The purpose of this prospective research study is to determine the best way to manage post-operative pain after a total hip arthroplasty. Currently, there is no standard of care for managing post-operative pain in these patients. The quadratus lumborum block (QLB) first described by Blanco in 2007, is a promising technique in this indication: recently, there is a growing evidence for the use of the QLB as an alternative technique for pain management after hip surgery.

A sample size of 100 patients (50 per group) was calculated based on 20% reduction in morphine consumption with 0.05% significance and a power of 0.8.

After ethical committee approval, eligible patients scheduled to have a fast-track total hip arthroplasty are screened during preoperative evaluation clinic. Informed written consent will be obtained from all patients during pre-anesthesia visit the day before surgery (J-1). Consenting patients will be randomized the day of surgery (J0) to undergo QLB with ropivacaine ("ropivacaine group") or normal saline ("saline group").

Before general anesthesia, all patients will have a needle-insertion posterior to the quadratus lumborum muscle avec injection of either ropivacaine in the "ropivacaine group" or normal saline in the "saline group".

After general anesthesia induction, dexamethasone and ketamine will be given to all patients.

Inclusion Criteria:

• American Society of Anesthesiologists Physical Classification Status (ASA) I to III
• Age ≥ 18 years
• Scheduled for fast-track total hip arthroplasty
• Written informed consent
• Patient covered by health insurance Exclusion Criteria
• Protected patients or patients incapable of giving written informed consent
• Pregnant or breastfeeding woman
• Vulnerable adult
• Contraindication for fast-track surgery
• Inability to comprehend or participate in pain scoring scales
• Allergy to study drugs
• Severe coagulopathy
• Chronic kidney disease with glomerular filtration rate (GFR) ≤ 30 mL/min (estimated by the Cockcroft & Gault formula)
• Chronic pain (treated by nonsteroidal anti-inflammatory drugs, opioids, neuroleptic drugs, antidepressant or anticonvulsants)
• Peripheral neuropathy