Abstract

Aims: The aim of the study was to compare the efficacy of 37% phosphoric acid, 12% nitric acid, 17% EDTA in endodontic smear layer removal and degree of erosion in the apical third of endodontic canals. Methods and Material: One hundred and four extracted single-rooted human teeth were randomized into four groups (n = 26) and instrumented using ProTaper Universal Ni-Ti rotary instruments. Each canal was irrigated with one of the following solutions: 37% phosphoric acid, 12% nitric acid, 17% EDTA, 5.25% NaOCl (control). All specimens were then irrigated with 5 mL distilled water and dried with sterile paper points. Kruskal-Wallis and Mann-WhitneyU statistical tests were used. Results: Scanning electron microscopy (SEM) evaluation showed no significant differences among test reagents in smear layer removal. However, the efficacy of 12% nitric acid and 17% EDTA in removing the smear layer was significantly greater than 5.25% NaOCl (control). Degree of erosion could not be evaluated. Conclusion: Protocols used in this study were not sufficient to completely remove the smear layer in the apical third of shaped root canals.

Keywords

EDTA; Nitric Acid; Phosphoric Acid; Irrigation; Smear Layer

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1. INTRODUCTION

Mechanical shaping of root canals was shown by many authors to create a smear layer covering the endodontic walls [1,2]. Smear layer contains inorganic and organic material1. Despite controversy over maintaining the smear layer, it was shown that the smear layer itself may contain bacteria and protect the bacteria within the dentinal tubules [3]. It has also been demonstrated that smear layer forms a barrier between the filling material and sound dentin that inhibits the penetration of irrigants into dentinal tubules, increases microleakage with commonly used sealers, and decreases the bond strength of resin based materials [4-6]. Currently, a final irrigation with chemicals such as ethylenediaminetetraacetic acid (EDTA) and sodium hypochlorite (NaOCl) is recommended to remove the inorganic and organic components of the smear layer [1,4,7]. However, there is concern that this combined irrigation regime causes inadvertent erosion of the intraradicular dentin [8-11]. Phosphoric acid is used to remove smear layer, both in restorative dentistry and endodontics [12-14]. A similar chelating agent, nitric acid, has been studied by few authors under its capacity to remove restorative dentistry smear layer [15]. Despite the relevant literature available concerning the effect of these agents on the smear layer removal, the small number of studies with similar parameters, such as time intervals, concentrations and methodologies limits the ability of valid comparisons between different treatment protocols. Therefore, the aim of this study was to assess the smear layer removal ability and the erosive effects of 37% phosphoric acid, 12% nitric acid, 17% EDTA through scanning electron microscopy (SEM) digital image analysis of the apical third of instrumented root canals.

2. METHODS AND MATERIAL

2.1. Tooth Selection

One hundred and four periodontally involved human maxillary central incisors with single straight root canal extracted from 28- to 64-year-old patients were selected, with the approval of the Ethics in Research Committee. The teeth were devoid of caries, cracks, endodontic treatments or restorations. Only teeth with intact and mature root apices were selected. After extraction, teeth were stored in 2% thymol solution at room temperature, and used within 1 week.

