Sagittal pelvic morphology and posterior instability in sacral fragility fractures: A CT-based morphometric analysis

Posterior pelvic instability in sacral fragility fractures varies considerably among elderly patients with comparable low-energy trauma and reduced bone quality. The role of intrinsic pelvic morphology in this variability remains insufficiently defined. This study investigated the association between sagittal pelvic parameters and posterior pelvic instability in sacral fragility fractures.

In this retrospective study, 154 patients with CT-confirmed sacral fragility fractures were included. Spinopelvic parameters, including pelvic incidence (PI), pelvic tilt, sacral slope, and sacral kyphosis, were measured on standardized midsagittal CT reconstructions. Fractures were classified according to the Fragility Fracture of the Pelvis (FFP) and OF-Pelvis systems. Associations between morphologic parameters and fracture severity were assessed using correlation analyses and multivariable logistic regression. CT-derived trabecular bone density was included to account for bone quality.

Pelvic incidence was positively associated with fracture severity according to the FFP classification. Patients with bilateral posterior pelvic involvement (OF-Pelvis type 4) exhibited significantly higher PI values than those with unilateral fractures. In multivariable analysis adjusted for age, sex, pelvic tilt, and CT-derived bone density, pelvic incidence remained independently associated with bilateral posterior instability. CT-derived bone density was not independently associated with posterior instability in the adjusted model.

Higher pelvic incidence is associated with increased posterior pelvic instability in sacral fragility fractures, independent of bone quality. As a fixed anatomical parameter measurable on routine CT, pelvic incidence may provide complementary morphologic information relevant to the assessment of posterior pelvic ring instability in elderly patients.