eMedicine Specialties > Radiology > Musculoskeletal

Ankylosing Spondylitis: Imaging

Author: Wilfred CG Peh, MD, MBBS, FRCP(Glasg), FRCP(Edin), FRCR, Clinical Professor, Faculty of Medicine, National University of Singapore; Senior Consultant Radiologist, Alexandra Hospital, Singapore
Contributor Information and Disclosures

Updated: Mar 25, 2009

Radiography

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 (see Image 1); these are followed by subchondral sclerosis and bony proliferation.

• With eventual bony fusion, the sclerosis resolves (see Image 2).
• At the end stage, the sacroiliac joint may be seen as a thin dense line, or it may not be visible at all.
• Sacroiliitis typically is symmetric, although it may be asymmetric in the early stages of the disease.
• 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 (see Image 3).

• Squaring of the vertebral body is another characteristic feature of ankylosing spondylitis; it is caused by a combination of corner erosions and periosteal new bone formation along the anterior aspect of the vertebral body. This is best seen in the lumbar spine, in which the anterior cortex of the vertebral body normally is concave.
• This is followed by syndesmophyte formation; this term refers to the process in which ossification of the outer fibers of the annulus fibrosis leads to bridging of the corners of one vertebra to another (see Image 4).
• 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 (see Image 5).

• The apophyseal and costovertebral joints frequently are affected by erosions and eventually undergo fusion (see Image 6).
• Complete fusion of the vertebral bodies by syndesmophytes and other related ossified areas produces bamboo spine (see Image 7).
• Calcifications of the disk may occur at single or multiple levels; they are usually associated with apophyseal joint ankylosis and adjacent syndesmophytes (see Image 8).

• For patients with established ankylosing spondylitis, fractures usually occur at the thoracolumbar (see Image 9) 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 (see Image 10).
• Pseudoarthrosis is seen radiographically as areas of diskovertebral destruction and adjacent sclerosis. The changes, which are referred to as the Andersson lesion, 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 (see Image 11).

• 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 (see Image 12), iliac crest, and femoral trochanters. Other locations include the coracoclavicular ligament attachment site to the inferior clavicle (see Image 13), 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 (see Image 14).

• Protrusio acetabuli develops in as many as one third of patients.
• 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 tuberosity.
• 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.

Degree of Confidence

Radiographs are a reliable means with which to make a diagnosis, particularly if typical radiographic features are present.

False Positives/Negatives

The sacroiliitis associated with ankylosing spondylitis should be differentiated from sacroiliitis caused by other diseases. Bilateral symmetric sacroiliac joint disease may be found in conditions such as psoriasis, Reiter disease, enteropathic arthropathy, hyperparathyroidism, and osteitis condensans ilii.

Ankylosing spondylitis also may present as a bilateral asymmetric sacroiliitis, mimicking psoriasis, Reiter disease, rheumatoid arthritis, and gouty arthritis. Unilateral distribution of sacroiliac disease also may be found in infective arthritis; therefore, it is important that radiographic features be carefully analyzed to look for signs of various diseases and that radiographic features be correlated with the overall pattern of bony involvement and the clinical findings.

Spinal abnormalities found in ankylosing spondylitis may be encountered in other diseases, such as enteropathic arthropathy, psoriasis, and Reiter disease. Careful analysis and classification of bony outgrowths in the vertebrae are helpful in differentiating the various conditions. In addition to ankylosing spondylitis, syndesmophytes are found in alkaptonuria. Flowing anterior ossification is a feature of diffuse idiopathic skeletal hyperostosis, whereas paravertebral ossification is present in both psoriasis and Reiter disease.

Spinal pseudoarthrosis in ankylosing spondylitis often produces marked diskovertebral destructive changes that may resemble infective spondylodiskitis. The presence of posterior element fracture or defect is an important distinguishing clue.

Computed Tomography

Findings

CT may be useful in selected patients in whom ankylosing spondylitis is suggested and in whom initial sacroiliac joint radiographs findings are normal or equivocal. Features such as joint erosions, subchondral sclerosis (see Image 15), and bony ankylosis are visualized better on CT than on radiographs; however, some normal variants of the sacroiliac joint may simulate the features of sacroiliitis.^10,11,12

CT supplements bone scintigraphy in evaluating areas of increased uptake, particularly in the spine. Bony lesions, such as pseudoarthrosis (see Images 16-17), fractures, spinal canal stenosis, and facet inflammatory disease (see Image 18), are detected well using CT, particularly with reformatted coronal, sagittal, or oblique images. Other useful applications include the assessment of atlantoaxial instability, costovertebral disease, manubriosternal disease, dural ectasia (see Image 19), and paraspinal muscle atrophy.

