The purpose of this study is to establish the effect of sub-anesthetic dosing of ketamine with spinal anesthesia during total knee arthroplasty on post-operative pain and narcotic consumption. We hypothesize that sub-anesthetic ketamine will decrease post-operative pain and narcotic consumption and may secondarily lead to shorter lengths of stay, faster rehabilitation, improved postoperative outcomes and patient satisfaction.

Inclusion Criteria:

• 18 - 85 years of age
• ASA I - IV
• Undergoing primary unilateral elective total knee arthroplasty with spinal anesthesia

Exclusion Criteria:

• BMI over 40*
• Contraindication or allergy to opioid pain medication or ketamine*
• Daily opioid use for pain control before surgery in excess of systemic morphine equivalent to 10 mg*
• Ejection fraction (EF) less than 30%
• Creatinine clearance less than 30 mL/min*
• History of chronic liver failure
• Desire for nerve block or general anesthesia
• Any neurologic or psychiatric disorder (including bipolar disorder, post traumatic stress disorder, schizophrenia)
• Prior surgery on ipsilateral knee within the last 6 months
• Alcohol abuse