Adolescent Idiopathic Scoliosis

Definition. Adolescent idiopathic scoliosis is a three-dimensional deformity of the spine: a lateral curvature with a Cobb angle of 10° or more, vertebral rotation toward the convexity of the curve, and onset between 10 years and skeletal maturity (approximately 18 years). "Idiopathic" means no underlying cause can be identified, so it is a diagnosis of exclusion. The Cobb angle measures the coronal plane, but the deformity involves the sagittal and axial planes as well.

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Prevalence. AIS affects 2-3% of adolescents, and the reported prevalence is broadly similar across world regions; apparent differences mostly reflect the screening method and the Cobb threshold used. Small curves of 10-20° are common, similar in prevalence across ethnic groups, and equally distributed between the sexes. The larger curves that need treatment are predominantly female, with a female-to-male ratio of 7:1 for curves over 30°, and Caucasian females have a higher prevalence of progressive curves requiring treatment. There is no significant urban/rural difference.

Age. Onset peaks with peak growth velocity during puberty: age 10-12 in girls, who are affected earlier than boys, and 12-14 in boys. The period of risk runs from the onset of puberty to skeletal maturity at Risser 4-5.

Natural History

What drives progression. The risk of a curve progressing depends on:

1. Skeletal maturity: Risser 0-2 (immature) is high risk
2. Curve magnitude: larger curves progress more
3. Curve pattern: double curves progress more than single
4. Age and pubertal status: premenarchal or early puberty is high risk

After maturity. The landmark natural history study is Weinstein's 50-year follow-up of 117 untreated patients against 62 matched controls. Curves under 30° at maturity remain stable lifelong. Curves of 30-50° may progress 10-15° over adulthood, usually without symptoms. Curves over 50° progress at approximately 1° per year throughout adulthood, and curves over 80° carry a risk of restrictive lung disease, dyspnoea and reduced quality of life.

Pulmonary function. Curves under 70° have minimal effect on respiratory function. Over 80-100° there is restrictive lung disease, reduced FVC and dyspnoea on exertion, and death from cor pulmonale occurs in severe untreated cases. The at-risk group, though, is defined by a conjunction rather than by Cobb angle alone. In the 50-year cohort, shortness of breath on daily activity was reported by 22% versus 15% of controls, a modest difference overall; it became dramatic only with the combination of a Cobb angle over 80° and a thoracic apex, which carried an adjusted OR of 9.75 (95% CI 1.15-82.98). A large lumbar curve does not carry the same pulmonary penalty as a large thoracic one, and the width of that confidence interval marks a small subgroup: do not quote the odds ratio as precise.

Survival. Survival in the cohort was similar to the general population (estimated probability about 0.55 versus 0.57 expected), which is the fact that reframes untreated idiopathic scoliosis as rarely life-shortening. Curves under 100° carry a normal life expectancy. Curves over 100°, rare and untreated, carry increased mortality from cardiopulmonary complications, and that is what justifies surgical intervention at the 45-50° threshold.

Back pain. Severe curves over 70° carry an increased prevalence of mechanical back pain in adulthood, from muscle fatigue and imbalance. Do not tell a patient with a modest curve that back pain is not increased at all: across the same 117 patients and 62 controls, chronic back pain was reported by 61% versus 35% (P=0.003). The reassurance lies in the severity rather than the prevalence. The pain was mostly little to moderate and these adults remained productive and functional, so the honest counselling line is that back pain is commoner, usually mild and rarely disabling, not that it is no different from the general population.

Psychosocial impact. Body image is the primary driver for seeking treatment, and a visible deformity (rib hump, shoulder asymmetry) has a negative effect on self-esteem. Taken together, the natural history makes AIS primarily a cosmetic and progressive deformity rather than an immediately life-threatening one; surgery aims to halt progression and prevent long-term cardiopulmonary decline.

A three-plane deformity. AIS is not just a coronal curve. In the coronal plane there is lateral curvature, which the Cobb angle measures; in the sagittal plane there is loss of the normal thoracic kyphosis, and hypokyphosis is common; in the axial plane the vertebrae rotate toward the convexity. The rib hump seen on the forward bend test is produced by that vertebral rotation, not by the lateral curvature alone: the ribs follow the rotated vertebrae and form a posterior prominence on the convex side.

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The Hueter-Volkmann principle. Compression inhibits growth and tension stimulates it. On the concave side of a curve the growth plates are compressed and grow more slowly; on the convex side they are under relative tension and grow faster. The result is a self-perpetuating vicious cycle for as long as growth continues.

Why idiopathic. No cause has been identified, but several factors are implicated. The genetic evidence is a 30% concordance in identical twins, implication of the CHD7, LBX1 and GPR126 genes, and a 10-fold increase in risk with a family history. The biomechanical theory is relative anterior overgrowth: hypokyphosis increases loading of the anterior column and progression becomes asymmetric. The neuromuscular theory is a proprioceptive deficit, from the subtle abnormalities of balance and postural control found in some patients.

