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Chapter Summary
• Context: Spinal deformities correspond to alterations in physiological spinal curvatures or the appearance of abnormal deviations in the coronal, sagittal, and axial planes. Although scoliosis, kyphosis, and lordosis are traditionally described as separate entities, modern spine surgery conceptualizes all clinically relevant spinal deformities as complex, three-dimensional (3D) spatial phenomena governed by vertebral morphology, axial rotation, skeletal growth, global spinopelvic alignment, and neuromuscular compensatory mechanisms. Deformities encompass a diverse spectrum of underlying etiologies: idiopathic (infantile, juvenile, adolescent [AIS], adult degenerative), congenital (segmentation failures, formation failures), neuromuscular (cerebral palsy, spinal muscular atrophy, Duchenne muscular dystrophy), syndromic/mesenchymal (Marfan syndrome, Ehlers-Danlos syndrome, osteogenesis imperfecta), neurofibromatosis (NF-1 dystrophic and non-dystrophic), post-traumatic, post-laminectomy, infectious, and neoplastic deformities. Identifying a spinal curve is merely the starting point of diagnostic synthesis: clinical history, physical inspection (Adam's forward bend test, trunk balance, skin stigmata), pain characterization, neurological examination, skeletal maturity assessment (Risser sign, Sanders bone age, open triradiate cartilage), and standardized standing 36-inch radiographs must be integrated to determine curve structurality, predict future progression risk, and guide individualized conservative versus surgical management.
• Chapter Objective: To present the fundamental concepts, standardized terminology, classification principles, and diagnostic clinical/radiographic evaluation of spinal deformities. The reader should be able to understand the three-dimensional nature of scoliosis and kyphosis; identify structural versus non-structural curves; recognize red-flag atypical curve patterns requiring advanced neuroimaging (MRI); assess skeletal maturity and progression risk; and utilize standardized radiographic parameters (Cobb angle, apical vertebra, neutral/stable vertebrae, pelvic parameters) to characterize spinal alignment.
• Spinal Deformity as a Three-Dimensional PhenomenonSpinal deformities alter normal regional alignment across all three dimensions simultaneously: 1) Coronal plane (lateral curvature measured by the Cobb angle; scoliosis is defined as a coronal Cobb angle >=10° with vertebral rotation); 2) Axial plane (vertebral axial rotation, causing the posterior spinous processes to rotate into the concavity while the vertebral body and attached ribs rotate toward the convexity, forming the diagnostic rib hump or lumbar prominence); 3) Sagittal plane (hypokyphosis/lordoscoliosis in thoracic adolescent idiopathic scoliosis, hyperkyphosis in Scheuermann disease, loss of lumbar lordosis in degenerative flatback deformity).
• Etiological Spectrum and PathogenesisDeformities are categorized etiologically: 1) Idiopathic: most common (80% of all scoliosis), divided by age of onset into Infantile (<3 years), Juvenile (3–9 years), Adolescent (AIS, 10–18 years), and Adult De Novo (>18 years); 2) Congenital: embryological malformations during weeks 4–8 (hemivertebrae, wedge vertebrae, unsegmented unilateral bars, block vertebrae); 3) Neuromuscular: long C-shaped collapsing curves with pelvic obliquity due to upper or lower motor neuron disease; 4) Syndromic and Connective Tissue: Marfan syndrome, Ehlers-Danlos, Down syndrome; 5) Neurofibromatosis Type 1 (NF-1): characterized by sharp, short-segment angular dystrophic curves with rib penciling and scalloping; 6) Secondary/Degenerative: asymmetric disc/facet collapse in adults.
• Structural vs. Non-Structural CurvesA fundamental diagnostic step is distinguishing structural from non-structural (compensatory) curves using supine maximum lateral bending radiographs: 1) Structural curves possess intrinsic rigidity, fail to correct below 20°–25° on dynamic bending films, and exhibit fixed axial vertebral rotation toward the convexity (Nash-Moe or Perdriolle grading); 2) Non-structural (compensatory) curves develop above or below a primary structural curve to maintain coronal head balance, lack fixed rotation, and demonstrate complete or near-complete flexibility (<20°–25°) on bending radiographs.
• Curve Characterization: Apex, Cobb Angle, and Key VertebraeStandardized terminology defines curve anatomy: 1) Apical Vertebra: the vertebra most deviated from the central sacral vertical line (CSVL) and exhibiting maximal axial rotation and horizontal tilt; 2) End Vertebrae (Superior and Inferior): the vertebrae at the cranial and caudal limits of the curve that are most tilted toward the concavity; 3) Cobb Angle: the angle formed between lines drawn along the superior endplate of the upper end vertebra and the inferior endplate of the lower end vertebra; 4) Neutral Vertebra: the most cranial/caudal vertebra with symmetric, non-rotated pedicles; 5) Stable Vertebra: the most proximal lumbar vertebra bisected by the CSVL.
• Skeletal Maturity and Progression RiskCurve progression risk depends directly on remaining skeletal growth and initial curve magnitude. Skeletal maturity is assessed via: 1) Tanner stage (secondary sexual characteristics, peak height velocity at Tanner 2–3 in girls); 2) Risser sign (iliac apophysis ossification from anterolateral [Risser 1] to posteromedial [Risser 4] and complete fusion [Risser 5]; Risser 0–2 represents high progression risk); 3) Triradiate Cartilage status (open acetabular triradiate cartilage indicates the peak adolescent growth spurt is yet to occur, carrying the highest progression risk); 4) Sanders Bone Age (hand/wrist radiograph grading 1–8, providing superior precision over Risser sign during peak growth).
• Comprehensive Spinopelvic and Coronal AssessmentFull-length 36-inch standing radiographs evaluate global balance: Coronal balance is assessed via the C7 plumb line (measured relative to the CSVL, normal <1.5–2.0 cm offset). Sagittal balance evaluates the Sagittal Vertical Axis (SVA), thoracic kyphosis (T5–T12 Cobb angle), lumbar lordosis (T12–S1), and pelvic parameters (Pelvic Incidence [PI], Pelvic Tilt [PT], Sacral Slope [SS]). Identifying atypical curves—such as left-sided thoracic curves, rapid progression, severe pain, or focal neurological deficits—mandates emergent whole-spine MRI to rule out syringomyelia, Chiari I malformation, tethered cord, or spinal tumors.
• Clinical Application: In outpatient spine practice, evaluate every patient presenting with spinal asymmetry through a structured step-by-step approach. Perform Adam's forward bend test with a Scoliometer: an Angle of Trunk Rotation (ATR) >=5°–7° warrants standing 36-inch spinal radiographs. Calculate the Cobb angle and determine skeletal maturity (Risser score and triradiate cartilage status). In a 12-year-old pre-menarchal girl with Adolescent Idiopathic Scoliosis (AIS), Risser 0, open triradiate cartilage, and a 25° right thoracic curve, recognize the high risk of rapid progression (>70% risk), mandating immediate rigid full-time bracing (TLSO / Boston brace worn 18–23 hours/day) and serial radiographs every 4–6 months. In any patient presenting with an atypical left-sided thoracic curve, unexplained nocturnal back pain, abnormal abdominal cutaneous reflexes, or foot asymmetry (cavus foot), obtain an urgent whole-spine MRI: up to 20–30% of left thoracic curves harbor occult neuroaxial abnormalities (Chiari I malformation, syringomyelia, or spinal cord tumors) that require neurosurgical decompression prior to any corrective spinal intervention.
