Inflammation of the Spinal Column

CT

CT is useful for documenting the presence of ankylosing spondylitis in patients whose initial sacroiliac joint radiographs are normal or equivocal. Features such as joint erosions, subchondral sclerosis, and bony ankylosis are visualized better on CT than on radiographs. However, normal variants of the sacroiliac joint may simulate the features of sacroiliitis. CT supplements bone scintigraphy in evaluating areas of increased radionuclide uptake, particularly in the spine. Bony lesions such as pseudarthroses, fractures, spinal canal stenosis, and facet inflammatory disease are well detected using CT, particularly with multiplanar reformatting into coronal, sagittal, or oblique images ( Fig. 20-1 ). Other useful applications of CT include the assessment of costovertebral disease, manubriosternal disease, dural ectasia, and paraspinal muscle atrophy. Multidetector CT (MDCT) is superior to radiographs and to MRI for showing the number of lesions and demonstrating fracture morphology. In patients with advanced disease, MDCT is the primary modality for evaluating cervical spine fractures ( Fig. 20-2 ). MDCT complements MRI, which better shows spinal cord and soft tissue injuries. CT is accurate for diagnosing complications of ankylosing spondylitis, such as spinal pseudarthrosis, fractures, and vertebral scalloping from dural ectasia.

MRI

MRI may contribute to the early diagnosis of sacroiliitis. MRI detection of synovial enhancement correlates with disease activity as measured by laboratory inflammatory markers. MRI is superior to CT for detecting abnormal cartilage, bone erosions, and subchondral bone changes. It is also sensitive for assessing disease activity early in the course of ankylosing spondylitis. Affected sites include the discovertebral junctions and peripheral joints. In general, areas of increased T2 signal intensity correlate with the presence of edema or vascularized fibrous tissue ( Fig. 20-3 ).

In established disease, MRI can assess the integrity of the intervertebral discs and spinal ligaments and display spinal fractures. MRI detects pseudarthroses, diverticula associated with cauda equina syndrome, and spinal canal stenosis. In patients with complications of fractures or pseudarthroses, MRI demonstrates any compromise of the spinal canal or spinal cord injury. MRI is very helpful in patients with neurologic symptoms, especially those with a symptom-free interval after initial diagnosis and those with neurologic deterioration after established spinal cord injury from trauma. Advantages of MRI include direct visualization of cartilage abnormalities, detection of bone marrow edema, improved detection of bone erosions, and lack of ionizing radiation.

Early in ankylosing spondylitis, the spinal lesions mainly involve the intervertebral discs, the adjacent edges of the vertebrae, and the rims. The cartilage and the subchondral bone are involved secondarily. Neither MRI nor CT is diagnostic of spondylitis/spondylodiscitis specifically early in the disease. However, it is possible to differentiate between the noninfective spinal inflammation of spondyloarthropathy and infectious spondylitis/spondylodiscitis if the bacterial infection has already spread to adjacent structures. Noninfectious spondyloarthropathy does not spread outside the bone and ligament, so evidence of such spread implicates infection. MRI is sensitive enough to detect early inflammation of the axial skeleton in both spondyloarthritis and rheumatoid arthritis.

Radiography

Radiographs are the single most important imaging technique for detection, diagnosis, and follow-up of patients with ankylosing spondylitis. Bony morphology, subtle calcifications, and ossifications are well demonstrated radiographically. Early spondylitis manifests as small erosions at the corners of the vertebral bodies with surrounding reactive sclerosis, findings designated the “shiny corner sign” or “Romanus lesion.” Squaring of the vertebral bodies is another characteristic feature of ankylosing spondylitis and is caused by a combination of corner erosions and periosteal new bone formation along the anterior aspect of the vertebral body. Squaring is most easily appreciated in the lumbar spine, where the anterior border of the vertebral body is normally concave. Further inflammation is associated with formation of syndesmophytes: ossifications of the outer fibers of the annulus fibrosus that form bony bridges between the inflamed corners of adjacent vertebrae. Ossification also occurs within the fibers of the adjacent paravertebral connective tissue. Ossification of the posterior interspinous ligaments links sequential spinous processes into a solid vertical midline “bone,” which causes a dense line on frontal radiographs. In similar fashion, the apophyseal and costovertebral joints are frequently affected by erosions and then ossification and eventually fuse. Complete fusion of the vertebral bodies by syndesmophytes and related ossifications produces the “bamboo spine” ( Fig. 20-4 ). The intervertebral discs may calcify at single or multiple levels, usually in association with apophyseal joint ankylosis and adjacent syndesmophytes.

