Changes in systemic immune-inflammation index predict periprosthetic joint infection after hemiarthroplasty in...

Background: Periprosthetic joint infection (PJI) remains a devastating complication after hemiarthroplasty in elderly patients with femoral neck fractures. Early identification of high-risk patients is important. The systemic immune-inflammation index (SII), calculated as platelet $\times$ neutrophil $/$ lymphocyte count, is a marker of systemic inflammation, although its role in this population remains unclear. Methods: This retrospective single-centre study included patients aged $\ge 65$  years who underwent hemiarthroplasty between 2015 and 2025. Preoperative and postoperative day-5 SII values were calculated from routine blood tests. $\Delta$ SII was defined as the difference between postoperative and preoperative values. The primary outcome was PJI within 1 year after surgery. Logistic regression and receiver operating characteristic (ROC) analyses were performed. Results: A total of 976 patients were included, of whom 47 (4.8 %) developed PJI. Postoperative SII values were significantly higher in patients with PJI, whereas preoperative SII was not associated with infection risk. In multivariable analysis, postoperative SII remained independently associated with PJI (OR 1.195, 95 % CI 1.144-1.248, $p<0.001$ ). $\Delta$ SII was also independently associated with PJI (OR 1.124, 95 % CI 1.085-1.164, $p<0.001$ ). ROC analysis demonstrated good discriminatory performance for postoperative SII (AUC 0.882), with an optimal cut-off value of 2118. In contrast, $\Delta$ SII showed moderate discriminatory performance (AUC 0.764), with an optimal cut-off value of 656. Conclusions: Postoperative SII and $\Delta$ SII were independently associated with PJI after hemiarthroplasty. SII may aid early postoperative risk stratification and clinical decision-making when interpreted alongside clinical findings.