Ankylosing spondylitis is the chronic inflammatory disease that affects primarily the axial skeleton and only secondarily the appendicular skeleton. It is seen predominantly in males between the ages of 15 and 35 years. Of all the inflammatory arthropathies, it is the least erosive and the most ossifying. Ankylosis of a joint is the predominant characteristic. The common radiographic findings are as follows:
- 1.
Normal mineralization before ankylosis; generalized osteoporosis after ankylosis
- 2.
Subchondral bone formation present before ankylosis
- 3.
Erosions—small, localized, and not a prominent part of the picture
- 4.
Absence of subluxations
- 5.
Absence of cysts
- 6.
Ankylosis
- 7.
Bilateral symmetrical distribution
- 8.
Distribution in sacroiliac (SI) joints and the spine, ascending from the lumbar to the cervical; then hips, shoulders, knees, hands, and feet, in decreasing order of frequency
The axial distribution and the predominant ankylosing features make the radiographic diagnosis relatively easy.
The sacroiliac joints
Although clinically ankylosing spondylitis is first suspected because of involvement of the costovertebral junction, radiographically the first involvement is seen in the SI joints. They are involved in a bilateral and symmetrical fashion ( Fig. 12-1 ). Small, succinct erosions are seen first on the iliac side and then on the sacral side, giving the joint margin the appearance of the perforated edge of a postage stamp ( Fig. 12-2 ). The erosions are surrounded by a small amount of bone repair. The erosions and sclerosis never become as extensive as those seen in the other spondyloarthropathies. The synovial aspect of the joint will ankylose relatively early. It is common for the entire SI joint, not only the true synovial aspect but also the ligamentous aspect, to ankylose. The ossification of the ligaments in the posterosuperior portion of the SI joint is seen radiographically as a “star” ( Fig. 12-3 ).
Other changes in the pelvis may be seen concurrent with the changes in the SI joints. There may be ossification of the ligamentous attachments (enthesopathy) to the iliac crest and ischial tuberosities, giving a “whiskered” appearance (see Fig. 12-4 ). The pubic symphysis may be involved, with small succinct erosions and reparative response adjacent to the erosions, followed by total ankylosis (Fig. 12-5 ). The pubic symphysis is involved in 23 percent of patients.
The spine
Initial involvement of the spine may be seen in the T12-L1 area. However, usually spine involvement is identified by the radiologist in the lumbar area. It is then seen to progress upward through the thoracic spine to the cervical spine. Initially there is erosion of the corner of the vertebral body with secondary reactive sclerosis. This gives a squared appearance to the vertebral body, and the reactive sclerosis is identified as the “ivory” corner ( Fig. 12-6 ). As the spine becomes immobilized, this reactive sclerosis disappears and one may identify nothing more than a squared vertebral body ( Fig. 12-7 ).
Ossification first takes place in the outer portion of the annulus fibrosus or in the Sharpey fibers. At first this ossification may not be visible radiographically, but lack of motion on physical examination and on flexion and extension films will indicate its presence ( Fig. 12-8 ). This ossification will extend from the Sharpey fibers into the deep layers of the longitudinal ligaments. This ossification is called a syndesmophyte and ossifies one vertebral body to the adjacent vertebral body in a succinct fashion ( Fig. 12-9 ). The syndesmophytes ascend the lumbar spine in a symmetrical fashion to eventually involve the thoracic spine and the cervical spine ( Fig. 12-10 ). Bone formation anterior to the vertebral body will result in loss of normal anterior concavity of the lumbar spine vertebral bodies ( Fig. 12-7 ). The disc spaces are generally preserved. Once ankylosis has taken place, disc calcification may develop ( Fig. 12-11 ).
