The Cobb angle is the most widely used measurement to quantify the magnitude of spinal deformities, especially in the case of scoliosis, on plain radiographs. Scoliosis is defined as a lateral spinal curvature with a Cobb angle of >10° 4. A Cobb angle can also aid kyphosis or lordosis assessment in the sagittal plane 7.
To measure the Cobb angle, one must first decide which vertebrae are the end vertebrae of the curve deformity (the terminal vertebrae) – the vertebra whose endplates are most tilted towards each other 4.
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 1,4.
Most PACS will have a dedicated angle tool to measure this without needing the lines to intersect or need to add the aforementioned lines at right angles.
Scoliosis is defined as a lateral spinal curvature with a Cobb angle of >10° 4.
A number of limitations of the Cobb angle are recognized and caution should be used in assuming that sequential measurements are correct when little change is evident. Some limitations include 1:
- intra-observer and inter-observer variation (at least 5-10° variation)
- rotation: minor rotation of patients between examinations can significantly change measurements (may be as high as 20° variation); consistent positioning must, therefore, be obtained
- diurnal variation: in the same patient on the same day, curvature increases during the day (~5° variation)
Overall, and despite the above-mentioned limitation, if a greater than 10° change in Cobb angle is measured, it is 95% likely to represent a true difference 1.
History and etymology
The Cobb angle was first described in 1948 by American orthopedic surgeon John R Cobb (1903-1967) 5.
- 1. Kim H, Kim HS, Moon ES et-al. Scoliosis imaging: what radiologists should know. Radiographics. 2010;30 (7): 1823-42. Radiographics (full text) - doi:10.1148/rg.307105061 - Pubmed citation
- 2. Eckalbar WL, Fisher RE, Rawls A et-al. Scoliosis and segmentation defects of the 2. vertebrae. Wiley Interdiscip Rev Dev Biol. 2012;1 (3): 401-423. Wiley Interdiscip Rev Dev Biol (full text) - doi:10.1002/wdev.34 - Pubmed citation
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- 4. Hefti F. Pediatric Orthopedics in Practice Read it at Google Books
- 5. The Hospital for Special Surgery 1955 to 1972: T. Campbell Thompson Serves as Sixth Surgeon-in-Chief 1955–1963 Followed by Robert Lee Patterson, Jr. the Seventh Surgeon-in-Chief 1963–1972. (2010) HSS Journal. 6 (1): 1. doi:10.1007/s11420-009-9136-5 - Pubmed
- 6. Malfair D, Flemming AK, Dvorak MF, Munk PL, Vertinsky AT, Heran MK, Graeb DA. Radiographic evaluation of scoliosis: review. (2010) AJR. American journal of roentgenology. 194 (3 Suppl): S8-22. doi:10.2214/AJR.07.7145 - Pubmed
- 7. Briggs AM, Wrigley TV, Tully EA, Adams PE, Greig AM, Bennell KL. Radiographic measures of thoracic kyphosis in osteoporosis: Cobb and vertebral centroid angles. (2007) Skeletal radiology. 36 (8): 761-7. doi:10.1007/s00256-007-0284-8 - Pubmed