Cobb Angle in Scoliosis

Definition and Historical Background

• Scoliosis is defined as an abnormal curvature of the spine in the coronal plane, typically presenting as an “S” or “C” shaped deformity.

• A spinal curvature greater than 10 degrees, measured using the Cobb method, is considered diagnostic of scoliosis.

• Hippocrates (460–377 BC) was the first to describe abnormal spinal curvature and introduced the term scoliosis, derived from the Greek word skolios, meaning crooked.

• Claudius Galen (131–201 AD) classified spinal deformities into scoliosis, kyphosis, and lordosis.

• The Growing Spine Study Group and the Children Spine Study Group define early onset scoliosis as any spinal deformity presenting before the age of ten years, irrespective of etiology.

• Adolescent idiopathic scoliosis refers to spinal curvature developing during the adolescent growth spurt, typically between 10 and 18 years of age, without an identifiable cause.

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Idiopathic Scoliosis

• Idiopathic scoliosis has no identifiable causal agent and is not associated with other systemic diseases.

• According to the Scoliosis Research Society, idiopathic scoliosis is classified into:

  1. Infantile idiopathic scoliosis: younger than 3 years

  2. Juvenile idiopathic scoliosis: 4 to 10 years

  3. Adolescent idiopathic scoliosis: 10 to 18 years

• Severity of curvature:

  • Mild: less than 25 degrees

  • Moderate: 25 to 50 degrees

  • Severe: greater than 50 degrees

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Epidemiology and Demographics

• Adolescent idiopathic scoliosis is the most common form of scoliosis.

• A positive family history is present in a significant proportion of patients.

• Prevalence is approximately 4 percent of adolescents.

• Incidence:

  • Curves between 10 and 20 degrees: approximately 3 percent

  • Curves greater than 30 degrees: approximately 0.3 percent

• Gender distribution:

  • Mild curves: male to female ratio of 1:1

  • Curves greater than 30 degrees: female predominance up to 10:1

• Thoracic curves are more common than lumbar curves.

• The most common pattern is a right-sided thoracic curve.

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Pathophysiology

• The exact etiology remains unknown.

• Proposed contributing factors include:

  1. Genetic predisposition

  2. Neurological abnormalities

  3. Hormonal and metabolic factors

  4. Skeletal growth imbalance

  5. Biomechanical influences

  6. Environmental and lifestyle factors

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Clinical Presentation and History

• Often detected due to cosmetic concerns raised by parents or caregivers.

• Frequently identified through school screening programs.

• A scoliometer reading greater than 7 degrees correlates with an approximate 20-degree coronal plane curve.

• Most patients are asymptomatic, with:

  • No significant back pain

  • No neurological complaints

• Important historical points include:

  • Age at first detection

  • Evidence of progression

  • Perinatal and developmental history

  • Family history

  • Menstrual history in female patients

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Physical Examination

General Inspection

• Attention to body symmetry is critical.

• Common findings include:

  • Uneven shoulders

  • Prominence of one scapula

  • Lateral deviation of the spine

  • Rib prominence on one side

  • Unequal hip levels

  • Asymmetric waistline

  • Head not centered over the pelvis

• Look for:

  • Café-au-lait spots or skin nevi, suggesting neurocutaneous disorders

  • Foot deformities such as cavovarus feet, which may indicate neural axis abnormalities and require magnetic resonance imaging

• Serial height measurements help identify peak height velocity, which correlates with curve progression.

• Limb length discrepancy should be excluded, as it can cause compensatory scoliosis.

Spine Examination

• Inspect for midline skin abnormalities such as hairy patches or dimples, suggestive of spinal dysraphism.

• Assess for rib rotational deformity.

• Adams forward bending test is the most important clinical screening test:

  • The patient bends forward while the examiner observes for rib or lumbar prominence.

  • Trunk asymmetry indicates a structural curve.

  • A scoliometer angle less than 7 degrees is considered normal.

• Forward bending in the sitting position helps eliminate limb length inequality.