Lenke is the current standard and exists for surgical planning: it determines which curves are fused. It has three components. The curve type (1-6) depends on which curves are structural; the lumbar spine modifier (A, B, C) describes where the lumbar curve apex sits relative to the CSVL; and the sagittal thoracic modifier (-, N, +) grades the T5-T12 kyphosis.

Lenke ClassificationLenke ModifiersKing-Moe (Historical)

Structural or not. The major curve, the one with the largest Cobb angle, is structural by definition and is always fused. The criteria decide the fate of the minor curves. A minor curve is structural if it meets either criterion:

• Residual Cobb of 25° or more on supine side-bending toward the curve's convexity (the defining flexibility test, applied to all regions)
• Junctional kyphosis of +20° or more: T2-T5 for the proximal thoracic curve, T10-L2 for the main thoracic or thoracolumbar/lumbar curve

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The Classic Inversion - Do Not Say Hypokyphosis Makes a Curve Structural

A widely repeated error holds that T5-T12 hypokyphosis renders the thoracic curve structural. Lenke's sagittal structural criteria run the other way: junctional hyperkyphosis of +20° or more (T2-T5 for the proximal curve, T10-L2 for the others) is what makes a minor curve structural. Hypokyphosis (T5-T12 under 10°) is merely the sagittal modifier "-": typical of AIS and important for planning sagittal restoration, but not a structural criterion.

The six types. Each type is a combination of structural regions, and the fusion follows the structural curves:

Type 1: Main Thoracic

Structural Curves

MT only

Typical Fusion Levels

T4/5 to T11/12/L1

Key Points

Most common, selective fusion, lumbar compensates

Type 2: Double Thoracic

Structural Curves

PT + MT

Typical Fusion Levels

T2-T12

Key Points

High left shoulder if PT not fused

Type 3: Double Major

Structural Curves

MT + TL/L

Typical Fusion Levels

T4-L3/4

Key Points

Both curves over 40°, largest curves

Type 4: Triple Major

Structural Curves

PT + MT + TL/L

Typical Fusion Levels

T1/2-L3/4

Key Points

Rare, all three regions structural

Type 5: TL/Lumbar

Structural Curves

TL/L only

Typical Fusion Levels

T10-L3

Key Points

Left-sided curves common

Type 6: TL-Main Thoracic

Structural Curves

MT + TL/L

Typical Fusion Levels

T3-L3

Key Points

Two structural curves, no lumbar compensation

Lenke Classification: The 6 Curve Types

Type	Structural Curves	Typical Fusion Levels	Key Points
Type 1: Main Thoracic	MT only	T4/5 to T11/12/L1	Most common, selective fusion, lumbar compensates
Type 2: Double Thoracic	PT + MT	T2-T12	High left shoulder if PT not fused
Type 3: Double Major	MT + TL/L	T4-L3/4	Both curves over 40°, largest curves
Type 4: Triple Major	PT + MT + TL/L	T1/2-L3/4	Rare, all three regions structural
Type 5: TL/Lumbar	TL/L only	T10-L3	Left-sided curves common
Type 6: TL-Main Thoracic	MT + TL/L	T3-L3	Two structural curves, no lumbar compensation

Type 1 is the most common, accounting for 50-60% of surgical AIS cases. Its principle is selective fusion: fuse only the structural main thoracic curve, spare the compensatory lumbar curve, which should not be fused (fusing it leads to imbalance), and rely on spontaneous lumbar correction after the thoracic curve is straightened.

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Lumbar spine modifier. Measured as where the CSVL (centre sacral vertical line) falls relative to the apical lumbar vertebra:

• A: CSVL passes between the pedicles of the apical vertebra (minimal lumbar deviation)
• B: CSVL falls between the medial pedicle border and the lateral edge of the apical vertebral body
• C: CSVL falls completely medial to the apical vertebral body, missing the vertebra entirely. No millimetre threshold exists in Lenke's definition

Modifier C flags substantial lumbar deviation and weighs toward including the lumbar curve, but it does not automatically mandate lumbar fusion. Selective thoracic fusion of a 1C curve is a recognised strategy in properly selected patients: a residual and usually well-tolerated lumbar curve is accepted in exchange for motion, and the price is a higher risk of coronal decompensation if selection is wrong.

Sagittal thoracic modifier. The T5-T12 kyphosis on the lateral radiograph:

Minus (-)

Kyphosis Range

Under 10° (hypokyphotic)

Clinical Significance

Typical of AIS - plan sagittal restoration (DVR); NOT a structural criterion

Normal (N)

Kyphosis Range

10-40° (physiologic)

Clinical Significance

Normal sagittal profile - no specific modification needed

Plus (+)

Kyphosis Range

Over 40° (hyperkyphotic)

Clinical Significance

Rare in AIS - may need reduction rather than addition of kyphosis

Sagittal Thoracic Modifiers

Modifier	Kyphosis Range	Clinical Significance
Minus (-)	Under 10° (hypokyphotic)	Typical of AIS - plan sagittal restoration (DVR); NOT a structural criterion
Normal (N)	10-40° (physiologic)	Normal sagittal profile - no specific modification needed
Plus (+)	Over 40° (hyperkyphotic)	Rare in AIS - may need reduction rather than addition of kyphosis

Hypokyphosis, the flat back, is the rule in AIS and is addressed surgically with derotation (direct vertebral rotation). It does not decide which curves are fused. The modifiers describe the deformity; the structural criteria, a side-bending residual of 25° or more and junctional kyphosis of +20° or more, decide the fusion.

