Comparative short-time evaluation of microtensile bond strength of modified 8^th generation bonding agent to sound and simulated erosive dentin

Introduction

It is an arduous task to establish long-lasting adhesion between the dentin, enamel, and resin materials (1). The introduction of bonding agents and composite resin restoration has revolutionized restorative dentistry. Since the introduction of bonding agents, intensive research has been conducted by scholars that focus on the chemistry of the setting of material, simplification of clinical use, and increase in the bond strength (2). The current concepts of adhesive materials define a micromechanical bond of the adhesive to the restorative material and the tooth structure declining the chemical bonding concept (3).

Fusayama’s research demonstrated two distinct layers of the carious dentin (4): the outer layer (caries-infected dentin) comprising of microorganisms is characterized as being highly demineralized, which is physiologically unremineralizable. This shows irreversibly denatured collagen fibrils with a virtual disappearance of cross-linkages and needs to be removed before the restoration placement. Whereas, the inner layer (firm dentin) is uninfected, partially demineralized, and physiologically remineralized, therefore should be preserved during clinical treatment (5).

The newer adhesives were introduced and generations of bonding agents advanced with improvement in properties from the succeeding generations (6). The 8^th generation bonding agent was introduced by Voco Futurabond Germany, in the year 2010. This generation consists of nano-sized filler particles that tend to increase the hybrid layer thickness and the resin monomer penetration along with dual cure bonding agent, thereby leading to a bonding system with improved mechanical properties (7).

Recently, there has been an interest in modifying the bonding agents with antibacterial agents like arginine and chitosan. Use of various percentages have lately been under the focus of research in restorative dentistry (8,9).

Arginine is naturally secreted in saliva as salivary peptides and is found in certain foods. It is metabolized in oral cavity by certain oral bacteria via the arginine deiminase system (ADS) pathway to supply ammonia. This leads to the neutralization of glycolytic acids and contributes to an increase in the pH of oral biofilms (10-12). Chitosan is a natural polysaccharide biopolymer produced by alkaline partial deacetylation of chitin. Chitosan is antibacterial in nature and this property has been backed up by ample evidence in the literature. It is also biocompatible, non-toxic, and biodegradable in nature (12,13).

In minimally invasive dentistry, affected, that is firm dentin is retained and the restorative material is bonded to it with the application of bonding agents or adhesives whereas the infected or soft dentin is removed (14). The longevity of any adhesive restoration depends on its physical properties (15). Thus, properties like microtensile bond strength (µTBS) play a vital role in evaluating the bonding efficacy on firm dentin. There is a scarcity of literature on such research topics, with one such study reporting on the simulation of erosive dentin with the assistance of 10% citric acid (16). This study was aimed to evaluate and compare the µTBS of the 8^th generation bonding agent modified with 7% arginine and 0.12% chitosan on sound and simulated erosive dentin.

Methods

Healthy maxillary and mandibular premolars indicated for extraction due to orthodontic treatment or periodontal condition were included in the study. The sample size was calculated using G-Power 3.1.9.2 software keeping alpha at 0.05, power of the study 80%, and effect size of 0.50. A total of 42 extracted teeth were included using the convenience sampling technique and were allocated in three groups and six subgroups with 14 teeth in each group and seven in each subgroup as follows:

• Group I: 8^th generation bonding agent (control group) (G-Premio BOND, GC Corporation, Tokyo, Japan):
  • Group IA: µTBS on sound dentin;
  • Group IB: µTBS on simulated erosive dentin.
• Group II: 8^th generation bonding agent modified with 7% arginine:
  • Group IIA: µTBS on sound dentin;
  • Group IIB: µTBS on simulated erosive dentin.
• Group III:8^th generation bonding agent modified with 0.12% chitosan:
  • Group IIIA: µTBS on sound dentin;
  • Group IIIB: µTBS on simulated erosive dentin.

