Cobb Angle – Measurement, Significance and Limitations

Cobb angle is used to measure spinal deformity. The original Cobb 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.

Scolioisis is defined as Cobb angle more than 10 degrees.

Relevant Terms

Apical vertebra 

The apical vertebra is the vertebra that is displaced and rotated the most. Its end plates are least tilted.

End Vertebra

The most superior and inferior vertebrae of the curve are called end vertebrae. These vertebrae are least displaced and rotated.

These vertebrae appear laterally wedged, longer in the convex side, and compress

There is the maximal tilting of the endplates towards the curve.ed on the concave side.

Neutral Vertebra

The neutral vertebra is the first vertebra after end vertebra which is not rotated on the anteroposterior image.

How to Measure Cobb Angle

For measurement, 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 is 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 endplate of the superior end vertebra and a second line drawn along the inferior endplate 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 the three-dimensional spinal deformity.

As a general rule, a Cobb angle of 10 is regarded as a minimum angulation to define scoliosis.

 

Significance of Cobb Angle

Cobb’s angle is used to ascertain the presence of scoliosis and measure its severity.

It is not only helpful for diagnosis but also useful for planning treatment and predicting diagnosis.

When a Cobb angle is greater than 50 degrees, the patient almost always requires corrective surgery.

Limitations of Cobb angle

Scoliosis is a three-dimensional deformity but Cobb angle is the uniplanar measurement.

There is an interobserver variation of 5 to 10 degrees.

Moreover, the variations can occur with changes in supine and upright positioning, rotation, body shape etc.

Cobb’s method generally gives a higher value than those obtained  Values higher than those obtained with other measurement techniques.

Cobb Angle for Kyphosis

The angle for kyphosis is measured in a similar manner. See the image below.

 

For kyphosis, on the lateral view, a line is drawn along the superior endplate of the superior end vertebra and a second line drawn along the inferior endplate of the inferior end vertebra. The angle formed by the intersection of lines perpendicular to above-mentioned lines is Cobb angle for kyphosis.

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]