|Year : 2015 | Volume
| Issue : 4 | Page : 337-341
|Comparative evaluation of shear bond strength of three resin based dual-cure core build-up materials: An In-vitro study
Gaurav Jain1, Aditi Narad1, Lalit C Boruah2, Balakrishnan Rajkumar2
1 Department of Conservative Dentistry and Endodontics, Saraswati Dental College and Hospital, Lucknow, Uttar Pradesh, India
2 Department of Conservative Dentistry and Endodontics, BBD College of Dental Sciences, Lucknow, Uttar Pradesh, India
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|Date of Submission||03-Feb-2015|
|Date of Decision||13-Apr-2015|
|Date of Acceptance||20-May-2015|
|Date of Web Publication||1-Jul-2015|
| Abstract|| |
Aim: The in-vitro study compared the shear bond strength (SBS) of three recently introduced dual-cure resin based core build-up materials namely ParaCore, FluoroCore, and MultiCore.
Materials and Methods: One hundred twenty extracted permanent human mandibular molar teeth were taken and sectioned horizontally beneath the dentinoenamel junction to expose the coronal dentin. The specimens obtained were divided into three main groups based on the materials used and then further divided into four sub-groups based on time interval with ten samples each. The dentin surface was treated with the respective adhesives of the groups and then bulk filled with core build-up materials. The attained samples were than subjected to shear loading in Instron Universal Testing Machine. The data were tabulated and statistically analyzed using analysis of variance (ANOVA), Tukey's HSD, and Levene's test.
Results: The mean SBS was highest in MultiCore at all time periods as compared to FluoroCore and ParaCore and was also higher at 48 h thermocycling in all three groups studied.
Conclusion: MultiCore dual-cure resin based core build-up material showed the highest mean SBS as compared to FluoroCore and ParaCore. SBS was not negatively affected by thermocycling.
Keywords: Core build-up; dual-cure resin; shear bond strength; thermocycling
|How to cite this article:|
Jain G, Narad A, Boruah LC, Rajkumar B. Comparative evaluation of shear bond strength of three resin based dual-cure core build-up materials: An In-vitro study. J Conserv Dent 2015;18:337-41
|How to cite this URL:|
Jain G, Narad A, Boruah LC, Rajkumar B. Comparative evaluation of shear bond strength of three resin based dual-cure core build-up materials: An In-vitro study. J Conserv Dent [serial online] 2015 [cited 2021 Sep 21];18:337-41. Available from: https://www.jcd.org.in/text.asp?2015/18/4/337/159754
| Introduction|| |
Core build-up is one of the most important steps to restore a severely damaged, fractured or extensively carious tooth. As the core becomes an integral part of the load bearing structure of the tooth, it should provide resistance and retention form for the coronal restoration and possess sufficient strength to resist occlusal forces. 
An ideal core build-up material should have physical properties similar to those of tooth structure, as a restored tooth tends to transfer stress differently than an intact tooth, where the occlusal biting loads are transferred to dentin as compression that is distributed over a large internal volume of tooth structure reducing local stress.  Restored tooth, allows complex stress distribution pattern along the tooth and restoration interface, producing compression, tension or shear stress.  The process of mastication is basically related to shearing phenomenon and the true nature of the adhesive strength of materials at the tooth and restoration interface is described by the shear bond strength (SBS).  SBS test is the most common method to evaluate bond strength, as testing in shear mode is more clinically relevant and relatively simple, reproducible, and widely accepted test. ,
Resin composite core build-up materials were introduced for the restoration of severely damaged endodontically treated teeth and excellent bond was achieved with the tooth structure when they were used in conjunction with a suitable adhesive system. ,
Recently, various dual-cured versions of resin composite build-up restoratives that combine the advantages of light-curing and self-curing mechanism have been introduced, with the rationale to develop a material capable of reaching higher degree of polymerization in either the presence or absence of light, and overcome the limitations of reduced interlayer strength.  Thus, the aim of the study was to compare the SBS of three recently introduced dual-cure resin based core build-up materials namely ParaCore, FluoroCore, and MultiCore.
| Materials and Methods|| |
The materials used in the study were three dual-cure resin composites (ParaCore [Coltene Whaledent Inc., Ascot Parkway, Cuyahoga Falls, USA], FluoroCore [Dentsply Caulk Inc., W. Philadelphia Street, York, USA], MultiCore [Ivoclar-Vivadent Inc., Memorial Drive, Somerset, NJ, USA]) with their respective adhesive systems [Table 1].
