The goal of this clinical trial is to determine whether treating pain after surgery with ketamine is more effective when administered intravenously as a continuous infusion or through a patient-controlled analgesia (PCA) pump. This study focuses on adult volunteers following major orthopedic surgery. We have two main questions to address: * Is a continuous intravenous infusion of ketamine as effective as ketamine administered through a PCA pump for managing pain after major orthopedic surgery. * Does a continuous intravenous infusion of ketamine result in lower opioid requirements compared to ketamine delivered through a PCA pump for pain management after major orthopedic surgery? Participants will be divided into two groups. One group will receive ketamine via continuous infusion, while the other will receive it through a PCA pump. Both groups will also receive a placebo (a substance that looks like the drug but contains no active ingredients), ensuring that neither group knows whether they are receiving ketamine through the PCA pump (which always contains a painkiller) or as a continuous infusion. Participants will be assigned to one of the two groups and will receive their assigned treatment for 48 hours while hospitalized. Researchers will visit participants during their first three days in the hospital to assess their pain levels and monitor for any side effects.