Criteria)

Craniocervical

Primary Stabilizer

RA AAI Prevalence

Down Syndrome AAI

At a Glance

Cervical instability refers to abnormal motion between vertebrae that may cause neurological compromise, occurring at the craniocervical junction (OC-C1-C2) or subaxially (C3-C7). Key measurements include the ADI (less than 3mm adults, less than 5mm children) reflecting transverse ligament integrity, PADI less than 14mm indicating cord compromise risk, and Powers ratio greater than 1 for anterior OC-C1 dislocation. Rheumatoid arthritis patients have 25-80% prevalence of atlantoaxial instability and require preoperative screening before anesthesia. White-Panjabi criteria (score ≥5) diagnose subaxial instability. Surgical stabilization (e.g., Harms technique C1 lateral mass + C2 pedicle screws) prevents neurological deterioration.

Mnemonic

ADI Values - AC/DC

Adult

Less than 3mm is normal

Child

Less than 5mm is normal

Danger

PADI less than 14mm = cord at risk

Cord

Measure space available for cord

Memory Hook:AC/DC - Adult 3, Child 5, Danger at 14 (Cord)

Mnemonic

White-Panjabi Criteria - PADS

Points

Sum clinical + radiographic criteria

Angulation

Greater than 11 degrees = 2 points

Displacement

Greater than 3.5mm translation = 2 points

Score 5

Score of 5 or greater = unstable

Memory Hook:PADS - 5 Points And you have instability (greater than 5 = unstable)

Mnemonic

OC-C1 Measurements - POB

Powers Ratio

BC/OA greater than 1 = anterior dislocation

Occipital Condyle-C1

Distance greater than 2mm = abnormal

Basion-Dens

BDI greater than 12mm = dissociation

Memory Hook:POB - Powers, Occipital, Basion: Key craniocervical measurements

Overview/Epidemiology

Cervical instability is defined as abnormal motion of the cervical spine that results in neurological compromise, incapacitating pain, or structural deformity under physiological loads.

Epidemiology:

Trauma: Most common cause overall - MVA, falls, sports
Rheumatoid arthritis: 25-80% develop AAI over disease course
Down syndrome: 15-20% have AAI on screening radiographs
Congenital: Os odontoideum, basilar invagination, occipitalization of C1
Tumors/Infections: Destabilize through bone destruction

Clinical Significance:

OC-C1 dislocation has 70% pre-hospital mortality
Untreated AAI can progress to myelopathy and quadriplegia
Early recognition and stabilization prevents neurological decline

Anatomy

Upper Cervical Anatomy (OC-C1-C2):

Bony Structures:

Occiput: Occipital condyles articulate with C1 lateral masses
Atlas (C1): Ring-shaped, no vertebral body, lateral mass articulations
Axis (C2): Odontoid process (dens) is the pivot for rotation
C1-C2 articulation: Allows 50% of cervical rotation

Ligamentous Stabilizers:

Transverse ligament: Primary restraint to anterior C1 translation
Alar ligaments: Limit rotation and lateral bending
Apical ligament: Connects dens tip to basion
Tectorial membrane: Extension of PLL, covers dens posteriorly
Cruciate ligament: Transverse + vertical bands

Subaxial Anatomy (C3-C7):

Stabilizing Structures:

Anterior: ALL, disc, vertebral body
Middle: PLL, posterior disc, posterior body
Posterior: Facet capsules, ligamentum flavum, interspinous ligaments

Denis Three-Column Model applies but White-Panjabi criteria more specific for cervical.

Pathophysiology

Mechanisms of Instability:

Traumatic Instability

Mechanism:

High-energy trauma causes ligamentous disruption or fracture
Flexion-distraction injuries disrupt posterior ligaments
Extension injuries disrupt anterior structures
Rotation and lateral bending cause facet injuries

Patterns:

OC-C1 dislocation: Usually fatal; survivors have tectorial membrane disruption
Odontoid fractures: Type II most common, highest nonunion rate
Hangman's fracture: Bilateral C2 pars fracture
Facet dislocations: Unilateral (25% subluxation) or bilateral (50% subluxation)

All traumatic cervical instability requires urgent assessment and immobilization.

Neurological Compromise Mechanism:

The spinal cord at C1-C2 level is approximately 10mm in diameter. The canal normally provides approximately 20mm of space.

