Sample size calculation
At the start of the study, a significance level of α = 0.05, a relevant average difference in PD of 1 mm between study groups with a standard deviation of 1 mm and a power (1—α) of at least 0.8 were set to calculate the minimum number of necessary cases (at least 20 per group). Assuming any possible dropouts during the study period, the number of patients was increased to 24 in each group. A power calculation at the end of the study with the given number of cases yielded a power of 99.6%.