Preparation of adhesives (8)

Arginine modified adhesive was prepared by adding 7% of arginine (NANOSHEL^®, Sundran, Punjab, India) to 8^th generation bonding agent (G-Premio, GC Corporation, Tokyo, Japan) and chitosan modified adhesive was prepared by adding 0.12% (w/w) of chitosan (NANOSHEL^®) to the bonding agent and chitosan solution was prepared by dissolving chitosan powder in 1% (v/v). The experimental adhesive was prepared by using an equal volume of chitosan or arginine and the bonding agent (one drop each) in a dark room under controlled temperature and humidity.

Simulation of erosive dentin (16)

Simulation of erosive lesions was done by placing 21 extracted teeth in 10% citric acid solution (Ammdent Citocid, Ammdent Pvt Ltd., Mohali, Punjab, India) for 4 hours.

Study procedures (Figure 1)

Figure 1 Evaluation of microtensile bond strength: (A) horizontal sectioning of the teeth to expose dentin; (B) simulated erosive dentin; (C) application of bonding agent; (D) 4-mm composite build up; (E) vertical sectioning of the teeth; (F) testing microtensile bond strength using universal testing machine.

• Application of adhesive: a total of 42 healthy extracted teeth were sectioned horizontally mesio-distally approximately at the level of dentin-enamel junction (DEJ) to expose the dentin. Following this, two coats of adhesives (control/experimental) were applied on the dentinal surface of sectioned premolar teeth (sound and erosive dentin) with the help of a disposable brush. The adhesives were air-dried for 5 seconds and were then light-cured using a light cure gun (Woodpecker, Henan, China) of wavelength 470 nm for 10 seconds. The distance of light source was standardized to 5 mm from dentine surface.
• Composite build-up: succeeding adhesive application, a 4-mm thickness composite resin (3M^TM Filtek^TM Z350 XT, St Paul, MN, USA) build up was done with the help of mold using a layered technique and was cured by visible light of 470 nm wavelength for 40 seconds. These specimens were stored in distilled water at 37 °C, with 100% humidity for 24 hours before sectioning.
• Teeth sectioning and testing (17): the bonded teeth were then vertically sectioned into serial slabs and further into beams with a cross-sectional area of 1 mm² using a low-speed diamond saw under water coolant. The µTBS testing was performed using a universal testing machine (computerized, software-based; ACME Engineers, India. Model No. UNITEST-10). After aligning the jaws of the universal testing machine in a straight line, the specimens were attached to a jig (fixtures) with cyanoacrylate adhesive. The crosshead speed of 1 mm/min at room temperature was selected and the teeth were subjected to failure. Load required to debond the adhesive interphase of tooth-composite was recorded. The µTBS was then calculated by dividing the load at failure (F) by the cross-sectional area (A). The µTBS was calculated in MPa by using the formula: µTBS = F/A.

Statistical analysis

The data were subjected to normality using Kolmogorov-Smirnov test. The IBM-SPSS (SPSS, Inc., Chicago, Illinois, USA) version 26 software was used to analyze the data. An unpaired t-test was used to assess the difference between the two groups. One-way analysis of variance (ANOVA) was performed followed by a post-hoc Tukey’s test for intergroup comparison between three groups. Confidence intervals were set at 95%, and a P value <0.05 was considered statistically significant.

The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the institutional ethics committee of Dr. D. Y. Patil Dental College and Hospital, Dr. D. Y. Patil University (DPU) (No. DYPDCH/IEC/164/159/20) and individual consent for this retrospective analysis was waived.

Results

The mean values of µTBS in 8^th generation bonding agent group for sound dentin (Group IA) was 7.138 MPa and simulated erosive dentin (Group IB) was found to be 5.084 MPa. In 8^th generation bonding agent modified with 7% arginine in Group IIA was 8.300 MPa, whereas in Group IIB was 6.251 MPa. However, µTBS in 8^th generation bonding agent modified with 0.12% chitosan in Group IIIA (sound dentin) was 9.127 MPa and in Group IIIB (simulated erosive dentin) was found to be 7.682 MPa (Table 1).

Table 1 represents a statistically significant difference in µTBS between sound and simulated erosive dentin in all the three study groups. The µTBS was significantly more with sound dentin compared to simulated erosive dentin in groups with 8^th generation bonding agent (P<0.001, t=10.727), 8^th generation bonding agent modified with 7% arginine (P<0.001, t=8.408), and 8^th generation bonding agent modified with 0.12% chitosan (P=0.001, t=4.149).

