Evaluation of the apical sealing ability and adaptation to the dentin of two resin-based Sealers: An in vitro study

Abstract

Aim:

To quantitatively evaluate the apical sealing ability and adaptation of two resin-based sealers to dentin.

Materials and Methods:

Fifty freshly extracted mandibular first premolars were taken and sectioned at the cemento-enamel junction. Thirty teeth were subjected to a leakage study by the resin infiltration method with two groups of 10 teeth each. Group I teeth were obturated with methacrylate resin-based sealer (EnoRez) and Group II teeth were obturated with epoxy resin-based sealer (AH Plus). The remaining 10 teeth were used as controls (positive and negative of five teeth each). Twenty teeth were divided into two groups and obturated as in the leakage study and subjected to a scanning electronic microscopy (SEM) analysis for adaptation and resin depth penetration.

Results:

Both the sealers produced apical leakage to a certain extent. The adaptation and resin sealer penetration in the coronal and middle thirds was better than in the apical third of the root canal under SEM observation. The hybridized resin sealer tags in the coronal and middle thirds of Group I were much longer than that shown by Group II.

Conclusion:

We conclude that the physical integrity of the sealer matrix may also be important in providing resistance to leakage.

INTRODUCTION

Complete obturation of the root canal system with an impervious, biocompatible and dimensionally stable filling material is an important factor in achieving endodontic success.[1] Several materials and technique have been advocated for obturation, in which the most reliable method is the use of gutta-percha cones with sealer cement,[2] but it is almost impossible with the currently accepted materials and obturation techniques.

Root canal sealers leak to some extent, and most leakage occurs between the root canal walls and the sealer,[3] but its use was found to significantly reduce apical leakage. A wide variety of root canal sealers are available, such as cements based on zinc oxide eugenol, calcium hydroxide, glass ionomer and epoxy resins.[4] At present, sealers based on epoxy resin afford very good physical properties with excellent apical sealing,[5,6] and ensure adequate biological performance,[7] but had problems in working properties, radioopacity and retreatability.[1,8] It is thus clear that the development of an adhesive material specifically designed for endodontic purposes is required. Recently, EndoRez (Ultradent Products, South Jordan, UT, USA), a new resin-based sealer, has been introduced, its active ingredient being urethane dimethacrylate (UDMA). The UDMA is a monomer forming the organic matrix of composite resins.[4]

Establishing an accurate method of studying leakage to evaluate the quality of root canal treatment has been an important component of endodontic research. Many techniques have been advised, both in vivo and in vitro, including the linear dye leakage commonly used, resin infiltration, radioactive isotopes, neutron activation analysis, etc.[9] However, this determination was very subjective rather than based on statistical evaluation. “Resin infiltration method” using resorcinol–formaldehyde resin was used in this study to quantitatively measure the apical leakage.

Morphoanalysis of adhesive interfaces has been performed using scanning electron microscopy (SEM), transmission electron microscopy and confocal optical microscopy.[10,11] SEM determines the topographic appearance nature of the surface and is cost-effective.

MATERIALS AND METHODS

Fifty freshly extracted mandibular first premolars with mature apex stored in normal saline were taken for the study and sectioned at the cemento–enamel junction using a water-cooled diamond disc. The working length was visually determined by subtracting 1 mm from the length of a 15-size K-file at the apical foramen. All the canals were instrumented by the crown–down technique using a Race 0.06 taper 25 size (FKG Dentaire, La-Chaux-de Fonds, Switzerland). One milliliter of 5.25% NaOCl and 10 mL of 17% EDTA for 1 min were used as irrigants to remove the smear layer. Finally, the root canals were flushed with 3 mL of distilled water and then dried with sterile paper points.

Of the 50 teeth, 30 teeth were subjected to apical leakage evaluation and 20 teeth were subjected to SEM evaluation.

Study A: Evaluation of apical leakage

The 30 prepared teeth were randomly divided into two experimental groups of 10 teeth each and positive and negative control groups of 5 teeth each. Group I teeth were obturated using 25-size 06 taper gutta-percha cone and methacrylate-based resin as sealer (EndoRez) (Ultradent Products) and Group II teeth were obturated using epoxy-based resin as sealer (AH plus) (Dentsply DeTrey GmbH, Germany) and laterally condensed with accessory cones. The sealer was applied to the canal wall according to the manufacture's instruction. The access cavity was sealed with Cavit. All specimens were stored in distilled water at 37°C and 100% humidity for 72 h.

