Pelvic Inlet Morphology and Fracture Risk in Sacral Fragility Fractures: A Case–Control CT Study

Purpose Fragility fractures of the pelvis (FFP) are increasingly prevalent in aging populations. Beyond bone quality, pelvic inlet morphology may modulate load transmission and influence both the incidence and morphologic severity of sacral fragility fractures (SFF). This study evaluates the association between axial pelvic inlet geometry and gynecological pelvic types with (1) fracture incidence using a matched control group, and (2) fracture severity within the SFF cohort. Methods We retrospectively analyzed 226 patients aged ≥ 65 years. The fracture cohort comprised 154 individuals with computed tomography (CT)–confirmed sacral fragility fractures (SFF). The control group included 72 age-matched patients who underwent pelvic CT to exclude a fracture due to pelvic or lumbar pain, but in whom no fracture was detected. Axial CT morphometric measurements included the transverse pelvic diameter (TPD), anteroposterior pelvic diameter (APD), and the pelvic radius index (PRI), calculated as TPD/APD. Pelvic inlet morphology was categorized as android, gynecoid, anthropoid, or platypelloid, according to established obstetric dimensional thresholds. Fracture severity was graded using the Fragility Fracture of the Pelvis (FFP) classification (types I–IV) and by identifying H-type (“Honda sign”) fractures. Odds ratios (OR) for the presence of SFF were calculated for each pelvic morphology category, using the gynecoid type as the reference. Associations between pelvic type and fracture severity were analyzed using the χ² test. Results Pelvic type distributions differed significantly between fracture and control groups (χ² = 11.11, p = 0.011). Compared with gynecoid, platypelloid inlet shape showed significantly lower odds of fracture (OR 0.46; 95% CI 0.23–0.91). Anthropoid showed a higher proportion among fracture patients, though not statistically significant (OR 1.30; 95% CI 0.71–2.40). Within the fracture cohort, pelvic type showed a strong trend toward association with more complex patterns: H‑type fractures were most frequent in anthropoid inlets (χ² = 6.13, p = 0.069). Conclusion Pelvic inlet morphology is significantly associated with SFF incidence, with platypelloid shapes demonstrating a protective association. A strong trend suggests anthropoid inlets may predispose to complex H‑type configurations. Incorporating inlet typing into imaging‑based risk assessment may improve identification of elderly patients at risk of SFF. Level of Evidence: Level III.