Steel's Rule of Thirds at C1:

1/3 dens
1/3 cord
1/3 space (buffer)

When PADI (space available for cord) drops below 14mm, cord compression becomes likely. Myelopathy results from:

Direct mechanical compression
Vascular compromise (anterior spinal artery)
Dynamic cord impingement with flexion/extension

Classification

Upper Cervical Instability (OC-C1-C2)

Occipitoatlantal (OC-C1) Dislocation:

Type I: Anterior dislocation (most common in survivors)
Type II: Longitudinal distraction
Type III: Posterior dislocation

Atlantoaxial Instability (C1-C2):

Anterior subluxation: ADI greater than 3mm (most common)
Posterior subluxation: Rare, associated with dens fracture
Rotatory subluxation: Fielding-Hawkins Types I-IV
Lateral translation: Associated with Jefferson fracture

Fielding-Hawkins Rotatory Subluxation:

Type I: Rotatory fixation without anterior shift
Type II: Rotatory fixation with 3-5mm anterior shift
Type III: Rotatory fixation with greater than 5mm shift
Type IV: Rotatory fixation with posterior shift

Upper cervical instability patterns determine surgical approach and fusion levels.

Atlantoaxial rotatory subluxation CT imaging — CT imaging of atlantoaxial rotatory subluxation (Fielding type 1). Panel a: 3D CT reconstruction showing C1-C2 relationship. Panel b: Axial CT demonstrating unilateral facet subluxation with arrow indicating anterior displacement of C1 lateral mass relative to C2.Credit: Landi et al., Child's Nervous System (2020)

Clinical Presentation

History:

Mechanism of injury (trauma)
Neck pain, especially with motion
Myelopathic symptoms: weakness, numbness, gait disturbance
Electric shock sensation with flexion (Lhermitte's sign)
History of RA, Down syndrome, or connective tissue disorder

Physical Examination:

Neurological Assessment

Upper Motor Neuron Signs (Myelopathy):

Hyperreflexia
Hoffmann sign positive
Ankle clonus
Babinski sign positive
Spastic gait
Inverted radial reflex

Motor Examination:

Test all myotomes C5-T1
Assess grip strength
Intrinsic hand muscle wasting

Sensory:

Dermatomal assessment
Proprioception (posterior columns)
Cape-like sensory loss (central cord)

Gait:

Spastic, wide-based
Difficulty with tandem walking

A thorough neurological assessment is essential before any manipulation or reduction.

Investigations

Imaging Protocol:

Plain Radiographs

Standard Views:

AP, lateral, open mouth odontoid
Flexion-extension laterals (supervised, if neurologically intact)

Key Measurements:

Measurement	Normal	Abnormal
ADI (adult)	Less than 3mm	Greater than 3mm
ADI (child)	Less than 5mm	Greater than 5mm
PADI	Greater than 14mm	Less than 14mm
Powers Ratio	Less than 1	Greater than 1
BDI	Less than 12mm	Greater than 12mm
BAI	Less than 12mm	Greater than 12mm

Powers Ratio (BC/OA):

BC = basion to posterior C1 arch
OA = opisthion to anterior C1 arch
Greater than 1 = anterior OC-C1 dislocation
Less than 0.7 = posterior dislocation

Flexion-extension views are essential but should only be done with supervision if patient is neurologically intact.

C1-C2 lateral translation on X-ray — Open-mouth radiograph demonstrating pathological lateral translation at C1-C2. Arrows indicate asymmetric positioning of C1 lateral masses relative to C2, suggesting atlantoaxial instability.Credit: Landi et al., Child's Nervous System (2020)

Management

Pre-operative Planning:

RA patients need pre-op rheumatology optimization
Stop biologics/DMARDs as directed
Airway plan with anesthesia (awake fiberoptic if severe instability)
Prepare for prone vs lateral positioning
Have halo available if needed for intra-op traction

Non-operative Management

Indications:

Stable injuries (White-Panjabi less than 5)
Asymptomatic RA with minimal ADI increase
Asymptomatic Down syndrome with AAI on screening
Patient unfit for surgery

Treatment:

Rigid cervical collar (Miami J, Aspen)
Activity modification
Serial imaging to monitor progression
Neurological monitoring

Duration:

6-12 weeks for stable injuries
Indefinite monitoring for RA/Down syndrome

Limitations:

True instability rarely heals without surgery
High failure rate for type II odontoid in elderly

Conservative management is rarely definitive for true cervical instability.

