Abstract: Objective To determine the median effective dose (ED₅₀) and 95% effective dose (ED₉₅) of methylene blue as an adjuvant to ropivacaine in elderly patients undergoing fascia iliaca compartment block for postoperative 24-hour effective analgesia without affecting lower limb muscle strength after hip replacement surgery, and to observe adverse reactions within 24 hours postoperatively. Methods A total of 29 patients scheduled for elective total hip arthroplasty in Taizhou Jiangyan Hospital of TCM from July to November 2024 were enrolled. Prior to general anesthesia induction, an ultrasound-guided fascia iliaca compartment block was performed using 30 mL of 0.25% ropivacaine combined with varying doses of methylene blue. The first patient received 40 mg of methylene blue, and subsequent doses were adjusted using Dixon’s up-and-down sequential method. If a patient achieved quadriceps muscle strength ≥4, resting VAS score ≤1, and movement VAS score ≤3 within 24 hours, the next patient received a lower dose; otherwise, the dose was increased. The study terminated after seven crossover points. Probit regression analysis was used to calculate the ED₅₀, ED₉₅, and their 95% confidence intervals. Adverse reactions within 24 hours postoperatively were recorded. Results The ED₅₀ of methylene blue combined with ropivacaine for effective analgesia without motor block was 6.956 mg (95%CI:18.929-32.231 mg), and the ED₉₅ was 0.070 mg (95%CI: 33.878-84.708 mg). Two patients required additional analgesics, both occurring at doses below the ED₅₀. Only one case (20 mg methylene blue) experienced lower limb weakness preventing ambulation. No falls due to muscle weakness occurred. None of the patients developed nerve injury, puncture-site infection, drug toxicity, or other nerve block-related complications. Conclusions The ED₅₀ of methylene blue combined with ropivacaine for postoperative analgesia via fascia iliaca compartment block in elderly hip arthroplasty was 26.956 mg, with an ED₉₅ of 40.070 mg. Importantly, the addition of methylene blue did not potentiate the motor-blocking risk of ropivacaine, while maintaining a favorable safety profile with low incidence of postoperative adverse effects.    

Key words: methylene blue, iliac fascia block, hip replacement, aged, half-effective dose, 95% effective dose