After standard monitored, the saphenous nerve will be blocked in the ACB group under ultrasound guidance using linear probe (9 mHz, GE Healthcare®) with nerve stimulator (Payunk®) at midfemoral level. When the needle tip is positioned, the nerve stimulator is turned on and set to deliver a current of 0.5 mA at a frequency of 2 Hz and pulse duration of 0.1 ms. The presence of a quadriceps muscle twitch, if elicited, will be recorded, and the needle was re-positioned until the muscle twitch disappeared. Then, 0.5% levobupivacaine with adrenaline 5 µg/ml 15 ml will be injected.