Early sacroiliitis changes present primarily as subchondral bone marrow edema (BME), which is a sign of active inflammatory changes. The changes can be symmetrical, asymmetrical and present on one or on both sides etc., as shown below.
With progressing disease, the changes are usually characterized by a mixture of signs of disease activity in the form of BME and structural joint changes in the form of erosions in addition to fat deposition corresponding to areas previously affected by active inflammation.
In more advanced disease stages, structural changes often dominate, but active changes may occur until the development of complete joint ankylosis.
Below are examples of changes that can generally vary considerably depending on activity and the development of structural change.