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.

BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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. TRIAL REGISTRATION: This study was retrospectively registered in Chinese Clinical Trial Registry (ChiCTR2500112398) on 13 November 2025.