Inclusion Criteria:  adult (male and female) subjects aged 18 to 75, English speaking, are scheduled for primary total knee arthroplasty with planned spinal anesthesia with saphenous peripheral nerve block at the adductor canal.  Exclusion Criteria:  general anesthesia, hepatic & renal failure, history of diabetic peripheral neuropathic pain, chronic opioid use, concurrent use of antidepressants, triptans, and/or linezolid, allergy to the study medications, prior knee surgery, BMI > 40, bleeding disorders, history of recent falls, concurrent benzodiazepine use.