Materials and methods.
The compositions of the three chelator pastes used in the present study according to the manufacturersâ€™ information are as follows:
- Calcinase Slide (lege artis, Dettenhausen, Germany): 15% sodiumedetate,58-64% water and a water-soluble gel base, pH 8-9.
- RC-Prep (Premier-Dental, Norristown PA, USA): 15% EDTA and 10% urea peroxide in a water-soluble gel base, polyethylenglycol, cetylalcohol, propyleneglycol.
- GlydeFile (DeTrey/Dentsply, Konstanz, Germany): 15% EDTA,10% urea peroxide in an aqueous solution.
The investigations were performed on freshly extracted maxillary incisors and single rooted premolars with intact roots. Teeth showing signs of endodontic or restorative treatment of the root dentine were excluded from the study as were teeth showing root caries. Teeth were stored in distilled water and always kept wet. Changes of microhardness.
The roots of the extracted teeth were cut horizontally into slices of 2-mm height. For this part of the study, 100 slices were prepared and randomly distributed into 10 groups. For each chelator paste,10 dentine slices were covered with 0.1mL paste for 30, 60, or 120 s followed by an irrigation with 5 mL H2O2 (5%) and 5 mL NaOCl (3%).This procedure was repeated five times, simulating root-canal enlargement of five instrument sizes resulting in a total working time of the chelator pastes of 2.5, 5 and 10 min. Microhardness of root dentine was measured at four points of the dentine near the root canal, before and after treatment (Fig.1). Ten slices served as controls and were only irrigated with hydrogen peroxide and sodiumhypochlorite as described. For evaluation of Vickers hardness, a Durimeter (Leitz,Wetzlar, Germany) was used with 490.3 mN (50 p) and an impression time of 10 s. Preoperative markings on the slices enabled pre- and postoperative measurements to be taken in direct neighbourhood close to the root canal (Figs 1 and 2).
Figure 1. Localization of the pre- and postoperative measuring points for evaluation of changes in microhardness.Loss of weight.
Figure 2. SEM of the pre- and postoperative measuring points in direct neighbourhood and near to the root canal (magnification: 700x).
For the determination of weight loss, 200 dentine slices were prepared as described before, dried in a desiccator (Novus-Desiccator, Wertheim, Germany) for 5 days, weighed (R 160 P, Sartorius, Gottingen, Germany), and randomly distributed into10 groups.
For each of the three chelator pastes, 60 slices were covered with 0.1mL of the chelator, applied with a syringe, for 3,6, or 9 min. Then the slices were irrigated with 5 mL H2O2 (5%) and 5 mL NaOCl (3%) to remove the chelator. Twenty slices served as controls and were only irrigated. Finally, the slices were dried in the desiccator for 5 days and weighed again. The optimal time for desiccation had been evaluated in preliminary tests, showing that no further changes in weight of the slices occurred after 5 days. Canal cleanliness.
For the evaluation of root-canal cleanliness after use of a chelator paste, 35 maxillary incisors with intact roots, closed apices, and no signs of carious decay or endodontic treatment of root-canal dentine were selected. An access cavity was prepared and length was determined by inserting an endodontic file through the apical foramen. Working length was determined 1mm coronal to the root tip and the size of the first instrument binding at working length was determined. All root canals were enlarged five ISO-sizes using stainless steel Hedstroem-files in a filing motion with 0.1mL of the chelator paste for each instrument size resulting in a total of 0.5 mL paste for each root canal. Most of the preparations were finished with ISO-sizes 50-60.Working time for each file was 1min, resulting in an identical total working time of 5 min for instruments and chelators in all groups. After each instrument size, the root canals were irrigated with 5 mL H2O2 (5%) and 5 mL NaOCl (3%). Five teeth served as controls and were prepared as described but without use of a chelator.
Following enlargement, the teeth were split longitudinally and evaluated under the SEM (DSM 960, Zeiss, Oberkochen, Germany).The roots were coded and mixed so that during the SEM evaluation the type of chelating agent could not be identified by the evaluator (M.H.) nor the SEM operator (F.S.). The root half was placed in the SEMand the SEM-beam adjusted to the coronal, middle or apical third of the root under a 20x magnification by the SEM operator.
A transparent grid was placed on the SEM screen and cleanliness of the root-canal walls was evaluated for10 pre-selected squares of the grid separately for the coronal, middle and apical third of the canal wall under a 1000x magnification using a four-score system with reference photographs. The four scores were defined as follows:
- Score I : dentinal tubules completely opened;
- Score II : more than 50% of dentinal tubules opened;
- Score III: less than 50% of dentinal tubules opened;
- Score IV: all of the dentinal tubules covered with smear layer.
The results for weight loss and changes in microhardness were statistically analysed for differences between the different working times for each chelator using the Kruskal-Wallis test. In cases of significance pair tests were undertaken using the Wilcoxon-Mann-Whitney test. Comparisons between the results for the apical, middle and coronal parts of the roots were performed using the Friedmanntestand the Wilcoxon test. The level of significance for all of the statistical tests was P < 0.05.