Intracanal heating of sodium hypochlorite: Scanning electron microscope evaluation of root canal walls.

INTRODUCTION: The aim of this study is to evaluate the surface of root canals dentine using scanning electron microscope (SEM) after instrumentation with rotary Nickel-Titanium systems and two different protocols of activation of sodium hypochlorite (NaOCl) (extracanal heating at 50°C and intracanal heating at 180°C), to assess the presence/absence of smear layer and also the presence/absence of open dentinal tubules along the walls at the coronal, middle, and apical third of each sample.
MATERIALS AND METHODS: Thirty-six single-rooted teeth were selected, divided into three groups and shaped with ProTaper Universal instruments following irrigation protocols with 5.25% NaOCl. At the end of the preparation, three different protocols of activation were used: nonheated NaOCl in Group A, extra-canal heated NaOCl at 50°C for Group B and intracanal heated NaOCl at 180°C for Group C. Specimens were cut longitudinally and analyzed by SEM at standard magnification of ×1000. The presence/absence of the smear layer as well as the presence/absence of open tubules at the coronal, middle, and apical third of each canal were estimated using a five-step scale for scores. Numeric data were analyzed using Kruskal-Wallis and Mann-Whitney U statistical tests and significance was predetermined at P < 0.05.
RESULTS: Kruskal-Wallis analysis of variance (ANOVA) for debris score showed significant differences among the Ni-Ti systems (P < 0.05). Mann-Whitney test confirmed that Group A presented significantly higher score values than other Ni-Ti systems. The same results were assessed considering the smear layer scores. ANOVA confirmed that the apical third of the canal maintained a higher quantity of debris and smear layer after preparation of all the samples. DISCUSSION AND
CONCLUSIONS: Intra-canal heating of NaOCl at 180°C proved to be more effective in obtaining clean canal walls. On the other hand, extra-canal heating at 50°C of NaOCl left a higher quantity of debris and the smear layer was widely represented.

INTRODUCTION

The success of a root canal treatment is based on cleaning, shaping, and sealing the root canal system.[1] The aim of endodontic therapy is the reduction or elimination of pathogens from the root canal space and the prevention of any sort of recontamination after the treatment.[23] Irrigating solutions are necessary to simplify the debridement and the disinfection of the root canal space: they are indispensable for the endodontic treatment.[4] The root canal shaping cannot successfully eliminate bacteria from the endodontic space, and all modern Ni-Ti systems generate smear layer along canal walls.[5] Irrigating solutions are needed to improve the cleansing of the canals and the removal of debris.[6] Currently, sodium hypochlorite (NaOCl) is the most commonly used irrigant because of its numerous advantages (antimicrobial action, the ability of the solution to dissolve vital and necrotic tissue, lubricating action, mechanical flushing of debris from the canal, low cost, and availability).[1] Although NaOCl is a highly effective antimicrobial agent, it does not remove the smear layer from the dentin walls.[7] Instead, ethylenediaminetetraacetic acid (EDTA) is appreciated for its ability to chelate hard tissue and for its decalcifying action.[1]

Current literature suggests various techniques to improve the effectiveness of NaOCl as irrigating solution. For instance, the use of greater amount of irrigant and preheating of the irrigant.[89] Cunningham has shown that a NaOCl solution at body temperature allows to carry out the sterilization in considerably less time compared to the same solution at room temperature.[10] Preheated NaOCl solution has greater ability to dissolve pulp tissue and cleanse the canal.[11] Woodmansey has shown that hypochlorite at boiling temperature is able to disintegrate the pulp tissue at speed 210 times higher compared to the solution at room temperature: For this reason, the author proposed the intracanal heating of NaOCl using the System-B heat source (Analytic Endodontics, Orange, CA, USA).[12]

At present, the most used techniques for activation of irrigating solutions are: “Ultrasonic activation” which consists in the activation of irrigants by ultrasonic tips (25–40 KHz). This technique allows, through a phenomenon called acoustic streaming, an intense stirring of the irrigant, as a better antibacterial activity and a greater dissolution of the tissues.[13] Conversely, the limits of this technique are passivity and irrigation extrusion beyond the apex.[14]“Subsonic activation:” activation of irrigants by subsonic tips (frequencies lower than sonic activation). With this technique, lower results than the sonic and ultrasonic activation are obtained.[15]“Intracanal controlled heating irrigation:” heated NaOCl increases its antibacterial, dissolutive and sliding properties, it must be heated directly inside the canal by controlled heat sources.[16]“Laser:” satisfactory technique in the detersion of the endodontic space with some limits due to the high cost of the equipment and the risks of apical extrusion of the irrigant.[17]“Negative pressure:” this technique allows a better cleaning in the apical third compared to positive pressure systems. An additional advantage is the almost no apical extrusion of the irrigant.[18] The only limitation of this technique is the requirement for preparation of the apical diameter at least equal to 0.35 ISO, not always achievable, especially in narrow and curved long canals. “Mechanical activation” (different types of files): Activation of irrigants using different types of files mounted on micro-motor. In literature encouraging, results are being made using these techniques.[1920]“Manual activation” (Gutta-percha cone): activation of irrigants by up and down movements of Gutta-percha cones up to the working length. Very simple technique but not comparable to previous techniques in terms of effectiveness.[2021]

