Does a longer operative time increase perioperative trauma in robot-assisted total hip arthroplasty? A post hoc analysis of a multicenter randomized controlled trial.

Robot-assisted total hip arthroplasty (THA) improves component-placement accuracy but lengthens the operative time, an established risk factor for perioperative complications. Whether this additional operative time increases perioperative trauma has not been formally tested, particularly where the robot-assisted THA procedure is substantially longer. This was a post hoc analysis of a prospective, multicenter randomized controlled trial comparing seven-axis robot-assisted and conventional THA at 3 centers. The modified intention-to-treat population comprised 99 operated patients (47 in the robot-assisted THA group and 52 in the conventional THA group). We compared operative time, blood loss, intraoperative transfusion, postoperative day-7 C-reactive protein and composite cellular-immune ratios, and adverse events, with all comparisons adjusted for study center. Exploratory mediation estimated the model-based indirect association between treatment assignment and blood loss through operative time. Robot-assisted THA took substantially longer than the conventional procedure (median 150 vs. 106 min; center-adjusted difference 34.8 min; P < 0.001), yet no statistically significant between-group differences were detected in blood loss, intraoperative transfusion, postoperative day-7 inflammatory markers, or adverse events. Exploratory mediation identified an indirect association through operative time (+ 97.8 mL, bootstrap 95% CI 46.2 to 161.5), while the total association was not significant. In this post hoc randomized cohort, exploratory mediation identified an indirect association through operative time without establishing causal direction. The observed pattern supports further testing of a time-trauma decoupling hypothesis: additional time associated with robot-assisted THA may not be accompanied by proportionate perioperative trauma. Clinically important differences remain possible, and the hypothesis requires validation. Trial registration: ChiCTR2200060115, 19 May 2022, retrospectively registered.