Multidetector CT (MDCT) is superior to radiographs and MRI in showing more injuries, and it yields more information on fracture morphology. In patients with advanced ankylosing spondylitis, MDCT is the imaging modality of choice for the evaluation of fractures of the cervical spine. It complements MRI, which better shows spinal cord and soft tissue injuries.

Degree of Confidence

Features such as joint erosions, subchondral sclerosis, and bony ankylosis at the sacroiliac joint and the lumbar vertebral facet joints are visualized better on CT scans than on radiographs. CT is accurate for the diagnosis of complications such as spinal pseudoarthrosis, fractures, and vertebral scalloping from dural ectasia.

False Positives/Negatives

On CT, some normal variants of the sacroiliac joint may simulate the features of sacroiliitis.

Magnetic Resonance Imaging

Findings

MRI may have a role in the early diagnosis of sacroiliitis. Detection of synovial enhancement on MRI has been found to correlate with disease activity, as measured by laboratory inflammatory markers. MRI has been found to be superior to CT in the detection of cartilage changes, bone erosions, and subchondral bone changes. MRI is also sensitive for the assessment of activity in relatively early disease. Affected sites include the diskovertebral junction and the peripheral joints. In general, areas of increased T2 signal correlate with the presence of edema or vascularized fibrous tissue.^{13,14,15,16,17,18,19,20,21,22,23}

In established disease, MRI detects pseudoarthrosis, diverticula associated with cauda equina syndrome, and spinal canal stenosis. In patients with complications of fracture or pseudoarthrosis, MRI is useful for the assessment of spinal canal compromise and cord injury (see Images 20-21). MRI may be used to assess the integrity of intervertebral disks and spinal ligaments in cases involving spinal fractures. MRI is considered to be mandatory for patients with neurologic symptoms, especially for those who experience a symptom-free interval and for those who experience neurologic deterioration after established spinal cord injury.

MRI has been found to have a role in monitoring the treatment of patients with active ankylosing spondylitis. The degree of spinal inflammation may be detected before and after therapy with drugs such as the tumor necrosis factor-alpha receptor fusion protein etanercept and the interleukin 1 receptor antagonist anakinra.

Advantages of MRI include direct visualization of cartilage abnormalities, detection of bone marrow edema, improved detection of erosions, and the absence of ionizing radiation.

Degree of Confidence

Yu et al have found MRI to be more sensitive than either radiography or CT in detecting early cartilage changes and bone marrow edema of the sacroiliac joints.²⁴

False Positives/Negatives

Although sensitive in the detection of sacroiliitis, MRI is not specific for diagnosing ankylosing spondylitis as the cause of sacroiliitis.

Nuclear Imaging

Findings

Bone scintigraphy may be helpful for patients with suggested ankylosing spondylitis in whom radiographic findings are normal or equivocal. Qualitative assessment of the accumulation of radionuclides in the sacroiliac region may be difficult because of normal uptake in the location. Quantitative analysis may be more useful in these patients. Ratios of sacroiliac joint to sacral uptake of 1.3:1 or more are considered abnormal (see Image 22).^25,26

Degree of Confidence

Scintigraphy has high sensitivity but low specificity for the diagnosis of sacroiliitis. Several factors potentially affect calculation of the sacroiliac joint to sacrum ratio. These factors include a prominent sacral tubercle that may produce increased uptake in the sacrum and the influence of age and sex on radionuclide uptakes in the sacroiliac joints and sacrum. In patients with advanced disease, the radionuclide uptake may not appear abnormal.

Although sensitive for the detection of active disease in the spine, increased radionuclide accumulation is not specific for the diagnosis of ankylosing spondylitis.

False Positives/Negatives

Abnormal radionuclide uptake in the sacroiliac joints and other spinal locations may not be specific for ankylosing spondylitis. Correlation with other radiologic and clinical findings is important.

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