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Cobb Angle compared with ALDT to measure Scoliosis

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by , 05-09-2009 at 02:55 AM (45166 Views)
Cobb Angle has been predominantly used for the past 60 years as a measurement for scoliosis despite its shortcoming in terms of accuracy. It was therefore of interest that I read about a new way of measuring the spinal curvature in patients with scoliosis. A group of doctors in a Chinese medical college proposed a new way of measuring known as axis-line distance technique and then went on to evaluate its accuracy.

The axis-line distance technique ALDT works as follows:


Illustrating A) Cobb angle, B)
Axis-Line-Distance Technique (ALDT).
A line connecting the midpoint of the superior border of the pubic symphysis and the central point of the seventh cervical vertebra is denoted the axis line, and it is used as a baseline for measurement. The distance between the axis line and the center of the apex vertebra is used to assess the degree of scoliotic curvature. To determine the apex vertebra, another line parallel to the axis line is drawn along the convex side of the vertebra at the peak of a scoliotic curve, and this outermost vertebra is considered the apex vertebra.

For people without scoliosis, the center of the vertebrae in a normal x-ray image should be in alignment with the axis line.

Is this method more accurate than the common Cobb angle?

In their study [19], 6 physicians measured x rays taken on 65 patients and on two different occasions that were 3 weeks apart. (This is a standard practice for such studies). They measured both the Cobb angles and their new ALDT. Results revealed that their method was much more reliable i.e. it did not show as much variance between the measurements taken on two different occasions as compared to Cobb angle measurements. The variance between measurements taken by different doctors on the same occasion was smaller using the ALDT method, so was the variance in measurements made by the same doctor on different occasion.

They hence concluded that ALDT for measuring scoliosis was more reliable, reproducible and straightforward than Cobb angle measurement.

Personally I feel that the strength in Cobb angle lies in the fact that it is relatively easy to relate to as we more or less can picture what a 45 degree curve or a 90 degree curve look like. So, my thinking is, why not use both methods. While one would provide a guide that we can easily understand on the severity of the scoliosis curve, the other would reveal how much the spine has actually deviated sideways. One without the other will only provide half the story e.g if the Cobb angle remains the same over a period of time, it does not mean that nothing in the spine has changed. There is a chance that the apex could have shifted sideways without altering the Cobb angle.

The ALDT is definitely more sensitive in detecting changes in the curve either from natural progression or treatments.

19. He, Jia-Wei, Yan, Zhi-Han, Liu Jun, Yu Zhi-Kang, Wang Xiang-Yang, Bai Guang-Hui, Ye Xin-Jian, Zhang Xian.
Accuracy and Repeatability of a New Method for Measuring Scoliosis Curvature
Spine. 2009 Apr 20;34(9): E323-9.

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