Evaluation of post-surface conditioning to improve interfacial adhesion in post-core restorations.

AIM: To examine the influence of different post-surface treatments on the interfacial strength between epoxy resin-based fiber posts and methacrylate-based resin composites that are employed as core build-up materials.
MATERIALS AND METHODS: Forty clear posts were divided into four groups of 10 each. The different surface treatments used were etching with alkaline potassium permanganate, 10% hydrogen peroxide, 37% phosphoric acid, and silanization alone. After etching and thorough rinsing, a single layer of silane was applied to the post surface. Then the post was placed in a rectangular plastic matrix and core bulid-up was done using Multi Core, a dual cured composite resin. A slab of uniform thickness, with the post in the center and the core build-up composite on either side was created. The specimens were cut so as to obtain microtensile sticks that were loaded in tension at a cross-head speed of 1 mm/min until failure. The statistical analysis was performed using two-way ANOVA and the paired T test for post-hoc comparisons.
RESULTS: The results achieved with potassium permanganate had a significant influence on microtensile interfacial bond strength values with the tested material.
CONCLUSION: Surface chemical treatments of the resin phase of fiber posts enhance the silanization efficiency of the quartz fiber phase, so that the adhesion in the post/core unit may be considered as a net sum of chemical and micromechanical retention.

INTRODUCTION

Caries and subsequent endodontic treatment may lead to a significant reduction in the capability of a tooth to resist different conditions to which it is exposed in the oral environment. Several methods have been proposed to overcome the problems of corono - radicular stabilization, with post-and-core system being the most common treatment. Fiber posts are widely used to restore endodontically treated teeth that have insufficient coronal tooth structure to retain a core for the definitive restoration.[1]

The clinical success of a post and core restoration depends on the composite selected, and also on the quality of the post - core interface, where materials of different compositions are in intimate contact.[2] Studies on fiber-reinforced composite materials have led to the modification and improvement of the properties of the interface between the resinous matrix and the fibers.[3] In many cases, interfacial failure was attributed to chemical incompatibility or the plasticizing phenomenon where impurities penetrate the interface.[4] In dentistry, the durability of a composite core restoration depends on the formation of a strong bond between the resin composite and the residual dentin as well as between the composite and the fiber post, enabling the interface to transfer stress under functional loading.

Surface treatments are common methods for improving the general adhesion properties of a material, by facilitating chemical and micromechanical retention between different constituents. Advances in adhesive dentistry have resulted in the development of surface conditioning techniques for natural substrates (i.e. enamel and dentin) and restorative materials.[5] The latter includes the use of acids to condition the surface of non-noble alloys or ceramics to partially dissolve the substrate and generate a rough surface to enhance adhesion. With respect to post/core restorations, most studies were designed for improving the performances of these restorations via enhancing the mechanical properties of the composite core build-up materials.[6] However, from acid etching and silanization of the fiber phase of these posts, little is known on how the resin phase of the fiber posts may be improved for interfacial adhesion.[7] The application of a silane coupling agent as adhesion promoter in fiber post/core units was investigated.[8] Nevertheless, adhesion of composites to fiber posts was still inferior when compared with the results achieved on dental substrates.[7] This is probably due to the absence of any chemical interaction between methacrylate-based resin composite and the epoxy resin matrix of fiber posts.

The present study is aimed at examining the influence of different post-surface treatments on the interfacial strength between epoxy resin-based fiber posts and methacrylate-based resin composites that are employed as core build-up materials. The tested null hypothesis was that different types of post surface treatment do not affect the interfacial strength between fiber posts and composite core build-up materials.

MATERIALS AND METHODS

Forty clear post-taper plus (Dispodent, India) with a diameter of 2.5 mm were used in this study. The clear posts are made of unidirectional pre-tensed quartz fibers bound in an epoxy resin matrix. Four different chemical treatments were tested for their efficacy on etching the resin phase of the fiber post- surface for better bonding.

Group I : Etching with alkaline potassium permanganate for 10 min.

Group II : Etching with 10% hydrogen peroxide for 10 min.

Group III: Etching with 37% phosphoric acid for 5 min.

Group IV: Silanization of the post-surface for 60 sec without chemical treatment of the resin phase (control group).

The etching procedure of group 1 consists of three successive steps:

Initially, the post was immersed in a conditioning solution (60 vol% of methyl - pyrrolidone in deionized water) for 3 min at 50–60°C. Then the post was etched in an alkaline potassium permanganate solution (20 vol% in deionized water) for 2 min at 70–80°C. Finally, it was immersed in a sulfate neutralizer solution (10 vol% in deionized water) for 5 min at 40–50°C. Each surface-treated post was rinsed with deionized water for 3 min, followed by air-drying.

