How Do You Measure The Cobb Angle?

One of the most common angle used in the measure of scolisis progression is the Cobb angle. It measures the frontal deformation looking from the back to the front (or technically, it measures the deformation of the spine at the coronal plane on the anterior-posterior plane.

It is measured by finding the most displaced vertebra from the top, and the most displaced vertebra from the bottom. The physiotherapist will draw two parallel lines from the ends of these two vertebrea. He/she will then draw two perpendicular lines from these two lines. The angle at where the two perpendicular lines meet is the Cobb angle. (see figure).

Do note that there is some error in measuring the angle. Typically, expect about ± 5 degrees. There may be some variation, even if it is done by the same person.

The cobb angle does not measure the amount of rotation of the spine in the other two planes (sagittal and transverse planes). As scoliosis is a 3-dimensional deformation of the spine, it is possible for a visible improvement of the overall spine but not the angle.

Reference:

1. Variation in Cobb angle measurements in scoliosis, Volume 23, Number 7 / October, 1994
2. Reliability of the Cobb angle index derived by traditional and computer assisted methods., Australas Phys Eng Sci Med. 1989 Mar;12(1):16-23.