Bilaterally and unilaterally painful joints show no differences in size nor morphology-related mineralisation density
As a consequence of the low onset of joint morphology T3, only comparisons between T1 and T2 were made. In the bilaterally painful joints, no significant size difference was present between sexes (p = 0.08 sacra; p = 0.3 ilia) (Fig. 3B). Furthermore, no significant differences were found in mean sacrum and ilium mineralisation density when comparing sizes (ilia, small 682 ± 130 HU (95% CI:), large 768 ± 84 HU (95% CI:); sacra, small 575 ± 84 HU (95% CI:), large 648 ± 163 HU (95% CI:)), sexes (ilia, males 682 ± 130 HU (95% CI:), females 768 ± 84 HU (95% CI:); sacra, males 575 ± 84 HU (95% CI:), females 648 ± 163 HU (95% CI:)), or morphology T1 and T2 (ilia, T1 748 ± 103 HU (95% CI:), T2 727 ± 125 HU (95% CI:); sacra: T1 688 ± 137 HU (95% CI:), T2 555 ± 100 HU (95% CI:)).
In unilaterally painful joints, a significant size difference was observed between sexes (p < 0.03 sacra and ilia) (Fig. 3C). No significant difference was found in mean sacrum and ilium mineralisation density when comparing sizes (ilia, small 751 ± 173 HU (95% CI:), large 747 ± 140 HU (95% CI:); sacra, small 583 ± 134 HU (95% CI:), large 568 ± 98 HU (95% CI:)), sexes (ilia, males 722 ± 128 HU (95% CI:), females 779 ± 178 HU (95% CI:); sacra, males 554 ± 105 HU (95% CI:), females 598 ± 124 HU (95% CI:)), or morphology T1 and T2 (ilia, T1 730 ± 173 HU (95% CI:), T2 770 ± 133 HU (95% CI:); sacra: T1 531 ± 85 HU (95% CI:), T2 595 ± 122 HU (95% CI:)).
Unilateral SIJD yielded no size, sex nor morphology-related mineralisation difference to non-painful joints nor bilateral SIJD
Comparison of cases with unilateral pain with their contralateral non-painful side yielded no significant size differences (Fig. 4A,B,C). Mineralisation was indifferent when comparing sexes, sizes and morphology T1 and T2. No significant difference was found between bilateral and unilateral painful joints when comparing mean HU mineralisation between sex, size and morphology T1 and T2 (Fig. 4A,B,C).
Sacrum mineralization in females with SIJD pain is different when compared to the SIJ of healthy controls
Significant differences in mineralization was found between healthy controls and the painful joints from the SIJD cohort. These were found only in females, small joints and T2 joints only on the sacral side (Fig. 5). Furthermore, correlations between mean surface mineralisation and surface area reveal weak positive correlations on the iliac side in both males and females in the control cohort (Fig. 6A) and bilaterally painful joints (Fig. 6B).
Semi-quantitative assessment of all sacroiliac joint surfaces revealed four main patterns for the purpose of classification
Two of these were marginal patterns: M1 had mineralisation located around the anterior border which could include the superior corner and/or apex and pattern M2 had mineralisation scattered around the borders with no specific maxima, the centre region demineralised. The other two were non-marginal patterns: pattern N1 had mineralisation spread over the whole superior region, pattern N2 had mineralisation spread over the superior and anterior regions (Fig. 7).
Pattern M1 was subdivided into four sub-patterns (Fig. 8) based on the visual assessment of the highest mineralisation point (maxima) at the anterior border. Pattern A had mineralisation across the anterior border. Pattern B had a maximum located in the superior corner only. Pattern C had maxima in the superior corner and apex. Pattern D had a maximum at the apex.
Painful joints exhibit higher pattern conformity in corresponding sacra and ilia than those without pain
A joint is ‘conforming’ when the sacral and iliac articulating surfaces reflect the same pattern. For the painful joints, there was 46% conformity between patterns M1 to N2, and 6% with the M1 sub-patterns. Conformity was higher than compared to the controls (M1 to N2: 26%; M1 sub-pattern added: 5%). The majority of dysfunctional joints was of T2 morphology (59%), and was equally distributed between small (49%) and large joints (51%). The majority of non-painful joints was also of T2 morphology (53%) and had smaller surface areas (63%) (Table 1). From a different perspective, within each morphology type, T1 and T2 had a majority of painful joints (both > 60%), with T3 having close to equal painful and non-painful joints. Larger joints had the highest frequency of painful joints (72%).
Table 1
Distribution of painful and non-painful joints by morphology type and auricular surface size. Type 1 morphology: posterior angle (> 160°), type 2 morphology: posterior angle (130–160°), type 3 morphology: posterior angle (< 130°).
| Morphology | Small joints n = 58 | Large joints n = 50 |
| Type 1 n = 43 | Type 2 n = 61 | Type 3 n = 4 | Females | Males | Females | Males |
Painful joints n = 70 | 27 (38%) | 41 (59%) | 2 (3%) | 27 (39%) | 7 (10%) | 7 (10%) | 29 (41%) |
Non-painful joints n = 38 | 16 (42%) | 20 (53%) | 2 (5%) | 15 (39%) | 9 (24%) | 3 (8%) | 11 (29%) |
Regarding the prominence of sub-patterns, M1C was more prominent in painful joints (39%), whilst pattern M1A was more frequent in the controls (31%) (Table 2). When comparing ilia and sacra in painful joints, M1B sub-pattern was more common in the ilia (> 55%) whilst M1C was most common in the sacra (> 60%). Cramér’s Φ revealed strong associations between patterns M1A and M1C in all painful joints and associated sub cohorts as well as in the controls (Table 2). Unilaterally and bilaterally painful joints were similarly associated with all three sub- patterns (Table 2).
Table 2
Results of Cramér’s Φ association between sub-patterns, M1A, M1B and M1C in each category. Strong association Φ ≥ 0.5, moderate 0.5 > Φ ≥ 0.4, weak 0.4 > Φ ≥ 0.1.
Cohort | n | Sub-pattern distribution | Cramér’s Φ |
M1A | M1B | M1C | |
All painful joints | 44 | 34% | 27% | 39% | Φ = 0.73 p < 0.01 | | |
Controls | 171 | 31% | 14% | 25% | Φ = 0.71 p < 0.01 | |
Non-painful joints (but painful contralateral side) | 22 | 59% | 9% | 32% | | Φ = 0.73 p < 0.01 |
Unilaterally dysfunctional joints | 27 | 37% | 22% | 41% | Φ = 0.71 p < 0.01 | |
Bilaterally dysfunctional joints | 17 | 30% | 35% | 35% | | |