In established ankylosing spondylitis, fractures usually occur at the thoracolumbar and cervicothoracic junctions. Upper cervical spine fractures and atlantoaxial subluxations are rare. Spinal fractures are characteristically transverse (i.e., horizontal) rather than vertical or oblique and are called “chalk stick” fractures. They extend from anterior to posterior and frequently pass through the unossified disc.

Pseudarthroses are seen radiographically as areas of discovertebral destruction and adjacent sclerosis. These changes are referred to as the Anderson lesion and may resemble disc infection. Pseudarthrosis usually develops secondary to a previously undetected fracture or at an unfused segment. Therefore, an important imaging feature of ankylosing spondylitis is the involvement of the facet joints, which is seen as a linear hypodense area with sclerotic borders.

The spinal abnormalities found in ankylosing spondylitis may also be encountered in other diseases, such as enteropathic arthropathy, psoriasis, and Reiter syndrome. Careful analysis and classification of bony outgrowths in the vertebrae help to differentiate among these conditions. Flowing anterior ossification is a feature of diffuse idiopathic skeletal hyperostosis. Paravertebral ossification is present in both psoriasis and Reiter syndrome. Syndesmophytes are found in alkaptonuria as well as ankylosing spondylitis.

Spinal pseudarthrosis in ankylosing spondylitis may produce marked discovertebral destructive changes that resemble infective spondylodiscitis. The detection of posterior element fractures or defects is an important distinguishing clue in diagnosing these patients with ankylosing spondylitis. The spondyloarthritides often share common features of aggressive erosions, bone density preservation, and proliferative bony response to inflammation. These features along with asymmetric involvement of the appendicular skeleton, sacroiliitis, and spine abnormalities usually allow differentiation of ankylosing spondylitis from rheumatoid arthritis and other inflammatory arthritides based on imaging studies.

MRI

Psoriatic arthritis shares MRI features with rheumatoid arthritis and spondyloarthropathy. On MRI, the synovitis of psoriatic arthritis appears indistinguishable from the synovitis of rheumatoid arthritis. Although in some patients the synovitis observed conforms to a typical rheumatoid pattern, in others the inflamed tissue extends far beyond the joint capsule and involves adjacent structures, such as thickened collateral ligaments and periarticular soft tissue. The erosions themselves consist of a break in the cortical bone overlying a region of altered signal intensity with definite margins. Psoriatic arthritic erosions can be large, ill-defined areas in the subcortical bone with increased signal on short tau inversion recovery (STIR) and T2-weighted (T2W) imaging with fat saturation. They may enhance on postcontrast T1-weighted (T1W) fat saturation sequences.

Radiography

Radiographically, the spondylitis and reactive arthritis of psoriatic arthritis are chunky and asymmetric, as compared with the smooth, fine, symmetric changes of ankylosing spondylitis.

CT

Paravertebral calcifications may be seen as the initial manifestation of reactive arthritis, in contrast to the syndesmophytes seen in early ankylosing spondylitis. Erosions of the anterior corners of the vertebral bodies are rare in reactive arthritis.

MRI

There is no specific role for MRI in the diagnosis of reactive arthritis.

Radiography

In early reactive arthritis radiographs may show no abnormalities. Two typical findings are unilateral or bilateral sacroiliitis and asymmetric paravertebral comma-shaped ossification involving the lower thoracic and upper lumbar vertebrae.