Complications

Perioperative Complications

Neurological:

Cord injury (rare, less than 1%)
Root injury (C2 - occipital numbness)
Worsening myelopathy

Vascular:

Vertebral artery injury (2-4% with C1-C2 screws)
May be asymptomatic if contralateral dominant
Cerebellar stroke if bilateral injury

Airway:

Post-op swelling and airway compromise
Delayed extubation often needed
Consider ICU monitoring

Positioning:

Pressure sores (prone positioning)
Brachial plexus injury
Peripheral nerve injury

Careful attention to positioning and monitoring minimizes perioperative complications.

Complication Prevention:

Pre-operative CT angiography for vertebral artery dominance
Neuromonitoring (SSEPs, MEPs) intraoperatively
Meticulous technique with anatomical landmarks
Appropriate collar immobilization post-op

Evidence Base

Landmark

📚 White and Panjabi Clinical Biomechanics

Key Findings:

Established subaxial instability criteria
Combined clinical and radiographic checklist
Score of 5 or greater indicates clinical instability

Clinical Implication: White-Panjabi criteria remain the gold standard for assessing subaxial cervical instability.

Source: Spine 1990

Level IV

📚 Harms and Melcher

Key Findings:

C1 lateral mass + C2 pedicle screw technique
Fusion rate greater than 95%
Safer than transarticular screws for vertebral artery

Clinical Implication: Harms technique is now the standard for atlantoaxial fusion due to safety and high fusion rates.

Source: Spine 2001

Level IV

📚 Casey ATH et al - PADI Study

Key Findings:

PADI less than 14mm predicts poor neurological outcome in RA
PADI more prognostically useful than ADI alone
Early surgery before neurological decline improves outcomes

Clinical Implication: Use PADI (not just ADI) to guide timing of surgery in RA patients - operate before PADI drops below 14mm.

Source: J Neurosurg Spine 1996

Level III

📚 Pueschel SM and Scola FH

Key Findings:

AAI prevalence 15-20% in Down syndrome on screening X-rays
Majority are asymptomatic
Recommend screening before high-risk sports

Clinical Implication: Screen Down syndrome patients for AAI, especially before contact sports or anesthesia.

Source: Arch Pediatr Adolesc Med 1987

Level IV

📚 Goel and Harms Technique Evolution

Key Findings:

Goel-Harms technique widely adopted globally
Fusion rates exceed 95%
Lower vertebral artery injury than transarticular approach

Clinical Implication: Modern C1-C2 screw fixation provides reliable fusion with acceptable complication rates.

Source: J Craniovertebr Junction Spine 2015

Level IV

📚 Subaxial Cervical Spine Injury Classification (SLIC)

Key Findings:

Morphology, DLC, neurological status scoring
Score greater than 4 suggests surgical treatment
Score less than 4 suggests non-operative treatment

Clinical Implication: SLIC provides structured approach to subaxial cervical trauma management decisions.

Source: Spine 2007 - Vaccaro et al

Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

RA Patient with Neck Pain and Myelopathy

EXAMINER

"55-year-old female with 20-year history of rheumatoid arthritis presents with progressive neck pain, electric shock sensation down her spine when flexing her neck, and difficulty with fine motor tasks in her hands. She is on methotrexate and a TNF-inhibitor. How would you assess and manage this patient?"

EXCEPTIONAL ANSWER

This patient likely has **atlantoaxial instability (AAI)** from rheumatoid arthritis, given her Lhermitte's sign and hand dysfunction suggesting myelopathy.