Neurological Examination

• Motor strength and sensory testing of upper and lower limbs

• Deep tendon reflexes

• Assessment for abnormal abdominal reflexes, clonus, Hoffmann sign, and Babinski sign

• Gait analysis and evaluation of developmental milestones

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Curve Progression

• Major risk factors for progression include:

  1. Initial curve magnitude

  2. Curve pattern

  3. Remaining skeletal growth

Skeletal Maturity Assessment

• Tanner staging

• Risser staging based on iliac apophyseal ossification:

  • Risser stage zero indicates significant growth remaining

  • Higher stages indicate reduced growth potential

• After skeletal maturity:

  • Thoracic curves greater than 50 degrees progress by 1 to 2 degrees per year

  • Lumbar curves greater than 40 degrees progress at a similar rate

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Imaging

Plain Radiographs

• Standing posteroanterior and lateral views are recommended.

• Radiographs should include:

  • Cervical spine proximally

  • Iliac crests distally

• Supine side-bending views are used for preoperative planning.

• Key radiographic parameters assessed:

  • End vertebrae

  • Apical vertebra

  • Stable and neutral vertebrae

  • Curve location and direction

  • Curve magnitude

  • Risser sign

Measurement Techniques

• Cobb angle:

  • A curve greater than 10 degrees defines scoliosis

  • Interobserver and intraobserver error is approximately 3 to 5 degrees

• Coronal balance is assessed using the relationship between the cervical spine plumb line and the central sacral vertical line.

• Sagittal balance is evaluated from the cervical spine to the sacrum.

• Clavicle angle is a strong predictor of postoperative shoulder balance.

Vertebral Rotation

• Nash–Moe method is used to estimate vertebral rotation based on pedicle position.

• Provides a rough approximation and is limited by vertebral asymmetry.

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Magnetic Resonance Imaging

• Imaging should extend from the posterior fossa to the conus medullaris.

• Purpose is to exclude intraspinal pathology.

• Indications include:

  • Atypical curve patterns

  • Rapid progression

  • Excessive kyphosis

  • Neurological symptoms

  • Foot deformities

  • Asymmetric abdominal reflexes

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Classification Systems

• Schulthess classification

• Ponseti and Friedman classification

• King–Moe classification

• Lenke classification (most widely used)

Lenke Classification

• Based on:

  1. Identification of the primary curve

  2. Lumbar modifier

  3. Thoracic sagittal modifier

• Helps determine which curves require inclusion in the fusion construct.

• Allows standardized surgical planning.

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Management Considerations

• Remaining growth potential

• Curve magnitude and pattern

• Risk of progression

• Sex and genetic risk profile

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Non-Surgical Management

Observation

• Indicated for curves less than 25 degrees.

• Regular clinical and radiographic follow-up is essential.

• Frequency depends on curve magnitude and skeletal maturity.

Bracing

• Indicated for progressive curves between 25 and 40 degrees in skeletally immature patients.

• Goal is to prevent progression, not to correct the curve.

• Commonly used braces include:

  • Cervicothoracolumbosacral orthosis

  • Thoracolumbosacral orthosis

  • Night-time bending braces

• Compliance is critical for success.

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Surgical Management

Indications

• Progressive curves greater than 40 degrees in skeletally immature patients

• Curves greater than 50 degrees at skeletal maturity

Surgical Options

• Posterior spinal instrumentation and fusion

• Anterior spinal fusion

• Combined anterior and posterior fusion in selected cases

Goals of Surgery

• Correction of deformity

• Maintenance of coronal and sagittal balance

• Preservation of pulmonary function

• Minimization of pain and complications

• Preservation of lumbar spine function

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Complications

• Neurological injury

• Blood loss

• Pseudarthrosis

• Implant-related complications

• Infection

• Flat back syndrome

• Superior mesenteric artery syndrome

• Crankshaft phenomenon in skeletally immature patients

Courtesy: Harry Benjamin Laing, MRCS, UK