Adolescent Idiopathic Scoliosis

Definition

Adolescent Idiopathic Scoliosis (AIS) is a three-dimensional spinal deformity characterized by:

• Lateral curvature of the spine (Cobb angle 10° or greater)
• Vertebral rotation toward the convexity of the curve
• Age of onset: 10 years to skeletal maturity (approximately 18 years)
• Idiopathic: No identifiable underlying cause (diagnosis of exclusion)

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Not just coronal deformity: AIS involves complex 3D changes in coronal, sagittal, and axial planes.

Epidemiology

Prevalence

Key point: Small curves (10-20°) are common and equal between sexes. Larger curves requiring intervention are predominantly female.

Age Distribution

• Peak onset: Corresponds to peak growth velocity during puberty
• Girls: Age 10-12 (earlier than boys)
• Boys: Age 12-14
• Risk period: From onset of puberty to Risser 4-5 (skeletal maturity)

Geographic & Ethnic Variation

• Similar prevalence across ethnic groups for small curves
• Caucasian females have higher prevalence of progressive curves requiring treatment
• No significant urban/rural difference
• Global consistency: Reported prevalence (2-3% of adolescents) is broadly similar across world regions; apparent differences mostly reflect screening method and Cobb threshold

Natural History

Curve Progression Risk

Depends on:

1. Skeletal maturity: Risser 0-2 (immature) = high risk
2. Curve magnitude: Larger curves progress more
3. Curve pattern: Double curves progress more than single
4. Age/pubertal status: Premenarchal/early puberty = high risk

Weinstein 50-year follow-up (landmark natural history study):

• Curves under 30° at maturity: Remain stable lifelong (no progression)
• Curves 30-50° at maturity: May progress 10-15° over adulthood (usually asymptomatic)
• Curves over 50° at maturity: Progress approximately 1° per year throughout adulthood
• Curves over 80°: Risk of restrictive lung disease, dyspnea, reduced quality of life

Impact on Health

Key message for exams: AIS is primarily a cosmetic and progressive deformity rather than immediately life-threatening. Surgery aims to halt progression and prevent long-term cardiopulmonary decline.

Spinal Biomechanics in AIS

Three-Dimensional Deformity

AIS is NOT just coronal plane curvature. It is a complex 3D deformity:

1. Coronal plane: Lateral curvature (Cobb angle measurement)
2. Sagittal plane: Loss of normal thoracic kyphosis (hypokyphosis common)
3. Axial plane: Vertebral rotation toward convexity of curve

Key concept: The rib hump seen on forward bend test is due to vertebral rotation, NOT just lateral curvature. Ribs follow rotated vertebrae, creating posterior prominence on convex side.

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Hueter-Volkmann Principle

Asymmetric loading theory: Compression inhibits growth, tension stimulates growth.

• Concave side: Increased compression → slower growth
• Convex side: Relative tension → faster growth
• Result: Self-perpetuating vicious cycle during growth

Clinical implication: Curves under 30° may remain stable at skeletal maturity, but curves over 50° often progress 1° per year lifelong (even after growth completion).

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Etiology: Why Idiopathic?

Idiopathic = unknown cause, but multiple factors implicated:

Curve Progression Risk Factors

Sanders Skeletal Maturity System (alternative to Risser):

• Based on hand/wrist radiograph (distal radius, ulna physis)
• Stages 1-8: More granular than Risser for predicting growth
• Sanders 2-4 = peak growth velocity = highest curve progression risk

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History: Key Questions

Physical Examination

Adams Forward Bend Test

Gold standard screening test for scoliosis.

Technique:

1. Patient stands with feet together, arms hanging freely
2. Patient bends forward 90° at hips, knees straight
3. Examiner views from behind, looks for asymmetry

Positive test: Rib hump or lumbar prominence on one side (indicates vertebral rotation).

Measurement: Use scoliometer (inclinometer) - angle of trunk rotation:

• 5-7° ATR (angle of trunk rotation): Refer for radiographs
• Over 7° ATR: High likelihood of Cobb angle over 20°

Shoulder Height Asymmetry

Left shoulder elevation suggests proximal thoracic curve (Lenke Type 2).

Key point: If Type 1 curve mistaken for Type 2, failure to fuse proximal thoracic leads to persistent shoulder asymmetry post-op.

