The Angle of Trunk Rotation (ATR) is measured on the surface of the back with a scoliometer. The Cobb angle is measured on an X-ray of the spine. They describe different things, they are recorded in different settings, and one cannot be converted into the other.
Both numbers are given in degrees, both relate to scoliosis, and both are commonly quoted in the same conversation — which is exactly why they get confused. A parent told their child has “8 degrees” after a school screening is being given an ATR reading, not a diagnosis of an 8° spinal curve. Understanding the difference prevents both false reassurance and unnecessary alarm.
The difference in one table
| Angle of Trunk Rotation (ATR) | Cobb angle | |
|---|---|---|
| What it measures | Rotation of the trunk — how much one side of the back rises above the other | Lateral curvature of the spine between the two most tilted vertebrae |
| How it is measured | A scoliometer laid across the back during the Adam’s Forward Bend Test | Lines drawn on a standing spinal X-ray |
| Who can measure it | A nurse, physiotherapist, chiropractor, teacher or parent | A clinician, on radiographic imaging |
| Radiation involved | None | Yes — X-ray exposure |
| Purpose | Screening: deciding who should be examined further | Diagnosis, classification and treatment planning |
| Typical action point | Around 7° prompts professional referral | 10° or more is the usual definition of scoliosis |
| Repeatable often | Yes — weekly or monthly if useful | No — imaging is spaced out to limit exposure |
Why they are related but not interchangeable
Scoliosis is three-dimensional. The spine curves sideways and the vertebrae rotate at the same time. That rotation is what pushes the ribs and paraspinal muscles backwards on one side, producing the asymmetry a scoliometer detects when a person bends forward. Because the curve and the rotation arise from the same underlying deformity, larger curves do tend to come with larger surface rotation.
But the relationship is loose. How much rotation appears at the surface for a given curve depends on where the curve sits in the spine, the shape of the rib cage, body composition, the flexibility of the curve, and how the person is positioned during the test. Two people with the same Cobb angle can produce noticeably different ATR readings, and the same person can produce different ATR readings on the same day if the bend depth or tester changes.
For that reason, published conversion rules should be treated with caution. Screening research generally uses ATR to decide who to image, not to estimate a Cobb angle. Any specific relationship varies between individuals and between measurement techniques.
What each number is actually used for
ATR
- Deciding whether a school or clinic screening should lead to a referral.
- Monitoring between clinical appointments, particularly during a child’s growth spurt.
- Tracking whether trunk asymmetry is stable or increasing over months.
- Giving a repeatable, radiation-free number that can be recorded at home.
Cobb angle
- Confirming or excluding a diagnosis of scoliosis (10° or more is the usual threshold).
- Classifying the curve pattern and grading severity.
- Deciding between observation, bracing and surgical referral.
- Assessing progression across imaging intervals, alongside skeletal maturity.
Can ATR change while the Cobb angle stays the same?
Yes, and this surprises people. Surface rotation can shift with posture, muscle tone, weight change, breathing pattern and measurement technique while the underlying curve is unchanged. It can also change genuinely if the rotational component of the deformity progresses more than the lateral component. The reverse happens too: a curve can increase on imaging while surface rotation looks similar.
This is one more reason to treat a single ATR reading as a prompt for review rather than as a verdict, and to keep the technique identical between measurements so that a change in the number means something. See how to use a scoliometer for the technique that makes readings comparable.
Reading the two scales side by side
| ATR reading | Commonly interpreted as | Cobb angle | Commonly interpreted as |
|---|---|---|---|
| Below 5° | Low; routine monitoring | Below 10° | Not classified as scoliosis |
| 5°–6° | Worth attention; referral point for overweight children | 10°–24° | Mild; usually observed |
| 7° or more | Usual referral threshold | 25°–44° | Moderate; bracing often considered in growing patients |
| Rising by about 5° | Progression signal; seek review | 45° or more | Surgical opinion commonly discussed |
The two columns are not aligned with each other. They are placed side by side only to show what each scale is used for. A 7° ATR does not correspond to a 25° Cobb angle. Management decisions in every band depend on age, skeletal maturity, curve type, curve location, symptoms and individual factors, and are made by a clinician after examination. For a fuller explanation of the ATR side, see what a normal scoliometer reading is.
What this means in practice
If a screening produces a raised ATR, the next step is a professional assessment — not a conversion table and not a self-diagnosis. The clinician will examine the back, take a history, consider growth stage, and decide whether imaging is warranted. If imaging is taken, the Cobb angle from that image becomes the number that guides management, while ATR remains useful for low-cost monitoring in between.
When to seek professional review
Seek an assessment if an ATR reading reaches the referral threshold, if readings are rising across measurements, or if you notice uneven shoulders, a prominent shoulder blade, an uneven waistline or a visible rib hump on forward bending. Results and appropriate management vary between individuals, and only a professional examination can establish what a reading means for a specific person.
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Frequently asked questions
- Is ATR the same as the Cobb angle?
- No. ATR is trunk rotation measured on the surface of the back with a scoliometer. The Cobb angle is spinal curvature measured on an X-ray. They are related but describe different things.
- Can I convert a scoliometer reading into a Cobb angle?
- Not reliably. The relationship varies with curve location, body shape, flexibility and measurement technique. Screening thresholds exist to decide who should be imaged, not to estimate a curve size.
- What Cobb angle counts as scoliosis?
- A lateral curve of 10° or more on imaging is the usual definition. Below that, a curve is generally not classified as scoliosis, though monitoring may still be advised during growth.
- Why did my ATR change when my X-ray did not?
- Surface rotation responds to posture, muscle tone, body composition and technique, so it can move while the underlying curve is stable. Keep the technique identical between measurements and discuss any change with your clinician.
- Do I still need an X-ray if my scoliometer reading is low?
- That is a clinical decision. A low reading alongside visible asymmetry, pain or a family history still warrants professional review; a scoliometer cannot rule scoliosis out.
Reference: van West HM, Herfkens J, Rutges JPHJ, et al. The smartphone as a tool to screen for scoliosis, applicable by everyone. European Spine Journal 2022;31:990–995. This research assessed smartphone-based ATR screening as a method and did not evaluate any specific application.