The investigators propose to make a comparative trial between levobupivacaïne 0.5% ropivacaine and 0.5% under the control of the ultrasound as part of a sciatic nerve block. The use of ultrasound will reduce the variability of results because the changes would be linked exclusively to the local anesthetic. The investigators test the hypothesis that levobupivacaïne gives better results in terms of onset of action than ropivacaine.