Scoliosis — Cobb Angle & Surgical Indications

Overview & Classification

Scoliosis is a three-dimensional spinal deformity defined by a lateral curvature of the spine of 10° or greater (Cobb angle) in the coronal plane, almost invariably accompanied by rotation of the vertebral bodies (the rotational component causes the rib hump seen on Adam`s forward bend test) and an alteration of the normal sagittal profile. It is not simply a lateral bend — the rotational component is fundamental to its pathoanatomy and must be addressed in surgical planning. Understanding the various aetiological classifications, the measurement tools, the natural history by curve type and patient characteristics, and the evidence-based indications for surgical intervention is essential.

Type	Aetiology	Key Features	Most Common / Notes
Adolescent idiopathic scoliosis (AIS)	Unknown — likely multifactorial (genetic + biomechanical); positive family history in 30%	Onset 10–18 years; female predominance (larger curves — F:M ratio for curves requiring treatment is 10:1); right thoracic curve is the most common (70%); by definition, a diagnosis of exclusion — must exclude neuromuscular, congenital, and syndromic causes	MOST COMMON scoliosis type (~80% of all scoliosis); a single left thoracic or unusual curve pattern should prompt MRI to exclude an underlying cause (syrinx, tethered cord)
Congenital scoliosis	Failure of formation (hemivertebra) or failure of segmentation (unilateral unsegmented bar) during embryogenesis; may be combined	Present from birth; associated anomalies in 60% — cardiac (VACTERL — Vertebral, Anorectal, Cardiac, Tracheo-Esophageal, Renal, Limb defects); renal anomalies in 20–33%; Klippel-Feil syndrome; occult spinal cord anomalies in 20%	A unilateral unsegmented bar (failure of segmentation) opposite a contralateral hemivertebra = worst prognosis (certain progression); all congenital scoliosis requires full imaging workup (renal USS, cardiac ECHO, full spine MRI)
Neuromuscular scoliosis	Upper motor neuron (cerebral palsy, spinal cord injury) or lower motor neuron (spina bifida, spinal muscular atrophy, Duchenne muscular dystrophy, Charcot-Marie-Tooth); spinal cord tumours	Long C-shaped curves extending into the pelvis (pelvic obliquity); rapidly progressive; may affect respiratory function; pelvis often involved (Cobb from T1 to pelvis); Duchenne MD — 90% develop progressive scoliosis; treatment often includes pelvic fixation	Surgical thresholds often lower than AIS (operate earlier due to rapid progression and respiratory compromise risk); outcomes depend on underlying condition and non-ambulatory status
Degenerative (de novo) adult scoliosis	Asymmetric disc degeneration, facet joint OA, and vertebral body remodelling in adulthood; not present in childhood	Adults >50 years; pain-dominant presentation (unlike AIS which is predominantly cosmetic in mild-moderate cases); associated with spinal stenosis and neurogenic claudication; lumbar or thoracolumbar location	Increasingly common with ageing population; treatment goals in adults are pain relief and neurological decompression, not curve correction per se
Syndromic scoliosis	Marfan syndrome, Neurofibromatosis type 1, Ehlers-Danlos syndrome, osteogenesis imperfecta	Each syndrome has characteristic curve patterns and treatment challenges; Marfan scoliosis — long sweeping curves, ligamentous laxity, dural ectasia, high risk of pseudarthrosis after fusion; NF1 — short angular `dystrophic` curves that progress even after bracing, require early surgery	Syndromic scoliosis should be managed at specialist centres; associated systemic features complicate management

