INTRODUCTION

Despite biomechanical advantages, a relatively high rate of failure has been demonstrated in fiber post-retained restorations.^1,2 However, fewer irreparable failures have been associated with fiber posts than with traditional metallic cast posts.³ Post debonding is one possible failure caused by the complexity of bonding to root canals.^4,5 Another commonly reported failure is fracture of the post and/or core.⁶ In addition, debonding increases the risk of fracture.⁷ Thus, proper bonding of resin cement to the post and the dentin of the root canal is essential for improving the success of fiber post-retained restorations.

Most failures of post-retained restorations result from debonding, and the cement/dentin interface is the weakest link.^8‐10 However, bonding between the fiber post and core buildup composites is also essential to the success of the restoration. The organic component of fiber posts is generally composed of epoxy resin, which has a high degree of conversion and is highly crosslinked.^11,12 Thus, this polymer matrix is virtually unable to react with the monomers of resin cements. Among the various treatments proposed to improve adhesion to the fiber-post surface, silanization has been assessed in several laboratory studies.^13,14 However, silane coupling agents, commonly used in dentistry, react with glass fibers and may not bond well to the organic component.¹⁵ Therefore, treating the post in order to roughen the surface and expose the glass fibers has been suggested; it allows for micromechanical interlocking of the adhesive/cement with the post.¹² In addition, chemical bonding may be established by using silane.^12,15‐17

Among chairside procedures, post-surface treatment with hydrogen peroxide (H₂O₂) is a simple and effective method to improve the bonding of adhesive/cement to the fiber post. Selectively, H₂O₂ removes the surface layer of epoxy resin and exposes the glass fibers,^15,18 resulting in chemical bonding between the adhesive/cement and the exposed fibers due to the ability of silane to couple with hydroxide-covered surfaces (such as glass fibers).^19,20 A previous study demonstrated that immersing a fiber post in a solution of 24% H₂O₂ improves bond strength to the post in one minute, which is a feasible clinical time.¹⁸ Clinically, however, the application of H₂O₂ over the post is more favorable than immersion. Furthermore, limited information is available regarding the use of high-concentration bleaching agents on the post etching commonly available in dental offices.

Thus, the aim of this study was to evaluate the effect of concentrations of H₂O₂, including a high-concentration bleaching agent, and mode of use on the bond strength between resin composite and glass fiber posts. In addition, the surface of fiber posts was evaluated using scanning electronic microscopy (SEM). The null hypothesis tested was that neither the H₂O₂ concentration nor the mode used would affect bond strength.

METHODS AND MATERIALS

Microtensile Bond Strength Test

A glass fiber-reinforced epoxy post system (White Post DC3, FGM, Joinvile, SC, Brazil) was used. Polyvinylsiloxane impression material (Aquasil, Dentsply DeTrey, Konstanz, Germany) molds were obtained to standardize core buildup on the posts. Two plastic plates (10 mm long × 4 mm wide × 1 mm thick) were attached along the post surface; each plate was placed opposite the other, and both were located in the same plane using cyanoacrylate adhesive. The post attached to the plates was centrally positioned into a polyvinyl chloride (PVC) tube (20 mm inner diameter × 15 mm high) (Figure 1A), and the impression material was placed into the tube. The post attached to the plates was removed after polymerization of the polyvinylsiloxane (Figure 1B), leaving a space to insert the post and resin composite (Figure 1C).

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A commercially available 35% H₂O₂ bleaching agent (Whiteness HP Max, FGM) or a 24% H₂O₂ solution (Dinâmica, São Paulo, SP, Brazil) was used to etch the posts. The fiber posts (n=10) immersed in the solution/bleaching agent were placed in Eppendorf tubes, or the solution/bleaching agent was applied over the post surface using a microbrush. All etching protocols were performed for one minute. After etching with H₂O₂, the posts were rinsed with distilled water and air-dried. Ten posts were rinsed only with water and used as a control. A silane-coupling agent was applied in a single layer on the post surfaces and gently air-dried after 60 seconds. The adhesive resin Scotchbond Multi-Purpose Plus (3M ESPE, St Paul, MN, USA) was applied over the post surface and light-cured for 20 seconds using a light-emitting diode light-curing device (Radii-Cal, SDI, Bayswater, Victoria, Australia). The post was positioned into the corresponding space of the mold, and a microhybrid resin composite (Opallis, FGM) was incrementally inserted to fill the mold, with each increment being light-cured for 40 seconds. The samples were stored for 24 hours under 100% humidity conditions.

The specimens were serially sectioned using a low-speed saw (Extec, Enfield, CT, USA) to obtain 1-mm thick sections. The beams were attached to the flat grips of a microtensile testing device and tested in a mechanical testing machine (EMIC DL 2000, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/minute until failure. The average value of the beams in the same specimen was recorded as the microtensile bond strength (MPa) for that specimen. Data for the experimental conditions were analyzed using two-way analysis of variance, followed by the Tukey's post-hoc test (α=0.05). The factors evaluated were concentration of H₂O₂ and mode of use. The Dunnet's test was used to compare experimental conditions to control conditions (α=0.05).

