Backfill refers to intra-articular high signal intensity on T1 weighted images seen in axial spondylarthritis within the sacroiliac joint filling up excavated bone erosions 1. It has been characterized as a complete loss of the cortical bone within the sacroiliac joint at the anticipated location either on the sacral or iliac side replaced by high-signal intensity tissue on T1-weighted images clearly definable from the adjacent normal bone marrow signal by the irregular signal from the sclerotic border of the erosion 1,2. The histopathological correlate of this MRI feature is not yet clear 1. Backfill has been associated with the resolution of joint inflammation and the decline of bone erosions and is thought to represent new bone formation or synovial metaplasia 3,4. It has been shown to be of a very high diagnostic value for axial spondylarthritis with a high specificity 5.
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