Cobb's angle

*Cobb's angle, a measurement used for evaluation of curves in scoliosis on an AP radiographic projection of the spine (Fig.1).

*When assessing a curve the apical vertebra 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. 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.

*If the end plates are indistinct the line may be drawn through the pedicles. 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.