King-Moe is obsolete, replaced by Lenke, but it is still asked:

King I

Curve Pattern

S-shaped, lumbar larger than thoracic

Key Features

Crossing CSVL into lumbar spine

King II

Curve Pattern

S-shaped, thoracic larger than lumbar

Key Features

Most common King type

King III

Curve Pattern

Single thoracic curve

Key Features

No compensatory lumbar curve

King IV

Curve Pattern

Long thoracic curve extending to L4

Key Features

L5 centred over sacrum

King V

Curve Pattern

Double thoracic curve

Key Features

Upper thoracic T1 tilt to left

King-Moe Classification

Type	Curve Pattern	Key Features
King I	S-shaped, lumbar larger than thoracic	Crossing CSVL into lumbar spine
King II	S-shaped, thoracic larger than lumbar	Most common King type
King III	Single thoracic curve	No compensatory lumbar curve
King IV	Long thoracic curve extending to L4	L5 centred over sacrum
King V	Double thoracic curve	Upper thoracic T1 tilt to left

Why it was abandoned. Examiners like this question, and there are three flaws to give. King is two-dimensional and ignores the sagittal plane, unlike Lenke. Its inter-observer agreement was poor, with a kappa of 0.46. And it often led to sub-optimal fusion levels, over-fusing or under-fusing.

History. The deformity is usually asymptomatic and is noticed by a parent through clothing fit, at school screening or at a physician check-up. A first-degree relative with scoliosis carries the 10-fold risk described under aetiology and helps distinguish familial AIS from a secondary cause. Establish skeletal maturity: age at menarche in girls (menarche occurs at Risser 1-2), and voice change, growth spurt and parental heights in either sex, because these predict the growth remaining.

Red flags for a non-idiopathic cause. Any of these means an MRI of the whole spine before treatment:

• Left thoracic curve: unusual in AIS, so consider a neurological cause (syrinx, Chiari malformation, cord tumour, tethered cord)
• Age under 10 (infantile or juvenile onset): high risk of underlying pathology
• Male sex: males have a lower incidence of AIS, so a curve in a boy carries higher suspicion
• Pain, especially night pain: AIS is typically painless, so think tumour
• Neurological symptoms or signs: weakness, numbness, hyperreflexia, clonus, cavus feet, bowel or bladder change
• Skin findings of an underlying syndrome or dysraphism (see the skin examination below)
• Sharp angular kyphosis: congenital vertebral anomaly, infection, tumour
• Rapid progression, over 10° in 6 months: investigate aggressively

Adams forward bend test. The gold-standard screening test. The patient stands with the feet together and arms hanging freely, bends forward 90° at the hips with the knees straight, and the examiner views from behind for asymmetry. A positive test is a rib hump or lumbar prominence on one side, the sign of vertebral rotation. Quantify it with a scoliometer (inclinometer) as the angle of trunk rotation: an ATR of 5-7° is the threshold for radiographs, and over 7° carries a high likelihood of a Cobb angle over 20°.

Shoulders and coronal balance. Left shoulder elevation suggests a proximal thoracic curve (Lenke type 2). If a type 2 is mistaken for a type 1 and the proximal thoracic curve is left unfused, the shoulder asymmetry persists after surgery. Coronal balance is the plumb line from the C7 spinous process to the sacral crease: within 2 cm of the sacral midline is balanced, and an offset over 2 cm is decompensated, a poor surgical candidate without balance correction.

Sagittal profile. Inspect for thoracic hypokyphosis (flat back), which is common in AIS; lumbar hyperlordosis, a compensatory mechanism; and thoracic lordosis, seen in severe cases and a red flag.

Neurological examination. It should be completely normal in AIS, and any abnormality makes an MRI of the spine mandatory.

Skin. The stigmata to look for:

• Café-au-lait spots (6 or more, over 5 mm) = neurofibromatosis type 1
• Axillary freckling (Crowe's sign) = NF-1
• Hairy patch or dimple over the spine = tethered cord, spinal dysraphism
• Port-wine stain = consider a vascular malformation

Standing PA and lateral. Once the Adams test is positive or the scoliometer reaches its threshold, request a PA rather than an AP film, which reduces the radiation dose to breast tissue. Include C7 to the sacrum on a single 36-inch cassette with the patient standing and the arms forward on supports. On the PA film measure the Cobb angles, identify the apex and end vertebrae, and read the Risser sign. On the lateral measure the T5-T12 kyphosis (normal 20-40°; Lenke's N modifier band is the wider 10-40°) and the lumbar lordosis (normal 40-60°), and look for thoracic lordosis, which is a red flag.

