Three-dimensional analysis of enamel surface alteration resulting from orthodontic clean-up –comparison of three different tools

Thirty human third molars free from carious lesions, extracted for orthodontic reasons from patients aged 16–24 years were selected, based on the criterion of intact buccal surfaces free from cracks or restorations. They were stored in distilled water for 24 h before bonding, then cleaned using a low-speed bristle brush, rinsed for 10 s and dried with oil-free compressed air. For the purpose of 3D scanning, the experimental teeth were embedded in impression silicone (Bisico S1 Soft, Bisico, Germany) in order to prevent unnecessary movement during manipulation.

Following a 20 s etching with 35 % phosphoric acid (Ultra Etch, Ultradent, USA) molar tubes (ERA, Farfield, USA) were bonded directly, using chemical-cure orthodontic adhesive (Unite, 3M, USA), similarly to the clinical conditions: at the centre of each buccal surface, parallel to the long axis of the crown. The teeth with tubes bonded were then stored in 0.9 % saline solution for 24 h, rinsed with distilled water to prevent saline crystallization, dried with oil-free compressed air and debonded using ligature cutting pliers. The pliers were positioned similarly to in the clinical conditions, e.g. occlusally and gingivally in order to gently peel the tube from the enamel.

The clean up procedure was performed by the same operator under typical clinical conditions and continued until no macroscopically visible adhesive remnants could be found. Since macroscopic debonding patterns were different for individual molars, the authors decided not to assess the time needed to remove adhesive remnants. Three different tools were used for each set of ten specimens: a twelve-fluted tungsten carbide bur (123-603-00, Dentaurum, Pforzheim, Germany), a one-step finisher and polisher (inverted cone One gloss, Shofu Dental, Kyoto, Japan) and Adhesive Residue Remover (989-342-60, Dentaurum, Pforzheim, Germany).

All the specimens were scanned in blue-light technology, using a 3D optical scanner (Atos III, Triple Scan, GOM, Germany) before etching and after adhesive rest removal. Scanning was proceeded using a lens with a field of 170×130×130 mm, to the nearest 2 μm. The high scanner precision was maintained by a regular calibration procedure, as indicated by the manufacturer, thus an error study was not necessary. Two cameras observed the course of stripes projected on the teeth and point coordination was calculated for each pixel of the camera sensor. Scans of initial enamel surfaces were used as reference and those after adhesive removal – as virtual objects. Shape alteration of the enamel surface of each tooth was calculated using GOM Inspect software (GOM, Braunschweig, Germany). This procedure allowed the calculation of adhesive remnant height and enamel loss depth in every location of the buccal surface. Subsequently, the volume of adhesive remnant remaining and the volume of enamel lost were calculated for each tooth.

Data normality was tested using Shapiro-Wilk test at the level of significance α = 0.05. In order to compare variables between the groups of teeth, two tests were used at the level of significance of α = 0.05: a parametric ANOVA test for data of normal distribution and nonparametric ANOVA rank Kruskal-Wallis test - for data which did not show distribution normality.

The tools used should remove maximum residual adhesive with minimal enamel loss, thus an index of effectiveness and safety was calculated for every tool, according to the following equation:

where I is a weighted average, V_A is adhesive rest volume and V_E is enamel loss volume; these are divided by a sum of multiplicands where enamel loss has been rated to be twice as harmful as adhesive remnants (enamel damage is irreversible and thus more detrimental).

Comparisons were made for adhesive remnant height and volume, for enamel loss depth and volume as well as for the proposed index of tool effectiveness and safety.