Statistical analysis
Descriptive analyses were used to summarize the data, frequency (percentage) was used for qualitative data, and median (range) was used for quantitative data. A simple regression model was used to analyze the trends in the number of trials included, with P<0.05 representing a statistically significant difference, and the year of the trial was defined by the date of the first ethical approval. All statistical analyses were performed on a personal computer with the statistical package SPSS for Windows (version 22.0).