Adequate postoperative pain control reduce postoperative morbidity, multimodal analgesia techniques is used for achieve this goal. multimodal analgesia for bilateral total knee arthroplasty are peripheral nerve block, periarticular infiltration, NSAIDs, gabapentins and others. patients undergoing bilateral total knee arthroplasty in siriraj hospital receive spinal anesthesia, single shot bilateral adductor canal block and bilateral periarticular infiltration with bupivacaine so patient receive a large dose of bupivacaine. the investigators study the efficacy of postoperative pain of reduced dose of bupivacaine for periarticular infiltration. and study plasma level of bupivacaine.