The fiber posts in group 2 were immersed in 10% hydrogen peroxide for 10 min at room temperature and then rinsed with deionized water.

The posts in group 3 were immersed in 37% phosphoric acid for 5 min at room temperature and then rinsed with deionized water.

Before performing the composite core build-ups, an additional exhaustive rinsing procedure was performed. All the posts were ultrasonically cleaned for 10 min in deionized water, immersed in 95% ethanol and dried with an air stream.

A single layer of silane coupling agent (Monobond -S, Ivoclar – Vivadent, Liechtenstein) was then applied with a brush to the post-surface of each of the three experimental and one control group, and gently air-dried after 60 sec, according to manufacturer's recommendations.

Core build-up was performed using dual cure composite core material (MultiCore Flow, Ivoclar Vivadent, Liechtenstein). Each post was positioned perpendicularly on a glass slab and maintained with a needle holder at the apical end. A rectangular plastic matrix was placed around the post and adjusted so that the post would be exactly in the middle. The matrix was 4mm in diameter. In height, the matrix extended only to the cylindrical portion of the post (10mm in Clear Posts) and an incremental technique was followed to build up the core. Each 2mm increment of the composites was cured for 40 sec with a halogen light curing unit with an output of 600 mW/cm2. The material was polymerized directly from the upper side of the matrix. The matrix was subsequently removed after being filled completely with polymerized composite. This resulted in a rectangular slab of resin composite that was built up around the fiber post. The bottom side of the slab that was previously in contact with the glass slab was light cured for an additional 20 sec to ensure complete polymerization of the composite material.

After storing in distilled water for 24 h, each bonded specimen were mounted on the acrylic block and held in the holding device of a slow-speed diamond saw. (Leica1600, Buehler, Lake Bluff, USA). A slab of uniform thickness, with the post in the center and the core build-up composite on either side was created. Each slab was then serially sectioned to obtain 2-3 beams of 1mm in thickness. Each beam was secured to the metal jig with cyanoacrylate adhesive, and subjected to a tensile load at a cross head speed of 1mm/min until failure, in a universal testing machine (Autograph 4000, USA). Interfacial strength was calculated using the mathematical formula previously described by Bouillaguet et al.[9] The tensile bond strength of each slice was calculated as the force at failure divided by the bonded cross-sectional surface area and expressed in MPa.

RESULTS

The results of the mean microtensile bond strength values of control and experimental groups are presented in Table 1. The values were statistically analyzed using two –way ANOVA test and paired t-test.

Table 1

Mean values of the microtensile bond strengths measured in all experimental groups

Statistical analysis revealed that the post-surface treatment procedure had a significant influence on microtensile bond strength (P < 0.05). More precisely, the post-core strengths achieved following pretreatment with potassium permanganate and hydrogen peroxide (Groups 1 and 2) were comparable and significantly higher than those of Groups 3 and 4 in which the post surface had been treated with phosphoric acid and silane (control group), respectively. In the control group (Group 4), the lowest post-core strength was achieved, and the difference was statistically significant.

DISCUSSION

The restoration of endodontically treated teeth with fiber-reinforced post systems has been drawing the attention of a growing number of clinicians.[10] The fiber posts with the newer generation bonding systems provide an integrated tooth, post and core bonded restoration. This results in a metal free, physiochemically homogeneous material often referred to as the monobloc type of restoration.[11]

Adhesive post restorations rely for their retention on the strength of the bonds established at different interfaces. Among them, the interface between root dentin and resin cement has been the object of several studies, involving both bond strength tests and microscopic investigations.[8] Although it is the root cement bond which represents the weakest link, the post-cement and post-core interfaces also deserve attention.

It has been showed that the bond strength achieved with the fiber post and the core is not sufficient enough to withstand the occlusal stresses. So, the surface pretreatment of fiber post was commonly employed to improve the adhesion property of the material.[12]

Epoxy resin etching techniques are commonly employed in industrial and laboratory fields. It has been suggested that the use of chemical surface treatments influences the interfacial bond strength between fiber posts and core build-up materials.[1314]

Potassium permanganate is usually applied in an industrial process designed for conditioning epoxy resin surfaces for metal plating of printed circuits board.[15] This treatment, commonly defined as desmearing, is a process designed to remove the smeared epoxy resin byproducts from copper surfaces, thus providing superior topography for increased adhesion of direct metallization or electroless copper.[12]

In this study, the posts in Group I were immersed in a conditioning solution (60 volume% of methyl-pyrrolidone in deionized water) for 3 min at 50–600 C. This initial step enhances the removal ability of permanganate with the epoxy resin being swollen and its surface chemical structure altered. Following this, etching was done to oxidize and remove the epoxy resin matrix previously degraded by the solvent. Then the post was immersed in deionized water to reduce and neutralize the excess permanganate and clean the surface of the post.

