Cobb Angle Used to Measure Scoliosis Curves


The Cobb Angle is used as a standard measurement to determine and track the progression of scoliosis. Dr John Cobb invented this method in 1948.

Cobb Angle

How to calculate Cobb angle.

Cobb suggested that the angle of curvature be measured by drawing lines parallel to the upper border of the upper vertebral body and the lower border of the lowest vertebra of the structural curve, then erecting perpendiculars from these lines to cross each other, the angle between these perpendiculars being the ‘angle of curvature’.

The Cobb Angle helps a doctor to determine what type of treatment is necessary.

A Cobb Angle of 10 degrees

Is regarded as a minimum angulation to define Scoliosis.

Between 15 and 20 degrees:

Some studies say that the patient does not require any specific treatment, but just needs regular check-ups to see if the curve is progressing until bone-maturity.

A study from Weiss et al. says the patient requires physical therapy with treatment free intervals (6-12 weeks without physical therapy ). The physical therapy contains exercise sessions initiated at the physical therapist’s office, plus a home exercise program (2-7 sessions a week, after 3 months one session every 2 weeks may be enough).

Between 20 and 40 degrees:

An orthopaedic doctor will generally prescribe a back brace to keep the spine from developing more of a curve. There are several braces for sale in the market, some worn for 18 to 20 hours a day and others only at night. Which type of brace the doctor will prescribe depends on the patient’s lifestyle, discipline and the severity of the curve.

A study from Weiss et al. says a scoliosis intensive rehabilitation program is necessary. This includes a 3- to 5 week intensive program (4 to 6 hour training sessions a day).

40 – 50 degrees or more:

Surgery may be required to correct the curve. There are several surgical procedures. A frequent recurring procedure is the “spinal fusion”, to link the vertebrae together so that the spine cannot longer continue to curve.

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.2

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 doctor 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 doctor 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.
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