Statistical analysis
Statistical analysis was performed using the statistical package for social sciences (SPSS) software version 16 (IBM Inc, Chicago, Il). The Kolmogorov-Smirnov and Shapiro-Wilk tests were used to assess the normality distribution of continuous data. If the number of cases was larger than 30, the Kolmogorov-Smirnov test was used, while the Shapiro-Wilk test was used for data of less than 30 subjects. Normally distributed data were presented using mean and standard deviation (SD), while non-normally distributed data were presented using median and interquartile range (IQR). Comparison of normally distributed continuous variables between groups was performed using one-way analysis of variance (ANOVA) with Bonferroni post hoc test. Non-normally distributed variables were compared between groups using the Kruskal-Wallis test and Mann-Whitney test for comparing variables between two groups.