Measurement — Cobb Angle

Cobb angle measurement technique: on a standing PA radiograph of the spine, identify the upper end vertebra (the most tilted vertebra at the top of the curve — the end vertebra whose superior endplate is most tilted toward the concavity) and the lower end vertebra (the most tilted vertebra at the bottom of the curve — whose inferior endplate is most tilted toward the concavity); draw a line along the superior endplate of the upper end vertebra; draw a line along the inferior endplate of the lower end vertebra; the Cobb angle is the angle between these two lines (or between perpendiculars to these lines); reproducibility is moderate — intraobserver variability ±3–5°; a change of >5° is considered clinically significant on serial measurements
The Risser sign: an important indicator of skeletal maturity used to guide treatment decisions; assesses ossification of the iliac apophysis from lateral (Grade 1 — 25% ossification) to medial (Grade 4 — 100% ossification) and fusion to the iliac crest (Grade 5 — complete fusion); Risser 0 = immature skeleton (physis open, high growth remaining, high curve progression risk); Risser 5 = mature skeleton (curve progression risk very low); the Risser sign is used to predict curve progression and determine when to stop bracing
Other maturity markers: Tanner stage; Sanders hand-wrist maturity score (8-stage assessment of thumb metacarpal, phalangeal, and sesamoid ossification — more granular assessment of skeletal maturity than Risser); peak height velocity (PHV) — the period of maximum growth velocity is associated with the greatest risk of curve progression; PHV in girls approximately age 12, boys approximately age 14; growth remaining after the PHV decreases rapidly, as does curve progression risk; the `risser 0, Sanders Stage 2` patient (peak velocity phase) has the highest progression risk
The Lenke classification (AIS): the most widely used classification for surgical planning in AIS; classifies curves into 6 main types based on curve location (thoracic, thoracolumbar, lumbar), which curves are structural vs compensatory (on side-bending films — a structural curve does not correct to <25° on side-bending), and the sagittal profile modifier (T, N, L for thoracic kyphosis); determines which levels need to be fused; guides selection of fusion levels and implant strategy

Natural History & Progression Risk

Factor	Higher Progression Risk	Lower Progression Risk
Skeletal maturity	Risser 0–1 (immature); pre-menarche; Sanders Stage 2; high growth remaining	Risser 4–5 (mature); post-menarche; minimal growth remaining
Sex	Female — 10× more likely to progress to surgery than males with the same curve	Male — lower progression risk
Curve magnitude	Cobb >25° at presentation in a skeletally immature patient — high progression risk; curves >45° in the skeletally mature will continue to progress at approximately 1°/year in adulthood	<25° in skeletally mature patient — generally stable
Curve type / location	Double curve patterns; thoracic curves; longer structural curves	Lumbar curves; single minor thoracic curves
Rotation	High apical vertebral rotation (Nash-Moe Grade II–III) at presentation	Minimal rotation

Long-term natural history of untreated AIS: the SRS/Weinstein Iowa natural history study showed that most patients with AIS have a satisfactory quality of life in adulthood; back pain prevalence is similar to the general population for thoracic curves; cosmetic concerns remain the primary issue; severe untreated thoracic curves (>100°) may cause cardiopulmonary compromise but this is uncommon in the modern era of earlier detection; thoracic curves >45° and lumbar curves >30° tend to progress in adulthood at approximately 1–2°/year; pulmonary function is significantly compromised only in curves >90°

Management

Cobb Angle (AIS)	Skeletal Maturity	Treatment	Evidence / Notes
<25°	Any	Observation — clinical and radiological review every 4–6 months during growth; reassure patient and family; physiotherapy (Schroth method — scoliosis-specific exercise) may be offered; no active treatment indicated for small curves	Schroth physiotherapy — evidence for modest curve improvement and stabilisation; not proven to prevent surgery but may improve quality of life and delay progression
25°–45°	Skeletally immature (Risser 0–2)	Bracing — the BRAIST trial (Weinstein et al., NEJM 2013): the first high-quality RCT demonstrating that bracing significantly reduces the risk of curve progression to surgical threshold (>50°) vs observation alone; success rate (treatment success defined as Cobb <50° at skeletal maturity) ~72% brace vs 48% observation; brace must be worn ≥18 hours/day for maximum effectiveness (dose-response relationship); the Boston TLSO (thoracolumbar sacral orthosis) is the most widely used design; full-time wear during the growth spurt is critical	BRAIST trial landmark evidence; dose-response confirmed (more hours wearing = better outcome); brace stopped when Risser 5 or no further progression; night-time-only bracing (Providence nocturnal brace) — evidence supports for curves <35° and correction >50% in brace
>45–50° (skeletally immature) or >40–45° (skeletally mature and progressing)	Any	Surgical — posterior spinal fusion with pedicle screw instrumentation and deformity correction; the gold standard surgical treatment; fusion across the structural curves only (compensatory curves not fused — they correct spontaneously); selective thoracic fusion or selective lumbar fusion depending on the Lenke curve type	Surgery achieves approximately 60–70% curve correction; prevents further progression; improves cosmesis; long-term outcomes excellent for AIS; blood loss can be significant — cell salvage, antifibrinolytics (tranexamic acid), and controlled hypotension reduce transfusion requirements; intraoperative neurophysiological monitoring (IONM — MEPs + SSEPs) is mandatory to detect spinal cord injury early during correction

