Rotator cuff surgery is associated with moderate to severe postoperative pain. Among different analgesic strategies, continuous interscalene block is reported to be an efficient method, reducing opioid consumption and allowing rapid rehabilitation; however, patients may develop a transient neuropathy after surgery, which could have a significant impact on active patients. This study is designed to compare the incidence of brachial plexus injury in two groups of patients: one with a continuous interscalene block, and one with a patient control analgesia of morphine (self-iv administration of morphine). All patients will have a clinical neurological exam with a preoperative electromyogram in order to rule out a pre-existing neuropathy. Another clinical neurological exam with electromyogram will be performed between 4 and 6 postoperative weeks and, if pathological repeated at 6 months, 9 months and 12 months. The surgery will be done under general anesthesia for all patients.