From the case:
Cobb angle measurment
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To measure the Cobb 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.
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.