One hundred twenty extracted permanent human mandibular molar teeth were chosen, cleaned with ultrasonic scaler and stored in distilled water at room temperature, the preferred method of storage with least negative influence on the measured bond strength of resin composite to dentin, as suggested by Titley KC et al.  The teeth were sectioned horizontally with a carborandum disk beneath the dentinoenamel junction to expose the coronal dentin surface and later finished with 600-grit silicon carbide paper to create a uniform flat surface. The teeth were then mounted in custom-made wax molds (size 18 mm height, 12 mm length, and 12 mm breadth) by auto-polymerizing pink orthodontic resin, with coronal portion of the tooth exposed [Figure 1]a-c.
|Figure 1: Schematic diagram of specimen preparation. (a) permanent mandibular molar sectioned horizontally beneath dentinoenamel junction (b) exposed coronal dentin surface (c) tooth mounted in auto-polymerizing resin mold (size 18 mm height, 12 mm length, and 12 mm breadth) with coronal portion of tooth exposed (d) polyvinyl mold (5 mm internal diameter and 5 mm height) (e) polyvinyl mold placed on treated dentin surface and bulk filled with dual-cure resin (f) light-curing of sample to initiate polymerization (g) the prepared sample after de-assembling the polyvinyl mold|
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The teeth were then randomly assigned to three groups on the basis of material used (Group A - ParaCore, Group B - FluoroCore, Group C - MultiCore), each group consisting of forty samples. Groups were further divided based on time interval into four sub-groups of 24 h (with and without thermocycling) and 48 h (with and without thermocycling), with 10 samples each.
Group A: The dentin surface was treated with ParaBond NonRinse Conditioner, scrubbed for 30 s, followed by application of premixed adhesive A and adhesive B on conditioned dentin surface for 30 s and air drying for 2 s.
Group B: The dentin surface was treated using Xeno IV dual-cure self-etching dental adhesive, followed by light-curing for 10 s using curing light (550 mW/cm 2 ).
Group C: The prepared dentin surface was treated using Tetric N-bond self-etch followed by light-curing for 10 s using curing light (550 mW/cm 2 ).
Placement of dual-cure resin composite
Readymade polyvinyl molds of 5 mm internal diameter and 5 mm height, coated with nonreactant lubricant (petroleum jelly) on inner walls were placed on the treated dentin surface and subsequently bulk filled with respective dual-cure core build-up materials in each group at room temperature. These were then initially light-cured for 10 s per surface to initiate polymerization and to achieve final set, left for 8 min for auto-polymerization as a standard for all groups [Figure 1]d-g.
The molds were then deassembled and were stored at 100% humidity at 37°C. These were randomly divided into four sub-groups based on time interval of 24 h and 48 h without thermocycling and 24 h and 48 h with thermocycling (5°C-55°C for 125 cycles with a corresponding dwell time of 30 s and 10 s transit time between baths according to ISO #11405 standard). 
Finally, the samples were subjected to SBS test using Instron Universal Testing Machine (Model 3382, Instron Industries, USA) following the 2003 ISO technical specification #11405,  at a cross-head speed of 0.5 mm/min until the specimens fractured under stress load. In this study, the shear force was applied perpendicular to the tooth surface, to evaluate the bond strength of the core build-up materials.
The data were tabulated and statistically analyzed using analysis of variance (ANOVA, Tukey's HSD, and Levene's) test to compare the SBS between three main groups and also within groups at different time intervals. The selected level of variance was 0.05. Analysis was performed on SPSS 19 software (IBM Corporation, Chicago).
| Results|| |
ANOVA showed a significant difference in mean SBSs at P < 0.05. The mean SBS of all three groups was highest at 48 h with thermocycling followed by 24 h with thermocycling, then 48 h without thermocycling and 24 h without thermocycling being the least. Moreover, at all the periods, the mean SBS of Group C was observed the highest followed by Group B and Group A [Table 2].
Comparing the mean SBS of three groups at four different time periods together by ANOVA, revealed significant effect of both, groups (F = 34404.17, P < 0.001) and periods (F = 73.40, P < 0.001) on SBS. Further, the interaction (groups × periods) of both on SBS was also found to be significant (P < 0.001 with F = 6.39) [Table 3].
|Table 3: For each group, significance (exact P [up to four decimal]) of mean difference of SBS within the groups (i.e., between time periods) by Tukey's HSD test|
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| Discussion|| |
This study compared the SBS of three different, resin based dual-cure core build-up materials with respective dentin bonding adhesives as provided and recommended by the manufacturer, to achieve the maximum effect of bonding procedure.
