The Effect of Resin Cement Systems on the Push-Out Bond Strength of Fibre Posts. An in-vitro Study

Faisal Al-Sahaly1                               BDS
Mohammed Al-Khathami2           BDS
Saud Al-Harbi3                                 BDS
Muath Al-Qunayan4                       BDS

INTRODUCTION

The use of fiber-reinforced posts is prevalent in the restoration of root filled teeth with compromised crown structure1,2. These posts provide improved retention for the core buildup for indirect coronal restorations. Resin composite bonds to the tooth structure and fiber posts, through surface treatments to provide a mono-block, effectively resisting the loading forces on the coronal restoration during function. However, in case of clinical failure of restored endodontically treated teeth, debonding of the fiber post is a common finding3,4. A multitude of factors are implicated in the successful bonding of fiber posts to root dentin, these include the presence of smear layer, the orientation and density of dentinal tubules, the use of different root canal irrigants, chemical treatment of the posts with etchants and silanes, canal configuration and cements and luting agents5-9. In addition the workability in cementation of fiber posts pose challenges due to different canal configurations, limited vision and access to the canal, control of moisture and managing wet and dry bonding and cement quantity.

Various luting agents are available for the cementation of fiber posts. Based on the method of dentin conditioning, luting agents are divided into total-etch and self etch. Total etch bonding system is the traditional technique for adhesive bonding of resin to dentin. Self-etch adhesive cement simplifies the bonding technique and reduces technique sensitivity10. In addition they undergo less demineralization as compared to total etch procedures, resulting in a thinner hybrid layers with shorter and irregular resin tags11-13.

Several studies have investigated adhesive bonding of fiber posts to root dentin with conflicting results14,15.

Therefore in light of the presented literature, it is hypothesized that the adhesive bond strength of fiber posts to root dentin using self etch and total etch systems will be similar. Hence the aim of this study was to evaluate the effect of self-etch and total etch systems on bond strength of cemented fiber posts.

METHODOLOGY

Maxillary anterior teeth previously stored in 0.1% thymol solution were selected for the study. Sixty teeth were selected after exclusion of teeth with cracks or craze lines, using magnification. In addition teeth with caries, cervical lesions, endodontic treatment, fractures, posts, and root length less than 15mm were also excluded. The teeth were divided in three groups (n=20).

All teeth were sectioned 2mm incisal to the CEJ with a low speed diamond saw under copious irrigation and were embedded in acrylic block (Paladur, HeraeusKulzer, Hanau, Germany). A standardized endodontic procedure, included enlarging of the canals with Gates Glidden (bur sizes 1-3)as a crown-down technique, followed by canal enlargement to 11mm length, using slow speed drills provided by the post system (size #3 tapered fiber post, and 1.6mm diameter, 3M ESPE RelyX Fiber Post, St. Paul, MN,USA) was undertaken. Intermittent irrigation with 5.25% NaOCl was performed with drying with paper points. A single operator performed the endodontic procedure. Following this the fiber posts were cemented using three different luting agents.

After the root canal treatment was completed, the teeth were randomly assigned into:

• Group A: Luxacore Dual. Dual cure, Core BuildUp Material (DMG America, Englewood, New Jersey).
• Group B: RelyX-Unicem Self-Adhesive Universal Resin Cement (3M ESPE. St. Paul, MN).
• Group C: MultiCore Flow Dual cure, Core BuildUp Material (IvoclarVivadentInc, Amherst, NY, USA).

The fiber posts were cleansed with alcohol and were cemented to the prepared teeth using the bonding protocol suggested by the manufacturer guidelines as provided by the luting agents. For groups A and C root canals were etched using 37% phosphoric acid for 15 seconds (sec) followed by rinse of post space with distilled water. The canals were dried with air stream and paper points. In group A, canals were bonded using Luxabond Dual (DMG America, Englewood, New Jersey). A pre-bond was applied for 15 sec and excess was air blown and removed with paper point. Bond A and Bond B were then mixed and applied for 20 sec with a microbrush and excess was removed. In group C, ExciTEF, dual cure resin was applied to the dentin walls and root canal, after etching. The bonding agent is agitated on the dentin and excess removed carefully. Specimens in Group B did not require a bonding agent as and they were cenmented using self etch- self adhesive RelyX-Unicem (According to manufacturers instruction).Cementation of fiber posts in all groups was done using a standardized procedure. For details of the materials see appendix A.

