Randomized, double-blind comparison of the onset of action of Ropivacaine 0.75% versus a mixture [Ropivacaine 0.75% + Mepivacaine 1.5%] for subgluteal sciatic nerve block

INTERVENTION: Trade Name: NAROPEINE® Pharmaceutical Form: Solution for injection Trade Name: CARBOCAINE® 2% Pharmaceutical Form: Injection* CONDITION: Regional anesthesia PRIMARY OUTCOME: Main Objective: To compare the time to onset of a thermo-algesic sensory block obtained with Ropivacaine and with a mixture [Ropivacaine + Mepivacaine] for sciatic nerve block via the subgluteal route. Primary endpoint(s): The primary endpoint is the time to onset of complete epicritic and thermo-algesic anesthesia of the sciatic nerve (i.e., affecting both its branches, the tibial and common peroneal nerves), compatible with the surgical procedure. Secondary endpoints are: - the severity and time to onset of a motor block (disappearance of the motor response to commands); - The duration of the sensory block: This is the beginning of the decrease in numbness in the operated limb, which we will consider to be the end of the analgesic effect of the substance being evaluated. This will allow us to deduce the duration of the sensory block from the time the block was administered; - The duration of the motor block: The patient must indicate to the healthcare team when they are able to perform a voluntary movement of the big toe – if the motor block was complete – or a voluntary movement of greater amplitude, or a movement with greater ease. This will allow us to deduce the duration of the motor block from the time the block was administered; - The appearance of signs of general toxicity from the local anesthetics; - The total morphine consumption, as well as any potential side effects. Secondary Objective: To evaluate the intensity and onset of motor block, the duration of sensory and motor block, the occurrence of toxic accidents, and morphine consumption per 24 hours, as well as any potential side effects. INCLUSION CRITERIA: - Age over 18 years - Foot surgery usually performed under sciatic nerve block in the department. Are the trial subjects under 18? No. Number of subjects for this age range: F.1.2 Adults (18-64 years) Yes F.1.2.1 Number of subjects for this age range F.1.3 Elderly (>=65 years) Yes