The present study was done in-vitro, as the clinical functions and characteristics of dental materials are difficult to evaluate under in-vivo conditions, and clinical trials cannot estimate mechanical properties of restored teeth.  Whereas, in-vitro tests give the possibility to evaluate mechanical properties of restored teeth,  and are considered as a predictor of the possible clinical performance of a material. 
The coronal dentin surface was used to evaluate SBS as previous studies have shown that a reduction in bond strength occurs when resin composite is bonded to deep dentin, , which can be attributed to the complexities in the structure of deep dentin, such as increase in the number of tubules and their diameters with much lesser intertubular dentin matrix as compared to superficial dentin. 
The teeth were randomly divided into four sub-groups within the main groups based on a time interval. Time distribution was done as studies have shown that bond strength evaluations are performed in-vitro, 24 h after specimen preparation and is considered adequate time to test the adhesive capability of the material.  Here, thermocycling was done, as thermocycling is an artificial aging methodology that stimulates stresses caused by oral functions,  and subjects the specimens to altering temperatures that induce contraction and expansion stress between the resin restoration and tooth due to differences in the coefficient of thermal expansion.  In the present study, thermocycling was done for 125 cycles, which was in accordance with the study done by Levartovsky et al. 
The mean (± standard deviation [SD]) SBS within groups was found to be higher in all three groups at 48 h with thermocycling followed by 24 h with thermocycling as compared to both 24 h and 48 h without thermocycling. This can be due to the fact that dual-cured resins used in the study were further polymerized by the repeated exposure to the 55°C environment (hot bath) during the thermocycling process. This was in accordance with the results of studies done by Korkmaz et al. and Titley et al. who found that the SBS was not negatively affected by thermocycling.
Group A, showed the least mean (± SD) SBS at all-time intervals as compared to Group B. This difference can be attributed to the fact that adhesive used in Group A contains ethanol as an organic solvent in bonding agent. Previous studies have shown that ethanol (alcohol) does not chase water, , due to its high boiling temperature and less vapor pressure (43.9 mmHg at 20°C) as compared to acetone present in Group B adhesive which can efficiently remove surface water, and adding 10% acetone to water, vapor pressure increases more than 300%, leading to volatilization of surface water. 
Moreover, the highest mean SBS of Group C at all time periods as compared to Group A and Group B can also be attributed to the presence of nanofillers silicone dioxide (approximate 5 wt%) in adhesive used, while the other group adhesives lack the nanofillers. Studies by Miyazaki et al. also reported higher filled adhesives yield stronger physical properties because of their ability to flex and relieve polymerization stress.
Further, the mean SBS between the groups (Group A, Group B, Group C), the SBS was significantly (P < 0.001) different, and was observed higher in case of both Group B and Group C as compared to Group A. This can be attributed to less percentage (10-15%) of high-molecular-weight urethane dimethacrylate (UDMA) monomer in Group A composition as compared to Group B and Group C, which has high percentage (20%) of UDMA. Studies have shown that composites with a higher percentage of UDMA show increased monomer conversion.  Hence, replacement of some low-molecular-weight triethylene glycol dimethacrylate with UDMA, increases the molecular weight per reactive group thus decreasing polymerization stress. 
At all four time periods, the mean SBS of Group C was found significantly (P < 0.001) higher as compared to Group A and Group B which can be because of high filler content of 70 wt% in Group C. It has been shown that filler volume and filler weight level of the composites correlate with the material strength and elastic modulus.  Boyer et al. in their study found that higher fillers resulted in more bond strength with no effect on elasticity and a decrease in polymerization shrinkage.  According to Watts and others,  composites with relatively high filler content have a significant reduction in volumetric shrinkage, accompanied by lower contraction stress value. Thus, lower shrinkage combined with the decreased stress leads to high bond strength. 
| Limitations and Future Scope|| |
The present study being an in-vitro has following inherent limitations, such as relatively smaller sample size and inflexibility to use same bonding strategies for all the groups due to the rigidity of manufacturer's protocol. Despite these limitations with this research on dual-cure resin materials, in the present in-vitro study, ParaCore, FluoroCore, and MultiCore have shown satisfactory SBS under simulated clinical conditions, which support their use as core build-up material.
| Conclusion|| |
Based on the findings of the present in-vitro study, following conclusions were drawn:
- MultiCore (Group C) core build-up material showed the highest SBS as compared with ParaCore (Group A) and FluoroCore (Group B). Whereas ParaCore (Group A) showed the least SBS.
- All the tested materials showed increased SBS after 48 h of thermocycling, which was statistically significant as compared to other time groups.
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Lucknow Gaurav Jain
Department of Conservative Dentistry and Endodontics, Saraswati Dental College and Hospital, Lucknow - 227 105, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3]
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