Before the beginning of operation, under ultrasound control, a catheter will be implemented to one of the chosen position: the adductor canal (the middle or lower third of thigh) or near the femoral nerve (below the inguinal ligament). The local anesthetic solution of 0.2 % ropivacaine will be started with an elastomeric pump (5 mL per hour, up to 72 hours) as soon as a catheter in the right position.