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Measuring the Cobb angle. The steps:

1. Identify the end vertebrae: the most tilted vertebrae at the top and bottom of the curve, maximally tilted into the concavity
2. Draw a line along the superior endplate of the upper end vertebra and the inferior endplate of the lower end vertebra
3. Draw perpendiculars from each endplate line
4. The angle at which they intersect is the Cobb angle

Interobserver variability is ±5°, whether the same observer or different observers measure. A curve under 10° is spinal asymmetry, not scoliosis. Above that, 10-25° is mild scoliosis and is observed, 25-40° is moderate and is considered for bracing if the patient is immature, and over 40-50° is severe and is the surgical threshold; the full decision by curve and maturity is set out under the management algorithm.

Supine side-bending films. These measure curve flexibility and separate structural from compensatory curves. The patient lies supine, which eliminates gravity, and a PA film is taken with the patient maximally side-bent toward the convexity of the curve, then the opposite bend for the other curve. A curve that retains 25° or more on bending is structural; one that corrects to under 25° is flexible and compensatory. Order them for pre-operative planning when curves approach the surgical threshold (over 40°); they are essential for Lenke classification.

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Risser sign. Read on the PA spine radiograph from ossification of the iliac crest apophysis (Risser-US system):

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

The Risser-French system stages differently (it starts medially rather than laterally), so know which system your institution uses. The triradiate cartilage, the Y-shaped cartilage of the acetabulum, closes around age 12-13; closure indicates that puberty has begun but growth remains. Both predict growth, but Risser is key for curve progression risk.

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Sanders skeletal maturity system. An alternative to Risser, read from a hand and wrist radiograph (distal radius and ulna physes) in stages 1-8, which is more granular than Risser for predicting growth. Sanders 2-4 is peak growth velocity and the highest curve progression risk. It is optional; Risser is usually sufficient.

Putting maturity together. The high-risk immature patient is Risser 0-2, premenarchal, with an open triradiate cartilage. The low-risk mature patient is Risser 4-5, two or more years post-menarche, with a closed triradiate. Post-menarchal status in girls means lower risk.

Vertebral rotation. The Cobb angle captures only the coronal plane, and the axial rotation that drives the rib hump is graded separately. The Nash-Moe (pedicle) method, on the standing PA film, grades rotation by the position of the convex-side pedicle shadow within a vertebral body divided into segments:

• Grade 0: both pedicles symmetric (no rotation)
• Grade I: convex pedicle moves toward the midline but remains in the outer segment
• Grade II: convex pedicle in the middle segment; concave pedicle disappearing
• Grade III: convex pedicle at the centre of the vertebral body
• Grade IV: convex pedicle past the midline into the concave half

It is quick but only semi-quantitative. The Perdriolle method places a torsionmeter, a transparent overlay template, over the apical vertebra on the PA film and reads rotation in degrees from the pedicle and vertebral-body-edge positions; it is more quantitative than Nash-Moe and is the method quoted in surgical correction studies, including the rotational correction figures in the evidence base. Rotation determines the clinical rib hump and the scoliometer reading, is the target of derotation manoeuvres (direct vertebral rotation), and is now best quantified three-dimensionally on weight-bearing CT; Nash-Moe and Perdriolle remain the examinable plain-film tools.

Advanced imaging. The indications:

MRI of the whole spine is ordered for any of the red flags listed under Clinical Assessment, and before every surgical correction. The reason is prevalence: 5-10% of presumed AIS have a neural axis abnormality on MRI, syrinx being the most common.

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

Observation. Observation alone is appropriate for:

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

Progression is defined as an increase in Cobb angle of over 5° between visits, accounting for measurement variability. The interval between visits follows maturity:

Bracing

The evidence: BrAIST (2013). The Bracing in Adolescent Idiopathic Scoliosis Trial was a multicentre trial with randomised and preference cohorts, bracing against observation, in patients with typical bracing indications (curves 20-40°, skeletal immaturity). Its results:

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

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

Who to brace. The SRS indication is a Cobb angle of 25-40° (BrAIST's entry range began at 20°): under 25° is too small to justify treatment, and over 40° often requires surgery because bracing rarely prevents progression there. The patient must be skeletally immature, Risser 0-2 with significant growth remaining, an open triradiate cartilage or premenarchal, because bracing is ineffective once mature. Any curve type can be braced; thoracic and thoracolumbar curves respond best, while high thoracic and cervicothoracic curves are difficult to control with a standard TLSO. The patient and family need to be motivated, with realistic expectations and psychological readiness for brace wear, and compliance monitoring is essential.

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Brace types. The orthotic options:

How a brace works. It applies three-point pressure: at the curve apex on the convex side, with counter-pressure above the curve and below the curve on the concave side. The goal is not to correct the curve permanently but to halt progression during growth.

