Mechanical and morphological evaluation of the bond–dentin interface in direct resin core build-up method

Abstract

Objectives

The purpose of this study was to evaluate the interfacial adhesion between resin and root canal dentin in the direct resin core build-up method in terms of microtensile bond strength (μTBS) and dentin micro morphology.

Methods

Single-rooted human teeth were decoronated at the cementoenamel junction and endodontically treated. Post spaces were prepared in the roots to a depth of 10 mm. The spaces were then treated with a dual-cure bonding system, and filled with dual-cure resin composite. After 24-h storage in water at 37 °C, they were trimmed into approximately 1.0-mm 2 beams for μTBS. Bond strength was analyzed with one-way ANOVA and Tukey’s test. The fractured surfaces were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX). Sectioned specimens were observed by ultra-high-voltage transmission electron microscopy.

Results

The bond strength of root dentin decreased gradually from the coronal to apical side, and the bond strength of the coronal section was significantly higher than that of the radicular section. Moreover, the failure modes in the coronal and apical sides of the specimens differed. The apical specimens fractured within the core material, while the coronal specimens fractured at the bonding layer. SEM and EDX analyses revealed that the core material penetrated into dentinal tubules in the apical region.

Significance

In the direct resin core build-up method, the interfacial adhesion of resin to root canal dentin may be insufficient in the apical region of the root canal due to poor polymerization.

Introduction

Currently, the resin build-up method using a fiber-reinforced post with dual cure resin composite core materials is widely accepted and increasingly used for the restoration of endodontically treated teeth. These materials are reported to have some advantages. First, fiber posts and resin composite have similar elastic moduli to that of dentin and thus reduce the risk of root fractures . Second, the increased transmission of light through the root and the overlying gingival tissues has improved the esthetic properties of these materials. Finally, the remaining tooth structure can be preserved, leading to maintenance of physical properties as well as esthetics. However, resin bonding to root canal dentin is still considered unstable. Rasimick reported that the commonly reported cause of failure was debonding . Debonding of the material may occur due to the following reasons. The root canal space is long and narrow, making it extremely difficult to remove moisture completely. Furthermore, light inaccessibility is also a serious problem leading to dislodgement of the crown and post core . As a result, the unpolymerized bonding layer shows weak bonding strength. In addition, the C-factor (the ratio of the bonded to the unbonded surface areas of cavities) varies from 1 to 5 in coronal restorations; it can be higher than two hundred in the three-dimensional environment of the root canal .

Regarding adhesive systems, the dual-cure materials are polymerized by a chemical reaction, a photopolymerization process, or a combination of both. However, it has also been reported that when a dual-cure adhesive is chemically polymerized, bond strengths to root canal dentin are lower compared with light-activated polymerization .

Most failures occur at the interface between dentin and resin core materials. However, a detailed examination of the interface between dentin and adhesives inside the root canal has not been performed yet. Therefore, the purpose of this study was to evaluate the interfacial adhesion of resin to root canal dentin in a direct resin core build-up method in terms of both microtensile bond strength (μTBS) and dentin micro morphology.

Materials and methods

Tooth preparation and resin core build-up

A total of 16 caries-free human teeth including incisors and premolars with single and straight root canals, extracted due to periodontal reasons, were selected for this experiment. The teeth were collected with the patients’ consent and stored in Hank’s balanced salt solution (HBSS) at 4 °C. The experimental protocol was approved by the Ethics Committee of the Osaka University Faculty of Dentistry.

