Before induction of GA, all patients will receive sciatic, obturator, lateral femoral cutaneous and femoral nerve blocks.~The femoral nerve block will be performed using 15ml of ropivacaine which concentration relied on the response of the previous patient.~Based on biased-coin design up-down sequential method: when a patient has a negative response, the next patient will receive a concentration 0.01% w/v higher. However if he has a positive response, the next patient will be randomized to receive either the same ropivacaine concentration or a concentration 0.01% w/v less.~Patients with pain free recovery will be considered to have positive response.~Patients' responses will be analyzed to calculate the MEAC90
The aim of this study is to determine the minimal effective ropivacaine concentration required to provide adequate analgesic femoral nerve block in 90% of patients (MEAC90)