Comparative evaluation of three different methods of surface pretreatment on the depth of adhesive resin penetration in sandwich technique – An in vitro confocal laser scanning microscopy study

Abstract

Background:

Sandwich technique combines the use of resin-modified glass ionomer cement (RMGIC) and composite to reduce polymerization shrinkage and postoperative sensitivity. Effective bonding relies on adhesive penetration inside RMGIC enhanced by surface pretreatments such as acid etching and lasers ensuring improved bond strength and longevity. The present study evaluated the depth of adhesive resin penetration into RMGIC using three surface pretreatment methods in sandwich technique analyzed through confocal laser scanning microscopy (CLSM).

Materials and Methods:

Class I cavity preparation was done in 30 extracted maxillary premolars. RMGIC was applied and samples were categorized into three groups: Group 1 (control group, no pretreatment), Group 2 (etching was with 37% phosphoric acid for 15 s), and Group 3 (treated with erbium, chromium-doped yttrium, scandium, gallium, and garnet laser). Rhodamine B dye was mixed with adhesive resin and applied over RMGIC followed by composite restoration. Samples were sectioned buccolingually and evaluated under CLSM. Data analysis was done using analysis of variance and Tukey’s post hoc tests.

Results:

Statistical analysis revealed significant differences among the three groups. The laser-treated group exhibited the greatest depth of penetration compared to the acid-etched and control groups.

Conclusion:

Laser pretreatment significantly enhances adhesive penetration depth compared to acid etching or control suggesting its clinical efficacy in improving bond strength in the sandwich technique.

INTRODUCTION

In dentistry, there has been the quest for an optimal restorative material that mirrors the structural properties of tooth structure, provides adhesion to both enamel and dentin, and resists degradation within the oral cavity. Glass ionomer cement (GIC) was first introduced by Wilson and Kent in an effort to achieve these qualities.[1] A significant advancement in this type of material emerged with the advent of resin-modified GIC (RMGIC).[2]

A material such as GIC or RMGIC, which has a low resistance to elastic deformation, is applied beneath composite restorations in deep cavities. This approach, known as the “sandwich technique” otherwise known as composite laminated restoration, has greatly minimized postoperative sensitivity commonly associated with traditional composite restorations. This method synergistically combines the chemical bonding and sustained release of fluoride from RMGIC with advantage of low shrinkage reducing postoperative hypersensitivity and superior esthetic qualities of composite resin.[3,4] The long-term viability of the restoration depends on the interaction between RMGIC and composite resin. This bond relies on both mechanical interlocking and chemical interactions, which are influenced by factors such as the surface characteristics of RMGIC, the adhesive resin’s viscosity, and the curing process of the composite.[5,6]

Proper surface pretreatment of the RMGIC layer plays an essential role in enhancing adhesion and ensuring the longevity of the restoration. Various conditioning methods have been developed which include acid etching, air abrasion, photodynamic therapy, and laser treatments using erbium, chromium-doped yttrium, scandium, gallium, and garnet (Er, Cr: YSGG) laser. Acid etching improves micromechanical retention but is time-consuming and technique-sensitive. With the evolution of lasers in dentistry, laser treatments using Er, Cr: YSGG laser create rough, etched-like surfaces through ablation and microabrasion. Lasers are safe, efficient, and effective, providing promising outcomes for improving adhesion in clinical applications.[7,8]

Application of bonding agents improves the wettability of RMGIC to adhere to composite resin, thus promoting a strong shear bond between RMGIC and the resin composite.[9] The one-step self-etch adhesive system is a simplified adhesive technology, designed to reduce working time. Two-step self-etch adhesive system integrates the primer and adhesive into one solution, effectively removing the smear layer and enhancing bond strength.[10] The penetration of adhesive evidenced by the formation of tags caused by the filling in the microspaces is the key factor in bonding between RMGIC and composite.[11]

Confocal laser scanning microscopy (CLSM) has emerged as a valuable tool for evaluating the depth of adhesive resin penetration. This technique provides high-resolution, three-dimensional imaging of the resin–substrate interface, allowing for detailed analysis of bonding efficiency.[12] Despite its advantages, there is limited research comparing the effects of various surface pretreatment methods on the depth of resin penetration using CLSM. The aim of this study is to evaluate and compare three various surface pretreatment methods on the depth of adhesive resin penetration into RMGIC in the sandwich technique using CLSM.

