Statistical analysis
SPSS 17.0 software (SPSS Inc., Chicago, IL, USA) was used for statistical analysis. Values were shown as n (percentage, %) or mean ± SD. The differences between each group were derived from Mann–Whitney test or Two-way ANOVA followed by Tukey's multiple comparisons test. Fisher’ s exact test or Chi-square test was used for assessing distribution of observations between two groups. 