Ankylosing spondylitis is a distinct disease entity characterized by inflammation of multiple articular and para-articular structures, frequently resulting in bony ankylosis. Ankylosing spondylitis usually is classified as a chronic and progressive form of seronegative arthritis.
Radiographic findings:
- Sacroiliitis occurs early in the course of ankylosing spondylitis and is regarded as a hallmark of the disease. Radiographically, the earliest sign is indistinctness of the joint. The joints initially widen before they narrow. Subchondral bony erosions on the iliac side of the joint are seen which are followed by subchondral sclerosis and bony proliferation. With eventual bony fusion, the sclerosis resolves .At the end stage, the sacroiliac joint may be seen as a thin dense line or may not be visible at all (as seen in the image above).
- In the spine, the early stages of spondylitis are manifested as small erosions at the corners of the vertebral bodies. The areas are surrounded by reactive sclerosis and have been termed the shiny corner sign or Romanus lesion. Squaring of the vertebral body is another characteristic feature of ankylosing spondylitis This is followed by syndesmophyte formation, which refers to ossification of the outer fibers of the annulus fibrosis, giving rise to bridging of the corners of one vertebra to another .
- Ossification of the adjacent paravertebral connective tissue fibers also occurs. Posterior interspinous ligament ossification, combined with linking of the spinous process, produces an appearance of a solid midline vertical dense line on frontal radiographs.
- The apophyseal and costovertebral joints frequently are affected by erosions and eventually undergo fusion. Complete fusion of the vertebral bodies by syndesmophytes and other related ossified areas produces bamboo spine .
- Fractures in established ankylosing spondylitis usually occur at the thoracolumbar and cervicothoracic junctions. Upper cervical spine fractures and atlantoaxial subluxation rarely are seen. Fractures typically are transverse, extend from anterior to posterior, and frequently pass through the ossified disk. They have been termed chalk stick fractures or Carrot stick fractures.
- Pseudoarthrosis is seen radiographically as areas of diskovertebral destruction and adjacent sclerosis. The changes are referred to as the Andersson lesion and may resemble disk infection although pseudoarthrosis usually develops secondarily to a previously undetected fracture or at an unfused segment. Therefore, an important imaging feature is the involvement of the posterior elements, seen as a linear hypodense area with sclerotic borders
- Enthesopathy is seen radiographically as ill-defined erosions with adjacent sclerosis at the sites of ligamentous and tendinous attachments. With healing, sclerosis decreases and new bone proliferation occurs. Lesions typically are bilateral and symmetric in distribution. Enthesopathic changes are particularly prominent at certain sites around the pelvis, such as the ischial tuberosity, iliac crest, and femoral trochanters. Other locations include the coracoclavicular ligament attachment site to the inferior clavicle, humeral tuberosity, anterior patella, and plantar aspect of the calcaneum.
- Hip joint involvement typically is bilateral and symmetric. The hip joint space is narrowed uniformly, axial migration of the femoral head occurs, and a collar of osteophytes may be seen at the femoral head-neck junction.
- Bony ankylosis eventually may occur.
- At the glenohumeral joint, the joint space narrows uniformly, and a large erosion may be present in the upper greater trochanter. Knee changes consist of uniform joint space narrowing and surrounding bony proliferation. In the hands, the joints usually are involved asymmetrically, erosions are smaller and shallower, marginal periostitis is seen, and bony density usually is preserved.
- Lung manifestations of ankylosing spondylitis are seen as progressive fibrosis and bullous changes at the apices. On radiographs, chest lesions may resemble tuberculous infection. Lung bullae may be complicated by infection by Aspergillus species and other opportunistic infections. Lung changes usually are seen several years after joint disease develops.