Sample size: The literature reports a 30% incidence of postoperative sore throat with metal blades. The hypothesis is that this incidence could increase from 30% to 40% if plastic blades are used. Un sample size of 356 patients per group is required to detect this increase of 10% with a Chi2 test, a statistic power (1-beta) of 0.80 and alpha type I error probability of 0.05.