How Scoliosis Is Measured by the Cobb Angle

Cobb angle is a measurement of the degree of side-to-side spinal curvature, which is a deformity you may know as scoliosis. A Cobb angle describes the maximum distance from straight a scoliotic curve may be. 

Generally, it takes at least 10 degrees of deviation from straight before scoliosis is defined.

A Cobb angle of 20 degrees usually requires that a back brace is worn and that you or your child undergo intensive physical therapy. The goal of these treatments is to help halt the progression of the curve. Braces are generally worn between 18 and 20 hours per day. As for physical therapy, your healthcare provider will likely give you a referral, but many people report excellent results with the Schroth or other scoliosis specific exercise methods.

A 2017 study published in the Journal of Exercise Rehabilitation found that Cobb angles can be decreased in adolescents who have idiopathic scoliosis with the use of core stabilization exercise programs.

Once the Cobb angle reaches 40 degrees, surgery is considered. Often a spinal fusion is done to force the curve to stop developing.

If your or your child's curve is between 10 and 20 degrees, you may only need to visit the healthcare provider periodically to check the curve for progression.

How Is a Cobb Angle Measured?

To get your Cobb angle measured, you'll need to have an X-ray taken. These generally require you or your child to be in a standing position; side and back views are taken. After that's done, the healthcare provider or examiner views the films and locates the most affected vertebra in the curve. This is called the apical vertebra.

The apical vertebra in a scoliotic curve is the spinal bone with the greatest degree of rotation; it is also the bone in a curve takes the biggest detour away from straight.

Straight, in this case, refers to the center of a normal spinal column

The apical vertebra also has the least amount of tilt.

Then, to come up with a number for the Cobb angle, the top and bottom vertebrae of the side to side curve are identified. Unlike the apical vertebra, these bones have the most tilt, but the least amount of rotation and displacement. They are located above and below the apical vertebra, respectively.

Cobb Angle X-Ray and Interpretation

To interpret your X-rays, a line is drawn along the edge of the top and bottom bones of the curve. These lines are extended out. On the top bone, the line starts at the high side, is drawn along the top edge and slopes downward according to the angle of the vertebra.

Similarly, on the bottom vertebra, the line starts on the low side, is drawn along the bottom edge and will slope in an upward direction. The two lines meet to form an angle at the level of the apical vertebra (discussed above.)

The Cobb angle is found by measuring the angle of the two intersecting lines. 

Cobb angles are also used to measure kyphosis, which is an outward rounding deformity in the spine.

Is It an Exact Science?

Even with the above protocol widely in use, measuring scoliosis has yet to be made into an accurate science. Variations occur between people who do the measuring, as well as between tools used in the process (specifically, the protractor.) Variations also occur from clinic to clinic.

Just the same, scientists continue to work on developing more accurate ways to determine the degree of scoliosis. The trend seems to be moving towards computerized measurement. But one thing that remains a manual process is determining which upper and which lower vertebrae are the ones with the greatest tilt.

Who Created the Cobb Angle?

The Cobb angle is named, aptly enough, after orthopedic surgeon John Robert Cobb who led the Margaret Caspary Scoliosis Clinic at what was, in the early 20th century, known as the Hospital for Ruptured and Crippled in New York City. Today, the hospital is the Hospital for Special Surgery.