Coronal Balance

Measure plumb line from C7 spinous process to sacral crease:

• Balanced: Plumb line within 2cm of sacral midline
• Decompensated: Over 2cm offset (poor surgical candidate without balance correction)

Sagittal Examination

Inspect for:

• Thoracic hypokyphosis (flat back) - common in AIS
• Lumbar hyperlordosis - compensatory mechanism
• Thoracic lordosis - severe cases, red flag

Neurologic Examination

Should be completely normal in AIS. Any abnormality = MRI spine mandatory.

Skin Examination

• Café-au-lait spots (6+ spots over 5mm) = Neurofibromatosis-1
• Axillary freckling = NF-1 (Crowe's sign)
• Hairy patch, dimple over spine = Tethered cord, spinal dysraphism
• Port-wine stain = Consider vascular malformation

AIS is a diagnosis of exclusion. The following structural and non-structural causes of a spinal curve must be considered and confidently excluded before labelling a curve idiopathic.

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Radiographic Assessment

Initial Films

PA Standing Scoliosis SeriesSupine Side-Bending FilmsRisser Sign Assessment

Standard views for diagnosis and monitoring:

1. PA Spine (NOT AP): Reduces radiation to breast tissue

   • Include C7-sacrum on single 36-inch cassette
   • Patient standing, arms forward on supports
   • Measure Cobb angles, identify apex and end vertebrae

2. Lateral Spine: Assess sagittal profile

   • Measure T5-T12 kyphosis (normal 20-40°)
   • Assess lumbar lordosis (normal 40-60°)
   • Identify thoracic lordosis (red flag)

When to order: Any patient with positive Adams test or scoliometer over 5-7°.

Purpose: Determine curve flexibility - distinguish structural from compensatory curves.

Technique:

• Patient supine (gravity eliminated)
• Obtain PA film with patient maximally side-bent toward curve convexity
• Obtain opposite side-bend for other curve

Interpretation:

• Structural: Curve corrects to over 25° on side-bending
• Non-structural: Curve corrects to under 25° (flexible, compensatory)

When to order: Pre-operative planning for curves approaching surgical threshold (over 40°). Essential for Lenke classification.

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Evaluated on PA spine radiograph - iliac crest apophysis ossification.

Risser Stages (Risser-US system):

• Risser 0: No ossification - most growth remaining (1-2 years)
• Risser 1: Ossification 0-25% iliac crest
• Risser 2: 25-50% ossification
• Risser 3: 50-75% ossification
• Risser 4: 75-100% but not fused to ilium
• Risser 5: Complete fusion - growth complete

Risser-French system: Different staging (starts medially vs laterally). Know which system your institution uses.

Triradiate cartilage: Y-shaped cartilage of acetabulum. Closure indicates puberty onset but growth remaining.

Cobb Angle Measurement

Step-by-step technique:

1. Identify end vertebrae: Most tilted vertebrae at top and bottom of curve (maximally tilted into concavity)
2. Draw lines: Line along superior endplate of upper end vertebra, inferior endplate of lower end vertebra
3. Perpendiculars: Draw perpendicular lines from each endplate line
4. Measure angle: Angle of intersection = Cobb angle

Interobserver variability: ± 5° between measurements (same observer or different observers).

Clinical significance:

• Under 10°: Not scoliosis (spinal asymmetry)
• 10-25°: Mild scoliosis, observe
• 25-40°: Moderate, consider bracing if immature
• Over 40-50°: Severe, surgical threshold

Specialized Imaging

Key concept: MRI spine is mandatory before surgical correction if:

• Left thoracic curve
• Rapid progression
• Any neurologic signs/symptoms
• Male patient
• Painful scoliosis
• Age under 10 years

Prevalence of neural axis abnormalities: 5-10% of presumed AIS have MRI findings (syrinx most common).

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Observation Protocol

Indications for observation alone:

• Curves 10-25° in any patient
• Curves 25-40° in skeletally mature patients (Risser 4-5)
• Curves 25-40° in early immature patients where bracing compliance doubtful

Follow-Up Schedule

Progression definition: Increase in Cobb angle by over 5° between visits (accounting for measurement variability).

Bracing

BrAIST Study (2013) - Landmark Evidence

Key trial: Bracing in Adolescent Idiopathic Scoliosis Trial (BrAIST).

Design: Multicentre trial (randomised + preference cohorts), bracing vs observation, typical bracing indications (curves 20-40°, skeletal immaturity).

