What Is Backfill? New Bone Formation in Axial Spondyloarthritis

Purpose or Learning Objective: Several magnetic resonance imaging (MRI) findings of the sacroiliac joint space in axial spondyloarthritis (axSpA) were previously described, such as inflammation or fatty metaplasia inside an erosion (i.e., “backfill”). However, it is incompletely understood if one or all of these changes represent calcified bone matrix. This study aimed to one-by-one link the changes just described to computed tomography (CT) measurements and to understand which of those findings represents new bone formation.

Methods or Background: From 178 patients in two prospective studies that included CT and MRI of the sacroiliac joints, all the patients with the final diagnosis of axSpA were selected. MRI was screened by two senior musculoskeletal radiologists in consensus for MRI findings related to joint space and grouped into three categories: type A, hyperintense in short tau inversion recovery (STIR) and hypointense in T1 (inflammation inside erosion); type B, hyperintense in both sequences; and type C, hypointense in STIR and hyperintense in T1 (backfill). By using image fusion techniques and one-by-one comparison, the Hounsfield units of those lesions, as well as normal cartilage and spongious and cortical bone, were measured on CT.

Results or Findings: Ninety-nine patients with axSpA were identified, and 48 type A, 88 type B, and 84 type C lesions were assessed. Fig. 1 shows the CT measurements. The HU values of cartilage were 73.6 ± 15.0, spongious bone 188.0 ± 69.9, cortical bone 1,086.0 ± 100.3, type A 341.2 ± 96.7, type B 359.3 ± 153.5, and type C 446.8 ± 123.0, respectively. The lesion values were significantly higher than cartilage and spongious bone but lower than cortical bone (p < 0.001). Types A and B showed similar Hounsfield units (p = 0.93), whereas type C lesions were denser (p < 0.001).

Conclusion: All joint space lesions (types A–C) show calcified matrix and thus resemble new bone formation with gradually more calcified matrix in type C lesions, that is, typical backfill. Therefore, the terminology of those lesions should be critically reassessed.

Publication History

Article published online:
02 June 2022