Seventy caries-free extracted human molars were used for this experimental study. The collection of the teeth was approved by the ethical committee of Witten/Herdecke University (116/2013). Informed verbal consent was obtained from the patients before the use of their teeth. The teeth were stored in 0.9 % NaCl containing 0.1 % thymol until use.
From the 70 teeth, 3-mm-thick dentin discs were prepared using a saw microtome (Leica 1600, Leitz Wetzlar, Germany). The discs were randomly divided into 7 groups of 10 discs each and etched with lemon juice (Hitchcock, Mönchen Gladbach, Germany) for 5 min, and one half of each disc were covered with aluminum tape. Following this, the discs were placed into a tooth-brushing machine, and a tooth brushing time of 6 months was simulated. The brushing time was calculated as follows: 28 teeth per oral cavity assuming a vestibular and an oral surface = 56 surfaces. A recommended brushing time of 360 s per day results in a brushing time of 6.4 s per tooth surface. This is multiplied by 182.5 days (6 months) and results in a total brushing time per tooth surface of 19 min 33 s. The used toothbrush has an active brushing field of 28 mm length which would cover two tooth surfaces at one time in the oral cavity, therefore, the brushing time was again doubled and resulted in a total bushing time per surface of 39 min and 6 s. As toothbrush the American Dental Association Standard Toothbrush was used. The toothbrush load was 2 N. The standard toothbrush of the American Dental Association was used with 120 linear strokes per min. The toothpastes and the active ingredients that were used are summarized in Table 1. One group served as a negative control and was brushed with tap water only. After tooth brushing, the aluminum tape was removed, and the height differences between the covered halves and the brushed halves of the discs were determined using an optical profilometer (Infinite focus G3, Alicona, Germany). Twenty measurements per disc were made, and the mean value was calculated for each disc.
Sample size calculation was carried out (Axum 7, Mathsoft, Cambridge, Massachusetts, USA) with data obtained in a preliminary experiment with a power of 0.8 and a significance level of α < 0.05, revealing a minimum number of 8 specimens per group. The mean values of the height differences were compared between the different toothpastes and the negative controls using a Wilcoxon-Mann–Whitney test for independent variables and post hoc Bonferroni adjustment, which resulted in a final p value of 0.0083. The correlation between abrasivity and RDA value was calculated with the nonparametric Spearman-Rho test. Descriptive statistics are presented as boxplots. All calculations were performed with SPSS (IBM Corporation, Armonk, NY, USA; Rel. 21) statistical software.