Weaning. Start at skeletal maturity, Risser 4-5 and two or more years post-menarche, and wean gradually:

1. Reduce to 16 hours/day for 3 months
2. Reduce to 12 hours/day (night-time only) for 3 months
3. Discontinue the brace and obtain PA and lateral films
4. Review at 6 and 12 months after weaning

Expect a rebound: the curve may increase 5-10° after the brace is discontinued, which is acceptable if it stays under 50°.

Compliance. Adolescent non-compliance is common, driven by social stigma and discomfort. Temperature sensors in the brace log the hours worn; follow-up is frequent, every 4-6 months with radiographs; peer support groups connect patients with others in braces; and positive reinforcement, emphasising success stories and avoided surgery, helps.

Scoliosis-Specific Physiotherapy Exercises (PSSE)

What PSSE is. A family of active, curve-specific exercise programmes, of which the Schroth method is the best known (others include SEAS, BSPTS, Side Shift, DoboMed and FITS). They combine three-dimensional auto-correction, curve-specific postural training, rotational or asymmetric breathing (Schroth's "rotational angular breathing") and stabilisation of the corrected posture during daily activities. They are taught by specifically trained physiotherapists and differ fundamentally from generic, symmetrical back exercises.

What it is for. To slow or reduce curve progression during growth, improve posture and aesthetics and balance the trunk musculature, and to complement bracing, both during brace wear and after weaning. PSSE does not claim to reverse an established structural curve.

The evidence and where practice diverges. The 2016 SOSORT guidelines endorse PSSE, with one PSSE recommendation reaching grade-A / level-of-evidence-I status alongside bracing, on the strength of several single-centre randomised trials. SOSORT and many European and Asian centres actively prescribe PSSE, whereas the US evidence-appraisal tradition regards the exercise evidence as weaker, the practice tension set out in the guidelines section below.

Where it fits. PSSE is used for mild curves to slow progression, as an adjunct to bracing for moderate curves, and to maintain correction after weaning. It does not replace bracing for high-risk moderate curves, where the BrAIST-level evidence for bracing is stronger, or surgery for curves beyond threshold.

The decision. Once the diagnosis is confirmed on standing films and the red-flag screen is clear, treatment is determined by curve magnitude and skeletal maturity together:

When to escalate. Progression of over 10° in 6 months despite bracing warrants surgical referral and an MRI to rule out a secondary cause. Discovery of a red flag, a left thoracic curve, pain or neurological signs, means immediate MRI of the spine and brain, with neurosurgical consultation if indicated. Severe body-image distress, depression or social withdrawal warrants psychology referral, and earlier surgery can be considered if the patient meets the criteria.

The referral pathway. Across health systems AIS follows a common structure:

• Primary care or opportunistic/school detection, then referral to a paediatrician or orthopaedic/spine surgeon
• Specialist assessment in a paediatric orthopaedic spine clinic, ideally multidisciplinary (surgeon, orthotist, physiotherapist)
• Bracing by an orthotist with scoliosis expertise (TLSO fabrication, fitting, compliance monitoring)
• Surgery by a paediatric spine surgeon at a centre with intra-operative neuromonitoring capability

Stable curves can be managed with routine monitoring, while rapidly progressive curves warrant expedited specialist review; the principle applies regardless of how a given system funds or schedules care.

Indications. There is no universally agreed cutoff, but a Cobb angle of 45-50° or greater is the consensus threshold, because curves over 50° progress lifelong and surgery prevents cardiopulmonary compromise. Documented progression of over 5-10° despite bracing in an immature patient is bracing failure, and surgery is needed to prevent worsening. Cosmetic deformity is a quality-of-life indication: patient-reported concern about the rib hump, waistline asymmetry or shoulder imbalance, measured on SRS-30 scores. Pain is atypical in AIS and should prompt a search for other causes, though back pain in large curves over 70° can be an indication in adult patients. The relative indications:

• 40-45° in an immature patient with rapid progression
• Over 50° regardless of symptoms, to prevent respiratory decline
• Over 70-80°, where cardiopulmonary compromise becomes a risk

Goals. Surgery sets out to:

1. Halt progression: arthrodesis prevents further curvature
2. Correct the deformity by 50-70% of the Cobb angle, not 100%, because over-correction risks neurological injury and junctional kyphosis
3. Maintain coronal and sagittal balance
4. Preserve motion by fusing only the structural curves and sparing compensatory segments
5. Improve cosmesis: reduce the rib hump and shoulder asymmetry

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Posterior Spinal Fusion

Posterior spinal fusion with pedicle screw instrumentation is the most common operation for AIS.

