Statistical analysis
The statistical analysis was performed using the SPSS software program (version 22.0; IBM Corp, Chicago, IL, USA). Continuous variables satisfying normal distribution were represented by mean ± SD. Variables that do not meet the normal distribution are represented by the median (25-75% percentile range). The statistical tests involved in this study are all unpaired tests. The tests of significance were 2 tailed at an α error of 0.05. Baseline comparisons were performed using the chi-square test (to test for equal proportions) for categorical data, an independent t test for continuous normally distributed variables and the Wilcoxon rank-sum test for data that does not satisfy a normal distribution.