Fiber posts provide acceptable esthetic, adhesion to dentin and suitable distribution of functional stresses.1,2 The factors affecting the longevity of a post and core restoration are directly related to core material, post type and bond strength of the post to the core.3,4 It has been suggested that the post surfaces should undergo special treatments in order to improve retention. A large number of studies have been evaluated various treatment procedures by using chemical and mechanical techniques to improve the bond between quartz fiber posts and core materials and tooth structures.5-7 It has been reported that mechanical techniques including the use of air-abrasion with silica particles or sandblasting improve the bond efficacy between fiber posts and composite resin cores, but decrease post fitness within the root canal, because particles inflict damage on fibers and post shape modifies.8-15 Chemical treatments lead to a greater mechanical retention by roughening the post surface.16 Some of the chemical solutions used for post surface treatments are potassium permanganate, hydrofluoric acid, silane and hydrogen peroxide.17-19

Hydrogen peroxide dissolves the resin matrix and exposes the fibers, resulting in an improved adhesion between the post and composite resin.2 One of the most important problems of hydrogen peroxide use is the accumulation of free oxygen radicals on the micro porosities of post surface due to hydrogen peroxide degradation. These free radicals can interfere with adhesion between the post and the core. Therefore, with the use of agents which can neutralize the effect of free oxygen radicals it might be possible to overcome this problem. A large number of research studies have been performed on antioxidants.7,20,21

Antioxidants donate one of their electrons and stabilize free radicals.22 Ascorbic acid has antioxidant properties and acts as a reducing agent.23-25 Therefore; it is possible that free radicals on the post surface are removed by ascorbic acid. On the other hand, some agents can reduce the adverse effects of free oxygen radicals by removing and evaporating them from the surface; some of these agents include ethanol and acetone.26,27 Since sufficient and definite data is not available in relation to the effects of these agents on bond of quartz fiber post pre-treated with hydrogen peroxide to resin composite core, the purpose of the present study was to assess and compare the effect of ascorbic acid, acetone and ethanol on the microtensile bond strength between fiber posts treated with hydrogen peroxide and composite resin cores.

Twenty four N-2 white quartz fiber posts (DT Light-Post Radiopaque, RTD, Grenoble, France) were used in this study. The maximum diameter of each post was 1.8 mm. The posts were immersed in a solution of 24% hydrogen peroxide for 10 minutes at room temperature. Subsequently, the posts underwent a two-minute rinsing procedure with running water. Finally, the posts were dried under a gentle air. The posts were divided into 4 groups (6 in each group) based on antioxidant agents used. Group 1 was considered as the control group (without surface treatment). In Group 2, post surfaces were treated with fresh 10% ascorbic acid solution (Sigma Chemical Co., St. Louis, MO, USA) for 5 minutes. In Group 3, posts surfaces were treated with 70% ethanol solution (Merck KGaA, Germany) for 5 minutes. In Group 4, posts surfaces were treated with 70% acetone solution (Merck KGaA, Germany) for 5 minutes. The solutions were applied by brush on surfaces and refreshed every 1 minute. The post surfaces underwent a one-minute rinsing procedure under running water to remove excess agents used and then were gently air-dried. Subsequently, a core build-up procedure was carried out using a light-cured flowable composite resin (Elite Flow, Bisco, Inc, Shaumburg, IL, USA). The core was built up according to Gorracii's procedure.28 The posts were placed upright on a glassslab and secured using sticky wax. A cylinder-shaped matrix was fabricated, which measured 10 mm in diameter; the length of the cylinder was the same as that of the non-tapering part of the post, measuring almost 6 mm. Each post was surrounded by the cylinder in a manner in which the post was placed exactly at the center. Flowable composite resin was placed on the post using the incremental technique; each increment had 1 mm thick and was light-cured, separately. A halogen light-curing unit (Degulux, Degussa Dental, Hanau, Germany) was used for the curing procedure. Before removing the matrix, an additional 40-second light-curing step was carried out from the bottom of the cylinder. All the samples were kept in distillated water at 37℃ for a 24-hour period. A blade was used, under water cooling, in a sectioning machine (IsoMet, Buehler LTD., Lake Bluff, IL, USA) to prepare two sticks in a direction along the post long axis, followed by sectioning at right angle to the post long axis. The final sections consisted of the post sandwiched between composite resin core laterally (Fig. 1). The sections measured 1 ± 0.1 mm in diameter as determined by a digital measuring instrument (Mitutoyo CD15, Mitutoyo Co., Kawasaki, Japan). Two sticks were chosen from each post, which were fixed on the two freely gliding portions of a platform mounted on the microtensile test machine (EZ Test, Shimadzu Co., Kyoto, Japan). Failure rate was 0.5 mm/min. This design was used for measuring pure microtensile forces. With dividing the force at failure (N) to the bonding surface area (mm²) were calculated bond strength in Mpa.

