Last Updated on December 12, 2023
Cobb angle is used to measure spinal deformity in scoliosis. 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 the evaluation of curves in scoliosis, an anteroposterior radiograph is used. For evaluation of Cobb angle in kyphosis lateral view of the spine is chosen.
Scoliosis is defined as a Cobb angle of more than 10 degrees.
Relevant Anatomy and Terms
Tthe spine consists of 33 vertebrae that span across 5 regions and named differently
- Cervical (C1–C7)
- Thoracic (T1–T12)
- Lumbar (L1–L5)
- Sacrum (S1-S5)
- Coccyx (Co1–Co4)
A normal spine has lordosis [convex anterior curvature] in the cervical and lumbar regions whereas the thorax shows kyphosis. When viewed anteroposteriorly spine appears a straight line and is centered over the pelvis.
The spine does not normally show any curvature when viewed from the front or back.
Scoliosis is a condition where the spine has a lateral curvature [curved towards left or right]. A person’s spine with scoliosis will look like a C- or S-shaped curve as shown in Figure 2.
Cobb angle is the most common quantification method used and is named after John Robert Cobb who developed it. It also measures the severity of the scoliotic curve.
Image Credit: Radiopaedia
Various Terms Ued
- Apical vertebra:The apical vertebra is the vertebra that is displaced and rotated the most. Its end plates are the 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 curved on the concave side.
- Neutral Vertebra: The neutral vertebra is the first vertebra after the 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 is 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.
A Cobb angle of less than 10 degrees is considered to be a normal part of the spinal curve. Higher than that, it is scoliosis. The various degrees are clubbed as
- 0–10: Spinal curve
- 10–20: Mild scoliosis
- 20–40: Moderate scoliosis
- >40: Severe scoliosis
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.
Here are general treatment guidelines based on Cobb’s angle
- 10-20 degrees: Need to be monitored. Bracing if substantial growth is pending
- 20- 40 degrees: Bracing if growth left. Nothing but monitoring if skeletal maturity is already achieved
- 40- 50 degrees: Depending on the individual case, bracing or surgery
- >50 degrees: Surgery
These are general but not strict guidelines. Each case need to be taken on an individual basis.
Limitations of Cobb angle
Scoliosis is a three-dimensional deformity but the Cobb angle is the uniplanar measurement. Thus it fails to measure the rotational element of the condition.
There is an interobserver variation of 5 to 10 degrees.
Moreover, 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 similarly. 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 the 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.
