The objective of this clinical trial is to compare two regional analgesia techniques in patients undergoing total knee arthroplasty: IPACK (Infiltration of local anesthetic between the Popliteal Artery and Capsule of the Knee) vs selective tibial nerve block. The main question is whether one of these peripheral blocks is more effective in reducing postoperative pain than the other. All patients will have a spinal block with an adductor canal block. One group will receive an IPACK block and the other group, a selective tibial nerve block. Postoperative pain scores and morphine consumption, among others, will be compared between groups.

The aim of this randomized controlled double blinded trail is to compare an analgesic effect of an IPACK (Infiltration of local anesthetic between the Popliteal Artery and Capsule of the Knee) block vs selective tibial nerve block in patients scheduled for total knee arthroplasty under spinal anesthesia with an adductor canal block.

The investigators presume that association of a selective tibial nerve block with an adductor canal block provides better postoperative analgesia than a combination of an IPACK (Infiltration of local anesthetic between the Popliteal Artery and Capsule of the Knee) block with an adductor canal block. The research team plan on including 100 patients and randomly assign them to two groups of 50 patients each. After written informed consent, patients will be allocated to one of the groups, following a computer-generated list of randomization. All patients will receive central spinal anesthesia with 12.5 mg of hyperbaric bupivacaine with 100 mcg of morphine and an adductor canal block with 150 mg of ropivacaine. IPACK block group will then receive an IPACK block with 38 mg of ropivacaine and a tibial nerve group will receive a selective tibial nerve block with 37.5 mg of ropivacaine.

During the surgery, iv dexamethasone 8 mg, iv magnesium sulfate 40 mg.kg-1, iv ketorolac 30 mg, and iv acetaminophen 1000 mg. will be administered to all patients.

After surgery, all patients will be prescribed an iv patient-controlled analgesia (PCA) of morphine (boluses of 2 mg available every 10 min, maximum of 40 mg every 4 hours) along with oral acetaminophen (1000 mg every 6 h) and oral ibuprofen (400 mg every 8 hours). Oral ondansetron 4 mg every 8 hours will be available on request in case of nausea or vomiting.

The primary outcome will be the cumulative iv morphine consumption at 24 h postoperatively. Secondary outcomes will include pain- and functional-related outcomes. Pain related outcomes are cumulative morphine consumption in the recovery room, at day 1, day 2 and day 3 postoperatively, rest and dynamic pain scores, duration of peripheral nerve block defined as the time between the execution of the block and the first dose of iv morphine, incidence of postoperative nausea and vomiting and pruritus during 3 days following surgical intervention, incidence of peroneal nerve block and complications of peripheral nerve blocks.

Inclusion Criteria:

• American Society of Anesthesiologists classification I-III
• 18 yo or older
• patients scheduled for total knee arthroplasty under central spinal anesthesia
• informed consent signed

Exclusion Criteria:

• Refusal or inability for informed consent
• Known allergies to ropivacaine, acetaminophen, ibuprofen, ketorolac, morphine, ondansetron or dexamethasone
• Secondary surgical revision
• Opioid treatment such as morphine, hydrocodone, hydromorphone, tramadol, methadone, fentanyl, buprenorphine or codeine
• Bleeding diathesis
• Neurological deficit
• Known renal insufficiency (eGFR <45 ml/min)
• Known hepatic insufficiency (Child score B or C)
• Alcohol abuse

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