Patients undergoing major spinal surgery continue to experience moderate-to-severe pain during the first 2-3 days following the operative procedure. Several factors contribute to postoperative pain in this patient population. Many patients present to surgery dependent on relatively high doses of oral opioids; this daily administration leads to tolerance to the effects of these drugs as well as hyperalgesia (exposure to opioids makes subsequent pain worse). In addition, surgical procedures on the spine are very painful. Furthermore, most of the opioids used after surgery only produce analgesia (pain relief) for 2-4 hours, which leads to fluctuations in levels of pain control (patients have to push a button to deliver pain medication when they begin to feel discomfort). Recent data suggest that the use of a long-acting opioid like methadone in the operating room, which provides analgesia for 24-36 hours, may improve pain control after spinal fusion surgery. However, other pain treatment modalities are required in this patient population. Studies have demonstrated that ketamine, a drug that prevents pain by a mechanism different from opioids, is effective in reducing pain medication requirements when given in the perioperative period. Small-dose infusions not only provide analgesia, but also prevent opioid tolerance and hyperalgesia. In particular, the combination of methadone and ketamine may be especially effective in controlling pain in patients following major operations. The aim of this randomized clinical trial is to examine the effect of a low-dose perioperative infusion of ketamine, when given with methadone in the operating room, on postoperative pain medication requirements, pain scores, and clinical recovery characteristics after spinal fusion surgery.