Abstract
Aims:
To compare and evaluate the bonding ability of resin composite (RC) to three different liners: TheraCal LC™ (TLC), a novel resin-modified (RM) calcium silicate cement, Biodentine™ (BD), and resin-modified glass ionomer cement (RMGIC) using an universal silane-containing adhesive and characterizing their failure modes.
Materials and Methods:
Thirty extracted intact human molars with occlusal cavity (6-mm diameter and 2-mm height) were mounted in acrylic blocks and divided into three groups of 10 samples each based on the liner used as Group A (TLC), Group B (BD), and Group C (RMGIC). Composite post of 3 mm diameter and 3 mm height was then bonded to each sample using universal adhesive. Shear bond strength (SBS) analysis was performed at a cross-head speed of 1 mm/min.
Statistical Analysis Used:
Statistical analysis was performed with one-way analysis of variance (ANOVA) and post hoc test using Statistical Package for the Social Sciences (SPSS) version 20.
Results:
No significant difference was observed between group A and group C (P = 0.573) while group B showed the least bond strength values with a highly significant difference (P = 0.000). The modes of failure were predominantly cohesive in Groups A and B (TLC and BD) while RMGIC showed mixed and adhesive failures.
Conclusions:
Hence, this present study concludes that the bond strength of composite resin to TLC and RMGIC was similar and significantly higher than that of BD following application of universal adhesive.
Keywords: Biodentine™ (BD), failure modes, resin-modified glass ionomer, stereomicroscope, TheraCal LC™ (TLC), universal adhesive
INTRODUCTION
Extensive research has been taking place in generating bioactive restorative materials with a potential for remineralization.[1] Bioactivity refers to apatite-forming ability[2] while biomineralization is the ability to get anchored to the underlying dentin by the formation of a mineral-rich interfacial layer and a tag-like structure extending from the interfacial layer to the dentinal tubules.[3] Biodentine™ (BD) (Septodont, Saint-Maur-des-Fossés, Creteil, France) and TheraCal LC™ (Bisco Inc, Schamburg, IL, USA) are calcium silicate-based bioactive liners that are proposed as alternatives to glass ionomers (GIs).
The use of bioactive liners beneath resin composite (RC) would clinically be more advantageous than using GI liners as they are biologically well-tolerated by the pulp tissue[4] and have comparatively higher remineralizing ability.[5] The success of these laminate restorations depend not only on the bond strength of the liner to the dentin but also on the quality of bond between liner and overlying RC. Various studies suggest the application of resin-modified glass ionomer cement (RMGIC) instead of GI in the sandwich technique because of improved bond strength to RC due to its chemical bonding.[6,7] The bond strength of RMGIC to RC varies depending on the type of adhesive used and it has been proved that self-etch is better than total etch.[8]
Biodentine™ (BD) is recently being used as a dentin replacement material under composite restorations.[9] A study by Danya F. Hashem et al.[10] showed no significant difference in bond strength between BD and RC in either self-etch or total etch mode. They also suggested BD composite restoration is a two-stage clinical procedure that requires a minimum waiting period of 2 weeks for adequate maturation of the BD to attain the physicomechanical properties sufficient enough to withstand the contraction forces of RC.
TheraCal LC™ (TLC) is a novel light-cured mineral trioxide aggregate (MTA)-filled, resin-modified (RM) calcium silicate cement and was given approval as a liner under composite restorations aiming to achieve a bond between the different layers of materials and as a pulp protectant. Gandolfi et al.[11] studied the chemical and physical properties of TLC and reported more calcium release than ProRoot MTA and Dycal. It was reported that calcium silicate-based materials showed apatite formation at a faster rate than calcium hydroxide-based materials.[12] However, there are contradictory findings reported in the literature about the hydration characteristics of TLC.[13] The role of moisture drawn in from the pulp and dentin is also unclear. TLC shows physiochemical bonding to the dentin and is well-tolerated by immortalized odontoblast cells.[14] Recently, Cantekin[15] proved that the bond strength of TheraCal methacrylate-based composite was significantly higher than that with silorane-based composites and GI cement. Currently, there is limited information in the literature on the bonding ability of TLC to RC in comparison to other liners.
Recently, a new single bottle universal or multimode adhesive with silanes (Single Bond Universal™, 3M ESPE, St. Paul, MN, USA) was introduced that simplifies the bonding procedure as single adhesive and can be used in self-etch or total etch or selective etch mode and on any surfaces (enamel, dentin, any direct, or indirect restorative materials) without additional primer.
