Cobb angle

The Cobb angle is the most widely adopted technique to quantify the magnitude of spinal deformities, especially in the case of scoliosis, on plain radiographs. A scoliosis is defined as a lateral spinal curvature with a Cobb angle of 10° or more ⁴.

Measurement

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

Lines are then drawn along the endplates, and the angle between the two lines, where they intersect, measured. 

In cases where the curvature is not marked, then the lines will not intersect on the film/monitor, in which case a further two 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 need to add the aforementioned lines at right angles. 

Limitations

A number of recognised 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 recognised limitations include ¹:

• intraobserver and interobserver variation (at least 5-10° variation) 
• 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 Dr John R Cobb(1903-1967) ⁵.