Take the SEAS patient. She seemed to have some fixed bony curve. When they braced her, she never braced past that curve. So, if even force couldn't correct her past that point, I'm guessing that's because that part of the curve is made up of bone.
OK, so they take the brace off, and she stays more or less stable for awhile and then, over the years, she progresses to her *pre-bracing curve*. Then, they intervene with exercise, and they're able to get her back to around where her curve was when she was braced.
So, the curve she had when she was braced, and the one the was able to regain with exercise, is likely a bony curve.
But the curve *before* she was braced, and the progression she saw *after* she was braced - that had to have been something other than bone. Otherwise, she wouldn't have been able to reverse it with exercise.
So, if all that is true, then, for someone like my son, how much of his curve is actually fixed in bone? He had his rapid progression after he stopped growing, and I suspect at least some of that was his muscles - he just seemed to collapse into the curve. Maybe that's similar to the SEAS woman, and he could get back to his pre-rapid progression curve (around 35 degrees).
Of course, this could all be fanciful and all 50 something degrees of his curve could be etched in bone.