This is a prospective, randomized, double-blinded, placebo-controlled study designed to examine pain relief following intrathecal morphine sulfate (0.2mg) in patients undergoing total knee arthroplasty (TKA) under spinal anesthesia in addition to a femoral nerve catheter. The protocol consists of two parts: (1) a prospective patient recruitment study and (2) a retrospective assay for endocannabinoids on previously collected specimens.

This is a prospective, case-controlled randomized, double-blinded, placebo-controlled study expected to enroll up to 100 patients scheduled to undergo unilateral total knee arthroplasty (TKA) under regional anesthesia (defined as intrathecal spinal + femoral nerve blockade + femoral nerve catheter for post-operative analgesia) at Stony Brook Hospital. The patients were randomized to receive either IT morphine 0.2 mg (Group 1) or placebo (Group 2) in combination with their standard-of-care spinal anesthesia. Post-operatively, the patients were given an intravenous patient-controlled analgesic opioid for breakthrough pain.

Only the pharmacist preparing the drug/placebo has knowledge of how each subject was randomized. Study staff completed multiple questionnaires regarding subjects' pain scores and patient disability from pain to assess changes between different study periods (ie. pre-operative, post-operative and follow up). Biological samples were also taken from each subject to measure and analyze differences and changes in endocannabinoid levels.

Inclusion Criteria:

• Scheduled for unilateral TKA under regional anesthesia
• ASA Class 1, 2 or 3
• Able to give informed consent
• Able to understand English

Exclusion Criteria:

• Medical condition(s) that prevents use of regional anesthesia (e.g. infection at site of injection, coagulopathy, severe hypovolemia, severe aortic or mitral stenosis, increased intracranial pressure, severe spinal deformity, spinal cord hardware or stimulator implanted)
• Allergy to morphine
• Morbid obesity (BMI > 45)
• Respiratory compromise (difficult airway, severe emphysema or COPD)
• Obstructive sleep apnea (only if diagnosed in a sleep disorders clinic and CPAP was prescribed)
• Chronic pain with opioid usage over 100 mg morphine-equivalents po/day
• History of abuse of opioids or other drugs of abuse
• Scheduled for bilateral TKA
• Revision of knee arthroplasty
• Any medical condition that would affect the patient's ability to metabolize or excrete the study drugs (e.g. chronic kidney failure with patient on dialysis) or other medical condition that in the investigator's opinion would render the patient unsuitable for this research study