Sample Size Calculation The estimated number of cases was calculated based on potential differences in surgical time, which is the main outcome variable with the smallest difference to detect. Sample size was calculated assuming a hypothetical mean difference of 15 minutes with an 80% of statistical power, 95% of confidence, with a 5% of alpha error (Type I error) in a two-tied distribution hypothesis. Based on these assumptions a sample size of at least 75 subjects per group (arm) is necessary.