The main objective of the study is to demonstrate the efficacy of Dexmedetomidine on the prolongation of analgesia obtained by interscalenic bloc of the brachial plexus and intravenous injection of Dexamethasone (efficacy objective).

This is an interventional, comparative, randomized, controlled study versus reference treatment, in 2 parallel groups, double blind (patient and evaluator), multicenter, aiming to demonstrate the superiority of This is an interventional, comparative, randomized, controlled study versus reference treatment, in 2 parallel groups, double blind (patient and evaluator), multicenter, aiming to demonstrate the superiority of Dexmedetomidine over the absence of Dexmedetomidine, in adjuvantation of a loco regional anesthesia (interscalenic bloc of the brachial plexus + intravenous injection of Dexamethasone), on the prolongation of the analgesia, following a surgery of the rotator cuff of the shoulder under arthroscopy.

Patients will be randomized into 2 groups according to the treatment received:

• Dexmedetomidine;
• Nothing (control arm). The investigator (anesthesiologist) who will perform the randomization will not be blinded to the treatment.

Inclusion Criteria:

• Age ≥ 18 years;
• Patient undergoing arthroscopic shoulder surgery to repair the rotator cuff tendons;
• Patient capable of understanding the information related to the study, of reading the information leaflet and agreeing to sign the consent form.

Exclusion Criteria:

• Patient on oral morphines before surgery;
• Contraindication to performing a general anesthesia associated with a shoulder loco regional anesthesia in a single injection;
• Contraindication to take oral morphines;
• Any non-arthroscopic repair (intraoperative conversion to "open sky" technique);
• Any other loco regional anesthesia than an interscalenic block (change of intraoperative strategy);
• Pregnant or lactating woman;
• Intolerance or allergy or contraindication to one of the treatments under study;
• Patient participating in another clinical trial, or in a period of exclusion from another clinical trial.