The goal of this clinical trial is to compare postoperative pain score between perioperative intravenous ketamine and placebo in patients undergoing unilateral total knee arthroplasty under general anesthesia. The main question\[s\] it aims to answer are: * Can perioperative intravenous ketamine reduce postoperative pain score during rest and movement at 0, 2, 4, 8, 12, 18, 24 hours better than no administration of Ketamine in patients undergoing Unilateral total knee arthroplasty under general anesthesia? * Can perioperative intravenous ketamine reduce morphine consumption in postoperative 24 hours, length of hospital stay, first time to receive opioid and side effect or complication from ketamine and opioid better than no administration of Ketamine in patients undergoing Unilateral total knee arthroplasty under general anesthesia? Participants will receive intravenous ketamine intraoperative TKA and comparison group will receive placebo that is normal saline. Researchers will compare perioperative intravenous ketamine and placebo to see postoperative pain score, morphine consumption in postoperative 24 hours, length of hospital stay, first time to receive opioid and side effect or complication from ketamine and opioid.