To compare oral (PO) to intravenous (IV) acetaminophen with postoperative pain scores, and consumptions of opiates, among orthopedic surgery patients undergoing total knee replacement with spinal anesthesia.

Our hypothesis is that IV acetaminophen patients will have improved analgesia, less opioid consumption, a lower percentage of patients rescuing, or a longer time to first rescue with IV acetaminophen. The investigators will compare the efficacies of oral and intravenous acetaminophen for postoperative pain control, and utilization of opiates as rescue agents.

Intravenous acetaminophen has rapid onset and potent analgesic properties. Recent studies have shown several advantages of using intravenous acetaminophen perioperatively. However, due to its relatively high cost and limited clinical data concerning its efficacy compared with other agents, physicians are hesitant to use intravenous acetaminophen in the perioperative period.

174 adult patients from the orthopedic surgeons undergoing total knee replacement with spinal anesthesia will be enrolled. The study will be double blind, randomized; placebo controlled comparing oral and IV acetaminophen to each other and a control (placebo). At the time patients arrive in the post-anesthesia care unit (PACU), Group 1 will receive IV 1 gram acetaminophen, and PO placebo; Group 2 will receive PO 1 gram acetaminophen and IV placebo; Group 3 will receive IV placebo and PO placebo.

The primary outcome that will be assessed is pain scores, which will be recorded by using the numeric rating scale 1-10 every 15 minutes. Secondary outcomes include time to rescue analgesia (measured in minutes), time to PACU discharge (measured in minutes), and the total amount of hydromorphone (PO or IV) consumed within 6 hours postoperatively for adequate analgesia (total measured in milligrams) will be collected.

The difference in pain scores will be assessed by a chi-square test. A multivariable regression model will be performed with the clinical and statistically significant variables to remove confounding.

Inclusion Criteria:

• Patients undergoing total knee replacement
• Successful spinal placement
• ≥18 years old

Exclusion Criteria:

• Failed spinal anesthesia
• Chronic opiate users
• Liver disease patients
• Allergy/hypersensitivity to acetaminophen
• Patients with baseline dementia
• Patients weighing less than 50kg, history of alcohol dependence, renal impairment, opiate allergy, and pregnancy.

174 adult patients from the orthopedic surgeons undergoing total knee replacement with spinal anesthesia will be enrolled. Patients will be enrolled for the study the morning of their surgery. Subjects will randomly receive one of the three treatments. The study will be double blind, randomized; placebo controlled comparing PO and IV acetaminophen to each other and a control (placebo). At the time patients arrive in the PACU, Group 1 will receive IV 1 gram acetaminophen, and PO placebo; Group 2 will receive PO 1 gram acetaminophen and IV placebo; Group 3 will receive IV placebo and PO placebo.