Fracture Type Modifies the Prognostic Impact of Acute Kidney Injury on In-Hospital Mortality after Hip Fracture Surgery in Elderly Patients: A Retrospective Cohort Study with Risk Stratification

Introduction: In-hospital mortality following hip fracture surgery remains a significant outcome in elderly patients, yet perioperative determinants - particularly those modifiable during hospitalization - are incompletely characterized. Whether the prognostic impact of acute kidney injury (AKI) on in-hospital mortality differs according to fracture type has not been previously examined. Materials and Methods: This retrospective cohort study included 342 patients aged ≥65 years who underwent surgical treatment for hip fracture at a tertiary referral center between January 2018 and January 2022. The primary outcome was in-hospital mortality. Independent predictors were identified using multivariable logistic regression. Model performance was assessed by ROC analysis, Hosmer-Lemeshow calibration testing, and bootstrap internal validation with 1000 iterations. A simple additive risk score was derived from the final model. Interaction analysis evaluated whether fracture type modified the association between AKI and mortality. Results: In-hospital mortality occurred in 29 of 342 patients (8.5%). Independent predictors were postoperative AKI (OR 5.46, 95% CI 2.20-13.51), lower preoperative serum albumin (OR per 1 g/dL increase 0.38, 95% CI 0.18-0.78), and lower admission oxygen saturation (OR per 1% increase 0.86, 95% CI 0.77-0.95). The model demonstrated acceptable discrimination (AUC = 0.766, 95% CI 0.655-0.877). A three-variable additive risk score stratified mortality from 3.4% (score 0) to 39.1% (score 2-3). Interaction analysis revealed that fracture type significantly modified the prognostic impact of AKI (interaction p = 0.016), with a substantially stronger effect in extracapsular fractures (OR 14.3, 95% CI 4.0-50.0) than intracapsular fractures (OR 1.32, 95% CI 0.30-5.88). Conclusions: Postoperative AKI, hypoalbuminemia, and reduced admission oxygen saturation independently predict in-hospital mortality after hip fracture surgery. The mortality risk conferred by AKI is substantially greater in extracapsular fractures, highlighting the need for fracture-type-specific perioperative vigilance and early renal protection strategies.