2.2. Root Canal Preparation

The teeth were decoronated to standardized root length of 15 mm and randomly divided into one of four groups (n = 26). The working lengths were measured by deducting 1 mm from lengths recorded when the tips of #10 or #15 K-files (Dentsply Maillefer, Ballaigues, Switzerland) were visible at the apical foramina. Hot glue was used to seal apexes of all teeth to prevent the flow of irrigants through them. The specimens were shaped using ProTaper Universal Ni-Ti rotary instruments (Dentsply Maillefer, Ballaigues, Switzerland) according to the manufacturer’s instructions until ProTaper F4 file reached the working length. Each instrument was only used for the preparation of four teeth. After using each file and before proceeding to the next, canals were irrigated with 2 mL of 5.25% NaOCl 37˚C (Chematek SpA, Rome, Italy). After instrumentation, all teeth underwent final irrigation as follows: (1) phosphoric acid Group, 1 mL of 37% phosphoric acid for 1 minute followed by 3 mL of 5.25% NaOCl 37˚C; (2) nitric acid Group, 1 mL of 12% nitric acid (Chematek SpA, Rome, Italy) for 1 minute followed by 3 mL of 5.25% NaOCl 37˚C; EDTA Group, 1 mL of 17% EDTA (Chematek SpA, Rome, Italy) for 1 minute followed by 3 mL of 5.25% NaOCl 37˚C; and (4) control Group, 1 mL of 5.25% NaOCl 37˚C for 1 minute followed by 3 mL of 5.25% NaOCl 37˚C. The irrigating solutions were delivered via a sterile 30-gauge NiTi needle (Stropko NiTi Flexi-Tip, SybronEndo, Orange, USA), which penetrated to within 1 to 2 mm of the working length. The root canals then were irrigated with 5 mL of distilled water and dried with sterile paper points.

2.3. Specimen Preparation

Scanning electron microscopy was used to evaluate endodontic smear layer removal (“cleanliness”) and erosion in the apical third of the instrumented root canals. To prepare the samples for imaging, the teeth/roots were usually split longitudinally in the bucco-lingual plane. To facilitate fracture into two halves, all roots were grooved longitudinally on the external surface with a diamond disc, avoiding penetration of the root canals. The roots were then split in two halves with a chisel. For each root, the half containing the most visible part of the apex was conserved and coded. The coded specimens were secured on metal stubs, desiccated, sputter-coated with gold, and viewed with SEM (Digital scanning microscope, DSM 950, Carl Zeiss, Oberkochen, Germany).

2.4. Scanning Electron Microscope Preparation

The cleanliness and degree of erosion was evaluated at 2 mm from the apical foramen of each canal wall and photographed at 2000× magnification. The views were divided into sixteen subareas by overlaying a grid. Two observers performed blind evaluation independently after the examination of 26 specimens jointly for calibration purposes. Intraexaminer and interexaminer reliability for the SEM assessment was verified by the Kappa test. Cleanliness was scored according to the following criteria [16]: score 0 = No smear layer (no smear layer on the surface of the root canals with all tubules clean and open); score 1 = Moderate smear layer (no smear layer on the surface of root canals, but tubules contain debris); score 2 = Heavy smear layer (smear layer covers the root canal surface and the tubules). The same observers scored the degree of erosion of dentinal tubules as follows: score 0 = No erosion (all tubules look normal in appearance and size); score 1 = Moderate erosion (peritubular dentin is eroded); score 2 = Severe erosion (intertubular dentin is destroyed, and tubules are connected to each other). Data were analyzed using Kruskal-Wallis and Mann-WhitneyU tests (software used: XLSTAT ver. 2012. 6.08); p values were computed and compared with statistical significance at the p = 0.05 level.

3. RESULTS

Kappa test results, with a significance set at 0.5, showed good intraexaminer and interexaminer agreement with values ranging from 0.90 and above for the different groups. Table 1 shows cleanliness and degree of erosion findings. Specimens treated with 17% EDTA (EDTA group) showed a thick smear layer at the apical third; virtually no erosion was seen in any specimen of the EDTA Group (Figure 1(a)). Samples treated with 37% phosphoric acid (phosphoric acid group) showed a heavy smear layer in the apical third similar to the control group (Figure 1(c)). Samples treated with 12% nitric acid (nitric acid group) showed presence of a smear layer (Figure 1(b)). The degree of erosion could not be statistically evaluated because of the few areas devoid of smear layer among the specimens. Samples in the 5.25% NaOCl 37˚C (control group) showed a heavy smear layer (Figure 1(d)).

Statistical Analysis

Table 1 shows results of the statistical comparison between groups for cleanliness. Significant values were between p < 0.05 and p < 0.0001. No significant difference in cleanliness was found between the control group

Conflicts of Interest

The authors declare no conflicts of interest.

References