The 24-Hour Window: How Time to Admission Reshapes the Impact of Surgical Delay on Mortality in Elderly Hip Fracture

Objective: This study investigated the modifying effect of time to admission (TTA) on the relationship between admission to operation time (ATO) and outcomes in elderly patients with hip fractures. Methods: This retrospective cohort study included elderly patients with hip fractures treated at the trauma center. We collected data on patient characteristics, comorbidities, TTA, ATO, and all-cause mortality. Statistical analyses were conducted using R and EmpowerStats software. Results: Two thousand three hundred and sixty patients were included, with a median follow-up duration of 39.00 (29.05–50.79) months. Patients were stratified into four groups by ATO: ATO ≤ 2 d; 2 < ATO ≤ 4 d; 4 < ATO ≤ 6 d; and ATO > 6 d. The median TTA for each ATO group was as follows: 24 (4–168), 8 (4–48), 10.5 (4–48), and 10 (4–48) h, respectively. During follow-up, 733 patients (31.07%) died. Interaction analyses further stratified patients by TTA: ≤24 h vs. >24 h. The results demonstrated that TTA exerted a significant modifying effect on the ATO–mortality association. In the TTA ≤ 24 h group, each 1 d ATO delay was associated with a 5% increased risk of mortality (HR = 1.05; 95% CI: 1.01–1.09, p = 0.0095). In contrast, ATO delay was not associated with higher mortality in the TTA > 24 h group (HR = 0.98; 95% CI: 0.93–1.02, p = 0.3050). Notably, the TTA–ATO–mortality interaction remained statistically significant across several high-risk subgroups: females, those with femoral neck fractures, age-adjusted Charlson comorbidity index ≤ 4, hypertension, coronary artery disease, arrhythmia, and absence of chronic obstructive pulmonary disease. Conclusions: TTA is an important effect modifier of the association between ATO and mortality among elderly patients with hip fractures.