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.

Inclusion Criteria:

1. A patient undergo unilateral total knee arthroplasty under General anesthesia at Chomthong Hospital Chiang Mai During 2024 to December 2025
2. Age is more than 18 years old
3. ASA (American society of anesthesiologist) status are I,I,III
4. A patient can cooperate in research

Exclusion Criteria:

• Patients allergic to local anesthetic agent
• Patient who has contraindication to use ketamine
• Patients has coagulopathy.
• Injection site is infected.
• Patients with chronic pain received painkillers for more than 3 months.
• The patient has a history of opioid use.
• The patient has a history of alcohol dependence.
• Patients allergic to fentanyl or morphine.
• Patient who has unstable cardiovascular disease
• Patient who has increase intracranial pressure and high ocular pressure
• Pregnancy
• The patient who has communication problems cannot describe the level of pain
• The patient refused to participate in the study.