Statistical methods
All statistical analyses were performed with SPSS statistical software version 21.0 (IBM Corp, Somers, NY, USA). Continuous values are expressed as mean ± standard deviation or standard error of mean (SEM) as indicated and were compared with Student’s t-test after confirmation of normal distribution. Otherwise, the Mann-Whitney-U-test was used. Categorical variables are displayed as frequency and percentages and were compared using the Chi-square-test or Fisher’s exact test in small sample sizes or when one or more of the cells had an expected frequency of five or less. Multi-group comparisons were performed using ANOVA test with Bonferroni post-hoc correction. For intra-group comparisons, a paired t-test was used. A p-value of less than 0.05 was considered to indicate statistical significance.