Someone is considered to have scoliosis when the spine has a sideways curvature of at least 10 degrees, which is measured by the Cobb angle. With this measurement, doctors are able to judge the severity of the scoliosis, assess the curve and recommend treatment options. Typically, curves under 20 degrees in adults are not worrisome.
In addition, comparing the patient’s Cobb angle to past exams can help determine whether or not the curve is getting progressively worse, as well as whether treatment is working or even needed.
How to Measure the Cobb Angle
The Cobb angle measurement process was developed in 1948 by Dr. John Robert Cobb. The measurement includes the following steps:
- The patient stands while a front view x-ray of the spine is taken.
- The doctor uses the x-ray to locate the apex vertebra, which is at the deepest part of the scoliosis curve, as well as the most-tilted vertebra above the apex and most-tilted vertebra below the apex.
- A perpendicular line extending from the most-tilted vertebra above the apex is drawn. The same is then done for the most-tilted vertebra below the apex.
- Where the two lines extending from the most-tilted vertebra above the apex and most-tilted vertebra below the apex join together gives the Cobb angle.
Sometimes the lines drawn from the most-tilted vertebrae can extend a relatively long distance and stray off the x-ray image. As such, it is common to see a clinician (or software) employ the use of perpendicular lines and right angles to measure and show the same Cobb angle but in a manner that stays closer to the spine on the x-ray image.
Cobb Angle Margin of Error
While the Cobb angle is more accurate than using a scoliometer, there can still be errors in its measurement. Doctors might disagree on which vertebrae are the most tilted, clinicians could follow different x-ray and measurement procedures from one appointment to the next, or the patient might stand with different posture—all of which could affect a Cobb angle measurement.
Due to possible variances in how Cobb angle measurements are taken, most doctors look for at least a 5-degree increment from one appointment to the next before considering scoliosis to be progressing, rather than run the risk of acting based on a measuring error.