Introduction

The use of disinfecting solutions prior to bonding procedures can interfere with the adhesion process, altering the properties of some hydrophilic resins (1, 2). Residues of these substances may also impede the deep penetration of the bonding agent and its intimate contact with dentin, also affecting the adhesion process (1).

The use of chemical agents for cleaning of root canals after retreatment and prior to the application of adhesive systems has been evaluated because these substances have been pointed as the responsible for the decrease of intra-radicular adhesion (3-5).

It is known that the smear layer and debris formed by the action of endodontic irrigants should be removed to increase the retention of intra-radicular dowels (6). Therefore, several chemical solutions and combinations of solutions have been proposed to promote cleaning and disinfection of root canals as well as removal of smear layer, among which ethylenediaminetetraacetic acid (EDTA), and sodium hypochlorite (NaOCl) associate with EDTA (7, 8).

The composition of adhesive systems may also affect the bond strength when NaOCl is applied to the etched dentin (9-11). It has been shown that 5% NaOCl or 3% hydrogen peroxide may decrease the adhesion quality due to the presence of residual oxygen produced by these substances, thus inhibiting the polymerization of the bonding agent. However, this effect seems to vary according to the composition of the adhesive system employed (3, 12). NaOCl also causes damage to the organic components of dentin, mainly collagen, which may affect the diffusion of resin monomers through the demineralized structure (13).

The use of chemical irrigants during root canal treatment may have a deleterious effect on binding of adhesive systems and resin cements to dentin since their residues and by-products can diffuse through the dentin tubules, affecting the polymerization of monomer in the demineralized dentin matrix and resin penetration into the tubular dentin structure (3, 12). Although it is known that the chemical substances used in the treatment of intra-radicular dentin may affect adhesion, but there is little research on their behaviour with respect to endodontically treated teeth that will receive fibre glass dowels (3, 12). Therefore, this study evaluated the effect of different endodontic irrigating solutions on the micro push-out bond strength of a fibre glass dowel to dentin.

Material and Methods

Seventy freshly extracted mandibular bovine incisors with straight roots and of similar shape and dimensions and closed apices were selected from a random collection and checked for absence of root caries, cracks and structural defects. The teeth were sectioned transversally below the cementoenamel junction with a water-cooled double-faced diamond disc (KG Sorensen, Barueri, SP, Brazil) to obtain 16-mm-long apical root segments, as measured with a digital caliper (Isomet 1000; Buehler Inc., Lake Buffer, IL, USA).

In order to have a standardized sample, only root segments in which #5 Largo drill (Ângelus Ind. Prod. Odontol., Londrina, PR, Brazil) did not penetrate into the canal were used in the study. This way, the diameter of all root canals would correspond to the diameter of this drill.

The root canals were instrumented and widened with #3 and #4 Gates Glidden drills (Dentsply Ind. Com. Ltda., Petrópolis, RJ, Brazil) with a silicone stopper to limit penetration to 15 mm. The instrumented canals were irrigated with 2.5% NaOCl, dried with absorbent paper points and filled by gutta-percha cones and an epoxy resin-based endodontic sealer (Sealer 26; Dentsply, Petrópolis, RJ, Brazil). In order to verify the obturation quality, periapical radiographs were taken. The root segments were cleaned with gauze soaked in alcohol and stored in distilled water at 37°C during 24 hours.

After this period, the root fillings were reduced to a length of 4 mm in the apical third using a #4 Gates Glidden drill (Dentsply Ind. Com. Ltda., Petrópolis, RJ, Brazil). A #5 Largo drill (Ângelus Ind. Prod. Odontol., Londrina, PR, Brazil) was introduced into the canal in a single movement to complete dowel space preparation. Following the dowel space preparations, the canals were irrigated with distilled water and dried with absorbent paper points. Presence of any residual gutta-percha in the walls of dowel space was checked by radiographic evaluation. The prefabricated fibre glass dowels (Reforpost; Ângelus Ind. Prod. Odontol., Londrina, PR, Brazil) were tried to ensure that the they would reach the bottom of the dowel space. The specimens were then randomly assigned to 7 groups (n=10) according to the type of irrigating solution used prior to the cementation of intra-radicular fibre glass dowels: (Table 1).

Prior to cementation, all dowels were cleaned with alcohol for 1 min and a layer of a silane solution (Silano prosil; FGM Prod. Odontológicos Ltda., Joinville, SC, Brazil) was applied with a microbrush tip (KG Sorensen Ind. Com. Ltda., Barueri, SP, Brazil) for 1 min, according to the manufacturer’s instructions. The dowels were cemented with self-adhesive resin cement (Rely-X U100; 3M/ ESPE, St. Paul, MN, USA) according to the manufacturer’s instructions. The cement was applied onto the surface of the dowels and into the orifice of the root canals. The dowels were inserted into the dowel space to full depth by using finger as pressure, and excess was immediately removed with dentin excavators. The teeth were placed in custom-made light-proof polymerization box that permitted photoactivation of the dual resin cement without interference of light. Photoactivation of the tooth-dowel set was performed for 40 seconds through the dowels, with the tip of a halogen light unit directly in contact with the coronal end of the dowels. All dowel-cemented roots were maintained by a moist gauze for 10 minutes and were then stored in individual, labelled light-proof plastic receptacles containing 10 mL of water deionized at 37⁰C for 24 hours.