Assessment:

Full neurological exam - document myelopathic signs (hyperreflexia, Hoffmann's, gait)
Lateral cervical flexion-extension X-rays - measure ADI (abnormal if greater than 3mm) and PADI (critical if less than 14mm)
CT cervical spine - bony detail, rule out vertical settling
MRI cervical spine - assess pannus, cord compression, cord signal change

Management:

If PADI less than 14mm or symptomatic myelopathy, surgical stabilization is indicated
Pre-op: Rheumatology consult to optimize disease control, stop biologics 2-4 weeks pre-op
Anesthesia consult - plan for awake fiberoptic intubation given cervical instability
Surgery: Posterior C1-C2 fusion using Harms technique (C1 lateral mass screws + C2 pedicle screws)

KEY POINTS TO SCORE

AAI is common in long-standing RA (25-80%)

PADI less than 14mm is the critical threshold - indicates cord at risk

Awake fiberoptic intubation for severe instability

Harms technique: C1 lateral mass + C2 pedicle screws

Stop biologics pre-operatively to reduce infection risk

COMMON TRAPS

✗Forgetting to measure PADI (more important than ADI for prognosis)

✗Not coordinating with rheumatology regarding DMARDs

✗Missing subaxial disease (staircase subluxation)

✗Underestimating anesthetic risk - must plan intubation carefully

LIKELY FOLLOW-UPS

"What is the normal ADI in adults?"

"What are the three patterns of RA cervical instability?"

"How would you position this patient for surgery?"

VIVA SCENARIOStandard

Down Syndrome Pre-Sport Screening

EXAMINER

"Parents of a 12-year-old boy with Down syndrome bring him for clearance to participate in school gymnastics. They have heard about neck problems in Down syndrome. How do you approach this?"

EXCEPTIONAL ANSWER

This is a common clinical scenario. Children with Down syndrome have a 15-20% prevalence of atlantoaxial instability due to ligamentous laxity and dens hypoplasia.

Assessment:

History: Any neck pain, torticollis, neurological symptoms (weakness, gait changes)
Examination: Full neurological exam, check for myelopathic signs
Imaging: Lateral cervical X-ray with flexion-extension views - measure ADI
Normal ADI in children: less than 5mm

Management:

If asymptomatic and ADI less than 5mm: Can participate in sports with activity modification
Avoid high-risk activities: diving, gymnastics floor exercises, contact sports
If ADI greater than 5mm but asymptomatic: Restrict from high-risk activities, annual monitoring
If symptomatic or ADI greater than 10mm: Surgical stabilization indicated

KEY POINTS TO SCORE

15-20% of Down syndrome patients have AAI on screening

Most are asymptomatic and do not require surgery

ADI threshold in children: less than 5mm is normal

Restrict from contact sports and gymnastics if ADI borderline

Surgery only for symptomatic patients or gross instability

COMMON TRAPS

✗Recommending surgery for all patients with radiographic AAI

✗Using adult ADI cutoff (3mm) for children

✗Missing associated atlantooccipital instability

✗Ignoring cardiac comorbidities that affect surgical risk

LIKELY FOLLOW-UPS

"What causes AAI in Down syndrome?"

"When would you fuse this patient?"

"What other pre-operative considerations in Down syndrome?"

VIVA SCENARIOStandard

Trauma - Suspected OC-C1 Dislocation

EXAMINER

"24-year-old restrained driver involved in high-speed head-on MVA. GCS 15, complaining of severe neck pain. Initial trauma CT shows 'possible craniocervical dissociation'. How do you assess and manage?"

EXCEPTIONAL ANSWER

Occipitoatlantal (OC-C1) dislocation is often fatal at scene. Survivors need urgent stabilization to prevent catastrophic neurological deterioration.

Immediate Management:

Maintain cervical immobilization - rigid collar, sandbags
Do NOT apply traction - can cause distraction injury
Full ATLS assessment for associated injuries

Imaging Review - Key Measurements:

Powers Ratio (BC/OA): greater than 1 = anterior OC-C1 dislocation
Basion-Dens Interval (BDI): greater than 12mm = abnormal
Basion-Axis Interval (BAI): greater than 12mm = abnormal
Condyle-C1 Interval (CCI): greater than 2mm = abnormal

Management:

MRI: Assess ligamentous injury, cord status
If craniocervical dissociation confirmed: Surgical stabilization indicated
Technique: Occiput to C2 posterior fusion (may extend to C3 if needed)
Consider halo for temporary immobilization while optimizing patient

KEY POINTS TO SCORE

70% of OC-C1 dislocations are fatal pre-hospital

Do NOT apply traction - can worsen distraction injury

Powers Ratio greater than 1 indicates anterior dislocation

BDI and BAI greater than 12mm indicate dissociation

Treatment is occiput to C2 (or C3) posterior fusion

COMMON TRAPS

✗Applying cervical traction (contraindicated!)

