I understand what you're saying about the difference of stiffness and immobility, usually the pain and stiffness occur during the fusion process and true immobility occurs when fusion has occured but due to the stiffness, the patient will usually have a decreased range of motion....as far as reading my statement in a medical book, you won't find it unless you're looking at a Physician's Diagnostic Procedure manual...but I don't recall saying that was a quote from any medical reference website, journal or book, it was just my opinion and I also don't recall saying that AS always starts in the SI joints but please read the statement below, it's from the AS diagnostic section of the Spondylitis Association of America.
The hallmark of AS is involvement of the sacroiliac (SI) joint. The x-rays are supposed to show erosion typical of sacroiliitis. Sacroiliitis is the inflammation of the sacroiliac joints. Using conventional x-rays to detect this involvement can be problematic because it can take 7 to 10 years of disease progression for the changes in the SI joints to be serious enough to show up in conventional x-rays. Another option is to use MRI to check for SI involvement. To diagnose ankylosing spondylitis, the sacroiliac joints (SI joints) at the back of the pelvis must be affected.
