Cobb Angle for Scoliosis

The American Scoliosis Research Society (ASRS) recommends the Cobb angle as the standard measuring technique for quantifying spinal curvature in the frontal plane to obtain uniform and comparable values.

It is named after John Robert Cobb, an American orthopedic surgeon who developed the Cobb method for measuring spinal curvature.

How to measure the Cobb Angle?

The cobb angle measurement is performed on a standing anteroposterior (AP) radiograph of the full spine.

It is determined by measuring the angle between the most tilted vertebrae above the curve and the most tilted vertebrae below the curve on an X-ray image of the spine. The vertebrae are identified by their superior and inferior endplates, which are the upper and lower surfaces of each vertebra.

To measure the scoliosis angle, draw lines parallel to the endplates of the most tilted vertebrae above and below the curve. The angle formed by lines drawn perpendicular to the reference lines is the Cobb angle.

The end vertebrae of a spinal curve may be identical with the neutral vertebrae. The neutral vertebra is defined as the vertebral body between two curves that is the least rotated and has parallel endplates. Often, the neutral vertebrae are located one or two vertebrae farther away from the apical vertebra than the end vertebrae. The apical vertebra, which forms the apex of the curve, is the vertebral body that is the most rotated and shows the greatest deformity.

See Also: Spine Movements

Accuracy

Variables that can lead to errors in measuring Cobb Angle include:

1. the positioning of the subject,
2. the position of the x-ray beam,
3. the clarity of the vertebrae on the radiograph,
4. the selection of the end vertebrae,
5. the placement of the end plate lines,
6. the width of the lines drawn,
7. the type of measuring device used.

The standard error of measurement using the Cobb method scoliosis has been reported to be 2.2 to 3.0 degrees.

Definition of Cobb angle

Cobb method is usually used for Measuring scoliosis severity as following:

Note

Kyphosis and lordosis, just as scoliosis, are measured by the Cobb technique also as recommended by the Scoliosis Research Society.

For this measurement, lines are drawn parallel to the superior end plate of the superior end vertebra and to the inferior end plate of the inferior end vertebra. Perpendicular lines to these end plate lines construct the Cobb angle. Measurements of thoracic kyphosis (T10–T12) and lumbar lordosis (L1–S1) are examples of measurements using regional coordinate systems.

A segmental Cobb angle, the sagittal angulation of one vertebra with respect to an adjacent vertebra, is an example of a measurement using a local coordinate system. Use of the C7 sagittal plumb line and the measurement of sacral inclination (SI), or of sacral slope (SS), with respect to the plumb line are examples of measurements using global coordinate systems.

References

1. Horng MH, Kuok CP, Fu MJ, Lin CJ, Sun YN. Cobb Angle Measurement of Spine from X-Ray Images Using Convolutional Neural Network. Comput Math Methods Med. 2019 Feb 19;2019:6357171. doi: 10.1155/2019/6357171. PMID: 30996731; PMCID: PMC6399566.
2. Cobb .JR- Outline for the study of scoliosis. In: American Academy of Orthopedic Surgeons. Instructional Course Lectures. Vol. 5. Ann Arbor, Mi: Edwards; 1948
3. Campbel’s Operative Orthopaedics 12th edition Book.
4. Millers Review of Orthopaedics -7th Edition Book.
5. Bridwell and Dewald’s Textbook of Spinal Surgery 4th Ed 2020