The purpose of this study is to compare the use of magnesium and bupivacaine to buprenorphine and bupivacaine in post-operative adductor canal blocks (ACB); the study will determine if one combination can result in decreased opioid consumption and improved pain management for patients after same-day discharge total knee arthroplasty (TKA) .

The investigators will assess whether the addition of magnesium to bupivacaine 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 buprenorphine is added to bupivacaine.

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.

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) versus adding buprenorphine to bupivacaine for the blocks.

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 buprenorphine is administered instead.

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