Statistical analysis
The data were analyzed based on the intention to treat method. Statistical analyses were performed with SPSS 17.0 (SPSS Inc., Chicago, IL, USA). The values were expressed as the mean ± standard deviation (SD). An unpaired Student’s t test was used to test continuous variables. The Chi-square test was used to compare categorical variables. Sample size calculation was based on a pilot study that showed the difference of VAS scores were 1.6 between the two groups within 24 h postoperative. Choosing a difference of 1 score as the minimum desired difference between the groups, 37 patients per group were required to achieve a significance level of 0.05 with a power of 95%. Combined with the number of operations in our department and to compensate for dropouts, we planned to recruit 67 patients in each group.