Total knee arthroplasty is a safe procedure with excellent outcome. In recent years, simultaneous bilateral total knee arthroplasty has become more popular. In views of enhanced recovery after surgery, intraarticular cocktail injection for pain control has been developed. However, the safety accumulation dose for simultaneous bilateral knee injection is still an issue. Recently, adding magnesium sulfate to intraarticular cocktail injection in unilateral total knee arthroplasty has been proved effective for prolong pain control. We tempt to conduct a double blinded study to evaluate that whether adding magnesium sulfate intraarticular cocktail injection to one of the knees in simultaneous bilateral total knee arthroplasty patient could effectively decrease pain score with less analgesia dose or not.

Inclusion Criteria:

• bilateral knee advanced osteoarthritis or osteonecrosis who tend to receive simultaneous bilateral total knee arthroplasty

Exclusion Criteria:

• hypersensitivity to magnesium sulfate
• hypermagnesemia
• severe cardiac, hepatic or renal compromised