Hydrogen peroxide is frequently used in immunological electron microscopy to partially dissolve the resin surface of epoxy resin-embedded tissue section, and expose tissue epitopes for immunolabeling enhancement.[16] The etching effect of hydrogen peroxide depends on its capacity to partially dissolve the resin matrix, breaking epoxy resin bonds through a mechanism of substrate oxidation. A similar hydrogen peroxide etching procedure is employed to improve the micromechanical retention between the epoxy resin matrix of fiber posts and methacrylate-based resin composites. According to Vano and Monticelli et al,[17] there is no statistically significant difference in results achieved between the use of 24% H 2 O 2 and 10% H2O2.

Phosphoric acid has been used for etching the tooth surfaces in concentrations ranging from 30 to 50%. Generally, 37% phosphoric acid is preferred for acid etching the tooth surface.[18] In this study also, 37% phosphoric acid was used for conditioning the post surface.

Silane coupling agents are hybrid organic-inorganic compounds that can mediate adhesion between inorganic and organic matrices through an intrinsic dual reactivity.[19] The silane coupling agent most commonly used for dental applications is a pre-hydrolyzed monofunctional methacryloxypropyl trimethoxysilane (MPS) diluted in an ethanol-water solution with a pH between 4 and 5. Monobond - S is a pre-hydrolyzed single component silanizing agent and contains 1wt% of 3-methacryloxypropyl trimethoxysilane (3-MPS) in an ethanol/water-based solvent.

Epoxy polymers exhibit a high degree of conversion and highly cross-linked structures.[20] Since MPS silane does not bond well with the epoxy matrix, the bond strength between the epoxy resin phase of the fiber post and the methacrylate-based resin composite is not enhanced.[21] With the removal of the superficial layer of epoxy resin via chemical treatment, more exposed quartz fibers in terms of surface area are available for reacting with the silane molecules. The increased chemical union between the silanized quartz fibers and the methacrylate-based core material would significantly improve the interfacial bond strength. When the surface of the post is etched and rinsed, a more reactive surface is thus generated for both chemical and micromechanical retention.[17]

The core build up material used in this study was MultiCore (Ivoclar Vivadent). MutiCore is a dual cure, radiopaque composite containing fluoride fillers that demonstrates excellent mechanical properties for core build-ups. It cures chemically without the use of light. Light curing is optional.

In this study, the core build up was extended only to the cylindrical portion of the post (about 10mm in clear posts), since for appropriate cutting of the micro tensile specimens, it is desirable that the post diameter be constant throughout the post length.[8] The micro tensile technique in this study was adopted as it is currently regarded as the most reliable method for bond strength testing. The small size of the specimens is the condition for a more uniform distribution of the stress on loading, which limits the chance of cohesive failures, thus allowing for an accurate assessment of the interfacial bond strength. In particular, the non-trimming variant of the technique was chosen as there are indications in the literature that it is less aggressive than the variant which involves trimming the specimen to an hourglass shape at the bonded surface.

According to the results of this study, the surface pretreatment of fiber post with potassium permanganate (Group I) showed the highest bond strength values among all the experimental methods. Thus, the null hypothesis is rejected.

The probable reason may be that the etching procedure affected the superficial part of the epoxy resin matrix of the fiber posts, leaving exposed smooth quartz fibers intact. It may also be due to the use of a specific neutralizing solution after conditioning in order to ‘clean’ the fibers from residual MnO2- ions, which improves the hydrophilicity of the post surface. So, an increased deposition of silane may take place in the presence of a surface with more hydroxyl groups.

Surface pretreatment of post with hydrogen peroxide (Group II) showed the second highest bond strength values. This may be due to the fact that by removing a surface layer of epoxy resin, a large surface area of exposed quartz fibers is available for silanization. The spaces between these fibers may provide additional sites for micromechanical retention of resin composites.

The bond strength values obtained with the surface pretreatment of fiber post with phosphoric acid (Group III) was the least among the experimental groups, even though higher than the control group (Group IV). This may be due to less removal of superficial layer of epoxy resin thereby leading to small amount of micromechanical retention.

CONCLUSIONS

Within the limitations of this study, it can be concluded that

Surface chemical treatments of the resin phase of fiber posts enhance the silanization efficiency of the quartz fiber phase, so that the adhesion in the post/core unit may be considered as a net sum of chemical and micromechanical retention.

Surface pretreatment of the fiber posts with potassium permanganate significantly enhanced the bond strength between the fiber post and core when compared to other groups tested.