The purpose of this ex vivo study was to investigate by scanning electron microscope (SEM) image the endodontic dentinal surfaces after canal shaping with a traditional NiTi rotary system, under irrigation with 5, 25% NaOCl solutions, but with different final activation protocols of NaOCl, to evaluate the presence/absence of smear layer and the presence/absence of open dentinal tubules at the coronal, middle, and apical third of each canal.

MATERIALS AND METHODS

Thirty-six single-rooted human teeth freshly extracted for periodontal reasons were selected for this study and placed in saline at room temperature immediately after extraction. The inclusion criteria were as follows: morphological similarity, single-canal roots, straight roots, the absence of root decay, the absence of previous endodontic treatment, root length of at least 13 mm and apical diameter of at least #20.

The crown of each tooth was removed at the level of the cemento-enamel junction in order to obtain root segments similar in length. Two longitudinal grooves were prepared on the palatal/lingual and buccal surfaces of each root with a diamond bur used with a high-speed water-cooled handpiece to facilitate vertical splitting with a chisel after canal instrumentation.

All the roots were randomly divided to three groups of 12 specimens each.

Samples were prepared by the same trained operator. The root canals were preliminary scouted using stainless steel #10 K-file (Dentsply Maillefer, Ballaigues, Switzerland) and then glide path was created with the 15/02 ProGlider single-use rotary Ni-Ti file (Dentsply Maillefer, Ballaigues, Switzerland) at 300 rpm and torque 2 N/cm. After preparing the samples were shaped with the Protaper Universal system using the complete sequence S1-S2-F1-F2-F3 at 300 rpm and torque 3 N/cm.

Root canals were irrigated during instrumentation with 6 ml of 5.25% NaOCl. The files were frequently cleaned to remove debris from their flutes, and the irrigating solutions were frequently replaced to maintain its effectiveness. Small 27G endodontic needles (Navitip Sideport, Ultradent, USA) were allowed to reach the apical third with the reflux of irrigating solutions.

At the end of the preparation phase, different protocols of activation of NaOCl were used:

Group (A) saline solution (control): endodontic needle reached 2 mm shorter than the working and 6 ml of saline were used

Group (B) extracanal heating at 50°C: endodontic needle reached 2 mm shorter than the working and 6 ml of NaOCl at 50°C were used

Group (C) intracanal heating at 180°C: Endodontic needle reached 2 mm shorter than the working, and 6 ml of NaOCl were used; System-B Heat Source (Analytic Endodontics, Orange, Ca, USA) set at 180°C was used with X-fine tip (30/04) at 3 mm shorter than the working length; the tip was not in contact with dentinal walls and was activated for 8 s and then left nonactivated for 10 s; this activation procedure was repeated 10 times and NaOCl was refreshed with new solution at each cycle.

Finally, all the canals were flushed with ethanol for 30 s and dried with calibrated paper points (Absorbent Paper Points, Denstply-Maillefer, Konstanz, Germany).

Scanning electron microscope preparation and examination

Each sample was dipped in liquid nitrogen immediately after canal preparation and split longitudinally into two halves with a stainless steel chisel. The sections were then prepared for SEM analysis: They were allowed to air-dry overnight in a desiccator at room temperature, sputter-coated with gold and prepared for SEM analysis (EVO MA 10 Carl Zeiss SMT AG, Germany).

SEM images were obtained at a standard magnification of ×1000 [Figures 1–3]. Six photomicrographs were taken in three different areas (a coronal, middle, and apical third of the root canal). Blindly, three trained operators scored the presence or absence of debris and smear layer on the surface of the root canal at the coronal, middle, and apical portion of each canal. The rating system used was proposed by Hulsmann et al.,[22] and the criteria for the scoring were reported as follows:

Figure 1

Representative samples of scanning electron microscope images of the root canal dentin surface after final irrigation with saline (Group A) at coronal (a), middle (b), and apical (c) third of the root (×1000)

Figure 3

Representative samples of scanning electron microscope images of the root canal dentin surface after final irrigation with intracanal heated Sodium hypochlorite at 180°C (Group C) at coronal (a), middle (b), and apical (c) third of the root (×1000)

Representative samples of scanning electron microscope images of the root canal dentin surface after final irrigation with sodium hypochlorite at 50°C (Group B) at coronal (a), middle (b), and apical (c) third of the root (×1000)

Scores of the debris

Score 1: Clean root canal walls, only a few small debris particles

Score 2: Few small agglomerations of debris

Score 3: Many agglomerations of debris covering <50% of the root canal walls

Score 4: >50% of the root canal walls covered by debris

Score 5: Complete or nearly complete root canal walls covered by debris.