Pre-operative workup. The steps before consent:

1. Classification: Lenke type (1-6), lumbar modifier (A/B/C) and sagittal modifier (-/N/+)
2. Side-bending films to separate structural from compensatory curves
3. MRI of the whole spine to rule out a neural axis abnormality
4. Optimisation: nutritional status, psychological preparation, expectations
5. Consent, quantifying the neurological injury, infection and revision risks set out under Complications

Fusion levels. The Lenke classification guides the levels, and the principle is to fuse the structural curves and spare the compensatory ones. The landmarks are the stable vertebra (SV), the first vertebra bisected by the CSVL, and the neutral vertebra (NV), the least rotated vertebra with the most parallel endplates. The distal level is critical to coronal balance and is generally the neutral or stable vertebra or one level distal; a lumbar modifier C usually directs the fusion into the lumbar spine, though selective thoracic fusion is a recognised option in selected 1C and 2C curves.

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Intra-Operative Neuromonitoring

SSEP. Somatosensory evoked potentials monitor dorsal column function (position and vibration sense): a peripheral nerve (tibial, median) is stimulated and the response recorded at the scalp. The alarm criteria are a 50% amplitude decrease or a 10% latency increase.

MEP. Motor evoked potentials monitor the corticospinal tract: the motor cortex is stimulated transcranially and the response recorded at muscle in the foot or hand. MEP is more sensitive than SSEP for detecting motor injury, and the alarm criterion is a 50-80% amplitude decrease. EMG monitors nerve root irritation alongside.

When the alarm sounds. The sequence:

1. Stop the surgical manoeuvre immediately
2. Check the anaesthesia: MAP over 70, haemoglobin over 8, temperature normal
3. If there is no improvement, release the correction by loosening the rods and screws
4. If the signals recover, attempt the correction more gradually
5. If they do not recover, perform a Stagnara wake-up test

The Stagnara wake-up test. Historical, and rarely needed now, but it is the fallback when neuromonitoring is unavailable or the signals do not recover. The anaesthetic is lightened mid-case, the patient wakes, squeezes the hands and moves the feet on command, and motor function is confirmed intact despite the neuromonitoring alarm.

Anterior Spinal Fusion

Where it stands. Anterior fusion is less commonly used since pedicle screw posterior instrumentation arrived, and its indications are limited: thoracolumbar and lumbar curves (T12-L4) where an anterior release improves flexibility, Lenke 5 curves with severe rigidity on side-bending films, and selective anterior fusion to spare fusion levels, which is controversial.

Approach and cost. The approach is an open thoracotomy or thoracoabdominal incision, which is invasive and painful, or video-assisted thoracoscopic surgery (VATS) as the less invasive option. Against the posterior approach it is more painful afterwards, carries a risk of pulmonary complications, does not address sagittal hypokyphosis as well as posterior DVR, and has generally been replaced by posterior-only techniques. Its current role is an anterior release followed by posterior instrumentation, a two-stage operation, for severe rigid curves over 80-90°.

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Minimally Invasive and Growth-Modulating Techniques

The emerging options. Anterior scoliosis correction (ASC) by vertebral body tethering (VBT); the ApiFix device, which corrects gradually through a ratcheting implant; and magnetically controlled growing rods (MCGR), which are for early-onset scoliosis rather than AIS.

Vertebral body tethering. The concept is growth modulation through the Hueter-Volkmann principle: a flexible tether is placed thoracoscopically on the convex side, restricting convex growth and allowing the concave side to catch up. The indication is a curve of 40-65° in a skeletally immature patient (Risser 0-2). It preserves motion, avoids fusion and is less invasive; against that stand tether breakage, over-correction and a lack of long-term data.

Quote the comparative figures, because the breakage rate is far higher than the commonly repeated 10-15%. In a matched comparison of 23 AVBT against 26 posterior spinal fusion patients (thoracic curves 40-67°, Risser 1 or less, ages 9-15) followed for 2-5 years, broken tethers occurred in 12 of 23 (52%), 4 of whom required revision. There were 9 revision procedures in the tethering group and none in the fusion group, including 3 conversions to fusion with 3 more pending. Correction was also less complete: from near-identical starting curves (53° AVBT versus 54° PSF), the final residual deformity was 33° versus 16° (P less than 0.001). Patient-reported outcomes after intervention were similar between the groups, and tethering did delay or avoid fusion in the majority, which is the genuine appeal.

How to frame it for a family. Tethering trades correction and reoperation risk for motion preservation and fusion avoidance; it does not achieve the same result without a fusion. The evidence is level III (a retrospective matched cohort, small numbers, single centre), so the percentages are indicative rather than definitive. It remains investigational in many countries and is not yet standard of care for AIS.

Intra-Operative

Neurological injury. The incidence is under 1% with neuromonitoring. The causes are pedicle screw malposition, over-distraction, hypotension and cord ischaemia; the prevention is SSEP/MEP monitoring, the wake-up test and gradual correction.

Dural tear. Occurs in 1-4% of primary AIS series, higher in revision surgery and series-dependent, and is caused by dissection around the laminae and facets. Repair primarily with a 4-0 suture and fibrin glue over a subfascial drain; the risks are CSF leak and pseudomeningocele.

Blood loss. Average loss for a posterior fusion is 800-1200 mL, and 5-10% of patients require transfusion. Large curves, long fusions and revision surgery raise it; controlled hypotension, antifibrinolytics (TXA), cell saver and pre-operative autologous donation reduce it.

