Assessing the Efficacy of Intravenous Acetaminophen Versus the Oral Formulation for Perioperative Pain Treatment

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.