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 Table of Contents  
Year : 2016  |  Volume : 13  |  Issue : 3  |  Page : 127-132

The effect of plasma surface treatment on push-out bond strength of fiber-reinforced posts to root canal dentin

1 Department of Dental Biomaterials, Faculty of Dentistry, Tanta University, Tanta, Egypt
2 Department of Endodontics, Faculty of Dentistry, Tanta University, Tanta, Egypt

Date of Submission25-Mar-2016
Date of Acceptance30-May-2016
Date of Web Publication29-Sep-2016

Correspondence Address:
Usama M Abdel-Karim
Department of Dental Biomaterials, Faculty of Dentistry, Tanta University, Tanta
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/1687-8574.191433

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Objectives: This study evaluated the effect of surface treatment with plasma or sandblasting on push-out bond strength of fiber post bonded to root canal dentin.
Methods: Crowns of 50 teeth were sectioned and their root canals were instrumented, obturated and received post-space preparations. The roots were equally divided into five groups according to post surface treatment. Posts were surface treated with oxygen plasma in Group 1 and argon plasma in Group 2. Post surfaces were treated with sandblasting in Group 3 and silanized after sandblasting in Group 4. In control group (Group 5), posts were cemented to the canal wall without surface treatment. After post cementation, push-out test was used to evaluate the bond strength of fiber post bonded to root canal dentin. Data were statistically analyzed using Kruskal-Wallis and Mann-Whitney tests (α = 0.05).
Results: Oxygen plasma revealed the significantly highest push-out bond strength of all groups (P < 0.01). No significant difference was found between argon plasma and control group. Sandblasting significantly improved the bond strength compared to control group. The difference between sandblasted and sandblasted-silanized groups was not significant.
Conclusions: Oxygen plasma treatment of fiber post recorded the highest bond strength to root canal dentin followed by sandblasting. Silanization and Argon plasma did not play a significant role in enhancing bond strength of fiber posts to canal dentin. Clinical significance: Oxygen plasma surface treatment offered the highest resistance to displacement of fiber posts. Clinical trials involving plasma technique are indicated.

Keywords: fiber posts, plasma, push-out bond strength, sandblasting, silanization

How to cite this article:
Abdel-Karim UM, Alhadainy HA. The effect of plasma surface treatment on push-out bond strength of fiber-reinforced posts to root canal dentin. Tanta Dent J 2016;13:127-32

How to cite this URL:
Abdel-Karim UM, Alhadainy HA. The effect of plasma surface treatment on push-out bond strength of fiber-reinforced posts to root canal dentin. Tanta Dent J [serial online] 2016 [cited 2023 May 27];13:127-32. Available from: http://www.tmj.eg.net/text.asp?2016/13/3/127/191433

  Introduction Top

Posts and cores are commonly used in endodontically treated teeth when significant coronal tissues are lost. Fiber posts, bonded to root canal dentin with resin cements, are increasingly used for this purpose [1]. Adhesion quality of fiber posts may be affected by conditions of canal wall such as degradation of dentin collagen, density of dentinal tubules, irrigating solutions, and eugenol-containing materials. Factors such as fluidity of the bonding materials and surface characteristics and composition of posts are determinant for adhesion quality of fiber posts [2]. Therefore, dislodgement of fiber-reinforced posts was reported to occur most frequently at the post–cement interface [1].

Mechanical and chemical surface treatments of fiber-reinforced posts have been tried to overcome post–cement adhesion failure. Mechanical surface treatment such as sandblasting involves spraying a stream of aluminum oxide (Al2O3) particles against the post surface under high pressure. It relies on particles abrasion with different particle sizes to remove superficial layer resulted in roughened surface that provides some degree of mechanical interlocking with the adhesive [3]. Airborne-particles abrasion was reported to significantly improve retention of fiber posts cemented with dual-polymerizing resin cements [4]. Enhancing a mechanical interlock in post–cement interface was also attempted through exposure of the post's fibers with sandblasting followed by surface treatment with a silane coupling agent [5]. However, sandblasting with Al2O3 particles [6] or chemical etching with hydrofluoric acid [7] may cause adverse effects on strength and fit of the fiber post when treatment is performed over a long period of time [6],[7].

