Some researchers have found a very consistent discrepancy in trunk rotation strength in idiopathic scoliosis. Working on rotation strength seems to slow the curve progression.
There have been two articles published by Vert Mooney (a retired orthopedist) et al on a continuing study he did using rotation strength conditioning with teenaged scoliosis victims. The apparatus was the MedX torso rotation machine. He got some encouraging results (quite a few reductions in Cobb curvature) but has unfortunately discontinued the study (so I'm told by a scoli-mom who called him to inquire). Search PubMed (Mooney scoliosis) for the abstracts.
After reading his first article I tried it myself. I had a significant imbalance of rotational strength which I evened out with the MedX machine at a local Gold's Gym in just a couple brief sessions per week for about two months. But the scoliosis animal is a wily beast that will not be tamed by just one exercise.
I am looking into starting some research on the rotation strengthening. I talked with Dr. Mooney and like scolip said he is not planning any new studies. The studies basically used a trunk rotation machine you would find in any gym.