After that, the dowel-cemented roots were fixed with autopolymerizing acrylic resin (Duralay; Reliance Dental Mfg. Co., Place Worth, IL, USA) to the self-adjustable vertical metallic shaft of a delineator to ensure that the sectioning procedures would be done perpendicularly to the long axis. The dowel-cemented roots were sectioned at the coronal portion with a Diamond Wafering Blade 3” (76 mm; Buehler Ltd., Lake Bluff, IL, USA) in a low-speed cutting machine (ELQUIP, São Carlos, SP, Brazil) under distilled water cooling. In each root, the first cut done at 1 mm from the most coronal portion of the root was discarded and 6 slices with thickness of 1.00 ± 0.05 mm were obtained from each tooth at the coronal, middle and apical root thirds (2 slices per third).

Each slice was marked on its coronal side with an indelible marker, and the thickness of the slices was checked by using a digital calliper accurate to the nearest 0.001 mm (Mitutoyo Sul Americana, São Paulo, SP, Brazil). For the push-out tests, each specimen was placed with its coronal side faced down on the centre of a stainless steel support with a 3-mm-diameter opening, which was connected to a universal testing machine (Kratos Equipamentos Industriais Ltda., São Paulo, SP, Brazil). The push-out tests were performed at a crosshead speed of 0.5 mm/min and using a 1 KN load cell. The push-out jig was placed on the test machine. Care was taken to centre the 1.0 mm-diameter stainless steel push-out pin on the centre of the dowel surface, without stressing the surrounding dowel space walls. The load was applied to the apical side of the root slice to avoid any limitation of dowel movement due to dowel space taper. The peak force at the point of extrusion of the dowel from the slice was taken as the point of bond failure. Data recorded in kgf were transformed into MPa and analyzed by analysis of variance and Tukey’s multiple variation test (LSMEANS) in a split-plot design with a 6x2x3 factorial scheme. The GraphPad Prism software v. 5.0 for Windows (GraphPad Software, San Diego, CA, USA). A significance level of 5% was set for all analyses.

Results

A total of 540 root dentin slices were subjected to the micro push-out bond strength test. The push-out bond strength means (MPa) and standard deviations according to the groups and root canal thirds (irrigating solution) are presented in Table 2. The group treated with alcohol (18.5 MPa) and the group treated with EDTA (8.6 MPa) presented the highest and the lowest push-out bond strength means among all groups, respectively. ANOVA revealed statistically significant differences (p<0.0001) for the factor “irrigating solution”, but no significant differences were found among the root thirds (p=0.0591) (Table 3).

Pairwise comparisons of the groups using the Tukey’s test as well as comparison of the six experimental groups with the control group (saline) using the Dunnet’s test are summarized in Table 4. G5 presented the highest bond strength and differed significantly from all groups, including the control group. G1, G4 and G6 did not differ significantly from each other and none of them was significantly different from the control group. In the same way, there was no significant difference among G2, G3 and G4, and they did not differ significantly from the control group.

The statistical analysis revealed that only alcohol was capable of increasing significantly the bond strength of dowel to intracanal dentin. The graphic presentation of the push-out bond strength means and standard deviations are presented in Figure 1.

Figure 1 Push-out bond strength means and standard deviations in the experimental and control groups

Discussion

Endodontically treated teeth with great coronal destruction might require the use of intra-radicular dowels to increase the retention of the coronal restorative material (14).

Some solutions are applied into the dowel space prior to dowel cementation to promote for cleaning of root dentin, remove debris from instrumentation and provide additional decontamination. Intra-radicular adhesion involves great deal of challenge due to limiting factors for the establishment of an effective resin-root dentin bond. These factors include the difficult access to the pulp chamber and root canals; anatomic variations of the root dentin; smear layer formation during root canal instrumentation and dowel space preparation; probable influence of sealer composition on the luting agent used for dowel cementation; difficulty in polymerizing resin materials in extremely deep cavities; use of an weak etching agent, which would promote insufficient removal of smear layer; and influence inherent to dowel systems (15).

Dentin adhesion can be affected by the use of irrigating solutions and medications during the root canal treatment (1, 2). The influence of irrigating solutions used on root canal cleaning prior to cementation of fibre glass dowels was evaluated in the present study.

The group irrigated with NaOCl presented the lowest micro push out bond strength mean together with the EDTA group, differing significantly from the groups treated with alcohol, NaOCl/EDTA and polyacrylic acid. NaOCl can decrease the quality of the adhesion due to the presence of residual oxygen produced by this substance, inhibiting adhesive polymerization. However, this effect seems to vary according to the composition of the adhesive system (3, 12).

