Frequency of fat infiltration as recorded by ≥ 2 readers. Data are no. subjects (percentage).
| Cohort | Cohort A (BP; n = 89) | Cohort B (AAU + BP; n = 88) | |||||
|---|---|---|---|---|---|---|---|
| Group | nr-axSpA | AS | NSBP | HC | nr-axSpA | AS | NSBP |
| No. subjects | 20 | 10 | 39 | 20 | 31 | 24 | 33 |
| FI per se | |||||||
| FI in ≥ 1 SIJ quadrant | 11 (55.0) | 9 (90.0) | 15 (38.5) | 4 (20.0) | 16 (51.6) | 24 (100.0) | 9 (27.3) |
| FI in ≥ 2 SIJ quadrants1 | 9 (45.0) | 9 (90.0) | 14 (35.9) | 2 (10.0) | 15 (48.4) | 24 (100.0) | 8 (24.2) |
| FI with additional MRI lesions | |||||||
| Fat w/o BME or erosion | 0 (0) | 0 (0) | 9 (23.1) | 1 (5.0) | 2 (6.5) | 2 (8.3) | 3 (9.1) |
| FI w/BME | 8 (40.0) | 8 (80.0) | 5 (12.8) | 2 (10.0) | 9 (29.0) | 21 (87.5) | 5 (15.2) |
| FI w/erosion | 6 (30.0) | 9 (90.0) | 0 (0) | 0 (0) | 10 (32.3) | 21 (87.5) | 3 (9.1) |
| Morphological features of FI | |||||||
| FI w/distinct border | 5 (25.0) | 6 (60.0) | 1 (2.6) | 0 (0) | 7 (22.6) | 16 (66.7) | 3 (9.1) |
| FI w/distinct border + BME | 4 (20.0) | 5 (50.0) | 1 (2.6) | 0 (0) | 4 (12.9) | 10 (41.7) | 3 (9.1) |
| FI w/distinct border + erosion | 3 (15.0) | 6 (60.0) | 0 (0) | 0 (0) | 7 (22.6) | 13 (54.2) | 3 (9.1) |
| Homogeneous FI | 4 (20.0) | 6 (60.0) | 0 (0) | 0 (0) | 8 (25.8) | 18 (75.0) | 3 (9.1) |
| Homogeneous FI + BME | 3 (15.0) | 5 (50.0) | 0 (0) | 0 (0) | 5 (16.1) | 12 (50.0) | 3 (9.1) |
| Homogeneous FI + erosion | 2 (10.0) | 6 (60.0) | 0 (0) | 0 (0) | 7 (22.6) | 14 (58.3) | 3 (9.1) |
| Subchondral FI | 7 (35.0) | 8 (80.0) | 7 (17.9) | 1 (5.0) | 12 (38.7) | 24 (100.0) | 5 (15.2) |
| Subchondral FI + BME | 5 (25.0) | 7 (70.0) | 2 (5.1) | 0 (0) | 9 (29.0) | 21 (87.5) | 5 (15.2) |
| Subchondral FI + erosion | 5 (25.0) | 8 (80.0) | 0 (0) | 0 (0) | 10 (32.3) | 21 (87.5) | 4 (12.1) |
| FI with ≥ 1 feature2 | 7 (35.0) | 9 (90.0) | 8 (20.5) | 1 (5.0) | 12 (38.7) | 24 (100.0) | 6 (18.2) |
| FI with ≥ 1 feature + BME | 5 (25.0) | 8 (80.0) | 3 (7.7) | 1 (5.0) | 9 (29.0) | 21 (87.5) | 5 (15.2) |
| FI with ≥ 1 feature + erosion | 5 (25.0) | 9 (90.0) | 0 (0) | 0 (0) | 10 (32.3) | 21 (87.5) | 4 (12.1) |
| FI with ≥ 2 features2 | 5 (25.0) | 6 (60.0) | 1 (2.6) | 0 (0) | 11 (35.5) | 18 (75.0) | 3 (9.1) |
| FI with ≥ 2 features + BME | 4 (20.0) | 5 (50.0) | 1 (2.6) | 0 (0) | 8 (25.8) | 13 (54.2) | 3 (9.1) |
| FI with ≥ 2 features + erosion | 3 (15.0) | 6 (60.0) | 0 (0) | 0 (0) | 9 (29.0) | 15 (62.5) | 3 (9.1) |
| Anatomical distribution of FI | |||||||
| ≥ 1 FI in all 4 sacral quadrants | 5 (25.0) | 7 (70.0) | 6 (15.4) | 2 (10.0) | 6 (19.4) | 19 (79.2) | 2 (6.1) |
| ≥ 1 FI in all 4 iliac quadrants | 0 (0) | 5 (50.0) | 3 (7.7) | 0 (0) | 4 (12.9) | 14 (58.3) | 1 (3.0) |
| ≥ 1 FI in all 4 upper quadrants | 0 (0) | 6 (60.0) | 4 (10.3) | 1 (5.0) | 6 (19.4) | 14 (58.3) | 0 (0) |
| ≥ 1 FI in all 4 lower quadrants | 1 (5.0) | 6 (60.0) | 3 (7.7) | 0 (0) | 9 (29.0) | 17 (70.8) | 2 (6.1) |
↵1 FI in ≥ 2 SIJ quadrants on the same slice or in ≥ 1 SIJ quadrant on ≥ 2 adjacent slices.
↵2 Morphological features of FI: FI with distinct border, homogeneous FI, subchondral FI. AAU: acute anterior uveitis; AS: ankylosing spondylitis; BME: bone marrow edema; BP: back pain; FI: fat infiltration; HC: healthy control; MRI: magnetic resonance imaging; nr-axSpA: nonradiographic axial spondyloarthritis; NSBP: nonspecific back pain; SIJ: sacroiliac joint.