Vascular injury. Rare, under 0.1%, from anterior perforation of the vertebral body by a pedicle screw or anterior instrumentation. The aorta, IVC and segmental vessels are at risk, and it is a life-threatening emergency.

Early Post-Operative

Superior mesenteric artery syndrome. Seen in 1-3%, it follows acute lordotic positioning on the Jackson table, which compresses the duodenum between the SMA and the aorta. It presents with intractable vomiting on POD 3-7 and an inability to tolerate oral intake, and an upper GI series shows duodenal compression or obstruction. Treat with NG decompression, TPN and prone positioning, which relieves the duodenal compression; it usually resolves in 1-2 weeks.

Late Post-Operative

Pseudarthrosis. Non-union of the intended fusion mass, in 1-2% with modern pedicle screw instrumentation and more with hooks and wires. The risk factors are smoking (rare in adolescents), long constructs over 12 levels, inadequate decortication, infection and poor nutrition. It is often asymptomatic; otherwise it presents as back pain at the fusion site, progressive loss of correction or implant failure with rod fracture. Diagnose it on fine-cut CT through the fusion mass or on dynamic flexion-extension films showing motion; a rod fracture on plain films implies pseudarthrosis. An asymptomatic pseudarthrosis in a well-balanced spine without progression is observed; a symptomatic one needs revision fusion, posterior with or without anterior, with bone graft and possible re-instrumentation.

Proximal junctional kyphosis (PJK). Kyphosis of over 10° at the junction between the fused and mobile segments, within two levels of the UIV, in 20-30% after posterior fusion for AIS, most of it mild and asymptomatic. The mechanisms are the stress riser at the proximal end of the construct, ligamentous injury during dissection, over-correction of the main curve creating compensatory kyphosis, and osteoporotic vertebral fracture, which is more common in adults. It presents as a visible kyphosis at the base of the neck or upper back, neck pain if severe, and rarely neurological compromise. Mild PJK under 20° is observed and usually stable; severe PJK over 30° or progressive PJK is revised with proximal extension of the fusion. Prevention: avoid over-distraction of the proximal screws, make the transition from corrected to uncorrected spine gradual rather than a sharp angle, and consider tethering the proximal screws as a semi-rigid connection.

Adding-on. Progression of the compensatory curve distal to the fusion into a structural curve, in 5-10%, and preventable with proper fusion level selection. The cause is under-fusion: a distal level chosen too proximal, leaving an unstable curve below. It is most common in Lenke 1 curves where the lumbar modifier was underestimated, a curve that should have been read as modifier C and fused into the lumbar spine. It presents with progressive coronal imbalance, shoulder asymmetry and back pain, and is managed by distal extension of the fusion to include the adding-on levels, or rarely proximal extension if the shoulders are imbalanced. Prevention is careful pre-operative assessment of the lumbar modifier, fusing to the neutral or stable vertebra rather than one level proximal, and considering lumbar fusion when the modifier is B or C.

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Crankshaft phenomenon. Continued anterior spinal growth after a posterior-only fusion in a very immature patient: the anterior vertebral bodies keep growing, the posterior fusion mass acts as a tether, and the deformity progresses despite a solid posterior fusion. The risk factors are an open triradiate cartilage at surgery, Risser 0 and pre-pubertal age under 10. It is rare in AIS and more common in early-onset scoliosis. Prevention is to delay surgery until triradiate closure if possible, consider anterior fusion if surgery is required in a very immature patient, and rely on modern high-density pedicle screw constructs, which may resist crankshaft, although that is debated.

Hardware failure and revision. Late hardware failure occurs in under 5%. The overall revision rate is 5-10% at 10 years, for infection, pseudarthrosis, severe PJK, adding-on and hardware prominence, and most revisions occur in the first 2 years.

The first 24 hours. In ICU or HDU, neuromonitoring continues with SSEP/MEP for the first 2-4 hours to detect delayed neurological change, with hourly motor and sensory checks (5/5 strength in all extremities) thereafter. Analgesia is multimodal (IV opioids, ketorolac, paracetamol), the MAP goal is over 70 mmHg to maintain spinal cord perfusion, and mobility is log-roll only for the first 24 hours.

Mobilisation. Day 1, sit at the edge of the bed with physiotherapy and occupational therapy assistance and assess orthostatic tolerance. Day 2, stand and walk 5-10 metres under BLT precautions (no bending, lifting or twisting). Day 3, walk to the bathroom independently and practise stairs if needed for discharge. The goal is independent walking by POD 3-4.

Analgesia. IV opioids (morphine or oxycodone PCA) on POD 0-2, transitioning to oral opioids on POD 2-3, with paracetamol 1 g four times daily and ketorolac 10 mg three times daily (if no renal concerns, maximum 5 days) as adjuncts, and gabapentin or pregabalin if there is a neuropathic component. The goal is oral medication only by POD 3-4, and current practice uses these techniques to minimise opioid use while keeping the patient comfortable.

