In some patients ankylosing spondylitis is restricted exclusively to an affection of the sacro- iliac articulation (sacroiliitis), best shown by MRI, as it causes bone marrow edema and cartilage changes. X-ray takes a long time to reveal this arthritis, because there only bone destruction and ankylosis are visible, these are not seen in the early stages of the disease. In the end sacroiliitis can lead to total destruction and ankylosis of the sacroiliacal joint, then of course it is clearly visible in x-ray (Figure 3). Active sacroiliitis can lead to local pressure pain and pain associated with movements of the pelvis