Results:

• 72% success with bracing vs 48% with observation in the combined analysis (success = curve staying under 50° / reaching maturity; propensity-adjusted OR 1.93)
• 75% vs 42% in the intention-to-treat randomised cohort (OR 4.11)
• Significant dose-response: more hours of brace wear correlated with greater success (P less than 0.001); brace prescribed for at least 18 hours/day
• Trial stopped early owing to the efficacy of bracing

Clinical impact: Bracing is now standard of care for appropriate candidates, and the benefit increases with longer daily wear time.

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Bracing Indications (SRS Guidelines)

Brace Types

Principle of corrective bracing: Apply three-point pressure:

1. Pressure at curve apex (convex side)
2. Counter-pressure above curve (concave side)
3. Counter-pressure below curve (concave side)

Goal: NOT to permanently correct curve, but to halt progression during growth.

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Weaning Protocol

When to start: Skeletal maturity (Risser 4-5, 2+ years post-menarche).

Gradual weaning:

1. Reduce to 16 hours/day for 3 months
2. Reduce to 12 hours/day (nighttime only) for 3 months
3. Discontinue brace, obtain PA/lateral films
4. Follow-up at 6 months, 12 months post-weaning

Rebound phenomenon: Curve may increase 5-10° after brace discontinuation (acceptable if stays under 50°).

Monitoring Compliance

Challenge: Adolescent non-compliance is common (social stigma, discomfort).

Strategies:

• Temperature sensors in brace - logs hours worn
• Frequent follow-up - every 4-6 months with radiographs
• Peer support groups - connect patients with others in braces
• Positive reinforcement - emphasize success stories, avoiding surgery

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Surgical Indications

Absolute threshold: No universally agreed cutoff. 45-50° is consensus surgical threshold.

Relative indications:

• 40-45° in immature patient with rapid progression
• Over 50° regardless of symptoms (prevent respiratory decline)
• Over 70-80° = cardiopulmonary compromise risk

Goals of Surgery

1. Halt progression: Spinal arthrodesis prevents further curvature
2. Correct deformity: Reduce Cobb angle 50-70% (not 100% - risks neurologic injury)
3. Maintain balance: Coronal and sagittal balance essential
4. Preserve motion: Fuse only structural curves, spare compensatory segments
5. Improve cosmesis: Reduce rib hump, shoulder asymmetry

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Posterior Spinal Fusion (PSF) - Standard Approach

Most common surgical technique for AIS.

Pre-Operative Planning

Key decisions:

1. Proximal fusion level: Guided by Lenke classification
   • Stable vertebra (SV) = first vertebra bisected by CSVL
   • Neutral vertebra (NV) = least rotated, most parallel endplates
2. Distal fusion level: Critical to avoid coronal imbalance
   • Generally to neutral/stable vertebra or one level distal
   • Lumbar modifier C (CSVL beyond apex) requires fusion into lumbar spine

Surgical Technique

Intra-Operative Neuromonitoring

SSEP (Somatosensory Evoked Potentials):

• Monitors dorsal column function (position/vibration sense)
• Stimulate peripheral nerve (tibial, median), record at scalp
• Alarm criteria: 50% amplitude decrease OR 10% latency increase

MEP (Motor Evoked Potentials):

• Monitors corticospinal tract function (motor)
• Stimulate motor cortex (transcranial), record at muscle (foot, hand)
• More sensitive than SSEP for detecting motor injury
• Alarm criteria: 50-80% amplitude decrease

Management of alarm:

1. Stop surgical maneuver immediately
2. Check anesthesia: MAP over 70, Hgb over 8, temp normal
3. If no improvement, release correction (loosen rods/screws)
4. If signals recover, attempt correction more gradually
5. If signals do not recover, perform Stagnara wake-up test

Stagnara wake-up test (historical, rarely needed now):

• Lighten anesthesia mid-case
• Patient wakes, squeezes hands, moves feet on command
• Confirms motor function intact despite neuromonitoring alarm

Anterior Spinal Fusion (ASF)

Less commonly used since advent of pedicle screw posterior instrumentation.

Indications (limited):

• Thoracolumbar/lumbar curves (T12-L4) where anterior release improves flexibility
• Lenke 5 curves with severe rigidity on side-bending films
• Selective anterior fusion to spare fusion levels (controversial)

Approach:

• Open thoracotomy or thoracoabdominal incision (invasive, painful)
• Video-assisted thoracoscopic surgery (VATS) - less invasive option

Disadvantages:

• More painful post-op (vs posterior approach)
• Risk of pulmonary complications
• Does not address sagittal hypokyphosis as well as posterior DVR
• Generally replaced by posterior-only techniques

Current role: Anterior release followed by posterior instrumentation (two-stage) for severe rigid curves over 80-90°.

