The term “Cobb Angle” is used worldwide to measure and quantify the magnitude of spinal deformities, especially in the case of scoliosis. It is used as the standard measurement to quantify and track the progression of Idiopathic Scoliosis. Cobb Angle was first expressed in 1948, by Dr. John R Cobb, where he outlined “how to measure the angle of the curve”. Hence, the term “Cobb Angle” came about, bearing his name.
When the Adam’s Test that is used to screen for scoliosis proves to be positive, an X-ray will be performed on the patient and the Cobb angle measured on the X-ray according to the method prescribed below:
To first locate both the end vertebras of the curve (the vertebras with the most tilted endplates). Then, at the top of the curve, draw a parallel line to the highest vertebral endplate, and at the bottom, a parallel line to the lower vertebral endplate. The angle between these two lines (or lines drawn perpendicular to them) is the Cobb Angle for that curve.
To track the progression or improvement of scoliosis treatment on the curve, pre and post x-rays have to be taken so that the respective Cobb angles can be measured and compared. It must be stressed that the same two vertebral segments in both pre and post x-rays are used for measurements.
The Cobb Angle however, is not without a setback, with studies indicating possibilities of Cobb Angles having a margin of error between 6°-7°. This is basically a result of trying to determine the spinal curvature of a three-dimensional structure using a two-dimensional basis for measurement. Nevertheless, the Cobb Angle still remains the most popular measurement among many doctors to express the magnitude of a scoliosis curve, as it is simplest concept to articulate, and patients to relate to.
How to Find the Cobb Angle
- Locate the most tilted vertebrae at the top and draw a parallel line to the vertebral endplate.
- Locate the most tilted vertebrae at the bottom and draw a parallel line to the vertebral endplate.
- Erect intersecting perpendicular lines from these.
- The angle formed is Cobb’s angle.