For many years, X-rays have been used to diagnose and measure scoliosis (curvature of the spine). No matter how young or old the patient was or where the curve was located, this technique was used because it was simple and reliable.
Then in the mid-2000s, researchers from the Scoliosis Research Institute in Korea took another look at the Cobb angle measurement. They compared two different starting and ending points used in measuring the angle of the curve and reported on the results. More recently, they compared using the Cobb method versus the centroid method in different age groups (teens versus adults 60 years old and older).
The Cobb angle is defined as the angle formed between a line drawn parallel to the superior endplate of one vertebra above the curve and a line drawn parallel to the inferior endplate of the vertebra one level below the curve. Superior means above and inferior refers to below. The endplate is a flat piece of cartilage that comes in direct contact with the disc as it sits in between two vertebrae. The Cobb method measures the maximum curvature.
The centroid angle is defined as the angle between the lines connecting the centroids of each vertebral body. The centroid is the center point of the vertebral body. It is the center point created by two diagonal lines drawn across the vertebra from one top corner to the opposite bottom corner of the bone. The two lines drawn form an X. The centroid is the place in the middle where the two lines cross.
Results of studies from the Scoliosis Research Institute have shown that the Cobb method is more reliable with older patients. The centroid method has greater reliability with younger patients. The centroid method is not as reliable with older adults. This is because of the way centroid measurements are taken. Distortions of the vertebra or endplates (present in the older degenerated spine) create too much variability and thus decrease reliability.
The Cobb method relies on selecting the most appropriate vertebrae to begin and end the measuring process. Degenerative changes causing thicker endplates present in older adults (but not in adolescents) make it easier to determine where to draw the lines that form the angles in the Cobb method of measuring.
The bottom line is that changes in the vertebrae between young and old with scoliosis are different enough to warrant different methods of measuring the severity.