Resin infiltration method

All the samples except the negative control were coated with two layers of nail polish to within 4 mm of the end of the apex, and the entire surface of the negative control teeth were coated with nail polish. The resin was prepared by dissolving 1.3 g of resorcinol powder in 2 mL of 40% formaldehyde solution[12] and the pH was adjusted to 8.2 with saturated aqueous potassium hydroxide (KOH) immediately before use. Because polymerization may occur at room temperature in about 1 h, the resin was placed at 4°C in the freezer. The teeth were immersed completely in the resorcinol–formaldehyde resin for 5 days at 4°C and the resin was allowed to polymerize completely for 4 days at room temperature.

The samples were then taken out of the resin and the nail polish was completely removed and embedded in epoxy resin and sectioned horizontally at 1.5 mm (level 1), 2.5 mm (level 2) and 3.5 mm (level 3) from the anatomic apex using a slow speed diamond disk with water coolant. The sections were then immersed in HCl for a few minutes in order to color the resin. All cross-sections were photographed under a light microscope at 10× magnification using a digital camera and the images were analyzed using AUTOCAD software. The dark brown-colored resin area at the gap between the canal wall and the gutta-percha at each level was measured. The ratio of the area of the resin to the total area of the canal was obtained as the mean leakage area.

Study B: SEM evaluation

The 20 obturated teeth were randomly divided into two groups of 10 teeth each and split longitudinally by grooving the roots with a diamond disc in the bucco-lingual direction and splinting them with a chisel. All samples were immersed in 6 mol L¹ HCl for 30 s for acid dissolution (inorganic part) and 1% NaOCl for 30 min (organic part) and dried for 24 h and mounted on aluminum stubs, gold sputtered (JEOL JFG-1600 Auto Fine Coater) of about 20 nm and then examined under SEM (JEOL JSM - 5610LV) for adaptation and penetration of the resin sealers into the canal walls from the apical to the coronal, and the findings were recorded as present or absent.

Statistical analysis

Mean ranks were estimated from the sample for each group. Mean values were compared by the Mann–Whitney U-test to identify significant differences between the three levels within the same group and as well as between the groups at each level. The Kruskal–Wallis test and one-way ANOVA were employed to identify significant differences between the three levels in each group, with P < 0.05 as significant.

RESULTS

Both the sealers produced apical leakage to a certain extent. The teeth in the positive control showed maximum (100%) penetration, and there was no penetration seen in the negative control.

In Group I [Graph 1], there was a significant difference between levels 1, 2 and level 1, 3, in which level 1 leaked more than levels 2, 3. In Group II [Graph 2], level 1 showed significantly more leakage than levels 2, 3 and level 2 also showed more leakage than level 3. There was no statistically significant difference between the two experimental groups.

Graph 1.

Leakage compared at each level in group I

Graph 2.

Leakage compared at each level in group II

The adaptation and resin sealer penetration in the coronal and middle thirds was better than that at the apical third of the root canal under SEM observation. The hybridized resin sealer tags in the coronal and middle thirds of Group I were much longer than that shown by Group II. Cracks were observed within the resin filled in the root canal.

DISCUSSION

Endodontics has reached greater heights due to the developments in the materials and techniques in the recent past. Three-dimensional sealing of root canals is one of the main goals of endodontic treatment by preventing the reinfection of the canal and for preserving the health of the periapical tissues. The most commonly used core filling material is gutta–percha, which does not seal the canal when used alone[13] and requires sealer cement.

An ideal root canal sealer should have low viscosity, good wetting properties, leaking resistance to promote improved sealing thus maintaining the bacteria inactive,[14] thin film thickness, low surface tension to flow into the irregularities and spaces between the gutta-percha cones and root canal and good adhesion (Grossman[15] and Branstetter and Fraunhofer[16]). Different sealer formulations have been subjected to extensive research with respect to their mechanical and biological properties, reflecting that the appropriate selection of a sealer and its clinical performance may influence the clinical outcome.[17] Although various types of sealers have been used, the development of adhesive, resin-based filling material with better properties could increase the potential for successful outcomes.

Moreover, 60% of endodontic failures can be attributed to incomplete filling of the root canal system;[18] this would seem to be a very important field of assessment for new endodontic materials and methods used for obturation of the root canal. At present, leakage studies are widely used in different degrees of subjective analysis and more objective means by spectrophotometry of radioisotopes and electrochemical methods. In addition, volume addition leakage has been measured quantitatively by gas chromatography. Thus, currently, there is no definitive method for the quantitative analysis of root microleakage.