MATERIALS AND METHODS

Sample collection

Thirty freshly extracted intact permanent maxillary premolars were collected for this study. The patients undergoing extraction were informed about the procedure and general consent was obtained for the use of the extracted teeth in research purpose. An ultrasonic scaler was used to clean the teeth. The teeth were then sterilized by autoclaving and subsequently preserved in 0.1% thymol solution to prevent dehydration and microbial growth.

Cavity preparation and restoration procedure

Class I tooth preparation was done on each tooth using a no. 245 carbide bur [Figure 1a], achieving dimensions of 3 mm depth and 3 mm width [Figure 1b and c]. RMGIC was mixed following the manufacturer’s instructions and placed in 1 mm thick layers into the cavity preparations [Figure 1d and e]. RMGIC was light-cured for 20 s using an LED-curing light with an output intensity of 1000 mW/cm² [Figure 1f].

Figure 1.

Preparation of samples: (a) Class I tooth preparation, (b) Measurement of depth 3 mm, (c) Measurement of width 3 mm, (d) Manipulation of resin-modified glass ionomer cement (RMGIC), (e) RMGIC base of 1 mm, (f) Light curing of RMGIC, (g) Group 1 – no surface pretreatment done, (h) Group 2 – acid etching, (i) Group 3 – laser pretreatment, (j) Application of bonding agent mixed with rhodamine dye, (k) Composite restoration, (l) Sectioning of tooth

Pretreatment of resin-modified glass ionomer cement surface

The samples were categorized into three groups randomly, each consisting of 10 samples (n = 10) according to the pretreatment on the surface of RMGIC.

• Group 1: No surface pretreatment done – control group [Figure 1g]

• Group 2: Etching was done for 15 s with 37% phosphoric acid [Figure 1h] (Eco-Etch Etchant, Ivoclar Vivadent)

• Group 3: Er, Cr: YSGG laser (Waterlase iPlus, Biolase Technology Inc., San Clemente, USA) was operated with a pulse energy of 1 W, set to 10% water and 11% air for a duration of 15 s. A 600-μm diameter G-type tip was used, positioned perpendicular to the area of interest at a distance of 1 mm from the surface. The size of beam spot was calculated at 0.282 mm², resulting in an energy density of 17.7 J/cm². This laser system emits photons with a wavelength of 2780 nm, has a pulse duration ranging from 140 to 200 μs, and operates at a repetition rate of 20 Hz [Figure 1i].

After a 30-s water rinse, the specimens were gently air-dried using compressed air free of oil. Adper Single Bond 2 adhesive (3M ESPE, St. Paul, MN, USA), mixed with 0.1% rhodamine B dye (HiMedia, Mumbai, India), was applied to the pretreated RMGIC surface using a microbrush and light-cured for 20 s [Figure 1j]. The restoration was done with Filtek Z250 composite resin (3M ESPE, St. Paul, MN, USA) in increments and each layer was light-cured for 20 s following the manufacturer’s recommendations. Finally, the composite restorations were finished and polished with a composite finishing kit (SHOFU, Japan) to achieve a smooth and glossy surface [Figure 1k].

Confocal laser scanning microscope analysis

All samples were sectioned using a Baincut low-speed saw under continuous water, and cutting was done in buccolingual direction to achieve 1 mm thick slices [Figure 1l]. The depth of penetration of adhesive resin was assessed for all the groups using a Leica Stellaris 5 Confocal Laser Scanning Microscope at × 10 [Figure 2a-c].

Figure 2.

