The purpose of this study is to evaluate whether adding ketamine to bupivacaine is superior to bupivacaine alone in terms of providing better pain control for 48 hours postoperatively after TKA.

Optimal pain relief is essential for functional recovery after total knee arthroplasty (TKA). Addition of femoral nerve block (FNB) to an analgesic regimen provides superior pain control and shortens hospital stay, in comparison with epidural or intravenous patient-controlled analgesia (PCA) alone. However, prolonged motor blockade from FNB is associated with a small (2%) but clinically important risk of fall. With FNB there will always be a compromise between the goals of adequate pain relief and muscle strength. An ideal nerve block would provide effective analgesia, minimize opioid use and side effects, and hasten mobilization by preserving motor strength. "Fast-track" total joint replacements are gaining popularity. Motor preservation with adequate analgesia has become the optimal postoperative pain goal in orthopedic surgeries to enable earlier physical therapy, faster recovery, and shorter hospital stays.Nerve blocks such as femoral nerve block, adductor canal block, and epidural block have been more prevalent in TKA postoperative analgesia due to their effectiveness, easy manipulation, and low rate of complications. Local anesthetics such as ropivacaine or bupivacaine have been commonly used in nerve blocks. However, the postoperative analgesic effects and duration of local anesthetics are not good enough, and sometimes have led to delayed ambulation and an increased risk of falling after TKA. To overcome these shortcomings and further improve the analgesic effect, additional endeavors should be devoted to exploring new and effective agents for nerve block.With the advent of ultrasonography, the adductor canal can be easily visualized at the mid-thigh level, allowing performance of adductor canal block (ACB) with a high success rate. In recent years, ACB has been successfully used for postoperative pain control after knee surgery. Anatomical study of the adductor canal demonstrated that the adductor canal may serve as a conduit for more than just the saphenous nerve, possibly including the vastus medialis nerve, medial femoral cutaneous nerve, articular branches from the obturator nerve, as well as the medial retinacular nerve. Thus, the sensory changes are not limited to the distribution of the saphenous nerve, but includes the medial and anterior aspects of the knee from the superior pole of the patella to the proximal tibia.

Inclusion Criteria:

1. The subject is scheduled for elective primary unilateral TKA.
2. The subject is ≥ 18 years and ≤ 80 years.
3. Both sexes.
4. The subject's primary anesthesia care team has planned for a spinal anesthesia.
5. The patient agrees to receive an adductor canal block.
6. ASA class 1-3.

Exclusion Criteria:

1. Subject is < 18 years of age or >80 years of age.
2. Subject is known or believed to be pregnant.
3. Significant pre-existing neuropathy on the operative limb.
4. Significant renal, cardiac or hepatic disease.
5. ASA class 4-5.