To study efficacy of postoperative pain control of different concentrations of bupivacaine for periarticular infiltration, part of multimodal analgesia, in bilateral total knee arthroplasty.

To study plasma concentration of bupivacaine in patient who received spinal anesthesia and single shot bilateral adductor canal block and periarticular infiltration with bupivacaine for safety level.

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.

Inclusion Criteria:

• Aged more than 18 years old
• American society of anesthesiologists physical status classification 1-3
• Undergoing bilateral total knee arthroplasty with spinal anesthesia and single shot bilateral adductor canal block

Exclusion Criteria:

• Allergy to bupivacaine
• Body weight less than 50 kilograms
• Hepatic disease
• Coagulopathy
• Creatinine clearance less than 60 ml/min
• Uncontrolled cerebrovascular disease
• Hematocrit less than 35%