Median effective concentration of liposomal bupivacaine for ultrasound-guided supra-inguinal fascia iliaca compartment block in older adults undergoing total hip arthroplasty: a dixon geometric up-and-down dose-finding study

This study aimed to determine the median effective concentration (EC50) of liposomal bupivacaine (LB) for ultrasound-guided supra-inguinal fascia iliaca compartment block (S-FICB) in elderly patients undergoing total hip arthroplasty (THA), and to evaluate the safety of LB within the tested concentration range. This prospective, double-blind, sequential dose-finding trial enrolled patients aged 60–90 years scheduled for unilateral THA under general anesthesia. Following anesthesia induction, ultrasound-guided S-FICB was administered with a fixed injection volume of 40 mL. The initial LB concentration was set at 0.040%, with subsequent concentrations adjusted using the Dixon geometric up-and-down method at a 2% step size (×0.98 for a successful block or ×1.02 for a failed block). The sequential dosing phase was terminated upon observation of seven success–failure crossover pairs. Block success was defined as a resting Numerical Rating Scale (NRS) score ≤ 2 and a movement NRS score ≤ 3 within postoperative 24 h, with no effective patient-controlled intravenous analgesia administrations and no rescue analgesia required. The EC50 and its 95% confidence interval (CI) were estimated using probit regression. EC90 and EC95 were estimated using isotonic regression with the pooled-adjacent-violators algorithm (PAVA), with 95% CIs derived from bootstrap resampling. Adverse events were recorded for safety assessment. Thirty-one patients completed the sequential trial; 24 observations (12 successes and 12 failures) were included in EC50 modeling. The EC50 of LB for S-FICB in older patients undergoing THA was 0.0347% (95% CI, 0.0344–0.0351). The total dose corresponding to 40 mL of 0.0347% LB was 13.88 mg and the body weight-normalized dose was approximately 0.237 mg kg− 1. No postoperative motor block was observed. In elderly patients undergoing THA who received ultrasound-guided S-FICB with a fixed 40 ml injection volume, the EC50 of LB was determined to be 0.0347%. This result provided a valuable empirical reference for the rational selection of LB concentrations in clinical fascia iliaca plane analgesia for geriatric patients. This study was retrospectively registered in Chinese Clinical Trial Registry (ChiCTR2500112398) on 13 November 2025.