Abstract / Summary
To investigate the association between combined deterioration in muscle mass and bone quality and the anatomical stability of fragility fractures of the pelvis (FFP) in elderly women, and to explore the Osteosarcopenic Index (OSI) as a quantitative composite measure for characterizing fracture phenotypes. This retrospective study included elderly female patients with FFP admitted between 2024 and 2026. Psoas muscle index (PMI) and L4 vertebral attenuation (L4 HU) were measured on admission CT images and integrated into an exploratory OSI. Fracture stability was assessed according to the Rommens classification. Mediation analysis was used as an exploratory statistical approach to examine whether concurrent variation in bone quality statistically explained part of the association between skeletal muscle status and fracture phenotype. Shannon entropy and the Simpson diversity index were used to characterize the distributional heterogeneity of fracture patterns across OSI-defined subgroups. Among 324 elderly women (mean age, 78.9 ± 8.35 years), patients with stable fractures (FFP I/II, n = 231, 71.3%) had lower PMI (4.16 ± 0.85 vs. 4.55 ± 0.86 cm²/m², p < 0.001) and lower L4 HU (66.7 ± 36.9 vs. 94.6 ± 51.0, p < 0.001) than those with unstable fractures (FFP III/IV, n = 93, 28.7%). PMI (OR = 1.375, 95% CI: 1.005–1.880, p = 0.046) and L4 HU (OR = 1.013, 95% CI: 1.007–1.020, p < 0.001) were independently associated with fracture instability. Exploratory mediation analysis suggested that concurrent differences in L4 HU statistically explained part of the association between PMI and fracture instability (indirect effect = 0.237, 95% CI: 0.099–0.338, p < 0.01). The OSI showed modest apparent discrimination between stable and unstable fracture phenotypes (AUC = 0.685, 95% CI: 0.618–0.752). At the exploratory cutoff of 101.0, the proportion of unstable fractures was higher in the high-OSI group than in the low-OSI group (58.2% vs. 21.0%, χ² = 35.49, p < 0.001). Shannon entropy (1.331 vs. 1.182) and the Simpson diversity index (0.726 vs. 0.650) were also higher in the high-OSI group. In elderly women with FFP, lower PMI and L4 HU were associated with anatomically stable fracture types, suggesting that radiographic stability does not necessarily indicate preserved musculoskeletal status. This counterintuitive relationship supports a potential “fragility paradox”: severe musculoskeletal depletion may lower the structural failure threshold of the pelvic ring and, together with restricted mobility and lower-energy exposure, favor localized, relatively stable fractures. Conversely, better-preserved musculoskeletal status and broader activity exposure may be associated with more heterogeneous fracture phenotypes. The exploratory OSI provides a quantitative framework for describing this relationship.