One-way ANOVA revealed a significant difference between the groups for sound dentin (P<0.001, F=25.755) and simulated erosive dentin (P<0.001, F=50.321). A post-hoc test revealed a significant difference in µTBS between 8^th generation bonding agent modified with 0.12% chitosan and 8^th generation bonding agent (P<0.001 for sound and simulated erosive dentin), between 8^th generation bonding agent modified with 7% arginine and 8^th generation bonding agent (P=0.002 for sound dentin, P=0.001 for simulated erosive dentin), and between 8^th generation bonding agent modified with 7% arginine and modified with 0.12% chitosan (P=0.02 for sound dentin, P<0.001 for simulated erosive dentin) (Table 2).

Discussion

The use of 8^th generation bonding agent is crucial due to its advantages of improved adhesion, enhanced penetration and hybrid layer formation, optimized mechanical properties, versatility of the material, and dual cure polymerization utilization (8,18,19). The 8^th generation bonding agent contains nano-sized fillers that increase the penetration contributing to the formation of a thicker and more effective hybrid layer and enhancing the bond strength between the tooth and the restorative material. This improved hybrid layer thickness and resin penetration lead to enhanced mechanical properties of the adhesive such as better resistance to wear, stress, and forces experienced during mastication. Moreover, it can bond with all the substrates allowing for a wide range of applications in restorative and cosmetic dentistry, thereby indicating its versatility (8,18,19).

The present study compared the 8^th generation bonding agent (G Premio BOND, GC Corporation, Tokyo, Japan) with the modified version of the same adhesive; one modified with 7% arginine and the other with 0.12% chitosan and demonstrated 8^th generation bonding agent modified with 0.12% chitosan and 7% arginine exhibited significantly higher µTBS compared to the unmodified bonding agent in case of both, sound and simulated erosive dentin. The results of the present study are in accordance with the reported literature. The study by Nakajima et al. evaluated tensile bond strength on caries affected firm dentin and the normal dentin. The one-step bonding system following etching with the 10% phosphoric acid presented significantly lower tensile bond strength with caries affected firm dentin over the sound dentin. However, the tensile strength became comparable when etching was performed with 32% phosphoric acid (20). Another study by Yoshiyama et al. focused on the effect of total-etch adhesive and an experimental self-etching adhesive in the bond strength when applied over sound, firm, and soft dentin. The study demonstrated that bond strength was significantly higher for the sound dentin followed by affected firm dentin and the least with the infected soft dentin (21). Similar was reported in a meta-analysis wherein bonding to sound dentin was considered better over bonding to firm dentin.

A contrast result was reported by Baena et al., who evaluated the effect of chitosan as a crosslinker on matrix proteinase activity and bond stability and stated that, 0.1% chitosan solution when used as an additional primer with etch-and-rinse adhesive and universal self-etch adhesive, did not influence the immediate bond strength. The study also presented no influence on the bond strength after thermocycling (22).

The possible reason for the difference in bonding strength can be attributed to the differences in chemical, morphological, and physical characteristics of sound and firm dentin (5). It should be noted that the firm dentin being partially demineralized and porous, is more susceptible towards the acid etching allowing easy diffusion of acid conditioners and thus forms deeper porosities and a thick hybrid layer (23). However, when it comes to bonding with the dentin to form resin tags, the results are not satisfactory, and thus in spite of the thick hybrid layer, the porosities were seen in the firm dentin. Additionally, the bond strength of the adhesive with the dentin can be affected by other factors like decrease in the modulus of elasticity, the presence of acid-resistant intra-tubular mineral deposits, and decreases in modulus of elasticity. However, the problem may not arise if the firm dentin after excavation of infected dentin is surrounded by normal dentin to provide high bond strength (23,24).