After cementation, the mounted specimen were stabilized and sectioned as cervical and apical specimens using a low speed diamond saw (Model 650, SBT South Bay Technology Inc, Arlington, VA).For push out test, the sections of mounted specimens were stabilized using a metallic device 8mm in height, 20 mm in diameter, and a central opening larger than the diameter of the root canal orifice. A plunger (1.2mm) with is slightly smaller diameter than the post was used to apply load at a rate of 1mm/min in an apical-coronal direction until the post de-cemented. The bond strength σ (MPa) was calculated using the following formula: σ=C/A
Where C is the rupture load of specimen (Newtons) and A is the bonded area (mm2).

The value of bonded area “A” was calculated using:A=2πrh

Where π=3.14 (constant), r=radius of post, and h=length of specimen measured by a digital caliper. After push-out testing, the failed specimens were evaluated under a 3D digital microscope (Hirox- KH7700, 100 Commerce Way, Hackensack, US). Statistical software (SPSS version 20; IBM Corp) was used to analyze data obtained and the significance level was set at p<0.05.Means and standard deviations of bond strengths were calculated using One-way analysis of variance (ANOVA) and t-test.

RESULTS

The highest mean push out bond strength was achieved by group B (19.07 ±3.16 Mpa) and the lowest was shown by specimens in group A (15.40±3.02 Mpa). ANOVA showed no statistical difference among the experimental groups (Groups A, B & C) (p=0.328), (Tables 1 and 2).

Table 2 presents the comparison of push out bond strength values among the groups based on the cervical and apical segments. The mean push-out bond strength in the cervical segments was found to be significantly higher for all three luting agents used (represented by groups A, B & C) (p<0.001). The independent t-tests results showed that the mean push-out bond strength values of the cervical segments was significantly higher than the apical segments in all three groups (P<0.05).
Most of the specimens (68.33%) failed predominately at the interface between the adhesive and the dentin surface. However 20% of failures were adhesive at the post-cement interface. Only 7 failures were mixed (Table 3).

DISCUSSION

In the present study push out bond strength of fiber posts cemented with three different resin luting agents to root dentin was investigated. In addition, push out bond strength to cervical and apical root dentin was compared in the experimental groups. The hypothesis that, total-etch bonding system and self adhesive luting agent would show similar push out adhesive bond strength for fiber posts to dentin was proven. Previous investigations concur with the finding of the present study, showing similar bond strength for fiber posts with self-etch and total etch luting agents16.

The present investigation involved assessing bond strength using push-out test. Push out test results in shear forces acting at the root-post interface, which is similar to stresses under clinical conditions17. In addition, push out testing regime allows for more homogenous stress distribution through the root dentin, lower failure rates compared to tensile test and provide multiple testing specimens from a single root18, 19. Moreover it is rendered the most reliable method of assessing the bond strength of fiber posts to root dentin20.

Three different luting agents were used in this study with their recommended bonding agents for the cementation of fiber posts. In a previous study comparing different luting agents like Unifil core and Calibra, showed no significant difference in bond strength values21. Similarly, a comparison of Multicore, Variolink II and Multilink for cementation of fiber posts showed no significant difference in bond strength values22. In the present study although the push out bond strengths were higher in the specimens bonded with RelyXU, there was no significant difference in the values among the groups (p>0.05). Higher bond strength values in group B (Rely XU) may be related to the simple workability and lack of interaction among materials (no bonding agents involved) used in bonding of fiber posts.

It is pertinent to mention that although the use of self adhesive cements simplify the bonding procedure of posts, the use of core builds up materials like Multicore (Group C) and Luxacore (Group A) as luting agents also simplifies the clinical procedure for build up of endodontically treated tooth preventing interactions of different materials. Therefore to determine the overall effect of using core build up materials as luting agents on the survival of endodontically treated teeth, studies assessing the failure loads of endodontically treated-restored teeth are recommended.

Interestingly, in the present study the push out bond strength values for fiber posts was significantly higher in the cervical region as compared to the apical root area in all experimental groups (p=0.05). Although previous studies have shown conflicting results in this regard23, the authors propose that the lower bond strength in the apical region could be due to, poor dentin conditioning, distribution of resin cement, thickness of luting cement, inability to adhesively bond to gutta-percha apically and poor distribution of dentinal tubules in the apical root canal24.

