Abstract

Ankylosing spondylitis (AS) is the prototypical form of the spondyloarthropathies, which at a prevalence of 2 % is among the most frequent rheumatic diseases. Spondyloarthropathy comprises the following five disorders: AS, reactive arthritis, psoriatic arthritis, enteropathic arthritis in Crohn's disease, and ulcerosing colitis as well as undifferentiated spondyloarthropathy. In 99 % of the patients with AS initial abnormal findings affect the sacroiliac joints. The radiographic changes required for diagnosing AS occur as late as 5 - 9 years after the onset of clinical symptoms. MRI of the sacroiliac joints reliably demonstrates both chronic inflammatory changes (erosions, sclerotic changes, bone bridges) and acute inflammatory changes (synovitis, capsulitis, osteitis) and allows for grading the chronicity and acuity of such changes. Enthesitis of the interosseous ligaments of the retroarticular space is a manifestation of AS. Spondylodiscitis (Andersson 1937) may occur as an inflammatory or non-inflammatory process (transdiscal fatigue fracture). Inflammations of the facet and costospinal joints developing into ankylosis are typical of AS. Changes of the vertebral bodies occur as anterior (Romanus 1952), posterior, and marginal spondylitis. All forms of spondyloarthropathies are furthermore characterized by asymmetrical synovitis of the large joints, particularly of the legs (gonarthritis, coxitis, tarsitis, peripheral oligoarthritis), rheumatic fibroosteitis (pelvic enthesitis, rheumatic calcaneopathy), and peri- and synchondritis of the pubic symphisis and sternal synchondrosis. Since early inflammatory changes of the spinal column and of the extravertebral localizations in AS are demonstrated by MRI before they become apparent on radiographs, and thereby the diagnostic gap could be closed, the early use of MRI for diagnostic and follow-up is commendable, when new therapeutical options like the so-called "biologicals" are employed.