[Sacroiliac joints: osteoarthritis or arthritis]

One of the most challenging aspects of treating the sacroiliac joint (SIJ) pain is the complexity of diagnosis. Imaging methods have gained importance for the diagnosis of SIJ diseases. CT and MR exams had equal efficacy superior to radiography in staging structural changes in the SIJ due to osteoarthritis or sacroiliitis. The diagnosis of spondyloarthropaty can be delayed for several years using certain radiography studies. MR imaging reveal early cartilage changes and active inflammatory changes in the subchondral bone and surrounding ligaments in spondyloarthropaties, as well as subperiosteal and transcapsular yuxtaarticular infiltrations characteristic of septic sacroiliitis, which could not be found by either CT of radiography. T1-WI with fat suppression (FS) and STIR images improve the demonstration of erosions and inflammatory changes respectively, on MR studies. Additional T1-FS after i.v. contrast has proven valuable in demonstrating the extension of inflammatory changes and abscesses in septic sacroiliitis, and in spondyloartropaties may be useful although this is debatable. Scintigraphy gives high sensibility only in early inflammatory changes and low specificity for the diagnosis of sacroiliitis due to high bone turnover in the SIJ, although specific radioprobes are useful in confirming the septic etiology and evaluating additional foci. This complex joint of very limited mobility shows a lot of structural variations and some anatomical degenerative changes due to age, which are necessary to know to an adequate image interpretation and diagnosis of disease.