Cobb Angle for Measuring Scoliosis

Cobb Angle Measurement
Cobb Angle Measurement. creative commons

Cobb Angle Definition

Cobb angle is a measurement of the degree of side-to-side spinal curvature, commonly referred to as scoliosis. This measurement is taken by first finding the most affected vertebra in the curve, called the apical vertebra. The apical vertebra is the spinal bone that has the most rotation and displacement from its ideal alignment. It also has the least amount of tilt, as measured by the angle of the end plates (top and bottom edges of vertebral body).

Related: What is the Normal Angle of a Lordosis?

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

Of course, to take a Cobb angle measurement, you need an x-ray.

Related: What is Scoliosis?

Cobb Angle X-Ray and Interpretation

When having an x-ray for the purposes of measuring the degree of your or your child's scoliosis curve, you'll usually be in the standing position.  The films are then taken from the back, as well as the side.

On the resulting film, a line is drawn along the edge of the vertebrae and 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. If the Cobb angle for your scoliosis measures 15 degrees or less, it's likely that all you will need will be regular check-ups to see if the curve is progressing.

If it measures between 20 and 40 degrees, you will probably need a back brace. And if the number is greater than 40, your doctor will likely recommend surgery.

Related: Spinal Curves

Cobb Angle Measurement - Is it an Exact Science?

Even with the above protocol widely in use, measuring a scoliosis has yet to be made into 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 work on developing more accurate ways to determine the degree of a 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 (and is today the Hospital for Special Surgery) in New York City.

Related: Scoliosis Treatments in the U.S.

Sources:

Sardjono TA, Wilkinson MHF, Veldhuizen AG, van Ooijen PM a, Purnama KE, Verkerke GJ. Automatic Cobb angle determination from radiographic images. Spine (Phila Pa 1976). 2013;38(20):E1256-E1262. doi:10.1097/BRS.0b013e3182a0c7c3.

GREINER, K., MD, MPH Adolescent Idiopathic Scoliosis: Radiologic Decision-Making. American Family Physician. March 2002. Accessed March 20, 2009.
Cobbs Angle. E-radiography.net. Accessed March 20, 2009. Scoliosis. Teens Health Kids Health website. Accessed March 20 2009.

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