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. 2023 Dec 1;62(12):3893-3898.
doi: 10.1093/rheumatology/kead142.

New bone formation at the sacroiliac joint in axial spondyloarthritis: characterization of backfill in MRI and CT

Torsten Diekhoff  1 , Denis Poddubnyy  2 , Fabian Proft  2 , Katharina Ziegeler  1 , Dominik Deppe  1 , Christoph Niedermeier  1 , Kay Geert A Hermann  1
Affiliations

New bone formation at the sacroiliac joint in axial spondyloarthritis: characterization of backfill in MRI and CT

Torsten Diekhoff et al. Rheumatology (Oxford). .

Abstract

Objective: MRI findings of the SI joint space in axial SpA (axSpA) include inflammation and fat metaplasia inside an erosion; the latter is also termed 'backfill'. We compared such lesions with CT to better characterize whether they represent new bone formation.

Methods: We identified patients with axSpA who underwent both CT and MRI of the SI joints in two prospective studies. MRI datasets were jointly screened by three readers for joint space-related findings and grouped into three categories: type A-high short tau inversion recovery (STIR) and low T1 signal; type B-high signal in both sequences; type C-low STIR and high T1 signal. Image fusion was used to identify MRI lesions in CT before we measured Hounsfield units (HU) in each lesion and surrounding cartilage and bone.

Results: Ninety-seven patients with axSpA were identified and we included 48 type A, 88 type B, and 84 type C lesions (maximum 1 lesion per type and joint). The HU values were 73.6 (s.d. 15.0) for cartilage, 188.0 (s.d. 69.9) for spongious bone, 1086.0 (s.d. 100.3) for cortical bone, 341.2 (s.d. 96.7) for type A, 359.3 (s.d. 153.5) for type B and 446.8 (s.d. 123.0) for type C lesions. Lesion HU values were significantly higher than those for cartilage and spongious bone, but lower than those for cortical bone (P < 0.001). Type A and B lesions showed similar HU values (P = 0.93), whereas type C lesions were denser (P < 0.001).

Conclusion: All joint space lesions show increased density and might contain calcified matrix, suggesting new bone formation, with a gradual increase in the proportion of calcified matrix towards type C lesions (backfill).

Keywords: CT; MRI; axial spondyloarthritis; new bone formation.

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Figures

Figure 1.
Figure 1.
ROI with image fusion. To align MRI and CT scans, colour-coded fusion images were generated to identify the lesions visible in MRI on CT. The ROI was placed manually, avoiding partial volume to the normal cartilage, the bone and its margin and other lesions with different imaging characteristics
Figure 2.
Figure 2.
Flow chart of study inclusion. Of 110 patients (SIMACT) and 68 patients (VNCa/SWI), 99 were diagnosed with axSpA and included in the study. In 198 joints, several lesions (one per type per joint) were identified. In some joints, normal cartilage could not be assessed (e.g. due to extensive ankylosis)
Figure 3.
Figure 3.
Different types of joint space lesions on MRI and CT. All lesions (arrowheads) show faint bone matrix calcifications, irrespective of their imaging characteristics on MRI
Figure 4.
Figure 4.
CT attenuation measurement of three types of joint space lesions and three normal tissue structures. All lesions show higher HU values compared with normal cartilage and spongious bone but lower HU values than cortical bone. Type C lesions have higher HU values than the other lesion types. Except for the difference in HU between type A and B lesions, all differences are significant
See this image and copyright information in PMC

References

    1. Sieper J, Poddubnyy D.. Axial spondyloarthritis. Lancet 2017;390:73–84. - PubMed
    1. Lambert RG, Bakker PA, van der Heijde D. et al. Defining active sacroiliitis on MRI for classification of axial spondyloarthritis: update by the ASAS MRI working group. Ann Rheum Dis 2016;75:1958–63. - PubMed
    1. Maksymowych WP, Wichuk S, Chiowchanwisawakit P, Lambert RG, Pedersen SJ.. Fat metaplasia and backfill are key intermediaries in the development of sacroiliac joint ankylosis in patients with ankylosing spondylitis. Arthritis Rheumatol 2014;66:2958–67. - PubMed
    1. Poddubnyy D, Sieper J.. Mechanism of new bone formation in axial spondyloarthritis. Curr Rheumatol Rep 2017;19:55. - PubMed
    1. Hoshi K, Ejiri S, Ozawa H.. Ultrastructural, cytochemical, and biophysical aspects of mechanisms of bone matrix calcification. Kaibogaku Zasshi 2000;75:457–65. - PubMed

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