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IntroductionKnowledge of the stress distribution on structures around the sacroiliac joint (SIJ) is required to treat or prevent SIJ disorders. The purpose of this study was to reveal the association between sacral morphology and SIJ conformity.Materials and methodsThis cross‐sectional study included 11 adult patients with unilateral SIJ pain who underwent computed tomography (CT) imaging of the pelvis. Bony coordinate systems for the sacrum and innominates were embedded using anatomical landmarks. Local coordinate systems for the auricular surfaces of the sacrum and innominate were also defined. Conformity of the SIJ was quantified by the offset of the coordinate systems between the auricular surfaces of the sacrum and innominate. Repeated measure ANOVA and multiple regression analysis were used for statistical analyses.ResultsThere were large variations across subjects in sacral morphology and SIJ conformity. There were no differences in any of the six degrees‐of‐freedom positions or orientations of the bilateral sacral and innominate auricular surfaces between the symptomatic and asymptomatic sides. The SIJ downward rotation on the asymptomatic and symptomatic sides were 0.0 [−1.0, 1.1]° and 2.1 [1.2, 3.0]°, respectively. Smaller downward rotation of the sacral auricular surface based on the sacral bony coordinate system had significant association with the greater SIJ downward rotation (standard partial regression coefficient: −.44, p = .043).ConclusionsThe results indicate that the morphology of the sacrum is associated with poor SIJ conformity and that separation of the superior portion of the SIJ can be a risk factor for SIJ pain.

3-dimensional pelvic models based on magnetic resonance images (MRI) can be used to investigate accuracy and specifics of changing pelvic alignment during pregnancy and after childbirth. Few studies have investigated changes of pelvic alignment during pregnancy and after childbirth using three-dimensional pelvic models. This case report documents the changes of pelvic alignment during late pregnancy and after childbirth using MRI-based three-dimensional (3D) pelvic models. This was a longitudinal observation case report. A woman was imaged with MRI at 28 and 39 gestational weeks, as well as 4 and 72 weeks after childbirth. Greater internal, anterior, and downward rotation of both innominates at week 39 was observed from that at gestation week 28. Decreased internal, anterior, and downward rotation of both innominates at week 4 after child birth was observed compared with that at gestation week 39. We report the first case in Japan of changes of pelvic alignment measured using an MRI-based 3D pelvic alignment model during pregnancy and after child birth. This case suggests that the small changes of pubic area and greater separation of anterior portions of sacroiliac joints. Internal, anterior, and downward rotation of both innominates was observed in a Japanese primipara woman having no pelvic pain.

Purpose This study aimed to compare the repositioning error (RE) of patients with unilateral sacroiliac joint pain (SIJP) to that of patients with low back pain (LBP) and a healthy control (HC) group. Differences between the symptomatic and asymptomatic sides were also investigated. Methods Sixty-six patients with SIJP, LBP, and HC were included in this study. An active straight leg-raising repositioning test (ASLR-Rt) was performed. ASLR was performed three times each on the left and right sides, targeting a set base angle. RE was calculated as the difference between the base angle and the participant’s attempt to adjust the target angle. RE was expressed as constant error (CE) and absolute error (AE). Results The CE of the SIJP group (median [interquartile range]) (6.9 [4.6‒10.4]) was significantly higher than that in the LBP group (3.2 [1.3‒7.1]) and the HC group (2.7 [0.3‒4.6]) (P = 0.009, d = 0.91, P < 0.001, d = 1.30). The AE of the SIJP group (7.3[5.0‒10.4]) was also significantly higher than that in the LBP (3.7[2.8‒7.1]) and HC groups (3.0[1.9‒4.2]) (P = 0.003, d = 1.04; P = 0.001, d = 1.57). Comparing the symptomatic and asymptomatic sides in the SIJP group, the symptomatic side (8.0[6.0‒10.6]) was significantly higher than the asymptomatic side (5.7[3.6‒8.1]) in terms of CE (P = 0.05, d = 0.51). Conclusion Patients with SIJP increased RE during ASLR, which may be related to impaired proprioception and decreased motor control.