In the present study, bonding agent incorporated with chitosan demonstrated higher µTBS over unmodified bonding agent. Chitosan used as a modifying agent in this study, can form chemical bonds with the collagen fibers present in dentin. This interaction creates a stable interface between the dentin and the resin adhesive, enhancing the bond strength by promoting adhesion at a molecular level. Chitosan also tends to decrease the enzymatic activity of collagenase as it is a calcium chelator (25). Further, chitosan nanoparticles can act as reinforcing agents in the adhesive, enhancing its mechanical properties by increasing its toughness and bond strength (26). These properties explain the higher µTBS of chitosan added bonding agent over non modifying bonding agent in the present study.

In a study by de Carvalho et al., chitosan when incorporated in the adhesive system at a concentration of 0.12% and 0.25% did not compromise the bond strength to the dentin when compared to the higher concentration of chitosan, i.e., 1% (12). Few other studies have evaluated the effect of Chitosan and Arginine on change in the physical properties of resin materials. Kim et al. in their study reported that the Vickers hardness and flexure modulus were comparable between modified and the unmodified resin materials. However, the flexure strength was low in the resin incorporated with chitosan over the unmodified resin (27). Further, an inverse relation of concentration of chitosan was reported in a study wherein, there was a decrease in the flexure strength of the resin material when concentration increased from 5% to 20% (28). Additionally, when the incorporation of other additives was studied, there was no effect of the modified experimental adhesives incorporated with methacrylate monomers and arginine at 5%, 7%, and 10% or no arginine (control) on the physical and mechanical properties of adhesive (10).

The addition of arginine in the bonding agent demonstrated higher µTBS over unmodified bonding agent in the present study. The results obtained can be attributed to the properties of arginine reported in literature. Arginine acts as a surface activator and conditioner for dentin. It helps to modify the dentin surface, making it more receptive to adhesive infiltration. This conditioning effect enhances the penetration of the adhesive into the dentin, resulting in a stronger bond (29). The studies have also documented use of Arginine containing toothpaste and its effect on the bond strength of adhesive systems to eroded dentin. After 1,000 tooth brushing cycles with arginine containing toothpaste, the dentin eroded specimens were compared to the adhesive systems applied over dentin treated with artificial saliva and arginine-free toothpaste. No significant difference was observed in the bond strength of adhesive applied soon after tooth brushing on eroded dentin between arginine treated groups to that of control groups hereby showcasing no interaction of arginine with the bond strength (30).

The results of the present study were in accordance with the study wherein 5%, 7% and 10% arginine incorporation into the adhesives did not significantly alter the µTBS (10). A mirrored evidence is also provided by a literature review on documented literature which states that, the addition of antimicrobial agents into the adhesive did not negatively impact the mechanical properties on the materials when compared to their counterparts with unmodified adhesives (8,31).

Limitations

The limitations of this study were: the small sample size and the in-vitro study design. Moreover, the effect on bond strength after thermocycling was not considered. The µTBS was evaluated after storing the specimen in distilled water for a short duration (24 hours). The results should be confirmed by employing a larger sample size, longer evaluation period, and thermocycling. Additionally, in-vivo studies should be done to simulate the oral environment and assess the bonding efficacy of modified adhesives against the (carious) firm dentin with the incorporation of chitosan and arginine in the bonding agents.

Conclusions

Mean µTBS was significantly more in sound dentin compared to simulated erosive dentin in all the three study groups, i.e., 8^th generation bonding agent, 8^th generation bonding modified with 7% arginine, and 8^th generation bonding agent modified with 0.12% chitosan. The 8^th generation bonding agent modified with 0.12% chitosan provided the highest µTBS on both sound and simulated firm dentin, while the 8^th generation bonding agent provided the least µTBS.

Acknowledgments

Funding: None.

Footnote

Data Sharing Statement: Available at https://fomm.amegroups.com/article/view/10.21037/fomm-23-50/dss

Peer Review File: Available at https://fomm.amegroups.com/article/view/10.21037/fomm-23-50/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://fomm.amegroups.com/article/view/10.21037/fomm-23-50/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the institutional ethics committee of Dr. D. Y. Patil Dental College and Hospital, Dr. D. Y. Patil University (DPU) (No. DYPDCH/IEC/164/159/20) and individual consent for this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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