The teeth were decoronated using a low-speed diamond wheel saw at the cementoenamel junction under copious water-cooling. Root canal preparation was performed using K-file (K-file, MANI, Tochigi, Japan). The canal was shaped with a size 80 K-file to the working length and obturated by lateral condensation using gutta-percha points and non-eugenol sealer (Canals N, Showa Yakuhin Kako, Tokyo, Japan). The teeth were then stored in distilled water at 37 °C for 24 h. After immersion, the root canals were enlarged with low-speed preparation drills (Tokuyama FR drill for post preparation, Tokuyama Dental, Tokyo, Japan) to a working length of 10 mm from the cementoenamel junction. Following preparation, the canals were rinsed with 3% EDTA solution (Smear Clean, Nipponshika Yakuhin Co., Ltd., Yamaguchi, Japan) for 2 min and sodium hypochlorite gel (AD gel, Kuraray Medical, Okayama, Japan) for 1 min. The canal was finally irrigated with distilled water, and then dried well with paper points. The materials used for post-core restorations are listed in Table 1 , and the restoration procedures are shown in Fig. 1 . A dual-cure one-step self-etch adhesive system-bonding agent (Clearfil DC Bond, Kuraray Medical) was used according to the manufacturer’s instructions for bonding to root canal dentin. Excess adhesive resin at the bottom of the canal was removed using a paper point. The adhesive was then light-cured for 20 s with a cordless light-emitting-diode curing light (Mini LED3, SATELEC, Merignac, France) which had a maximal light density of 2200 mW/cm 2 . All post spaces were filled with dual-cure resin composite core material (Clearfil DC Core Automix, Kuraray Medical). The coronal surface of the root was covered with a plastic strip to squeeze out any excess resin. The specimens were light-cured for 40 s and then stored in water for 24 h at 37 °C. After 24-h storage, the bonded specimens were cut with a low-speed diamond wheel saw and six slabs were serially cut perpendicular to the bonded interface under water-cooling. The interfaces were precisely checked under an optical microscope to examine whether gutta-percha and/or sealer remained. Each slab was then transversely sectioned through the middle part of the post into approximately 1 mm × 1 mm thick beams for tensile test. The flat dentin of the coronal part served as control.

Table 1
Materials for post-core restorations.
Materials Manufacturer Composition
Clearfil DC bond Kuraray Medical Liquid A MDP, hydrophobic dimethacrylates, HEMA, photoinitiator, chemical catalyst, nanofiller
Liquid B Water, ethanol, chemical catalyst
DC core automix Kuraray Medical Catalyst Bis-GMA, TEGDMA, silanized glass fillers, silica microfillers, chemical/photoinitiator
Universal TEGDMA, dimethacrylates monomers, silanized glass fillers, silica microfillers, chemical/photoinitiator
AD gel Kuraray Medical Base Sodium hypochlorite
Thickener Arminan microfiller
Smear Clean Nihon Shika Yakuhin EDTA (3%)

Fig. 1
Single-rooted teeth were decoronated using a low-speed diamond wheel saw at the cementoenamel junction. Then root canal preparation was performed using K-files. The teeth were filled by means of lateral condensation using gutta-percha points and sealer, and stored in distilled water for 24 h. The root canals were enlarged. The canals were rinsed with 3% EDTA solution for 2 min and hypochlorite gel for 1 min. A dual-cure bonding agent was used for bonding to root canal dentin. The adhesive was then light-cured for 20 s. Post spaces were filled with a dual-cure resin composite core material several times. Light exposure was performed for 40 s. All specimens were then stored in water for 24 h at 37 °C. After storage, six slabs were serially cut perpendicular to the bonded interface under water cooling. Each slab was then transversely sectioned at the middle part of the post into approximately 1 mm × 1 mm thick beams for microtensile test.

Microtensile bond strength test

The cross-sectional area of each beam was measured using digital calipers (Mitsutoyo CD15, Mitsutoyo, Kawasaki, Japan). The end of the beam and the remaining interface were attached to a testing device in a tabletop testing machine (EZ test, Shimadzu, Kyoto, Japan) using cyanoacrylate glue (Model repair, Dentsply Sankin KK, Tochigi, Japan) and subjected to tensile force at a crosshead speed of 1 mm/min. The values of bond strength, initially in kgf/mm 2 , were then converted into MPa by multiplying with a conversion factor of 9.807. The μTBS data were analyzed using one-way ANOVA and Tukey’s test. All statistical analyses were performed at a 95% level of confidence.

