This multicenter, double blind, parallel-group randomized study, therefore, is aiming to evaluate efficacy and safety of postoperative intravenous Parecoxib sodium followed by oral Celecoxib in OA patients undergoing TKA. The hypothesis is that compared to placebo with opioids as rescue treatment, sequential use of Parecoxib/Celecoxib can achieve not only less morphine consumption over postoperatively 2 weeks, but also better pain control, quicker functional recovery, and less opioid related adverse events over 6-week recovery phase.