In cartilaginous joints (discovertebral junction, symphysis pubis and manubriosternal joint), the process appears to be inflammatory. Ossification produces syndesmophytes that extend from one vertebral body to another.

Enthesopathy (abnormalities in entheses or ligamentous attachments) is also a typical feature. Radiographically, erosion and eburnation of the subligamentous bone with poorly defined erosive abnormalities and surrounding sclerosis are seen.

Imaging features

The radiographic features in synovial joints in ankylosing spondylitis are similar to those of rheumatoid arthritis. Both diseases exhibit some degree of osteoporosis, joint space narrowing and bone erosion. Proliferation about sites of erosion is more characteristic of ankylosing spondylitis, however.

Radionuclide bone imaging may be of value in detecting sacroiliitis, although qualitative analysis of the uptake of radionuclide preparations is difficult because of the normal uptake. Focal areas of augmented radionuclide accumulation in the spine may indicate the site of an acute fracture or chronic pseudarthrosis.

CT scanning may possibly delineate early changes in the sacroiliac joint. Among the CT indicators of sacroiliitis are nonuniform iliac sclerosis, focal joint space narrowing and bone erosions. CT may also be used to detect spinal fractures, spinal stenosis, thecal diverticula, atlantoaxial instability and manubriosternal and costovertebral joint disease.

MR imaging can be of value in imaging certain manifestations and complications of ankylosing spondylitis. An increase in signal intensity within the vertebral body marrow, adjacent to abnormal intervertebral discs, in T2-weighted spin-echo sequences may reflect oedema and indicate discovertebral inflammation. Its role in the early diagnosis of sacroiliitis is still being investigated.

Specific sites

The sacroiliac joint is among the first sites to show involvement in this disease. Changes are bilateral and symmetrical. Periarticular osteoporosis, superficial erosion and focal sclerosis of subchondral bone are observed, followed by fraying of the bone surface and widening of the interosseous space. Bone proliferation leads to the formation of irregular bone bridges that traverse the articular cavity and later result in complete ankylosis.

In the spine, the discovertebral junctions reveal osteitis, syndemophytosis, discovertebral erosions and destruction (Andersson lesions), and discal calcification. The apophyseal joints are narrowed and fused; abnormalities are observed in the lumbar, thoracic and cervical segments of the spine and are accompanied by reactive subchondral bone formation (Fig.1). In the cervical region apophyseal joint ankylosis can be very striking.

In the hip, one of the early findings is an osteophyte in the lateral aspect of the femoral head. Subsequently the osteophytes create a collar around the femoral neck, and diffuse joint space narrowing produces axial migration of the femoral head with respect to the acetabulum, which may sometimes progress to protrusio acetabuli.

Glenohumeral joint abnormalities ar