Statistical analysis

Data analysis was performed using SPSS for Windows software (version 17.0; SPSS Inc, Chicago, IL, USA). We explored the distributions of continuous variables using the Kolmogorov–Smirnov test. The Levene test was used to evaluate the homogeneity of variance. Data are given as mean ± standard deviation, or as median (minimum–maximum), for continuous variables. Categorical data are shown as number of cases with percentage. The mean differences between groups were compared using Student’s t-test. When two independent groups were compared, the Mann–Whitney U-test was used to compare medians; otherwise, the Kruskal–Wallis test was applied. Categorical data were analyzed using the chi-squared or Fisher’s exact test, as appropriate. We used the Wilcoxon Sign–Rank test to explore whether differences between pre- and postoperative clinical measurements were significant. Associations between continuous variables were evaluated by Spearman rank correlation analysis. A P-value <0.05 was considered to reflect statistical significance. When we performed multiple comparisons, the Bonferroni correction was applied to control for type I error.