Ankylosing spondylitis (AS) is an inflammatory condition of the large joints that generally presents in young men and women with insidious onset of back pain and stiffness. Traditionally, it is thought of as a disease affecting young men while rheumatoid arthritis is one of middle-aged women. Antigen studies however have indicated that there is not much of a male preponderance of AS as was previously believed and that sacroiliitis can be found in a large number of B27-positive women. However, it is true that the severest forms of AS are more common in men. The relationship between AS and the HLA-B27 antigen is well recognized with 97% of Caucasian spondylitics are B27 positive. However, AS is a rare condition (about 0.05% prevalence) and B27 is relatively common antigen (5% prevalence). HLA typing, therefore, is most effective as an exclusionary test used to rule-out the presence of AS.
Upon initial presentation, most patients will have abnormal sacroiliac joints. While the sacroilitis may be unilateral or asymmetrical, it is most commonly symmetrical. There are two components to each sacroiliac joint: the synovial part and the ligamentous part. The normal synovial joint appears on the anteroposterior radiograph as two curvilinear, parallel thin cortices with a uniform cartilage space between. With sacroiliitis there is loss of cortex, irregular cartilage space width, focal erosions, sclerosis, and ultimately ankylosis. The ligamentous portion of the joint also becomes abnormal, as both sacral and iliac cortices undergo early erosive changes, then develop productive bone (whiskering) and finally undergo ankylosis. During the early stages CT or MRI may be helpful to identify initial changes.
Spinal involvement is generally already present when sacroiliitis is documented or follows shortly thereafter. The spine changes consist of vertebral body squaring, production of syndesmophytes, development of ligamentous mineralization, and apophyseal joint ankylosis. The spinal involvement may begin at either the lumbosacral or thoracolumbar regions, then progress to other areas of the spine. In women, sometimes the thoracolumbar area is spared while cervical spondylitis and sacroilitis are present. Vertebral body squaring, a characteristic feature, is produced by two mechanisms. First, osteitis and erosion adjacent to vertebral end-plate margins cause ‘shiny’ or ivory corners and contribute to the square appearance. Second, anterior longitudinal ligament mineralization may fill in the normal anterior concavity of the vertebral body. Syndesmophytes, the hallmark of spinal involvement, are gracile ossifications of the outer fibers of the annulus fibrosis which extend from the edge of one end-plate to the edge of the next. With maturation, these create the classic ‘bamboo spine’. Apophyseal joints go through the same sequential stages of synovial inflammation and eventually ankylose. As in rheumatoid arthritis and juvenile chronic polyarthritis, synovitis at the atlanto-axial joint may lead to subluxation which frequently becomes fixed. Eventually, the entire spine tends to become osteopenic.