Cobb Angle
Grades scoliosis severity from the Cobb angle measured between the most tilted end vertebrae.
The Cobb angle is the standard radiographic measure of spinal curvature, taken between the superior endplate of the most tilted upper end vertebra and the inferior endplate of the most tilted lower end vertebra of a curve.
📐 Formula
<10° not scoliosis; 10-24° mild; 25-39° moderate; ≥40° severe
🎯 When to Use
- Diagnosing and grading scoliosis on standing PA spine radiographs.
- Monitoring curve progression and guiding bracing or surgical decisions.
- Reporting spinal deformity severity.
⚙️ How It Works
- Identify the end vertebrae — the most tilted vertebrae at the top and bottom of the curve.
- Draw lines along the superior endplate of the upper and inferior endplate of the lower end vertebra.
- The Cobb angle is the angle between perpendiculars to these lines.
- Severity is banded by degrees of curvature.
📊 Interpreting the Result
- <10°: not classified as scoliosis (spinal asymmetry).
- 10-24°: mild scoliosis; observation with serial radiographs.
- 25-39°: moderate scoliosis; bracing typically considered in skeletally immature patients.
- ≥40-50°: severe scoliosis; surgical correction is often considered.
⚠️ Limitations & Pitfalls
- Measurement variability of ±5° is expected; changes below this may not be real progression.
- Two-dimensional projection does not capture rotational (three-dimensional) deformity.
- End-vertebra selection is observer-dependent.
📚 References
- Cobb JR. Outline for the study of scoliosis. Instr Course Lect. 1948;5:261-275.
- Scoliosis Research Society. Standardised terminology and severity thresholds for spinal deformity.
⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.