Clinical calculator
Cobb Angle
Cobb Angle for Scoliosis
The Cobb angle measures the magnitude of a scoliotic curve as the angle between the superior endplate of the upper end vertebra and the inferior endplate of the lower end vertebra, where 10 degrees or more defines scoliosis and 45 to 50 degrees is the conventional threshold for discussing surgery.
What the Cobb Angle score means
Cobb Angle scores run from 0 to 100, where a lower score is better. The bands below are conventional reporting categories, not diagnostic cut-offs.
- Not scoliosis0–9.9
Below 10° the curve does not meet the Scoliosis Research Society definition of scoliosis. Commonly reported as spinal asymmetry rather than scoliosis.
- Mild scoliosis10–24.9
Meets the definition of scoliosis. Usually observed with serial radiographs; bracing is not generally indicated at this magnitude in a growing child unless progression is documented.
- Moderate scoliosis25–39.9
In a skeletally immature patient this is the range in which bracing is conventionally offered, because the risk of progression is substantial.
- Severe scoliosis40–200
Surgical correction is conventionally discussed from around 45–50° in adolescent idiopathic scoliosis, because curves beyond this magnitude tend to progress into adulthood.
Published criteria and reliability
The published cut-offs that define this instrument, and the measured agreement between observers using it. Each is reported with the population it was derived in.
| Type | Value | Subscale | Population | Follow-up | Source |
|---|---|---|---|---|---|
| Criterion | ≥ 10° defines scoliosis | — | Scoliosis Research Society definition | — | Scoliosis Research Society 2018 |
| Criterion | 25–40° is the conventional bracing range in a skeletally immature patient | — | Adolescent idiopathic scoliosis | — | Weinstein SL 2013 |
| Criterion | ≥ 45–50° is the conventional threshold for discussing surgery | — | Adolescent idiopathic scoliosis | — | Weinstein SL 2008 |
| Criterion | Interobserver measurement error is approximately 3–5°; a change under 5° is not progression | — | Manual measurement on plain radiographs | — | Morrissy RT 1990 |
Criterion is a published cut-off, definition or performance figure belonging to a classification or decision rule — it is not a change score and should not be read as one.
How the score is calculated
Stated in full so the calculation can be independently checked, and reproduced by hand if needed.
A line is drawn along the superior endplate of the upper end vertebra and another along the inferior endplate of the lower end vertebra. The Cobb angle is the angle between these two lines, which is equal to the sum of their individual tilts from horizontal when they tilt in opposite directions — hence the two-measurement method offered here. The end vertebrae are the most tilted vertebrae at each end of the curve. A curve of 10 degrees or more meets the Scoliosis Research Society definition of scoliosis.
Instrument profile
- Developer
- John Robert Cobb
- Institution
- Hospital for Special Surgery, New York
- First published
- 1948
- Response format
- Angle in degrees, entered directly or derived from two endplate tilts
- Score range
- 0 to 100
- Direction
- Lower score = better
- Completion time
- About 2 minutes
- Age range
- Children, adolescents and adults
- Regions
- Spine, Paediatric
- Conditions
- Scoliosis, Adolescent idiopathic scoliosis, Congenital scoliosis, Neuromuscular scoliosis, Adult degenerative scoliosis
- Procedures
- Bracing, Posterior spinal fusion, Growing rod instrumentation
- Observer agreement
- Intraobserver error approximately 3°, interobserver error approximately 5°
- Validity
- The reference standard for quantifying scoliosis for more than seventy years, and the measure on which every bracing and surgical threshold in the literature is based.
- Reliability
- Measurement error is the dominant limitation. Choice of end vertebra accounts for much of the variability, so serial films should be measured using the same end vertebrae, ideally by the same observer. The Cobb angle is a two-dimensional projection of a three-dimensional deformity and does not capture vertebral rotation or rib prominence.
Normative values
Reference values from published cohorts, for comparison against the score you calculated.
| Population | Age | Mean | SD |
|---|---|---|---|
| General adolescent population | — | Roughly 2–3% have a curve of 10° or more | — |
| Curves reaching 30° or more | — | Roughly 0.3% of adolescents | — |
When to use the Cobb Angle
Well suited to
- Diagnosing and grading scoliosis on a standing posteroanterior radiograph.
- Serial monitoring of curve progression during growth, where the 5° threshold defines meaningful change.
- Determining eligibility for bracing or surgical referral against published thresholds.
Limitations and poor fits
- On supine radiographs, CT or MRI without noting the position, since the curve reduces when the patient lies down.
- As a standalone predictor of progression — skeletal maturity, curve pattern and sex all matter alongside the magnitude.
- To assess the three-dimensional deformity, including rotation and rib hump, for which it gives no information.
Cobb Angle compared with other instruments
Instruments measuring a related construct, with the practical difference between them.
| Instrument | Items | Range | What it adds |
|---|---|---|---|
| Meyerding | — | 0–100 | The Meyerding classification grades spondylolisthesis by the percentage of anterior translation of one vertebral body over the one below, measured on a standing lateral radiograph, from grade I at 25% or less through to grade V, spondyloptosis, beyond 100%. |
| ODI | 10 | 0–100 | The Oswestry Disability Index measures how far low back pain restricts ten everyday activities, scoring each section from 0 to 5 and expressing the total as a percentage from 0% (no disability) to 100% (maximum disability). |
Copyright and permitted use
Published openly because it is the question every clinician and researcher asks before using an instrument, and almost nobody answers it.
A published radiographic measurement method. Freely reproducible; no licence is required.
References
- Cobb JR. Outline for the study of scoliosis. Instructional Course Lectures, American Academy of Orthopaedic Surgeons. 1948.
- Morrissy RT, Goldsmith GS, Hall EC, Kehl D, Cowie GH. Measurement of the Cobb angle on radiographs of patients who have scoliosis: evaluation of intrinsic error. Journal of Bone and Joint Surgery (American). 1990.PMID 2312527
- Weinstein SL, Dolan LA, Cheng JC, Danielsson A, Morcuende JA. Adolescent idiopathic scoliosis. The Lancet. 2008.PMID 18456103
- Weinstein SL, Dolan LA, Wright JG, Dobbs MB. Effects of bracing in adolescents with idiopathic scoliosis. New England Journal of Medicine. 2013.PMID 24047455
- Scoliosis Research Society. Scoliosis: definition and classification. SRS reference material. 2018.
Editorial status
This page has not yet been signed off by a named clinical reviewer. The scoring logic is stated in full above so it can be independently verified, and every threshold is traceable to its source. See our editorial methodology.