No study till date has compared the bonding ability between TLC, BD, and RMGIC to RC using universal adhesive. Hence, in the present study the shear bond strength (SBS) of BD/TLC/RMGIC to composite using universal adhesive was compared and the null hypothesis was that there is no difference in the SBS within each substrate (TLC/BD/RM-GIC). The study also aimed to identify the specific modes of failure.
MATERIALS AND METHODS
The materials used are shown in Table 1. Thirty human intact molars extracted for periodontal reasons were collected for the study and the teeth were cleaned with ultrasonic scalers and stored in saline. The occlusal surfaces were grinded perpendicular to the long axis of the tooth with a high-speed diamond disc to obtain a flat surface. Then a cavity of 6 mm width and 2 mm depth was prepared to retain the liner. These teeth were mounted in acrylic resin blocks using a rectangular aluminium mould that was 15 mm/25 mm in dimension such that the occlusal surfaces were flush with the resin surface. These 30 samples were randomly divided into three groups: Group A — TLC; (TheracalLC™, Bisco Inc, Schamburg, IL, USA), Group B — BD; (Biodentine™, Septodont, Saint-Maur-des-Fossés, Creteil, France) and Group C- RMGIC; (Fuji II LC™, GC Corporation, Tokyo, Japan) and the cavities were filled as per manufacturer's instructions [Table 1] and their surfaces were not finished to mimic the clinical scenario.
Table 1.
Materials used in the study
Universal adhesive, (Single Bond Universal™, 3M ESPE, St. Paul, MN, USA) was applied on TLC/BD/RM-GIC surface with a bristle brush, rubbed for 20 s followed by gentle air drying with oil-free compressed air for approximately 5 s to evaporate the solvent and was light cured for 10 s after placing the polyethylene tube (2-mm diameter, 2-mm height) as per the manufacturer's instructions.
RC (Filtek™ Z-350 XT, 3M ESPE, St. Paul, MN, USA) was placed in the tube and light-cured with a light-emitting diode light-curing unit (Bluephase, Ivoclar Vivadent, Schaan, Lichtenstein) with an intensity of 1,200 mV/cm2 for 20 s. After the completion of RC build-up, the polyethylene tubes were removed with a sharp knife. All specimens were stored at 37°C in water for 24 h.
Measurement of shear bond strength
The specimens were attached to the universal testing machine (Instron 8500, Instron Corporation, Canton, OH, USA). A chisel with knife edge was gently held flush against the RC - TLC/BD/RM-GIC interface and loaded at a cross-head speed of 1.0 mm/min until bond failure occurred. The load at failure was recorded in N/mm square and then converted to MPa.
Fracture analysis
The fractured test specimens were examined under a stereomicroscope (Swift Stereo SM80, Tokyo, Japan) at a magnification of × 25 and fractures were classified as follows: Cohesive failure — Failure with in TLC/BD/RMGIC or RC, adhesive failure — Failure at RC- TLC/BD/RM-GIC interface, and mixed failure — When two modes of failure occur simultaneously. Fracture analysis was performed by a single observer who was completely uninformed about the experimental groups.
Statistical analysis
Statistical analysis was performed by using MS Excel 2007 and Statistical Package for the Social Sciences (SPSS) version 20.0 (SPSS Inc, Chicago, USA). Descriptive statistical data was presented in the form of mean and standard deviation. One-way analysis of variance (ANOVA) was performed to assess the mean significant difference between the different groups and LSD post hoc tests were used for multiple group comparisons. P value less than 0.05 was considered to be statistically significant.
RESULTS
The mean SBS values and standard deviations (SDs) are shown in Table 2 and were analyzed using ANOVA test; the post hoc test was used for intergroup comparison. Groups A and C (TLC, RMGIC) in comparison to Group B (BD) showed a very high significant difference (P = 0.000). Group A (TLC) showed no significant difference in bond strength compared to Group C (RMGIC) (P = 0.573). Group B (BD) showed the least bond strength values. The observed modes of failure were predominantly cohesive in Groups A and B (TLC and BD) while RMGIC showed mixed and adhesive failures [Figure A].
Table 2.
Mean shear bond strength in study groups (n = 10), (P < 0.05)
Figure A.
