Measuring the Cobb Angle

One of the most common angle used in the measure of scoliosis progression is the Cobb angle. The Cobb angle is the measure 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.

The Cobb angle is measured by finding from the top, the most displaced vertebra and from the bottom, most displaced vertebra. The two lines are drawn parallel from the ends of these two vertebrae. Two perpendicular lines are drawn 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 cobb angle. Typically, expect about ± 5 degrees. Even when the measurement is done by the same person, same variation is expected.

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 cobb 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.