✗Removing collar before adequate imaging

✗Missing associated injuries (vertebral artery, C-spine fractures)

✗Not checking for survival with intact neurology (good prognostic sign)

LIKELY FOLLOW-UPS

"How do you calculate Powers Ratio?"

"What fusion levels would you include?"

"How would you assess vertebral artery injury?"

VIVA SCENARIOStandard

Subaxial Instability - Facet Dislocation

EXAMINER

"32-year-old male dove into shallow water and now has severe neck pain and bilateral arm weakness. X-ray shows C6 anterolisthesis on C7. How do you manage this patient?"

EXCEPTIONAL ANSWER

This patient has a likely bilateral facet dislocation at C6-C7 with incomplete spinal cord injury (arm weakness suggests central cord component).

Initial Management:

Immobilize in rigid collar
Complete neurological assessment and document (ASIA score)
CT cervical spine - confirm facet dislocation, assess for fractures
MRI - assess cord compression, disc herniation, cord signal change

Key Decision - MRI First:

Obtain MRI before reduction to rule out disc herniation
If large disc herniation, may need anterior discectomy before posterior reduction
If no significant disc, can proceed with closed or open reduction

Treatment:

Closed reduction with Gardner-Wells tongs and sequential weights (if awake, cooperative)
If closed reduction fails or disc herniation: Anterior discectomy and reduction, then anterior fusion
Alternatively: Posterior open reduction and lateral mass screw fixation
Consider combined approach for 3-column instability

KEY POINTS TO SCORE

Bilateral facet dislocation causes approximately 50% anterolisthesis

MRI before reduction to assess for disc herniation

Closed reduction possible if awake and cooperative

Anterior approach for disc herniation, posterior for facet reduction

Document neurological status before and after reduction

COMMON TRAPS

✗Attempting closed reduction without MRI (risk of cord injury from disc)

✗Missing associated vertebral artery injury

✗Incomplete neurological documentation

✗Forgetting to check for associated injuries (head, chest)

LIKELY FOLLOW-UPS

"What is the White-Panjabi score for this injury?"

"How much weight would you use for closed reduction?"

"What is your preferred approach - anterior or posterior?"

MCQ Practice Points

Exam Pearl

Q: What is the normal atlantodental interval (ADI) in adults? A: Less than 3mm. ADI 3-5mm indicates transverse ligament incompetence; greater than 5mm indicates both transverse and alar ligament failure. In children, normal ADI can be up to 5mm due to ligamentous laxity.

Exam Pearl

Q: What PADI measurement indicates the spinal cord is at risk? A: Posterior Atlantodental Interval (PADI) less than 14mm predicts neurological deficit. PADI represents the space available for the cord (SAC) at C1-2 and is more predictive of myelopathy than ADI.

Exam Pearl

Q: How is the Powers Ratio calculated and what does a ratio greater than 1 indicate? A: Powers Ratio = BC/OA where B = basion, C = posterior arch C1, O = opisthion, A = anterior arch C1. Ratio greater than 1 indicates anterior occipito-atlantal dislocation. Normal ratio is 0.77 (range 0.55-1.0).

Exam Pearl

Q: Which patient populations require routine screening for atlantoaxial instability? A: Down syndrome (trisomy 21) and rheumatoid arthritis patients require screening. Down syndrome patients have 15-20% incidence of atlantoaxial instability due to ligamentous laxity. RA patients develop pannus erosion of the transverse ligament.

Australian Context

Epidemiology: Cervical instability in Australia is most commonly caused by trauma (motor vehicle accidents, falls) and rheumatoid arthritis. Indigenous Australians have higher rates of rheumatoid-associated cervical instability.

Trauma Management: Major trauma centres across Australia follow established protocols for cervical spine clearance. Tertiary spinal units manage complex cervical instability requiring surgical stabilisation.

Rheumatoid Arthritis: RA patients on disease-modifying agents require coordinated perioperative management with rheumatology. PBS-subsidised biologics have reduced the incidence of severe cervical involvement.

Surgical Care: Complex craniocervical and cervical fusion performed at tertiary spine centres. Intraoperative neuromonitoring is standard practice for upper cervical procedures.