Scanning electron microscopy and energy dispersive X-ray spectrometry

After μTBS testing, the fractured beams, both dentin and resin sides of beams, were analyzed with a field-emission scanning electron microscope using energy dispersive X-ray spectrometry (EDX) at an accelerating voltage of 20 kV and magnification of 3000×. Samples were mounted with carbon adhesion tape on a specimen holder for scanning electron microscopy (SEM). The samples were then coated with osmium to 5-nm thickness.

Evaluation of the interface using ultra-high-voltage transmission electron microscopy

Another block was fixed in 4% paraformaldehyde and 5% glutaraldehyde over night. Samples were then dehydrated in a graded ethanol series, embedded in epoxy resin (Quetol812 NissinEM, Tokyo, Japan), and sectioned with a diamond knife (Nanotome thick, Sakai Advanced Electron Microscope Research Center, Saitama, Japan) in an ultramicrotome (Ultrotome V, LKB, Stockholm, Sweden). Sections of 2-μm thickness were prepared for observation using a ultra-high-voltage transmission electron microscope (H-3000, Hitachi Co, Tokyo, Japan). The sections were mounted on copper grids, and stained with 2% uranylacetate and 0.4% Sato’s lead stain. The H-3000 equipped with a normal specimen holder was operated at 2 MeV.

Materials and methods

Tooth preparation and resin core build-up

A total of 16 caries-free human teeth including incisors and premolars with single and straight root canals, extracted due to periodontal reasons, were selected for this experiment. The teeth were collected with the patients’ consent and stored in Hank’s balanced salt solution (HBSS) at 4 °C. The experimental protocol was approved by the Ethics Committee of the Osaka University Faculty of Dentistry.

The teeth were decoronated using a low-speed diamond wheel saw at the cementoenamel junction under copious water-cooling. Root canal preparation was performed using K-file (K-file, MANI, Tochigi, Japan). The canal was shaped with a size 80 K-file to the working length and obturated by lateral condensation using gutta-percha points and non-eugenol sealer (Canals N, Showa Yakuhin Kako, Tokyo, Japan). The teeth were then stored in distilled water at 37 °C for 24 h. After immersion, the root canals were enlarged with low-speed preparation drills (Tokuyama FR drill for post preparation, Tokuyama Dental, Tokyo, Japan) to a working length of 10 mm from the cementoenamel junction. Following preparation, the canals were rinsed with 3% EDTA solution (Smear Clean, Nipponshika Yakuhin Co., Ltd., Yamaguchi, Japan) for 2 min and sodium hypochlorite gel (AD gel, Kuraray Medical, Okayama, Japan) for 1 min. The canal was finally irrigated with distilled water, and then dried well with paper points. The materials used for post-core restorations are listed in Table 1 , and the restoration procedures are shown in Fig. 1 . A dual-cure one-step self-etch adhesive system-bonding agent (Clearfil DC Bond, Kuraray Medical) was used according to the manufacturer’s instructions for bonding to root canal dentin. Excess adhesive resin at the bottom of the canal was removed using a paper point. The adhesive was then light-cured for 20 s with a cordless light-emitting-diode curing light (Mini LED3, SATELEC, Merignac, France) which had a maximal light density of 2200 mW/cm 2 . All post spaces were filled with dual-cure resin composite core material (Clearfil DC Core Automix, Kuraray Medical). The coronal surface of the root was covered with a plastic strip to squeeze out any excess resin. The specimens were light-cured for 40 s and then stored in water for 24 h at 37 °C. After 24-h storage, the bonded specimens were cut with a low-speed diamond wheel saw and six slabs were serially cut perpendicular to the bonded interface under water-cooling. The interfaces were precisely checked under an optical microscope to examine whether gutta-percha and/or sealer remained. Each slab was then transversely sectioned through the middle part of the post into approximately 1 mm × 1 mm thick beams for tensile test. The flat dentin of the coronal part served as control.

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Mechanical and morphological evaluation of the bond–dentin interface in direct resin core build-up method
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