3.1. Study selection and characteristics
We identified a total of 1602 potentially relevant studies. After adjusting for duplicates and reviewing title/abstract, the remaining 39 full-text manuscripts were reviewed. Consistent with the study protocol, 6 trials were excluded due to lack of outcome of interest (n=4) and combination with regional nerve block (n=2). In total, 33 RCTs (comprising 1711 patients) were identified. The process of literature selection is listed in Figure. 1 .