Statistical analysis
The sample size was calculated on the basis of a pilot study taking a mean value of 8.0 h and SD 1.78 h for the duration of postoperative analgesia in 10 patients who received IBPB with ropivacaine. A 20% difference in the duration of the postoperative analgesia was considered a clinically relevant difference. For a 2-group Statistic tests were performed using SPSS 16.0 for windows (SPSS 16, Chicago, IL, USA). Continuous numerical data were expressed as mean and standard deviation or median and interquartile range. Categorical data were expressed as frequencies and percentages. Normally distributed numerical data between groups were analyzed using the independent 2-sample 