The anesthesiologist will insert the interscalene catheter before the beginning of anesthesia for surgery. The usual medication will be used to make the installation of the catheter comfortable. Standard non-invasive monitoring will be used. The exact location where the catheter will be put in place will be identified using an ultrasound machine. This device will allow the anesthesiologist to identify the various anatomical structures. A specially designed needle will be inserted next to the brachial plexus and its position will be confirmed with a neurostimulator. Once the adequacy of needle position is confirmed, the anesthesiologist will introduce 2 cm (Group 1) or 6 cm (Group 2) of the catheter through the needle, and then will withdraw the needle. The catheter will be fixed in place to avoid displacement.