Scores of the smear layer

Score 1: No smear layer, orifices of dentinal tubules open

Score 2: Small amount of smear layer, some dentinal tubules open

Score 3: Homogenous smear layer covering the root canal walls, only a few dentinal tubules open

Score 4: Complete root canal wall covered by a homogenous smear layer, no open dentinal tubules

Score 5: Heavy, homogenous smear layer covering the entire root canal walls.

Statistical analysis

Statistical analysis was performed with Stata 12.0 software (Stata, College Station, Texas, USA). Debris and smear layer scores were separately recorded. Descriptive statistics for ordinal data, including the median, minimum, and maximum values were calculated for all groups.

Data were analyzed with a nonparametric analysis of variance (Kruskal–Wallis ANOVA) and Mann–Whitney U-test was performed for post hoc comparisons among the groups tested and among the thirds of the canals. Significance for all statistical tests was predetermined at P < 0.05.

RESULTS

Data derived from scoring are presented in Tables 1 and 2, respectively, for debris and smear layer scores, with a median, minimum, and maximum. Kruskal–Wallis ANOVA for debris score showed significant differences among the groups tested (P < 0.05). Mann–Whitney U-test post hoc test confirmed that Groups A and B presented significantly higher score values than Group C (P > 0.05). No significant differences were obtained between Groups A and B when considering scores about debris (P > 0.05). Similar results were achieved when considering the residual amount of smear layer on the surface of the root canal, as reported in Table 2. Comparing the three thirds of the root canal, ANOVA and Mann–Whitney test confirmed that the apical third maintains a higher quantity of debris and smear layer after preparation (P < 0.05), while middle and coronal thirds present lower amount of debris and smear layer (P > 0.05) [Tables 3 and 4].

Table 1

Summary scores of the debris

Table 2

Summary scores of the smear layer

Table 3

Median scores of the debris for the coronal, middle and apical third of the canals and overall average score

Table 4

Median scores of the smear layer for the coronal, middle and apical third of the canals and overall average score

DISCUSSION

When it comes to the irrigating solutions, it is well known that NaOCl appears to be the widely used solution due to its bactericidal activity, excellent lubrication, ability to dissolve organic components, bleaching capacity, and low surface tension.[123] However, adverse effects of NaOCl irrigation on dentin's mechanical properties have been reported, including a decrease of microhardness, flexural strength, and elastic modulus.[16]

NaOCl promotes loss of dentinal organic components because this solution is a nonspecific proteolytic agent. Organic phase depletion may change mechanical properties, because dentin has 22% organic material, especially type I collagen.[24] Collagen plays an important role during dentin structural formation, and it is responsible for sustaining dentinal mineral content. By losing this organic content, dentinal elastic moduli and flexural strength decline.[25]

It has been demonstrated that the antibacterial effectiveness of NaOCl is affected by its concentration, volume, contact time and temperature in the root canal.[26] Although results from previous studies have shown that high concentrations of NaOCl are needed for the elimination of bacteria, 2.5% NaOCl is still the most preferred concentration used in routine endodontic procedures.

Other than changing its concentration, one alternative approach to improve the effectiveness of NaOCl irrigants could be to elevate the temperature of the solutions. This appears to improve its immediate tissue-dissolution capacity as well as its effectiveness in removing organic debris.[171827] In addition, the systemic toxicity of preheated NaOCl irrigants, once they have reached body temperature, should be lower than nonheated counterparts with similar efficacy in the root canal.

Previous studies have reported that the use of several activation systems/techniques showed improvement in cleaning efficacy of irrigation solutions, in particular with NaOCl.

SEM is commonly used for the identification of organic/inorganic debris and smear layer on the root canal walls after endodontic preparation, allowing to obtain detailed pictures with higher magnification imaging of the dentinal tubules.[2829]

CONCLUSIONS

Within the limitations of this study, the final irrigation protocol based on intracanal heated NaOCl at 180°C with System-B Heat Source shows to be better than preheated NaOCl at 50°C in obtaining clean canal walls. Without the use of EDTA as an irrigating solution, the heated NaOCl at 50°C alone left a higher quantity of debris and the smear layer was widely presented along root canal walls.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.