Clinical procedures
Propofol 2-3mg/kg and 0.1-0.15 μ g / kg sufentanil were used for general anesthesia induction, and a laryngeal mask airway was placed at proper position. Volatile anesthetics sevoflurane was used to maintain anesthesia, with end expiratory sevoflurane concentration above 0.7MAC(minimum alveolar concentration) and ETCO2 between 35 and 45mmHg. During the operation, the anesthesiologist would use 0.1ug/kg sufentanil intravenously if any sign showing the insufficient anesthesia. All patients received PONV prophylaxis droperidol IV before emergence. Patients reached discharge criteria when (1) pain was controlled with a score less than 5, (2) airway remained patent with oxygen saturation greater than 95% on room air, (3) heart rate and systolic and diastolic blood pressures were within 20% of preanesthetic levels, and (4) minimal levels of nausea occurs when taking clear liquids. 1 g paracetamol was given every 6 h in postoperative analgesia and droperidol was used to prevent systematic postoperative nausea and vomiting (PONV). An independent observer who knew nothing about the study concentration of ropivacaine recorded the VRS (Visual Analogue Score) in the post-anaesthesia care unit (PACU), then once upon arrival in the ward, and finally at the end of the study 24 hours after the surgery.
Sensory and motor blockade in the operated foot was evaluated every 5min after local anesthetic injection by an observer blinded to the concentration of local anesthetic injected. Sensory block was evaluated as an absence of sensation to pinprick in the common femoral and sciatic nerve nerve distributions, and classified as follows: 1= normal sensation within the nerve distribution (no block); 2 = blunted sensation (analgesia); 3 = absence of sensation (anesthesia). Motor block was assessed for voluntary motor response by asking the patient to plantar-flex or dorsi-flex the foot, and was classified as follows: 1 = normal movement; 2 = decreased movement; 3 = absence of movement (complete motor block). The time between local anesthetic injection and successful complete block was considered the onset, and thereby registered