2.4 Statistical analysis
The normality of the variables was tested with the Shapiro-Wilk, and Skewness/Kurtosis tests. Continuous variables are expressed as the mean and standard deviation or median with the 25th to 75th percentile interval. Univariable analysis was applied with appropriate test (Fisher’s exact tests or Chi squared tests), depending on the observed frequency. A paired t-test was used for the difference between two variables for the same subject. Possible risk factors for a poor outcome with p< 0.25 on Cox univariable analysis were assessed by competing risk analysis using the Stata software package (version 14.1; Stata Corp, Texas, USA). A p-value < 0.05 was considered to indicate statistical significance. Actuarial survival, freedom from reoperation and reintervention were estimated by the Kaplan-Meier method.