Introduction / Definition

Adolescent idiopathic scoliosis (AIS) is a structural, three-dimensional torsional deformity of the spine and trunk occurring in children between the age of 10 and skeletal maturity. By definition, it requires a coronal plane Cobb angle of ≥ 10° measured on a standing whole-spine radiograph, accompanied by vertebral rotation and loss of normal sagittal contours (typically thoracic hypokyphosis or lordosis).

Epidemiology

• Prevalence: Approximately 2% to 3% of the adolescent population have curves >10°, while curves >30° affect roughly 0.2% to 0.3%.
• Sex distribution: While mild curves (10°–20°) demonstrate an equal female-to-male ratio (1:1), curves progressing to treatment thresholds (>30°) exhibit a marked female predominance (ranging from 5:1 to 10:1).
• Aetiology: Multifactorial with strong genetic susceptibility (polygenic inheritance). Genome-wide association studies (GWAS) have linked variants near LBX1 and GPR126 to susceptibility. Biomechanical factors involve relative anterior spinal overgrowth causing anterior column lengthening relative to the posterior elements, yielding hypokyphosis and compensatory rotational torsion (Heuter-Volkmann law).

Relevant Anatomy

• Vertebral wedging and rotation: Torsional rotation directs the vertebral bodies towards the convexity of the curve while spinous processes rotate towards the concavity.
• Rib hump deformity: Rotation of the thoracic vertebrae causes projection of the attached ribs backwards on the convexity, forming the characteristic clinical rib prominence.
• Apical, end, and neutral vertebrae:
  • Apical vertebra: The most rotated vertebra with greatest horizontal displacement from the central sacral vertical line (CSVL).
  • End vertebrae: Most cephalad and caudal vertebrae maximally tilted into the concavity of the curve, defining the Cobb angle limits.
  • Neutral vertebra: The most cephalad and caudal vertebrae with symmetrical pedicles (no rotational deformity) on standard posteroanterior views.
  • Stable vertebra: The most proximal vertebra below the Cobb curve bisected by the CSVL.

Classification

Lenke Classification System

The standard operative classification system comprises three components: Curve Type (1–6), Lumbar Spine Modifier (A, B, C), and Sagittal Thoracic Modifier (-, N, +).

1. Curve Types (1 to 6)

Determined across three regions: Proximal Thoracic (PT), Main Thoracic (MT), and Thoracolumbar/Lumbar (TL/L). The largest Cobb angle is designated the Major curve (always structural). Minor curves are deemed Structural if they meet either:

• Residual coronal Cobb angle ≥ 25° on maximum side-bending radiographs, or

• Sagittal kyphosis (T2–T5 for PT, T10–L2 for MT or TL/L) ≥ +20°.

• Type 1 (Main Thoracic): MT is major/structural; PT and TL/L are non-structural.

• Type 2 (Double Thoracic): MT is major; PT is structural; TL/L is non-structural.

• Type 3 (Double Major): MT is major; TL/L is structural; PT is non-structural. (MT ≥ TL/L).

• Type 4 (Triple Major): All three regions (PT, MT, TL/L) are structural.

• Type 5 (Thoracolumbar/Lumbar): TL/L is major/structural; PT and MT are non-structural.

• Type 6 (Lumbar-Main Thoracic): TL/L is major (≥ 5° larger than MT); MT is structural.

2. Lumbar Spine Modifier (A, B, C)

Based on the relationship of the Central Sacral Vertical Line (CSVL) to the apex of the lumbar curve:

• A: CSVL passes between the lumbar pedicles up to the apical vertebra.
• B: CSVL touches the medial border of the apical lumbar vertebral body or pedicle.
• C: CSVL falls completely medial to the lateral edge of the apical lumbar vertebral body.

3. Thoracic Sagittal Modifier

Measured from T5 to T12 on the standing lateral radiograph:

• Minus (-): Hypokyphosis (<10°).
• Normal (N): Kyphosis between 10° and 40°.
• Plus (+): Hyperkyphosis (>40°).

Clinical Presentation & Examination

Patients typically present asymptomatically due to posture asymmetry identified by family or school screening programmes.

Physical Examination

• Inspection & Alignment: Evaluate shoulder height balance (coronal balance), scapular asymmetry, trunk shift, flank creases, and pelvic obliquity.
• Adams Forward Bending Test: Diagnostic test highlighting rotational deformity; measured with a Scoliometer. An Angle of Trunk Rotation (ATR) ≥ 7° reliably correlates with a coronal Cobb angle ≥ 20° and warrants radiographical investigation.
• Plumb-line Assessment: Coronal balance assessed via a plumb line dropped from C7 spinous process (should bisect the gluteal cleft).
• Neurological & Stigmata Screen: Exclude secondary causes (neurofibromatosis, Marfan, spinal dysraphism) by inspecting for café-au-lait spots, axillary freckling, sacral dimples, and hairy patches. Detailed neurological examination must assess abdominal reflexes (asymmetry suggests syringomyelia or Chiari malformation), deep tendon reflexes, and foot alignment (e.g. pes cavus).

