Analgesia effect of premixed nitrous oxide/oxygen during the rehabilitation after total knee arthroplasty: a study protocol for a randomized controlled trial oleh Ya-Liang Dai, Xiao-Min Chai, Ning Zhu, Kai-Bin Wang, Wen-Qiang Bao, Xue-Sen Zhang, Lu-Lu Gao, Qiang Liu, Dong-Mei Bao, Li-Ting Wang, Yi-Ling Wang, Jun-Jun Zhang, Yu-Xiang Li, Jian-Qiang Yu