Failure patterns of the specimens were evaluated under a stereomicroscope (Nikon Eclips E600, Tokyo, Japan) at a magnification of ×20 and scored as cohesive (fracture within the post or core material), adhesive (fracture at post/core interface), or mixed (a combination of adhesive-cohesive). Two fractured samples were randomly selected from each group to be sputter-coated with gold-palladium for observation under a SEM (JSM-5310, JEOL, Tokyo, Japan) using different magnifications. One-way ANOVA and a post hoc Tukey test were used for statistical analysis. Statistical significance was defined at α=.05.

Means and standard deviation of microtensile bond strength values of the specimens are presented in Table 1. Results of one-way ANOVA (Table 2) demonstrated significant differences between the groups (P<.001). In addition, paired comparisons of groups using a post hoc Tukey test showed statistically significant differences between all groups (P<.001). Evaluation of fracture patterns under a stereomicroscopic showed adhesive failure in all the specimens. Table 3 indicates modes of failure within different groups. SEM micrographs indicated failure in fiber post-composite resin core interface in all the groups. However, in Group 4, there was more epoxy resin values on fiber post surfaces compared to Group 1 (Figs. 2-4).

In the present study, the effect of 10% ascorbic acid, 70% ethanol and 70% acetone on microtensile bond strength of fiber post to composite core was evaluated and compared subsequent to treating the post surface with hydrogen peroxide.

The results of the present study demonstrated that bond strength in groups 2, 3, 4 was significantly different from that of the control group. Higher bond strength in the ascorbic acid group compared to the control group might be attributed to the antioxidant effect of this agent, inhibiting the effect of residual free oxygen radicals derived from hydrogen peroxide degradation on post surfaces. Ascorbic acid plays a role as a reducing agent.22-25 Electron transfer from ascorbate to free oxygen radicals can result in the formation of semi-dehydroascorbate, which is a more stable material.

Generally, three major mechanisms of action are considered for antioxidant materials: Breaking or scavenging of chains, catalyzing of molecules of oxidant substrate, and preventing Fenton reactions and sequestering transition metal ions.29 Ascorbic acid had a function similar to the first group. However, this finding of the study supports the results of previous studies which have shown that ascorbic acid and related salts reverse the reduced bond strength after bleaching.30-32

Higher bond strength in G3 and G4 compared to the other two groups is probably attributed to higher surface wetting compared to their effects on removing surface free oxygen radicals. A recent study showed that dentin surfaces treated with acetone or ethanol become dehydrated and, therefore, might provide a suitable substrate for an adhesive procedure with hydrophilic self-etch adhesives, including one-bottle self-etch ones.31

This result is consistent with the results of a study by Hashimoto et al.33 Similar studies have been performed in this field on tooth structure, but there is no evidence available for application of antioxidants on fiber posts. The results showed that acetone can increase bond strength even more than ethanol, which might be attributed to more evaporating power of acetone in comparison to other materials used. Previous studies have shown differences in the capacity of either experimental solvents or commercial formulations to evaporate. Nihi et al. reported that the acetone-based materials presented a greater evaporation capacity compared to ethanol-based products.22 Adhesive fracture was the predominant pattern in all the groups in stereomicroscopic assessment, indicating that post core interface still is the weakest area. SEM observations confirmed stereomicroscopic results. Although, SEM views showed that in Group 4, there were more epoxy resin values on fiber post surfaces compared to Group 1, which might explain the better adhesion of fiber post to composite resin core in Group 4. However, it is necessary that more accurate techniques were applied for observation of post-composite resin interface. It is obvious that reinforcement of the adhesion of post to resin is required for better services of restorations. The use of microtensile test for bond strength measurement is advised because of its reliable results for specimens with a surface area of 1 mm². It seems that flowable composite resins, due to their low viscosity, are a good choice for core build-up because they provide a more homogenous penetration and void-free surface for bonding. In order to better simulate the oral cavity conditions, it is suggested that fatigue tests be carried out along with the use of antioxidants. In addition, in the present study, one fiber post type and composite resin material were evaluated; it is suggested that future studies evaluate other fiber post types and composite resin core materials. Clinical trials on bond strength are yet to be carried out.

Within the limitations of the present study, it is concluded that application of ascorbic acid, ethanol and acetone can increase microtensile bond strength between fiber posts treated with hydrogen peroxide and composite resin cores. The use of acetone resulted in a higher increase in bond strength than the other materials used.