The resorcinol–formaldehyde resin was used by Robinson et al.[19] as a cariostatic material in enamel lesions, and Kajimoto[12] undertook an investigation of the efficiency of this resin as a tracer of microleakage between the composite resin and the surrounding walls of cavities prepared in bovine incisors. We assumed that the resin might take up the spaces left behind by the canal-filling material being evaluated and the prepared canal wall. The amount of resin could be calculated using AUTOCAD software to give a mathematical total area. Similarly, by calculating the existing root canal wall in the same plane [Figure 1a], a ratio could be developed that would result in a specific numeric value rather than such descriptions used in previous studies as “slight,” “moderate” or “extreme” leakage.[20]

Figure 1a.

Leakages were analyzed using AUTOCAD software

Leakage shown in Group I could be due to incomplete removal of smear layer at level 1 and other factors such as presence of accessory canals, fins or oval-shaped canals that are difficult to prepare and fill adequately.

Similarly, in Group II, leakage could be due to the faster setting and its more pronounced shrinkage stresses, leading to earlier debonding from the dentinal walls; the resin is viscous, such that a thick layer was formed at the apical most region of the root canal, resulting in inability to obtain a homogeneous layer on the dentin surface.[21,22]

The leakage observed within the mass of the material [Figure 1b] may be the result of the presence of voids. The structure of dentin in the apical region of the human teeth[23] may contribute to leakage in all specimens in both groups.

Figure 1b.

Leakage observed within the mass of the resin sealer

Because of the hydrophilic nature of the phosphate esters of UDMA as the ingredient in EndoRez sealers, these can adhere to the dentin walls in the presence of moisture and expansion during setting by AH Plus sealer.[24] Assessment of leakage by cross-sectioning of the roots have been performed rather transversely, and quantitatively measured the leakage areas. Moreover, penetration depth measurement studies neglect the fact that penetration does not follow only one line to a certain depth but can cover a entire surface.

This study considered the apical 4 mm of the root for leakage evaluation, which is the most critical part of the root with many lateral canals. Additionally, even the slightest color intensity was defined as “penetration” and was barely visible, and its clinical relevance must be reviewed.

Because of the lateral force applied during the condensation technique and incomplete removal of the smear layer in the apical region, the adaptation and resin sealer penetration in the coronal and middle thirds was better than the apical third of the root canal. Similar findings were observed by Sevimay et al.[25] and Saurabh Chandra et al.[11]

The hybridized resin sealer tags in this study in the coronal and middle thirds of Group I were much longer [Figure 1c] than that shown by Group II [Figure 1d]. This is because of the hydrophilic property,[26] thin film thickness[27] and small particle size of the filler in the EndoRez sealer. AH Plus sealer contains silicone oils and other ingredients that could prevent complete wetting of the root canal wall. Cracks were observed within the resin [Figure 1e], which were attributed to polymerization contraction of the resin, which is inevitable, and less bond strength of the resin to dentin.[8]

Figure 1c.

Long hybridized resin sealer tags in group I

Figure 1d.

Fewer resin sealer tags in group II

Figure 1e.

Cracks within the mass of the resin sealer

Methacrylate resin-based sealer (EndoRez sealer) can be used in in vivo situations as it has shown similar characteristics as that of the AH Plus sealer with better resin penetration into the dentinal tubules and good adaptation to the canal wall.

The dye/resin infiltration was not a good indicator of clinical endodontic success or failure although the length of penetration was related to treatment outcome (Oliver and Abott[28]). De Almeida[3] et al. have stated that the results obtained in in vitro apical sealing studies cannot be directly extrapolated clinically, but they allow comparisons.

This leakage study used single-rooted teeth that have been obturated in vitro under optimal conditions. Clinically, it may be difficult to perform an obturation that gives a thin sealer layer. Comparative clinical studies with verified adhesion properties are therefore needed to establish the relevance of adaptation to canal walls as a requirement for root canal sealer.

CONCLUSION

Under the conditions of this in vitro study, we can conclude that:

• There was a significant difference in leakage between the three levels in each group, and level 1 showed the highest leakage when compared with level 2 and level 3

• There was no significant difference in leakage between Group I and Group II

• SEM observation showed good adaptation and resin sealer penetration at the coronal and middle thirds when compared with the apical third

• Resin tags seen in Group I were much longer than those in Group II

• Thus, the findings obtained from both the microleakage test and the SEM observation suggest that the physical integrity of the sealer matrix may also be important in providing resistance to leakage.