Sacroiliac Joint Anatomical Variants and Their Association with Spinopelvic Alignment: A Whole-Spine CT Study

Background/Objectives: Sacroiliac joint (SIJ) anatomical variants and spinopelvic alignment have each been studied extensively, yet in separate studies, and whether the two are related remains unknown. Exploiting a single-acquisition CT protocol that captures the entire spinal axis together with the pelvis, we examined the association between SIJ anatomical variants and spinopelvic parameters. Methods: In this retrospective single-center study, 239 trauma patients who had undergone craniocervical-to-pelvic CT were analyzed. Each sacroiliac joint was scored separately for six SIJ variants (Prassopoulos classification), and at the patient level, a variant was considered present when identified on at least one side. Lumbosacral transitional vertebrae (LSTV) were also assessed, and pelvic incidence (PI), sacral slope (SS), pelvic tilt, and the spinosacral angle (SSA) were measured. Associations were tested by univariable comparison and multivariable logistic regression, and interobserver reliability was quantified with the intraclass correlation coefficient and Cohen’s kappa. Results: SIJ variants were common, occurring in 62.8% of left and 65.3% of right joints and in 68.2% of patients; these prevalences derive from a single-center trauma cohort and may not reflect the general population. The presence of any variant, considered collectively, was unrelated to the CT-derived spinopelvic parameters. When the six variants were examined individually, however, one of them—the iliosacral complex—was associated with higher SS, PI, and SSA (all p ≤ 0.016), although only the association with SSA persisted after correction for the false discovery rate. On multivariable analysis, female sex (adjusted OR 6.35), LSTV (adjusted OR 3.02), and the SSA (adjusted OR 1.05 per degree) were independently associated with the iliosacral complex. Interobserver agreement was excellent for the quantitative parameters and perfect for the iliosacral complex (κ = 1.000). Conclusions: Among the six variants examined, only the iliosacral complex was associated with sacropelvic morphology, being independently related to the spinosacral angle, female sex, and a lumbosacral transitional vertebra. Given the modest effect sizes and the retrospective, single-center, supine-CT design, the iliosacral complex is best regarded as an associated anatomical feature rather than a determinant of spinopelvic morphology, and prospective studies, ideally with upright imaging, are needed to establish whether this association has any functional consequence.