The original Cobb’s angle was used to measure lateral curve severity in scoliosis but also has subsequently been adapted to classify deformity in kyphosis.
For evaluation of curves in scoliosis, an anteroposterior radiograph is used.
For measurement, When assessing a curve the apical vertebra [The vertebra most deviated laterally from the vertical axis that passes through the patient's sacrum] is first identified. This is the most likely displaced and rotated vertebra with the least tilted end plate.
The end/transitional vertebra are then identified through the curve above and below.
Image Credit: http://www.e-radiography.net/radpath/c/cobbs-angle.htm
The end vertebra are the most superior and inferior vertebra which are least displaced and rotated and have the maximally tilted end plate.
A line is drawn along the superior end plate of the superior end vertebra and a second line drawn along the inferior end plate of the inferior end vertebra.
The angle between these two lines or lines drawn perpendicular to them is measured as the Cobb angle.
In S-shaped scoliosis where there are two contiguous curves the lower end vertebra of the upper curve will represent the upper end vertebra of the lower curve. Because the Cobb angle reflects curvature only in a single plane and fails to account for vertebral rotation it may not accurately demonstrate the severity of three dimensional spinal deformity. As a general rule a Cobb angle of 10 is regarded as a minimum angulation to define scoliosis.
Cobb’s angle for kyphosis is measured in a similar manner. See the image below.
References
- Kado DM, Christianson L, Palermo L, Smith-Bindman R, Cummings SR, Greendale GA. Comparing a supine radiologic versus standing clinical measurement of kyphosis in older women: the Fracture Intervention Trial. Spine. 2006;31:463-7. [PMID: 16481959]
[...] is defined as an increase of 5 degrees or more measured by the Cobb’s angle over two or more [...]