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Explore This IssueApril 2015
Currently, little is known about the sequence of events leading to new bone formation in patients with ankylosing spondylitis (AS). In rheumatoid arthritis, there is an association between inflammation and erosive bone destruction, which is not followed by repair.
For patients with AS, it has been proposed that a similar sequence of events takes place, with the modification that erosive bone destruction is followed by tissue repair, which develops into mature bone when inflammation has resolved.1 Testing this hypothesis is difficult because AS is a slowly progressing disease, and there is a lack of sensitive clinical and biochemical measures of new bone formation. Further, the anatomical location and complicated anatomy of the sacroiliac joints and spine limit the availability of biopsies for histopathological analyses.
Therefore, indirect methods, such as magnetic resonance imaging (MRI), have increasingly been used to investigate the relation between inflammation and new bone formation. This review provides a summary of the current knowledge on the relationship between inflammation and new bone formation assessed by MRI and radiography of the spine and sacroiliac joints (SIJs) of patients with AS.
MRI & Radiography
In contrast to conventional radiography, magnetic resonance imaging (MRI) can visualize inflammation in the bone marrow (i.e., bone marrow edema or osteitis), synovium, ligaments, entheses and soft tissue. Moreover, MRI can visualize other types of lesions, such as fat infiltration, erosion, sclerosis and ankylosis. However, when new bone formation (i.e., syndesmophytes and ankyloses) is assessed in the spine, conventional radiography still provides superior information to MRI because the longitudinal ligaments and the cortex of the vertebral bodies are all black and, therefore, difficult to differentiate from each other.2 Thus, MRI is the gold standard for assessment of inflammation and fat, and conventional radiography of the spine is the gold standard for assessment of new syndesmophytes and ankyloses.
MRI is the gold standard for assessment of inflammation & fat, & conventional radiography of the spine is the gold standard for assessment of new syndesmophytes & ankyloses.
On MRI, the T1-weighted sequence is superior for detection of fat, and it provides detailed information on the anatomy. Fat-suppressed sequences, such as short tau inversion recovery (STIR), are water sensitive and superior for detection of inflammation. MRI is based on a tomographic (i.e., slice-based) technique, which makes it possible to assess different types of lesions at different locations of the spine and SIJs, and compare these with findings in the same areas assessed with other imaging modalities.