Low BMI and postoperative outcomes in elderly hip fracture patients: a Japanese nationwide database study

Introduction Hip fractures are common in elderly individuals and contribute to significant morbidity and mortality. Low body mass index (BMI) is associated with osteoporosis and frailty, yet its impact on postoperative outcomes after hip fracture surgery remains unclear. Materials and methods We conducted a retrospective cohort study using Japan’s Diagnosis Procedure Combination (DPC) database. Patients aged ≥ 65 years undergoing surgery for hip fractures from April 2016 to March 2022 were included. Low BMI was defined as < 17.0 kg/m 2 , and < 18.5 kg/m 2 was used for sensitivity analyses. Patients < 65 years or treated non-operatively were excluded. Primary outcomes included venous thromboembolism (VTE), pneumonia, urinary tract infection, cognitive dysfunction, in-hospital mortality, transfusion volume, and length of stay. Propensity score matching (1:1) was performed, followed by logistic regression. Results Of 474,293 patients identified, 63,761 matched pairs were analyzed. Compared to the non-low BMI group, the low BMI group had higher rates of urinary tract infection (3.6% vs. 3.0%; OR 1.203, 95% CI 1.131–1.280), pneumonia (5.3% vs. 3.0%; OR 1.850, 95% CI 1.746–1.961), and in-hospital mortality (3.4% vs. 1.6%; OR 2.233, 95% CI 2.068–2.411). Perioperative transfusion volume was higher in the low BMI group, while VTE was less frequent. Sensitivity analyses using < 18.5 kg/m 2 confirmed these findings. Conclusion Low BMI is associated with increased complications and mortality following hip fracture surgery in the elderly. These findings emphasize the prognostic relevance of BMI in perioperative risk assessment.