According to our preliminary data, the standard deviation of the mean duration of analgesia for the first 48 postoperative hours was 108 minutes. We chose to set the equivalence margin at 90 minutes, as it seemed clinically relevant to describe a difference in duration of analgesia for the patient. With a risk alpha = 5% and beta = 10% and 10% of patients expected not assessable, the estimated number of patients to include was 72 (36 in each group).