A possible limitation of the present study is the in-vitro design of the experiment, which although similar, does not completely simulate oral environment. In addition, intra-oral loads on the posts are conventionally non axial, however push out bond strength was assessed in the present study, which is in the long axis of the tooth, being different from a clinical scenario. Therefore, it is recommended that clinical trials with controlled cohort and inclusion and exclusion criteria to assess the efficacy of core materials as luting agents are performed.

Almost 70% of the interfacial failures took place at the dentin-adhesive interface in all specimens. Similar results for push out studies assessing bond strength of posts are shown in previous studies25. Adhesive failure at the dentin verifies the validity of push out bond strength tests and indicating the bonding interface between the adhesive and dentin to be the weakest link in the bonded post tooth assembly. A possible reason for the predominant failures at the adhesive-dentin interface, could be the residual NaOCL in the root dentin forming a oxygen layer and compromising the resin polymerization26.

The findings of the present study suggests that materials based on total etch adhesive technique which are marketed for core build-ups and also act as luting agents provide similar experimental bond strength as compared to self etch luting agents in the bonding of fibre posts. Therefore, these core build up materials are clinically recommended for luting as well as core build-up for teeth with compromised coronal tooth structure.

CONCLUSION

In summary, it was shown that:

1. The use of self adhesive cement (RelyXU) and total etch dual cure cements (core build-up materials) adhesives for the luting of fiber posts has similar push out adhesive bond strength.
2. Among the different luting materials, bond strength in the cervical area of roots was significantly higher as compared to the apical root segments.

CONFLICT OF INTEREST

None declared.

AUTHORS’ CONTRIBUTION

FS: Designed study and collected data.
MQ: Wrote manuscript and data statistics
SH: Laboratory experiment and material testing. And designing of experiment
MK: Manuscript writing and laboratory experiments.

APPENDIX A (COMPOSITION OF MATERIALS AND EQUIPMENT)

•  MultiCore Flow (IvoclarVivadentSchaan Liechtenstein)

Composition: Microhybrid resin composite, the monomer matrix consists of dimethacrylate (29 wt %). The inorganic fillers are barium glass, ytterbiumtrifluoride, Ba-Al-fluorosilicate glass and highly dispersed silicon dioxide (70 wt %). Additional contents are catalysts, stabilizers and pigments (1 wt %).

• LuxaCore Dual (DMG America, Englewood, New Jersey). 

Composition: Microhybrid resin composite, Barium glass 69%, pyrog. silica 3% in a Bis-GMA based matrix of dental resins.Composition: Microhybrid resin composite, Barium glass 69%, pyrog. silica 3% in a Bis-GMA based matrix of dental resins.

• RelyX-Unicem Self-Adhesive Universal Resin Cement (3M ESPE. St. Paul, MN).

Composition:

•  LuxaBodn Dual (DMG America, Englewood, NewJersey).

Composition: Ethanolic aryl sulfonate solution, Bis-GMA based resin matrix catalyst and benzoyl peroxide.

• ExciTE F DSC: (IvoclarVivadentSchaan Liechtenstein).

Composition: ExciTE F contains HEMA, dimethacrylate, phosphonic acid acrylate, highly dispersed silicone dioxide, initiators, stabilizers and potassium fluoride in an alcohol solution. The ExciTE F DSC applicator is coated with initiators.

• Universal testing machine (Model 4411; Instron Corp, Canton, Mass).
• Light curing device (Bluephase ® C8, IvoclarVivadent, Schaan, Liechenstein).
• 3D digital microscope (Hirox- KH7700, 100 Commerce Way, Hackensack, US).

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1. General Practitioner, Department of Restorative Dentistry, Riyadh, Saudi Arabia
2. General Practitioner, Department of Restorative Dentistry, Riyadh, Saudi Arabia
3. General Practitioner, Department of Restorative Dentistry, Prince Sultan Medical Military City
4. Intern, College of Dentistry, King Saud University, Riyadh, Saudi Arabia
Corresponding author: “Dr. Faisal Al-Sahaly” < academicksa2@gmail.com >