Total hip arthroplasty (THA) is one of the most common orthopedic surgical procedure and is associated with severe pain in the immediate postoperative period, thus limiting early recovery.

Postoperative pain management in THA requires multimodal analgesia, combining drugs and injection of a local anesthetic (LA). But, the best anesthesia strategy to provide optimal postoperative analgesia in THA remains controversial. Opioid free anesthesia could limit the episodes of hyperalgesia as well as tolerance and addiction to opioids.

The hypothesis of this study is that an opioid free anesthesia using dexmedetomidine could improve analgesia after THA.

The main objective of this monocenter, prospective, randomized, triple-blind, controlled trial is to assess the interest of opioid free anesthesia using dexmedetomidine on morphine consumption after THA.

In the pre-anaesthesia room, after the implementation of classical monitoring and a peripheral venous catheter, all patients will receive an antibioprophylaxis according to SFAR (French Society of Anesthesia & Intensive Care Medecine) recommendations and injection of 10 mg of IV dexamethasone.

The patients will be then randomized in 2 groups:

Control group:

• Pre-operative normal saline infusion 100 ml over 30 minutes
• Injection of sufentanil 10 µg in normal saline 2 ml on induction of anesthesia

If needed after incision :

• Per-operative normal saline infusion 100 ml
• Per-operative injection of sufentanil 5 µg in normal saline 1 ml

OFA group (experimental group):

• Pre-operative dexmedetomidine infusion 1 µg/kg in normal saline 100 ml over 30 minutes
• Injection of normal saline 2 ml on induction of anesthesia

If needed after incision :

• Per-operative dexmedetomidine infusion 0.4 µg/kg in normal saline 100 ml
• Per-operative injection of normal saline 1 ml

In the operating room, general anesthesia will be induced by intravenous ketamine (0.4 mg/kg) + propofol (3 mg/kg) +/- cisatracurium (0.1 mg/kg) for the introduction of laryngeal mask airway. Anesthesia will be maintained with propofol.

Postoperative analgesia protocol :

• Multimodal analgesia will be instituted during surgery by the administration of paracetamol (1 g), nefopam (20 mg) and ketoprofen (100 mg) and the infiltration of the surgical wound with 100 ml of ropivacaine 0.2%.
• In post-anesthesia care unit (PACU): oxycodone titration if NRS (pain score) >3 according to the centre's usual care.
• In ward and at home: systematic per os analgesia with paracetamol (1 g, 4 times a day) and ibuprofen (400 mg, 3 times a day); oxycodone (10 mg, lockout interval: 4 h) if NRS (pain score) >3.

Inclusion Criteria:

• 18 years and older,
• Undergoing outpatient primary total hip arthroplasty under general anesthesia with laryngeal mask,
• Consent for participation,
• Affiliation to a social security system

Exclusion Criteria:

• Heart rate < 60 bpm
• Chronic pain syndrome requiring preoperative morphine use (class 3)
• Contraindication for: paracetamol, ketoprofen, nefopam, oxycodone, propofol, ketamine, cisatracurium, sufentanyl, ropivacaine.
• Contraindication for dexmedetomidine: hypersensitivity to the active substance or to any of the excipients, advanced cardiac block (grade 2 or 3) unless paced ; uncontrolled hypotension ; acute cerebrovascular conditions
• Contraindication to laryngeal mask : old insulin-dependent diabetes, gastro-oesophageal reflux, morbid obesity (defined by BMI > 35)
• Pregnant or breastfeeding women
• Women of child bearing potential without contraception (any contraceptive method used regularly and appropriately with a low failure rate: <1% per year),
• A mental or linguistic inability to understand the study,
• Patient under protection of the adults (guardianship, curators or safeguard of justice),
• Patient included or planning to be included in another clinical trial relating to medications