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Minimally Invasive Techniques

Emerging approaches:

• Anterior scoliosis correction (ASC): Vertebral body tethering (VBT)
• ApiFix device: Gradual correction via ratcheting implant
• Magnetically controlled growing rods (MCGR): For EOS, not AIS

Vertebral Body Tethering (VBT):

• Concept: Modulate growth via Hueter-Volkmann principle
• Thoracoscopic placement of flexible tether on convex side
• Restricts convex growth, allows concave "catch-up"
• Indication: Curves 40-65°, skeletally immature (Risser 0-2)
• Advantages: Preserves motion, no fusion, less invasive
• Disadvantages: Tether breakage (10-15%), over-correction, long-term data lacking

Status: Investigational in many countries, not yet standard of care for AIS.

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Immediate Post-Operative Period (POD 0-3)

ICU/HDU Monitoring (First 24 Hours)

Standard protocol:

• Neuromonitoring: Continue SSEP/MEP for first 2-4 hours post-op (detect delayed neurologic changes)
• Neurologic checks: Hourly motor/sensory assessment (5/5 strength all extremities)
• Pain control: Multimodal analgesia (IV opioids, ketorolac, paracetamol)
• Hemodynamic monitoring: Goal MAP over 70 mmHg (maintain spinal cord perfusion)
• Log-roll only: No independent bed mobility for first 24 hours

Ward Care (POD 1-3)

Hospital Discharge (POD 4-6)

Discharge criteria:

• ✓ Adequate pain control on oral medications
• ✓ Independent ambulation (no assistive device needed)
• ✓ Tolerating regular diet
• ✓ Bowel function restored
• ✓ Neurologic exam normal (5/5 strength, intact sensation)
• ✓ Wound clean/dry/intact, no signs of infection
• ✓ Patient/family educated on home precautions

Discharge medications:

• Oral opioids (oxycodone 5mg PRN, 1-2 week supply)
• Paracetamol 1g QID
• Bowel regimen (docusate, senna)
• ± Gabapentin if neuropathic pain

Activity restrictions:

• No BLT (bending, lifting over 5kg, twisting) for 6 weeks
• No contact sports for 6 months (until fusion solid)
• Return to school at 2-3 weeks (light activities, no PE)
• Swimming allowed at 6 weeks (wound fully healed)

Outpatient Follow-Up Schedule

Red Flags for Early Return

Long-Term Monitoring

Years 1-5:

• Annual clinical + radiographic follow-up
• Monitor for PJK, adding-on, hardware failure
• SRS-30 outcomes questionnaire (assess QOL, self-image, pain)

After skeletal maturity + solid fusion:

• Discharge from routine follow-up
• PRN follow-up if develops pain or concerns
• Counsel: Curves should remain stable lifelong after solid fusion

Pregnancy counseling (for female patients):

• AIS surgery does NOT contraindicate pregnancy
• Spinal fusion does NOT affect ability to deliver vaginally
• Epidural anesthesia usually possible (inform anesthetist of fusion levels)
• Discuss at skeletal maturity/pre-pregnancy planning

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Radiographic Outcomes

Curve Correction

Key point: Goal is NOT 100% correction (risks neurologic injury). 50-70% correction with balanced spine = excellent outcome.

Sagittal Profile Restoration

Modern DVR technique (direct vertebral rotation):

• Restores physiologic thoracic kyphosis (20-40°)
• Avoids flat back syndrome (complication of older hook systems)
• Maintains lumbar lordosis (40-60°)

Sagittal balance more important than coronal correction for long-term patient satisfaction.

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Patient-Reported Outcomes

SRS-30 Questionnaire

Five domains (score 1-5, higher = better):

1. Pain: Minimal change (AIS rarely painful pre-op)
2. Self-image: Largest improvement (major driver of satisfaction)
3. Function: Returns to baseline by 6-12 months
4. Mental health: Improves post-op (body image confidence)
5. Satisfaction: Over 90% satisfied at 2+ years post-op

Key finding: Self-image improvement is primary benefit of surgery (not pain relief, which was minimal pre-op).

Return to Activities

Practical note: Return-to-sport clearance commonly requires written approval from the treating surgeon (for school/club liability) in many health systems.