Surface modifications using plasma applications include adhesion promotion, enhancing surface wettability and reducing surface friction [8]. Plasma is a partially or wholly ionized gas with a roughly equal number of positively and negatively charged particles. It can be referred to as the fourth state of matter that consists of energetic species including ions, electrons, free radicals, meta-stable particles, and photons in the short-wave ultraviolet range [9],[10].

Dielectric barrier discharge (DBD) is a simple, unexpressive and ideally suited system for plasma surface treatment at atmospheric pressure while maintaining a low treatment temperature of heat-sensitive material [11]. Surfaces in contact with plasma are bombarded by these energetic species that transferred from plasma to solid surfaces producing various effects on the polymer surfaces. Plasma effect on polymer surfaces including removal of organic contaminants, degradation of polymer chains, formation of radicals on polymer surfaces, alteration of the tactility of polymer chains, creation of a thin cross linking layer, and formation of chemical groups on the stabilized surfaces [12].

Surface modification of fiber post is expected to change its adhesive behavior to root canal dentin. Therefore, this study aimed to compare between the effects of surface treatment with two gases plasma or sandblasting with and without silanization on the push-out bond strength of fiber posts to root canal dentin.

  Materials and Methods Top

Specimen preparation

Extracted human single-rooted teeth with completely formed roots of approximately similar size were collected and stored in formalin. All patients were informed and signed a written consent for approval of using their extracted teeth in this study according to the ethics committee of Faculty of Dentistry, Tanta University. Fifty teeth with no root curvature, caries, or cracks were selected. Periapical radiographs were taken for all teeth to assist in selecting teeth with no radiographic evidence of canal curvature, previous root canal treatment, internal resorption, or calcification. The selected teeth were cleaned, washed with tap water and stored in normal saline. Crown was sectioned transversally using a high-speed carbide bur and water spray to obtain ∼ 15 mm-long root segments. Working length was established by inserting a K-file #10 (Mani Inc., Tochigi, Japan) to the root canal terminus and subtracting 1 mm from this measurement. Each root was embedded in transparent acrylic resin, except the coronal 1 mm, forming a circular block to facilitate its handling, treatment procedures and testing.

Instrumentation of root canal was performed using ProTaper Universal rotary instruments (Dentsply Maillefer, Ballaigues, Switzerland) at 300 rpm with 4: 1 reduction rotary hand piece powered by a torque-limited electric motor (Endo-Mate TC; NSK, Tokyo, Japan). Irrigation was performed with 2.5% sodium hypochlorite followed by 17% EDTA for 1 min. Each canal was flushed with 10 ml saline and dried with paper points (Dentsply Maillefer). All canals were filled with gutta percha (Dentsply Maillefer) and epoxy resin-based sealer (AH 26; Dentsply Detrey GmbH, Konstanz, Germany) using a lateral condensation technique. Root specimens were stored at 37°C and 100% humidity for 7 days to allow for complete setting of the filling materials.

Post space was prepared according to manufacturer's instructions with size #2 Easy Post drill (Dentsply Maillefer) using a low speed hand piece (1000–2000 rpm), leaving 5 mm apical gutta percha to create a standard post space of 10 mm in length.

Before post cementation, post space was conditioned with 17% EDTA for 1 min followed by sodium hypochlorite irrigation and water rinsing and then dried with oil-free oil and paper points.

Surface treatment of fiber posts

The 50 root specimens were randomly divided into five equal groups (n = 10) according to the surface treatment of the glass-fiber Easy Post size #2 (Dentsply Maillefer) as follows.

