Well, here's what I was thinking about the issue with unstable SI joints, my first symptom was pain, stiffness and a decreased range of motion in the lumbar area, I definitely didn't have instability in any part of my lumbar region...maybe some people present differently, I don't know but I don't think that I've ever read that SI joint instability was a symptom of AS, I could be wrong though, either way, I'm just trying to help you figure this out....