RESULTS

Microtensile Bond Strength Test

The statistical analysis showed a significant effect for concentration of H₂O₂ (p<0.001), application mode (p=0.02), or the interaction between the factors (p=0.02). The results are shown in Table 1. For immersion of the post into H₂O₂ solutions, there was no difference between concentrations, although using 35% H₂O₂ resulted in higher bond strength when the solutions were applied over the post surface. The mode of use did not affect bond strength for 35% H₂O₂. Immersion into the solution resulted in higher values when 24% H₂O₂ was used. Except for the application of 24% H₂O₂, the other experimental conditions resulted in higher bond strength than the control (11.0 ± 4.1 MPa).

Table 1: Means (SD) for Tensile Bond Strength (MPa)^a

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Surface Topography

Representative SEM images of each experimental condition and the control are shown in Figure 2. Figure 2A shows epoxy resin covering the glass fibers of the post without treatment and some areas with exposed fibers and flaws. The application of 24% H₂O₂ on the post surface did not effectively expose the glass fibers (Figure 2B). More exposed fibers were observed when the post was etched by immersion in H₂O₂ (both concentrations) or when 35% H₂O₂ was applied (Figures 2C through 2F).

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DISCUSSION

Several studies have evaluated chairside protocols to allow a chemo-mechanical bonding of resin-based material to fiber post surfaces.^{12,13,15‐18,21‐23} Sandblasting procedures with alumina particles^21,22 or aluminum oxide particles modified by silica²³ (Rocatec System, 3M ESPE) increase the roughness of the post surface and permit the mechanical bonding of resin-based materials to the fiber post. The use of Rocatec also allows chemical bonding by silanization due to the presence of the silica-coated surface.²³ However, these devices are not available in some dental offices. Using H₂O₂ over the post surface has also demonstrated mechanical-chemical bonding of resin-based material to the fiber post.^15,18 Frequently, H₂O₂ is available in dental offices as a result of bleaching procedures.

In the current study, fiber-post etching was performed for one minute, which is a feasible clinical time. Furthermore, a previous study demonstrated that immersing a post into 24% peroxide for one minute effectively improves the bond strength of resin composite to the post.¹⁸ However, for this period, the effect of the mode of use of H₂O₂ on the bond strength of resin composite to fiber post was concentration dependent. Thus, the null hypothesis tested was rejected. Only in the case of 24% H₂O₂ did the mode of use affect bond strength, whereas immersing the post into the solution resulted in higher values. The application of 24% H₂O₂ failed to effectively improve bond strength, which showed similar values to the control (without treatment). On the other hand, the mode of use for 35% H₂O₂ did not affect bond strength nor did it improve values compared with the control.

The SEM analysis of the surface topography showed that the 24% H₂O₂ application did not increase the amount of exposed glass fibers. However, all experimental conditions increased the exposure of glass fibers without damaging them. It has been demonstrated that H₂O₂ solutions can partially dissolve epoxy resin, thus exposing the fibers. This dissolution is related to an electrophilic attack of H₂O₂ on the cured secondary amine.²⁴ The spaces created between the fibers provide conditions for the micromechanical interlocking of the resin adhesive with the post.^15,18 Furthermore, the exposed fibers bond chemically to the adhesive through the silane agent.

Based on the results of the current study, it is reasonable to assume that the ability of H₂O₂ to affect fiber post etching is related to the concentration and the application mode. The etching effect of H₂O₂ is based on oxidation of the post surface, thus breaking epoxy resin bonds.^24,25 Although the oxidizing effect of H₂O₂ depends on the concentration of the radicals that are released, it dissociates and releases oxygen-free radicals, hydrogen-free radicals, water, and peridroxyl.^26,27 A higher H₂O₂ concentration results in a higher oxidizing effect. This explains why 35% H₂O₂ is effective in improving bond strength independent of the application mode.

To the contrary, 24% H₂O₂ increased the bond strength of resin composite to fiber post compared with the control only when the post was immersed in the solution. The application of 24% H₂O₂ resulted in similar values to those observed by the control. A possible explanation is that a single layer of peroxide applied on the post only reacted superficially with the epoxy resin once there was no replacement of radicals by oxidation. This replacement probably occurred when the post was immersed in 24% H₂O₂ solution, thus increasing the potential for etching. Based on the results of this study, the application of high-concentration bleaching agents using a microbrush, which is frequently used in the dental practice for dental bleaching, is a feasible clinical procedure to improve the bond strength of resin composite to glass fiber posts.

It is important to emphasize that the current study only evaluated the bonding of a resin composite to a fiber post, simulating a core buildup. In this instance, an adhesive is applied over the post before the composite is inserted. Further studies are necessary to confirm that this protocol is also effective for resin cements applied directly over etched posts without using an intermediate adhesive layer. Differences in the composition and viscosity of materials can alter the results.²³ Additionally, high stress generated by shrinkage of the resin cement during fiber-post cementation due the high C-factor of the root canal hinders bonding of the cement to the post.²⁸ Stability of the bonding obtained with post etching, using a commercial high-concentration bleaching agent, must also be evaluated in future studies. It has been demonstrated that thermal cycling can alter the bond strength of resin-based material to fiber posts.²⁹

CONCLUSIONS

Within the limitations of the current study, the following conclusions can be made:

• The mode of use of peroxide affected bond strength of the resin composite only when 24% H₂O₂ was used, where immersion resulted in the highest mean.

• The use of a high-concentration bleaching agent over the fiber post improved bond strength of the resin composite to the post surface.