The Cobb angle is the most widely used measurement to quantify the magnitude of spinal deformities, especially scoliosis, on plain radiographs. Scoliosis is a lateral spinal curvature with a Cobb angle of >10° ⁴. The Cobb angle technique can also assess the degree of kyphosis or lordosis in the sagittal plane ⁷. This article is focused on the Cobb angle technique in scoliosis.

Measurement

To measure the Cobb angle, one must first decide which vertebrae are the end vertebrae of the curve deformity (the terminal vertebrae) – the vertebra whose endplates are most tilted towards each other ⁴. 

Lines are then drawn along the endplates (or the pedicles if the endplates are not properly visualised ⁸), and the angle between the two lines, where they intersect, is measured. 

In cases where the curvature is not marked, the lines will not intersect on the film/monitor; in this case, two further lines can be plotted, each at right angles to the previous lines ^1,4. 

Most PACS will have a dedicated angle tool to measure this without needing the lines to intersect or to add the aforementioned lines at right angles. 

Interpretation

Scoliosis is a lateral spinal curvature with a Cobb angle of >10° ⁴.

A number of limitations of the Cobb angle are recognised and caution should be used in assuming that sequential measurements are correct when little change is evident. Some limitations include ¹:

• intra-observer and inter-observer variation: a 5° measurement error is well established ⁹

• rotation: minor rotation of patients between examinations can significantly change measurements (may be as high as 20° variation); consistent positioning must, therefore, be obtained

• diurnal variation: in the same patient on the same day, curvature increases during the day (~5° variation)

Overall, and despite the above-mentioned limitation, if a greater than 10° change in Cobb angle is measured, it is 95% likely to represent a true difference ¹. 

History and etymology

The Cobb angle was first described in 1948 by American orthopaedic surgeon John R Cobb (1903-1967) ⁵.

See also

• vertebral centroid angle