Anaesthesia and surgical procedure
femoral nerve block
All surgeries were performed according to the standard hospital protocol. Before the induction of general anaesthesia, patients in the 0.1% ropivacaine group received 20 ml of 1 mg/ml ropivacaine, while patients in the 0.2% ropivacaine group received 20 ml of 2 mg/ml ropivacaine for ultrasound-guided femoral nerve block. Laryngeal mask airways were inserted after patients were induced with 2 mg/kg propofol, 0.2 mg/kg etomidate, 0.15 mg/kg cisatracurium, and 0.3 μg/kg sufentanil. Inhalation of 1–2% sevoflurane and continuous infusion of 2–4 mg kg