Surgical Principles & Growing Spine Strategies

Posterior spinal fusion (PSF) with pedicle screws: the standard surgical treatment for AIS; pedicle screws provide three-dimensional control of the vertebral body through the pedicle; the Ponte osteotomy (posterior element releases — resection of the posterior facet joints and ligamentum flavum at the apex of the curve) is commonly performed to improve curve flexibility and facilitate correction; the rods are contoured to the desired corrected shape and connected to the pedicle screws; derotation manoeuvres (direct vertebral rotation) are used to correct the rotational deformity; selective fusion avoids unnecessary motion segment sacrifice
Growing rod techniques (early-onset scoliosis): for children <5 years (or <8 years in severe early-onset scoliosis) where definitive fusion would unacceptably restrict thoracic growth and cause `thoracic insufficiency syndrome` (small thorax, restrictive lung disease); growing rods are implanted with proximal (above the curve) and distal (below the curve) anchors, and a rod that can be lengthened periodically; traditional growing rods — lengthened every 6 months under general anaesthesia; MCGR (magnetically controlled growing rods) — lengthened non-invasively using an external magnet in clinic (the EOS MAGEC system); eliminates repeated GA for lengthening; ultimately requires definitive fusion at skeletal maturity
Vertebral body tethering (VBT): a newer motion-preserving alternative to spinal fusion for AIS; anterior screws are placed in the vertebral bodies of the curve; a polyethylene cord (tether) is placed under tension between the screws on the convex side; the tether generates compressive force on the convex (right) side and allows the concave (left) side to grow — a Hueter-Volkmann principle applied to the spine; gradual correction with growth while preserving motion segments; used in skeletally immature patients (Risser 0–1) with curves 40–65°; not yet FDA-approved in the UK as a standard of care — evidence still accumulating; risk of overcorrection if patients continue growing after tethering

Exam Pearls

Scoliosis definition: ≥10° Cobb angle lateral curvature + rotation; right thoracic most common in AIS; left thoracic = atypical → MRI to exclude syrinx/tethered cord; AIS = diagnosis of exclusion
Cobb angle: upper end vertebra superior endplate line + lower end vertebra inferior endplate line; reproducibility ±3–5°; change >5° = significant; measured on standing PA X-ray
Risser sign: iliac apophysis ossification 1–5; Risser 0 = high progression risk; Risser 5 = mature, no progression; Risser 0 + Sanders Stage 2 = peak velocity = highest risk period
BRAIST trial (NEJM 2013): bracing significantly reduces risk of curve progression to surgical threshold; success 72% brace vs 48% observation; ≥18 hours/day = dose-response benefit; Boston TLSO most common brace
Surgical indication (AIS): Cobb >45–50° in skeletally immature; >40–45° in mature with documented progression; posterior spinal fusion with pedicle screws; 60–70% curve correction; IONM (MEPs + SSEPs) mandatory
Bracing indication: 25–45° in Risser 0–2 immature skeleton; Boston TLSO; ≥18 hours/day; stop at Risser 5; avoid in >45° or mature skeleton
Congenital scoliosis: hemivertebra (failure of formation) vs bar (failure of segmentation); unilateral bar + contralateral hemivertebra = worst prognosis; full workup — renal USS, cardiac ECHO, full spine MRI; VACTERL association
Neuromuscular scoliosis: long C-curve to pelvis; rapidly progressive; involves pelvic obliquity; lower surgical threshold; often requires pelvic fixation; Duchenne MD — operate while FVC >50% predicted (respiratory function assessment)
Growing rod / MCGR: early-onset scoliosis <5–8 years; preserve thoracic growth; MCGR (MAGEC) — non-invasive lengthening in clinic; eventually fuse at maturity; avoids thoracic insufficiency syndrome
Adam`s forward bend test: rib hump (angle of trunk rotation — ATR); scoliometer >5–7° = refer for X-ray; used in school screening programmes

Scoliosis — Cobb Angle & Surgical Indications

References

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