The Scoliosis Research Society has given us some guidelines to use. Pain and a spinal curve that's getting worse instead of better are the two most common indications.
Curves that measure more than 65 degrees are considered for surgical correction. This is a measurement of the upper back's forward curve. The normal curve is between 20 and 40 degrees.
There isn't much debate about when to do surgery. The real decision is on what kind of surgery to do. Posterior fusion alone is linked with loss of correction.
Surgeons are starting to look at combining a release in the front of the spine with a fusion in the posterior vertebrae. There may be a better correction with less loss this way.