MRI of the sacroiliac joints and/or spine is the most appropriate diagnostic test to perform next in this patient with suspected spondyloarthritis, considered in patients with chronic inflammatory back pain beginning before the age of 45 years. It is important to establish the diagnosis of spondyloarthritis even if it will not change immediate management because it requires life-long monitoring for the development of cardiovascular and other major organ damage. A positive HLA-B27 can be supportive of this diagnosis, but a negative result does not rule it out. Conventional radiographs can demonstrate sacroiliitis (erosive changes and sclerosis) but may be normal in early disease. If there is high suspicion for axial inflammation and conventional radiographs are normal, MRI of the sacroiliac joints and/or spine should be considered to further evaluate for inflammation. MRI is the most sensitive imaging technique for detecting early inflammation in the spine and sacroiliac joints. Although his radiographs and HLA-B27 testing were negative, this 25-year-old patient has probable inflammatory back (morning stiffness lasting 90 minutes) and sacroiliac pain, making spondyloarthritis, specifically ankylosing spondylitis, a likely diagnosis. Advanced imaging is often needed to show sacroiliac joint abnormalities.
CT of the sacroiliac joints can provide evidence of erosive changes in the bone but has limited ability to detect soft-tissue inflammation of the spine and may be normal until bony changes are present.
When injected intravenously, technetium-99m binds to hydroxyapatite crystals. Increased uptake reflects increased bone turnover related to infection, cancer, trauma, and arthritis. Because of these characteristics, a positive scan is a sensitive but nonspecific indicator of bone, joint, and periarticular disorders and may be most useful when other first-line imaging modalities are negative but the suspicion of disease remains high.
Ultrasonography is relatively inexpensive, poses no radiation hazard, can scan across three-dimensional structures, and may be used concurrently with physical examination to evaluate moving structures (for example, tendon evaluation). Musculoskeletal ultrasonography can be helpful in detecting evidence of peripheral enthesitis and arthritis but has not demonstrated usefulness in detecting axial involvement such as sacroiliitis.