The purpose of this study is to determine whether the addition of magnesium to bupivacaine for the post-operative adductor canal blocks (ACB) can decrease opioid consumption and improve pain management for patients after same-day discharge total knee arthroplasty (TKA).

The investigators will assess whether the addition of magnesium will decrease visual analog scale (VAS) pain scores, decrease post-operative total opioid consumption (oral morphine equivalents), decrease the incidence of post-operative nausea and vomiting (PONV), and improve patient satisfaction in comparison to when magnesium is not administered.

Total knee arthroplasty (TKA) is one of the most common elective surgical procedures in the United States . The number of TKA procedures has doubled in each of the past two decades and is expected to top 3.4 million per year in the United States by 2030.

Within the last 10-15 years significant changes have taken place in the anesthetic management of patients undergoing total knee arthroplasty (TKA). In the past, the majority of patients underwent general anesthesia and were managed post-operatively with a PCA (patient controlled analgesia) pump.

More recently anesthesiologists have begun to use neuraxial anesthesia (spinal anesthesia) combined with regional anesthesia (peripheral nerve block (PNB)) techniques combined with monitored anesthesia care (MAC) for many of these surgeries.

Regional anesthesia is used to block the sensation in a specific part of body during and after surgery. It offers numerous advantages over conventional general anesthesia, including faster recovery time, fewer side effects, no need for an airway device during surgery, and a dramatic reduction in post-surgical pain and reduction in opioid use following surgery. The use of local anesthetic peripheral nerve blocks for surgical anesthesia and postoperative pain management has increased significantly with the advent of ultrasound-guided techniques.

However, the duration of traditional amide-based and ester-based regional anesthesia is normally limited to only a few hours. Techniques including continuous catheter placement or serial injections can be used to enhance the duration and effect of regional anesthesia for postoperative pain control. But these approaches can increase the risk of infection, toxicity, and cost. Therefore, alternative methods of extending the clinical duration of nerve blocks have been a topic of significant interest.

Magnesium sulfate, an NMDA receptor antagonist, exerts it analgesic effects by at least two mechanisms: it acts as a physiological calcium antagonist by blocking NMDA receptors and it inhibits the inflammatory response through the reduction of inflammatory cytokines.

Magnesium has also been shown to decrease peripheral nerve excitability. Addition of magnesium to intravenous regional anesthesia for chronic limb pain management has demonstrated improvement in quality of blockade and prolonged the duration of analgesia. Magnesium decreased intraoperative opioid consumption and tourniquet pain. Magnesium improved the quality of anesthesia and prolonged the time for first postoperative analgesic requirement. Overall, the addition of magnesium to local anesthetic is effective both for perineural (nerve blocks) and intravenous regional anesthesia.

In a recent meta-analysis of randomized controlled trials, Li et al. evaluated seven trials involving 493 patients. Investigators concluded that the addition of magnesium as an adjuvant to PNB prolonged the postoperative duration time of analgesia, sensory, and motor block.

Studies have consistently shown that addition of magnesium to local anesthetic significantly prolongs peripheral nerve blocks, including femoral nerve blocks with bupivacaine, interscalene blocks with bupivacaine, and axillary blocks with prilocaine and levobupivacaine. All of these papers denied adjuvant-related toxicity or side effects; however, nausea was two to three times more likely in the first 12 hours after interscalene blocks with 200mg magnesium in the study by Lee et al. This side effect was not reported in studies using 150mg magnesium.

The purpose of this study is to determine whether the addition of magnesium to bupivacaine for the post-operative adductor canal blocks can decrease opioid consumption and improve pain management for patients after total knee arthroplasty (TKA).

The investigators will assess whether the addition of magnesium will decrease visual analog scale (VAS) pain scores, decrease post-operative total opioid consumption (oral morphine equivalents), decrease the incidence of post-operative nausea and vomiting (PONV), and improve patient satisfaction in comparison to when magnesium is not administered.

Inclusion Criteria:

- Patients undergoing unilateral primary total knee arthroplasty with spinal anesthetic and MAC followed by a unilateral adductor canal block with ultrasound guidance.

Exclusion Criteria:

• Patients on chronic anticoagulation upon admission
• Patients with significant genetic or acquired clotting/bleeding disorders (hemophilia, DIC, etc.) or significant platelet dysfunction
• Patients with prior back surgery or leg surgery that precludes spinal or regional anesthesia
• Infection at sites for regional/spinal anesthesia
• Allergy to local anesthetics