Statistical analysis
STD
Statistical analyses were performed using the software IBM SPSS (version 22.0, Armonk, New York, USA). Sample size calculation was based on a few internal pilot cases independently performed before the beginning of this protocol; the primary endpoint was a continuous variable (postoperative morphine consumption), alfa was set 0.05 and power 0.9, resulting in a total of 60 procedures to be performed. Data are reported as mean with standard deviation (std. dev.), median with interquartile range (IQR), or number and percentage depending on the underlying distribution. After normality distribution (Shapiro-Wilk) and homoscedasticity (Bartlett’s) tests, Mann-Whitney tests were used for statistical analyses of the samples which were non-parametric. Results were considered to be statistically significant if p ≤ 0.05.