Inclusion Criteria:  ASA physical status I or II undergoing elective primary total knee arthroplasty surgery under spinal anesthesia.  Exclusion Criteria:  Patients with a known history of significant hepatic, renal, heart disease, autoimmune disease, any known convulsive disorder, any psychiatric disorders, chronic pain, pregnant females, regular use analgesics, anti-depressants, or opioids in the previous 2 months, revision surgery, morbid obesity, allergy to local anesthetics or morphine