Group 1

Glass-fiber posts were surface treated by oxygen plasma using DBD plasma system according to Garamoon et al. [13]. DBD cell consists of two stainless steel electrodes each of them is fixed to a Perspex disc. The lower electrode is connected to earth while the upper is connected to high voltage power supply of 20 kHz frequency and a voltage of 20 kV. A dielectric material is pasted to the upper electrode and both electrodes are collected to each other via O-ring making a gap distance of 5 mm between dielectric glass and the lower electrode. The cell is fed by gas via an inlet where the gas fills the gap space and is exhausted through an outlet.

Before any treatments oxygen was left to flow in the cell for 5 min. to sweep away any impurities in the gap space. The cell was fed by oxygen in atmospheric pressure and a limited resistance (250 kΩ) was used to limit the discharge current. The applied voltage was oscillated via a resistive potential divider (500: 1) and the waveforms of the voltage, current and charge were measured by a digital storage oscilloscope to control the consumed power in the discharge (3 W). Ten posts were exposed to oxygen plasma for 5 min.

After plasma treatment, each post was kept in a glass vial sealed with a rubber cap at atmospheric pressure and room temperature. Glass-fiber posts were cemented into root canals within an hour of plasma treatment.

Group 2

Surface treatment of the glass-fiber posts were performed as the same way as in group 1 using argon gas plasma instead of oxygen.

Group 3

Glass-fiber posts surfaces were treated with sandblasting (Co-Jet; 3M ESPE, St Paul, Minnesota, USA) using 50 μm silica-coated Al2O3 particles at 2.5-bar pressure for 5 s. The posts were held to the incoming particle stream at a distance of 30 mm. After surface treatment of each post, it was cleaned with oil-free air and kept in a glass vial sealed with a rubber cap at room temperature to be cemented into root canal within an hour of sandblasting.

Group 4

Each glass-fiber post was sandblasted as in group 3 followed by surface treatment with Silane (Rely X Ceramic primer; 3M ESPE AG, Seefeld, Germany) and air dried after 60 s before cementation.

Group 5 (control group)

Ten glass-fiber posts were cemented to the canal wall without any surface treatment.

For all groups, each post was cemented to the canal wall using self-adhesive dual-cured universal resin cement according to manufacturer's instructions (RelyX Aplicap Self-Adhesive Resin Cement; 3M ESPE AG). Light curing of the cement around and through the fiber post was done for 40 s. Excess cement was removed with a scaler after light curing. All specimens were stored in distilled water at 37° for 48 h before the push-out test.

Bonding test

All specimens were prepared for push-out test to evaluate bond strength of glass-fiber post cemented to canal dentin. Two horizontal sections with a thickness of 2 mm each were cut using a water-cooled diamond disc. The first section (coronal) was cut at 1 mm from the coronal surface and the second section (middle) was cut 5 mm from the coronal end. The diameter of adhesion area was measured apically and coronally for each root section. A compressive load via a Universal Testing Machine (LIoyd LRXplus; LIyod Instruments Ltd, Fareham, UK) was applied on the post without stressing the surrounding root canal walls at a crosshead speed of 1 mm/min using a 0.8 mm diameter stainless steel cylindrical plunger. The push-out force was applied in an apicocoronal direction until bond failure occurred, which was manifested by extrusion of the post and a sudden drop along the load deflection. The maximum failure load was recorded in Newton and was used to calculate the push-out bond strength in megapascals (MPa), considering the surface area of the post segments according to the following formula:

where A=area of the post surface that was determined as follows:

where π is the constant 3.14, r1 is the small apical post radius, r2 is the large coronal post radius and h is the section thickness (2 mm). Two sections (coronal and middle thirds) were tested from each specimen and the average of push-out bond strengths of the two sections was used as one value for each specimen.

