Cobb angle of 10 degrees is diagnosed scoliosis, but 8 is not. Why is that? What difference does that 2 degrees have besides makings the curve bigger?As far as your spine and body is concerned, none. There is nothing magical happening or a tipping point at 10 degrees. The only “magic” behind 10 degrees is a nice even number used as a cut of point to define scoliosis, nothing more and nothing less.

For a long time, me and my colleagues have pondered over this question. Nobody quite understood why 10 degrees used to define scoliosis. So, for years we just accepted it, without quite understanding it.

I finally came across the research paper that held the answer. Back in 1977, an orthopedic surgeon, Kane [2], looked at reported prevalence studies on scoliosis and realized that there was a huge difference in the reports of how common scoliosis was. Reports ranged from 0,3 % all the way to 16 %, which is a huge difference. The discrepancy was caused by what cut of point was used to diagnose scoliosis.

But why 10 degrees?

Kane saw a need for consistency for the diagnostic criteria of scoliosis and wrote a paper “Scoliosis prevalence: a call for a statement of terms.” He presented a few points for his argument and based on those criteria he proposed that a 10 degree Cobb angle should be used as the “cut of point” for diagnosing scoliosis.

His main points were:

1. Yearly cost of x-raying all skeletally immature persons with scoliosis. Back then, in 1977 he estimated that that cost would be $ 84 millions per year.
2. The number of doctors able to deal with the issue at hand, that is the number of members of Scoliosis Research Society at the time. They only had 130 members then.
3. At what degree would patients be deserving treatment, he believed it to be 20 degrees.

Kane did admit that his points were chosen arbitrarily and could be contested, but he wanted at lest to have a clear-cut basis for further discussion.

It seems that his suggestion was never contested…

2. Kane WJ.
Scoliosis prevalence: a call for a statement of terms.
Clin Orthop Relat Res. 1977 Jul-Aug;(126):43-6.