|ESSR 2019 / P-0092||
|Sacroiliac joint. How to differentiate inflammatory and non inflammatory conditions|
In 2009 the criteria for axial SpA were stablished, giving rise to a high demand od sacroiliac joint studies Fig. 1 .
Bone Marrow Edema (BME) was stablished as the hallmark of sacroiliitis,is a marker of inflammation and needs to be significant in degree before a diagnosis of sacroiliitis can be made. BME needs to be visible on at least two contiguous MR images, or at two separate locations on the same image before it can be considered significant.
The recognition of inflammatory-type low back pain helps segregate patients with axial spondyloarthritis (SpA) from those with more common mechanical low back pain. About 20% of patients with low back pain have inflammatory-type pain while about 20% of these patients with inflammatory-type pain will have axial SpA. The prevalence of axial SpA is about 1%.
However edema is a non specific finding, being comon to inflammatory and non inflammatory conditions. BME in mechanical (degenerative and overloading disease) is common all over the body and also in SIJ.
Care must be taken when interpreting a study according to ASAS criteria especially when there are mild changes without estructural alterations. The prevalence of edema by MRI is 10% in degenerative SIJ disease (up to 21% in controls and in almost a third of patients with non-SpA inflammatory low back pain or young athletes) and 50% in SpA, given that degenerative SIJ disease is about 60 times more prevalent than sacroiliitis, then a patient with SIJ edema by MRI may be 12 times more likely to have degenerative SIJ disease than spondyloarthritis.