Failure mode of the samples were assessed using a stereomicroscope (Nikon Eclips E600; Nikon, Tokyo, Japan) at ×20 magnification and recorded as cohesive (failure within dentin, post or luting cement), adhesive (failure at the interface between post and luting cement or between dentin and luting cement), and mixed failure (adhesive–cohesive failure).

Statistical analysis

Raw data were collected, tabulated and statistically analyzed using a statistical package SPSS 17.0 for Windows (SPSS Inc., Chicago, Illinois, USA). A test of normality was performed to determine the suitable statistical analysis used for inferential statistics. Ryan–Joiner normality test revealed that the data of this study do not follow normal distribution (R=0.94, P<0.01), therefore, the data were treated as nonparametric and Kruskal–Wallis test was used in testing the statistical significance for the effect of surface treatment on push-out bond strength. Mann–Whitney test was used for comparison between each two groups at P equal to 0.05.

  Results Top

[Table 1] represents mean ± SD and confidence intervals for push-out bond strength of different surface treatments groups for the fiber-reinforced posts to root canal dentin. Statistical analysis using Kruskal–Wallis test indicated a significant difference between the tested groups (P<0.01). Pairwise comparison of the studied groups using Mann–Whitney test exhibited oxygen plasma with the significantly highest push-out bond strength of all groups (P<0.01). No significant difference between surface treatment with argon plasma and the control group. Surface treatment of fiber post with sandblasting using Al2O3 significantly improved the push-out bond strength compared with the control group (P<0.05). However, the difference between sandblasted and sandblasted-silanized groups was not significant.
Table 1: Mean±SD and confidence intervals for push-out bond strengths in MPa between fiber posts and root canal dentin of the tested groups

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Stereomicroscopic evaluation revealed that all failures occurred as adhesive failure at the interface between post and luting cement. No cohesive or mixed failures were observed.

  Discussion Top

Although several studies compared various types of surface treatment of fiber posts, there were no agreements in the literature confirmed the best surface treatment for optimum bonding of fiber posts [1]. Gas plasma application could alter the surface characteristic of fiber posts. Significantly high push-out bond strength was obtained in the current study with oxygen plasma surface treatment. This finding could be attributed to the chemical and etching (physical) effects of oxygen plasma that generates highly reactive radicals and molecules such as atomic oxygen, ozone and/or excited oxygen molecules. Oxygen-containing groups of C–O and C=O were effectively introduced onto the polymer surface by bond breaking or degradation due to the highly reactive radicals of oxygen plasma [10]. Consequently, the hydropholicity of the post surface is improved and chemical interaction between free radicals on the post surface and the functional groups in the cement material may occur, thereby resulting in a significant increase in bond strength [8]. Oxygen plasma treatment can also improve resinous surface wettability because the atomic ratio of C: O changed from 1: 0.20 to 1: 0.29 and the percent of C–O in C–C increased with subsequent increase in bond strength [14]. Moreover, oxygen plasma treatment usually creates microroughness on the treated surface due to an etching effect. This etching was reported to create microporosities that increased micromechanical interlocking [15]. Thus, the spread and penetration of the luting cement into the irregularities of the post surface were enhanced by etching and chemical interaction between polar functional groups of C–O and C=O induced on the surface of the fiber post. The functional groups in the adhesive cement might result in push-out bond strength to be significantly improved [14].

Argon surface treatment in the present study did not significantly improve the bond strength. Yavirach et al. [16] reported that Argon plasma treatment significantly enhanced the tensile-shear bond strength between fiber posts and composite core build up. They attributed these results to polymer chain scission caused by the bombardment of energetic high-molecular-weight argon particles that can react with other surface radicals of the post or with other chains in the chain-transfer reactions of polymers. However, argon, as inert gas, limited the plasma treatment effect on physical modifications and formation of free radical on the polymer surface [14]. In addition, this disagreement could be attributed to different plasma machine parameters. Yavirach et al. [16] generated plasma under vacuum with pressure for longer exposure time (10 min). It seems that argon plasma treatment in the present study was not sufficient to achieve efficient bombardment effect.

