Patients undergoing total knee replacement arthroplasty are randomly assigned to six bupivacaine dose group (6, 7, 8, 9, 10, 11 mg). Combined spinal-epidural anesthesia would be performed. The dose would be considered as successful if no epidural supplement was required during surgery. A probit analysis will be performed to calculate the ED50 and ED95 of intrathecal bupivacaine with or without epinephrine for total knee replacement arthroplasty.