Investigations

Radiographs

• Standing PA and Lateral 36-inch Spine Films: Confirm diagnosis, measure Cobb angles, and calculate coronal/sagittal balance.
• Supine Maximum Side-Bending Radiographs: Crucial for assessing flexibility of curves and distinguishing structural from non-structural components under the Lenke classification.
• Skeletal Maturity Assessment:
  • Risser Sign: Measures ossification of the iliac apophysis from lateral to medial (stages 1 to 4), followed by fusion (stage 5).
  • Sanders Maturity Scale: Validated system based on hand/wrist epiphyses on a Tanner-Whitehouse (TW3) model; superior to Risser in predicting the peak height velocity (PHV) and curve progression risk.

Magnetic Resonance Imaging (MRI)

Routine screening MRI of the whole neural axis is indicated when red flags exist:

• Age <10 years at presentation.
• Left-sided thoracic curve (atypical curve pattern).
• Rapid progression (>1°/month).
• Neurological signs or asymmetric abdominal reflexes.
• Severe intractable pain or headache.
• Excessive thoracic hyperkyphosis (>40°) or atypical foot deformities.

Management

Non-Operative Treatment

1. Observation:
   • Indicated for skeletally immature patients with Cobb angle <25° or mature patients with curves <40–45°.
   • Follow-up serial radiographs every 6 months during periods of rapid skeletal growth.
2. Orthotic Bracing:
   • Indications: Skeletally immature patients (Risser 0–2, Sanders stage ≤ 4) with curve magnitudes between 25° and 40–45°, or documented rapid progression (>5°) in curves <25°.
   • Brace Types: Rigid thoracolumbosacral orthosis (TLSO, e.g. Boston brace) or night-time bending braces (Charleston/Providence) for isolated, flexible single curves.
   • Evidence: The landmark BrAIST Trial (Weinstein et al., NEJM 2013) demonstrated that bracing significantly reduced progression to surgical threshold (50°), showing a direct dose-response relationship between daily wear time (>12–18 hours/day) and success rate.

Operative Treatment

• Indications: Progressive structural curves exceeding 45^–50° in skeletally immature adolescents, or progressive curves >50^–55° in skeletally mature individuals.
• Goals: Prevent adult progression, achieve 3D coronal and sagittal realignment while preserving maximum motion segments, and ensure solid arthrodesis without neurological compromise.

Surgical Techniques & Principles

• Posterior Spinal Fusion (PSF) with Instrumentation:
  • Gold-standard modern procedure. Employs segmental pedicle screw fixation constructs providing superior 3D rotational control and sagittal restoration compared to hook/hybrid systems.
  • Level Selection:
    • Upper Instrumented Vertebra (UIV): Guided by shoulder height. When the left proximal thoracic curve is structural or the left shoulder is elevated, instrument to T2; if the right shoulder is elevated, T3 or T4 is selected.
    • Lower Instrumented Vertebra (LIV): Chosen based on the Stable Vertebra (SV) and Touched Vertebra (TV). In non-selective thoracic fusion, LIV is usually the vertebra touched by the CSVL (Last Substantially Touched Vertebra - LSTV).
• Ponte (Smith-Petersen) Osteotomies: Posterior column chevron osteotomies resecting spinous processes, interspinous ligaments, and facet joints bilaterally over multiple levels to restore thoracic kyphosis and mobilise stiff structural curves.
• Intraoperative Neuromonitoring (IONM):
  • Standard of care utilizing continuous Somatosensory Evoked Potentials (SSEPs) and Transcranial Motor Evoked Potentials (tc-MEPs).
  • An amplitude drop >50% in MEPs or >50% drop with >10% latency increase in SSEPs constitutes an alert.
  • Action protocol: Halt traction/distraction, correct mean arterial pressure (>80–85 mmHg), reverse recent correction/implants, and perform a Stagnara wake-up test if signals fail to recover.

Complications

• Neurological Injury: Incidence approx. 0.3% to 1.0%; caused by direct cord compression, vascular ischaemia, or distraction.
• Infection: Deep surgical site infection (1–2%); commonly caused by Cutibacterium acnes (formerly Propionibacterium acnes) or Staphylococcus aureus.
• Add-on Phenomenon and Distal Junctional Kyphosis (DJK): Progressive angulation or translation of vertebrae immediately adjacent to the LIV, frequently resulting from inappropriately short fusion levels.
• Crankshaft Phenomenon: Progressive rotational deformity and lordosis following posterior fusion alone in very immature patients (open triradiate cartilage, Sanders 1–2).
• Superior Mesenteric Artery (SMA) Syndrome (Wilkie's Syndrome): Compression of the third part of the duodenum between the superior mesenteric artery and aorta after spine lengthening, presenting with intractable post-operative bilious vomiting.

Exam Pearls

• Always inspect triradiate cartilages on radiographs: open triradiate cartilage identifies high risk for crankshaft phenomenon if posterior-only fusion is performed without anterior epiphyseodesis.
• An Angle of Trunk Rotation (ATR) ≥ 7° corresponds to a 20° coronal Cobb angle.
• The BrAIST study proved a direct relationship between daily brace wear hours and treatment success; >12.9 hours/day achieved >90% success in halting curve progression.
• In Lenke Type 1B and 1C curves, scrutinise whether selective thoracic fusion can be performed without fusing the lumbar spine: check lumbar flexibility on side bending (<25° residual curve confirms non-structural lumbar curve).