NaOCl also cause damage to the organic components of dentin, mainly, collagen. This action may influence the penetration of resin monomers through the demineralized structure (13). The use of EDTA alone has been shown to considerably decrease dentin microhardness (16), and may promote alterations in the inorganic matrix of root dentin, producing areas of demineralization (17). It has been reported that 15% EDTA at pH 7.3 after 5 minutes produced a 20- to 30-μm zone of demineralization, which is attributed to its chelating effect (18). A previous study compared the depth of demineralization of some commercial brands of acid etchants and found that the depth of demineralization varied between 0.5 and 5.8 μm, using an etching time of 15 s (19). Demineralization and deproteinization of dentin facilitate the penetration of resin tags into the dentin tubules and contribute to the higher adhesion forces observed when “wet bonding” technique was used¹². However, excessive demineralization caused by irrigation of root canals with EDTA or NaOCl should be avoided when self-etch adhesives are intended to be employed, since there is a decrease of adhesion with the use of these substances (12).

Since the self-adhesive sealer used in this study per se has the capacity of promoting micromechanical retention to root dentin, over-etching produced by EDTA could decrease bond strength. This is because the extent of demineralization is deeper than the diffusion and impregnation of cement monomers. If the infiltration depth is less than the demineralization depth, a zone of hydroxyapatite-depleted collagen fibrils is left exposed and unsupported and collapses, preventing adhesive infiltration into the demineralized dentin matrix and compromising bond strength. This naked collagen zone which is not impregnated with resin components can be unstable and susceptible to hydrolysis, thus compromising adhesion (20).

It has been shown that 5.25% NaOCl combined or not with EDTA decreases the bond strength, which is not observed when root canals are irrigated with 2% CHX solution and gel (21). The use of 5.25% NaOCl alone or combined with 17% EDTA can promote alterations in the collagen mesh¹⁷. In spite of this, in the present study, the groups in which NaOCl was used in association with EDTA presented higher push-out bond strength than the group in which NaOCl was used alone.

The CHX group presented intermediate bond strength means, which were lower than those of the control group (saline) though without statistical significance. Some studies have shown that disinfecting agents such as CHX did not promote alterations in the shear bond strength to dentin (22, 23). Phosphoric acid etching, used in some bonding protocols, can remove products left by the chemical substances used on root dentin treatment. In the present study, it could not be taken into consideration because the self-etch sealer does not require this step. Therefore, although some studies have reported that CHX does not interfere on adhesion, others have claimed that the application of 2% CHX negatively affects shear bond strength of adhesive systems to dentin (1, 24).

One of the great expectations of increasing of the adhesion to root dentin with self-adhesive cements relied on the pretreatment with polyacrylic acid, which would have the capacity of removing the smear layer and improving the adhesion pattern by cement penetration into dentin. However, in the present study, the group treated with this acid did not differ significantly from the control group, in spite of having higher bond strength values. The application of the polyacrylic acid produces a good visualization of the tubular structure, but dentin areas with smear layer and smear plugs can be left because the polyacrylic acid is little aggressive and does not have the capacity of removing the debris completely (25), though its smear layer removal capacity is considered efficient. As a weak acid, it cannot produce significant pulpal alterations (26), mainly because of its high molecular weight, which hinders its diffusion through the dentin tubules. The efficacy of polyacrylic acid can be related to its capacity of cleaning, producing adequate surface moistening and being chemically absorbed, promoting chelating effect (27). Because of its cleaning action, it was expected that the polyacrylic acid would optimize the self-etch action promoted by the cement, improving the adhesion, which, however, did not occur. This result might be attributed to a decrease of surface energy due to exposure of the mineral portion of Ca and P from dentin surface by the action of polyacrylic acid (28).

It is important to emphasize that, regardless of the higher or lower dowel-dentin push bond strength means; none of the groups treated with the different cleaning solutions differed significantly from the control group (saline), except for the group treated with alcohol. Therefore, it is assumed that the use of alcohol alone was capable of increasing the adhesion of the self-adhesive resin cement Rely-X U100 to the intracanal root dentin and that all other tested solutions (0.2% CHX, 17% EDTA, 11.5% polyacrylic acid, 5.25% NaOCl and NaOCl + EDTA) did not interfere with the adhesion.

The deepest areas of the dowel space, for example, may suffer influence from the difficulty of performing adequate photoactivation of the resin materials. Although previous studies have shown that adhesion in deeper thirds is lower than that in the coronal third (12), no significant difference among the root thirds was observed in the present study.

Conclusions

The use of 70% alcohol for cleaning of the root canal system increased the bond strength of the fibre glass dowel to the intracanal dentin walls; The push-out bond strength between the dowel and the root dentin was not affected by the root third (cervical, middle and apical).