Drains and catheter. The subfascial drain is monitored every 4-6 hours and removed when output falls under 30 mL per 8 hours, typically POD 2-3; it reduces the risk of haematoma and seroma, and minimising its duration reduces infection risk. A urinary catheter, if placed, is removed on POD 1-2 once the patient is mobile, watching for retention afterwards.

Nutrition and bowels. Clear fluids on POD 0-1 once awake and free of nausea, a regular diet on POD 1-2 as tolerated, and high-protein, high-calorie nutrition for healing, because fusion formation over the following months depends on it. Opioid-induced ileus is common on POD 1-3: laxatives (docusate, senna) as needed, a bisacodyl suppository if there is no bowel movement by POD 3, and vigilance for intractable vomiting on POD 3-7, which raises SMA syndrome.

Discharge (POD 4-6). Hospital stay is typically 4-6 days. The criteria:

• Adequate pain control on oral medication
• Independent walking without an assistive device
• Tolerating a regular diet, with bowel function restored
• Normal neurological examination (5/5 strength, intact sensation)
• Wound clean, dry and intact with no sign of infection
• Patient and family educated on home precautions

Discharge medication is oral opioid (oxycodone 5 mg as needed, a 1-2 week supply), paracetamol 1 g four times daily, a bowel regimen (docusate, senna) and gabapentin if there is neuropathic pain.

Activity. BLT precautions (no bending, no lifting over 5 kg, no twisting) continue until 6 months, and return to activity is staged:

Return-to-sport clearance commonly requires written approval from the treating surgeon, for school or club liability, in many health systems.

Follow-up. The schedule after discharge:

The long term. Through years 1-5 the annual clinical and radiographic review is paired with the SRS-30 questionnaire to track quality of life, self-image and pain. After skeletal maturity with a solid fusion the patient is discharged from routine follow-up, returning only for pain or concern, and can be counselled that the curve should remain stable lifelong after a solid fusion.

Pregnancy. Raise it at skeletal maturity or in pre-pregnancy planning. AIS surgery does not contraindicate pregnancy, spinal fusion does not affect the ability to deliver vaginally, and epidural anaesthesia is usually possible provided the anaesthetist is told the fusion levels.

Sagittal profile. Direct vertebral rotation restores a physiological thoracic kyphosis of 20-40° and maintains the lumbar lordosis of 40-60°, avoiding the flat back syndrome that complicated the older hook systems. Sagittal balance matters more than coronal correction for long-term patient satisfaction, and correction, once achieved, is maintained with minimal loss over time.

Patient-reported outcomes. The SRS-30 scores five domains from 1 to 5, higher being better:

1. Pain: minimal change, because AIS is rarely painful before surgery
2. Self-image: the largest improvement, and the major driver of satisfaction
3. Function: returns to baseline by 6-12 months
4. Mental health: improves after surgery with body-image confidence
5. Satisfaction: over 90% satisfied at 2 or more years

The primary benefit of surgery is therefore the improvement in self-image, not pain relief, which was minimal to begin with.

Cardiopulmonary function. A curve corrected to under 50° gives normal pulmonary function lifelong and no increase in mortality against the general population.

Back pain. SRS adult outcome data show that treated AIS, whether by surgery or brace, has a back pain prevalence similar to the general population at 20-30 years, and a fusion does not cause increased back pain if the spine is balanced. The untreated large curve is the exception, as set out under natural history.

Adjacent segment degeneration. A fusion increases stress on the adjacent mobile segments. Mild degenerative change is seen on MRI in 30-40% at 10 or more years, asymptomatic, and symptomatic adjacent segment disease requiring surgery occurs in under 5% at 20 years; the risk factors are fusion to L3 or more distal, with its higher lumbar stress, and sagittal imbalance. Selective fusion, sparing the lumbar spine when possible, reduces adjacent segment problems, and the overall complication and revision figures are set out under Complications.

Predictors of a good outcome. The favourable factors:

• A pre-operative curve of 45-70°, the optimal surgical range
• A balanced spine after surgery (C7 plumb line within 2 cm of the sacrum)
• Restoration of thoracic kyphosis (DVR technique)
• Selective fusion, sparing lumbar segments when appropriate
• Patient satisfaction with the cosmetic improvement

The unfavourable factors:

• A pre-operative curve over 90°: rigid, difficult to correct, higher complication rate
• Sagittal imbalance after surgery (flat back, positive sagittal balance)
• Over-fusion, fusing compensatory lumbar curves unnecessarily
• Persistent shoulder asymmetry from failure to address a structural proximal thoracic curve

Prognosis. For appropriately selected and treated patients the prognosis is excellent: normal life expectancy and quality of life, low complication and revision rates with modern techniques, and full participation in activities including sport and pregnancy. AIS surgery succeeds when it is done for the right indications with meticulous technique and proper fusion level selection.

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, including FDA Class II/III pathways in the USA, CE-marked Class III pathways in Europe, and comparable national or regional implantable-device frameworks elsewhere; 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.

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