Magnetic resonance imaging (MRI) can be used to evaluate the bone marrow, joint space, neurovascular structures, and soft tissues around the sacroiliac joint accurately. MRI is an important part of diagnosing the chronic inflammation found in axial spondyloarthritis (axSpA) because MRI, unlike plain radiographs, can reveal inflammatory changes, fatty changes, and subtle structural abnormalities.1

In 2009, the Assessment of SpondyloArthritis International Society (ASAS)/Outcome Measures in Rheumatology (OMERACT) MRI working group defined active sacroiliitis based on active inflammatory lesions of the sacroiliac joints. These lesions appear as bone marrow edema (BME) on T2-weighted images with fat saturation. Active sacroiliitis is diagnosed if at least 2 BME lesions appear within the same slice, or if there is 1 lesion in the same quadrant on 2 consecutive slices.1  

Results of MRI have shown that patients with axSpA are at increased risk for low bone mineral density (BMD), osteoporotic fractures, and neurologic compromise secondary to spinal changes.2 It is thought that inflammatory changes in axSpA may be associated with bone loss, resulting in low BMD and osteoporosis. AxSpA disease activity influences the rate of bone loss directly; thus, osteoporosis is considered a manifestation of the axSpA disease activity rather than a comorbidity.3

To establish whether an association existed between inflammatory lesions found on MRI in the sacroiliac joint and low BMD, Ha Neul Kim, MD, and colleagues, from The Catholic University of Korea, Seoul, South Korea, recruited 76 patients diagnosed with axSpA according to ASAS criteria.4 In this cross-sectional study, each patient was evaluated for the presence of joint inflammatory structural lesions on MRI, and BMD was measured in the lumbar spine, femoral neck, and total hip. AxSpA disease activity, C-reactive protein level, erythrocyte sedimentation rate, and serum collagen type 1 cross-linked C-telopeptide (CTX-1) level were assessed concurrently with MRI and BMD. CTX-1 was used as a surrogate marker for active bone resorption.   

Acute inflammatory lesions in axSpA were defined as BME measured on MRI. BMD was measured by dual-energy x-ray absorptiometry scan.

Researchers found low BMD in 18% of those with axSpA (mean age 33 ± 12 years). Patients with low BMD had greater BME and deep BME scores on MRI compared with those who had normal BMD (P < .047 and P < .007, respectively).  

Sacroiliac joint deep BME found on MRI, increased C-reactive protein level, and active sacroiliitis on x-ray were found to be associated with low BMD after multivariate analysis (odds ratio: 5.6, 14.6, and 2.5, respectively).

Summary and Clinical Applicability

This cross-sectional study found an association between MRI-detected sacroiliac joint inflammatory lesions and low femoral neck and total hip BMD in patients with axSpA. Sacroiliac joint active bony inflammation was positively correlated with CTX-1 and decreased BMD in those with axSpA, suggesting that active sacroiliitis may be involved in the pathogenesis of osteoporosis through the process of bone resorption. Thus, bone marrow inflammation may be a critical component of the pathogenesis of spinal osteoporosis.

This study suggests that radiographic sacroiliitis on x-ray and sacroiliac joint BME on MRI are risk factors for low BMD, and therefore clinicians treating patients with these radiographic findings should consider assessment for osteoporotic risk factors.

“The severity of sacroiliitis on X-ray may reflect cumulative inflammation at the SIJ, suggesting that cumulative bone inflammation at the SIJ, as well as acute sacroiliitis, may be associated with low BMD”, the authors concluded.

A longitudinal study looking at inflammatory markers over a longer period after MRI measurement could provide more information about the extent to which sacroiliitis of the sacroiliac joint is associated with low BMD.

References

1. Hermann KG, Baraliakos X, Van der Heijde DM, et al. Descriptions of spinal MRI lesions and definition of a positive MRI of the spine in axial spondyloarthritis: a consensual approach by the ASAS/OMERACT MRI study group. Ann Rheum Dis. 2012;71(8):1278-1288.

2. Forien M, Moltó A, Etcheto A, Dougados M, Roux C, Briot K. Bone mineral density in patients with symptoms suggestive of spondyloarthritis. Osteoporos Int. 2015;26(5):1647-1653.

3. Rosenbaum J, Chandran V. Management of comorbidities in ankylosing spondylitis. Am J Med Sci. 2012;343(5):364-366.

4. Kim HN, Jung JY, Hong YS, Park SH, Kang KY. Severe bone marrow edema on sacroiliac joint MRI increases the risk of low BMD in patients with axial spondyloarthritis. Sci Rep. 2016;6:22158.