Stereomicroscopic images representing modes of failure. 1. Cohesive failure in TLC. 2. Cohesive failure in BD. 3A. Adhesive failure in RMGIC. 3B. Mixed failure in RMGIC
DISCUSSION
TLC/BD releases calcium and silicon ions into the underlying dentin.[11,16] According to Saito et al.[5] silica is a stronger inducer for dentin matrix remineralization than fluoride ions of RMGIC. Cytotoxicity studies showed that TLC/BD is well-tolerated by immortalized odontoblast cells. These are the cells that retained their ability to divide with stable phenotypic protein expression profiles and ability to produce mineralized dentin extracellular matrix under in vitro conditions.[4,14] The conclusions from these studies bear relevance to the use of TLC/BD as a liner and an alternative to RMGIC in laminate restorations, provided the bond to composite is adequate to withstand polymerization stresses (at least 17 MPa).[17] Bond strength between TLC/BD/RMGIC liners and composite depends on their physicochemical properties, nature of the bond between liner and RC, and the types of adhesive used.
In the present study, methacryloyloxydecyl dihydrogen phosphate (MDP)-based, universal adhesive with silanes was selected. This self-etch 10-MDP-based adhesive shows chemical bonding to Ca ions, and Al and zirconium oxides.[18,19] The bifunctional silane molecule bonds chemically to silica-containing materials and has methacrylate functionality that allows chemical union with resinous substrate. Silanes also act as adhesion promoters by enhancing the wetting ability of th eadhesive system.[20] This adhesive was selected in our study, aiming for additional chemical bonding with Ca releasing bio active liners.
In this study, Groups A and C (TLC and RM-GIC) showed significantly higher bond strengths than Group B (BD) as TLC and RMGIC are resin-based light cure cements that attain early cohesive strength on photo activation. Group B (BD) showed the least SBS means (5.666 MPa), which may have been due to low early strength of the material per se and this was in agreement with previous studies.[10] BD is a porous material that needs at least 2 weeks time for crystallization of hydrated calcium silicate gel to attain bulk strength adequate to withstand the polymerization stresses.[21] In the present study, bonding was performed to BD immediately after 12 min to depict a single appointment clinical procedure. This could be the reason for low bond strength and cohesive failures in BD.
Groups A and C (TLC and RM-GIC) showed statistically similar SBS means (18.249 and 18.656 MPa, respectively) and were adequate to withstand contraction stresses of RC. This could be due to similar resin chemistry promoting chemical adhesion with RC as proposed for RMGIC. Hydroxyethyl methacrylate (HEMA) incorporated into the TLC and RM-GIC forms a chemical bond with the resin of the composite. Additional chemical union is due to copolymerization of unreacted methacrylate groups present in the oxygen-inhibited layer of TLC/RMGIC with those of composite resin.[6,22] The resin bonding agent intermixes with both composite and TLC/RMGIC by true chemical bonding to create a strong interface.
Though the SBS of Groups A and C (TLC and RMGIC) were similar, the failure modes were predominantly cohesive in Group A (TLC) while Group C (RMGIC) showed 60% mixed failures and 40% adhesive failures. Cohesive failure in TLC could have been due to its low bulk strength. TLC, a resin-modified (RM) calcium silicate cement is a combination of a HEMA/TEGDMA-based resin and calcium-silicate powder. On light activation, HEMA and TEGDMA monomers create a polymeric network that is able to stabilize the outer surface of the cement. Thus formed poly-HEMA is hydrophilic and favors the absorption of moisture and triggers a second setting reaction that is hydration of calcium silicate particles with liberation of calcium ions.[2] TLC releases more Ca ions than RMGIC. Hence, a strong chemical bonding among adhesive and Ca, Al, Zr, and silicon ions of TLC could have resulted in similar bond strength values as RMGIC group in spite of its low bulk strength.
CONCLUSION
Within the limitations of this in vitro study, the following conclusions are made:
RMGIC and TLC achieved adequate bond strength to withstand contraction forces from overlying composite resin due to the presence of a resin matrix.
Composite restoration can be placed immediately over TLC and RMGIC, completing the procedure in single appointment.
BD showed significantly lower bond strength values when immediately bonded to RC. The mode of failure was cohesive within BD, indicating it as a weak material in its early setting phase.
This highlights the importance of leaving BD to mature for longer time period before the application of overlying composite restoration.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
Acknowledgment
We would like to thank Mr. Govinda Raju, In-Charge, MSME Testing Station, Hyderabad, India for letting out the institute's facilities for bond strength testing, and Mr. Ganapati, Assistant Professor of Community Medicine, GSL Medical College, Rajahmundry, Andhra Pradesh, India for helping in statistical analysis.
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