Long-Term Health Outcomes

Cardiopulmonary Function

Post-surgical curves:

• Curves corrected to under 50° = normal pulmonary function lifelong
• No increased mortality vs general population
• Justifies surgical intervention at 45-50° threshold (prevents progression to over 80°)

Back Pain

SRS adult outcomes data:

• Treated AIS (surgery or brace): Similar back pain prevalence to general population at 20-30 year follow-up
• Untreated AIS over 70°: Increased mechanical back pain (muscle fatigue, imbalance)
• Fusion does NOT cause increased back pain if balanced

Adjacent Segment Degeneration

Concern: Fusion increases stress on adjacent mobile segments.

Evidence:

• Mild degenerative changes on MRI in 30-40% at 10+ years (asymptomatic)
• Symptomatic adjacent segment disease requiring surgery: Under 5% at 20 years
• Risk factors: Fusion to L3 or more distal (higher lumbar stress), sagittal imbalance

Key principle: Selective fusion (sparing lumbar spine when possible) reduces adjacent segment issues.

Complication Rates

Predictors of Good Outcome

Favorable factors:

• ✓ Pre-op curve 45-70° (optimal surgical range)
• ✓ Balanced spine post-op (C7 plumb line within 2cm of sacrum)
• ✓ Restoration of thoracic kyphosis (DVR technique)
• ✓ Selective fusion (sparing lumbar segments when appropriate)
• ✓ Patient satisfaction with cosmetic improvement

Unfavorable factors:

• ✗ Pre-op curve over 90° (rigid, difficult correction, higher complication rate)
• ✗ Sagittal imbalance post-op (flat back, positive sagittal balance)
• ✗ Over-fusion (fusing compensatory lumbar curves unnecessarily)
• ✗ Persistent shoulder asymmetry (failure to address proximal thoracic curve if structural)

Prognosis Summary

Excellent prognosis for appropriately selected and treated patients:

• Over 90% patient satisfaction at 2+ years
• Maintained curve correction with minimal loss over time
• Normal life expectancy and quality of life
• Low complication and revision rates with modern techniques
• Ability to participate in full activities including pregnancy and sports

Key message: AIS surgery is highly successful when performed for appropriate indications with meticulous technique and proper fusion level selection.

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Quick Reference MCQ Points

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OrthoVellum is a worldwide resource: this section gives the global standard of care plus the key regional differences a candidate may be examined on, drawing on the major society guidelines as cited evidence rather than any single country's health system.

Global Epidemiology

AIS is the most common form of paediatric spinal deformity, defined as a coronal Cobb angle of at least 10° in children aged 10-18 with no identifiable cause (per the US Preventive Services Task Force review). Population estimates are consistent across high-income settings:

Prevalence figures are broadly similar across world regions where school or population screening has been performed; reported variation largely reflects differing screening methods and curve thresholds rather than true biological difference. The female-to-male ratio rising with curve size is the most reproducible epidemiological signal.

Major Guidelines, Side by Side

The two dominant international frameworks are the Scoliosis Research Society (SRS) surgical/clinical tradition and the International Society on Scoliosis Orthopaedic and Rehabilitation Treatment (SOSORT) conservative-care guideline, which genuinely differ in emphasis. National screening policy is the clearest area of divergence.

A genuine practice tension exists: SOSORT and many European/Asian centres actively promote scoliosis-specific exercises and intensive conservative care, whereas the US evidence-appraisal tradition (USPSTF) regards both routine screening and exercise therapy as having insufficient evidence. Both nonetheless agree that bracing slows progression of moderate curves in skeletally immature patients - the one conclusion supported by Level I data (BrAIST).

Screening Policy Variation

Devices, Implants and Resource Setting

Spinal instrumentation systems are regulated as high-risk implantable devices across major jurisdictions (e.g. FDA Class II/III in the USA, CE-marked Class III in Europe, TGA-approved in Australia); AVBT/vertebral body tethering has more limited and evolving regulatory approval and remains a selected-patient option rather than standard care. There is no dedicated international AIS implant registry equivalent to the arthroplasty joint registries; outcome evidence instead comes from prospective society cohorts and multicentre studies such as those cited above.

In limited-resource settings, presentation is often later and with larger, more rigid curves because screening and timely bracing are less available, shifting management toward primary surgery (sometimes staged anterior release plus posterior fusion for rigid curves) and away from the brace-led pathways feasible where early detection is routine.