2.7 Sample size
The sample size of the study was calculated using the PASS 15 based on a pilot study with 10 patients in each group. The mean tramadol consumption was 281.50 ± 14.53 mg in the SAB group and 256 ± 36.57 mg in the RIB group. Assuming an α error = 0.01 (two tailed) with a power of 0.90, at least 27 participants were needed per group. With consideration of potential patient drop-out, we decided to include 30 patients in each group.