The current study indicated that sandblasting of the post surface using Al2O3 could improve the push-out bond strength compared to the control (no surface treatment). It is well accepted that sandblasting with Al2O3 increased the surface roughness and surface area of the post. The mechanical action of sandblasting depends on removal of the smooth superficial layer of resinous matrix and creating microretentive spaces on the post surface. In addition, sandblasting with silica-coated Al2O3 particles creates a silica layer on the post surface that allows a penetration of the particles about 15 μm [17] resulting in improving the bond strength between the quartz fiber-reinforced post and adhesive resin cements [18],[19]. Radovic et al. [19] reported a significant increase in surface retention of sandblasted methacrylic fiber posts to dual-cured resin composite. However, bonding of fiber post was found to be affected by the type of cement used and only a little influence of sandblasting was recorded on the bond strength between posts and resinous cements [4].

Generally, sandblasting effect depends on application time, particles size and pressure used. However, sandblasting with Al2O3 was considered to be aggressive for surface treatment of fiber posts and it may significantly modify their shape and fit into the root canals [6]. Therefore, the present study performed sandblasting with 50 μm silica-coated Al2O3 at 2.5-bar pressure for 5 s and a distance of 30 mm. This regimen of sandblasting did not produce visible changes to the post form but resulted in increased surface area and micromechanical interlocking with the resin cement [18],[19],[20]. However, the main problem with sandblasting technique is the lack of selectivity; both the resin matrix and the fibers of the post might be affected [3].

In the present study, adding single bottle silane to the sandblasted post surface could not significantly increase the push-out bond strength. This finding agrees with several authors who indicated that silane does not bond well with the epoxy matrix of fiber posts [21],[22],[23],[24]. As fiber post used in this study composed of 60% fiber glass and 40% epoxy resin, 3-methacryloxypropyltrimethoxysilane, is used as a specific coupling agent to bond methacrylate resin monomers to fillers forming dimethacrylate composite filling [25]. Accordingly, the silane used in this study might bond well with the methacrylate-based Rely X unicem luting cement and glass of the post while, it did not bond to the epoxy resin in the post. This might explain the adhesive failure mode between resin cement and fiber post. Ferrari et al. [22] found that post–resin interfacial strength is still relatively low due to the absence of chemical union between resin adhesive and the epoxy matrix of fiber post. Hooshmand et al. [24] reported that silane treatment did not enhance the bonding between fiber posts and different resinous cements. However, Aksornmuang and colleagues [26],[27] reported that silane application enhanced the bond strength of a resin core material to translucent fiber posts. They claimed that silicate layer could adhere to the sandblasted surface resulted in combining chemical and micromechanical retention, and silane could increase surface wettability leading to chemical bridges formation with OH-covered substrates such as glass or quartz fibers.

Silane coupling is considered to be technically sensitive and factors such as prehydrolyzed (one bottle which is less stable) or nonhydrolyzed (two bottle), solvent content, solvent evaporation, molecule size, application mode and pH could significantly affect the bond strength [28]. The contradictions on the effect of silanization drove Monticelli et al. [3] to state that even if silanization proved to be significant in terms of bond strengths to fiber posts, the clinical relevance of the differences have been considered of minor importance. They mentioned that chemical bond may be achieved only between the exposed fibers of the post and the resin materials but bond strength between the epoxy resin-based fiber posts and methacrylate-based resin could not be fully enhanced by silanization.

Further studies is required to investigate the effect of using epoxy-based resin adhesive/luting cement and 3-glycidoxypropyltrimethoxysilane to bond epoxy resin-based fiber post to root dentin and silorane composite (epoxy resin-based composite filling) core build up. Clinical trials involving plasma technique are indicated.

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Conflicts of interest

There are no conflicts of interest.

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