Confocal laser scanning microscopy image with measurements of depth of penetration: (a) No etching, (b) Acid etching, (c) Laser pretreatment

Statistical analysis

Statistical analyses were done with IBM SPSS Statistics for Windows, version 21.0 (IBM Corp., Armonk, NY, USA). The significance level was set at 5% and 95% study power was employed. The normal distribution data were confirmed with Shapiro–Wilk test. Descriptive statistics, specifically the mean and standard deviation, were computed to summarize the data. A comparison of the depth of penetration between the groups was done with analysis of variance followed by Tukey’s post hoc test. Statistical significance was determined at P < 0.05.

RESULTS

The results revealed significant variations in penetration depth across various RMGIC surface pretreatment methods. Notably, the surface pretreated with laser exhibited the greatest depth of penetration, showing a statistically significant difference compared to counterpart groups [Table 1]. The post hoc Tukey showed that there was a significant difference between laser and control groups as well as laser and acid-etching groups while there was no significant difference between acid-etching and control groups [Table 2].

Table 1.

The mean depth of penetration

	n	Mean (μm)	SD (μm)
No etching	10	61.7550	24.19672
Acid etching	10	69.9619	18.43558
Laser	10	218.0904	94.09737

SD: Standard deviation

Table 2.

Multiple group comparisons

Tukey HSD
(I) group	(J) group	Mean difference (I−J)	SE	Significant	95% CI
					Lower bound	Upper bound
No etching	Acid etching	−8.20697	25.53386	945	−71.5161	55.1021
	Laser	−156.33540*	25.53386	<0.001	−219.6445	−93.0263
Acid etching	No etching	8.20697	25.53386	945	−55.1021	71.5161
	Laser	−148.12843*	25.53386	<0.001	−211.4375	−84.8193
Laser	No etching	156.33540*	25.53386	<0.001	93.0263	219.6445
	Acid etching	148.12843*	25.53386	<0.001	84.8193	211.4375

*The mean difference is significant at the 0.05 level. HSD: Honestly significant difference, SE: Standard error, CI: Confidence interval

DISCUSSION

The sandwich technique is a widely recommended restorative procedure that leverages the esthetic qualities of composite resins and the enhanced physicochemical properties of RMGIC. This approach provides practitioners with several benefits of RMGIC, including stress absorption, fluoride release, shrinkage compensation during setting, strong ionic adhesion, and a low modulus of elasticity.[13] Composite resin was laminated over RMGIC as the bond strength between resin composites, and RMGIC is significantly higher than that with conventional GIC.[14,15] This increased bond strength is attributed to free-radical-initiated curing mechanism.[16]

The longevity of the restoration heavily depends on the interaction between RMGIC and both dentin and composite resin. The importance of pretreating enamel and dentin in the depth of adhesive penetration inside dentinal tubules is well-documented, while the required adhesive penetration depth after RMGIC surface pretreatment before composite resin application in sandwich restorations remains unclear.[17]

Pretreating the RMGIC surface generates microporosities, improving micromechanical retention by increasing surface irregularities, roughness, and porosity. The bonding relies mainly on these micromechanical interlocking features. Dental bonding agents have progressed through several generations, evolving from nonetch to total-etch systems (fourth and fifth generations) and subsequently to self-etch systems (sixth generations onward).[18] In this study, composite resin was bonded to pretreated RMGIC using a fifth-generation dentin bonding agent that integrates the primer and adhesive into a single bottle.

Watson and Wilmot were the first to propose fluorescent confocal microscopy for the examination of the interface between the tooth surface and restorative materials using fluorescent dyes included in adhesive agents to highlight the bonded interfaces.[19] Rhodamine B, a fluorescent dye, was incorporated into the adhesive resin at a concentration of 0.1%. Compared to methylene blue, the particle size of rhodamine B is smaller and has more surface-active molecules, making it a preferred choice in such applications.[10]

McLean et al. proposed a 60-s etching protocol to enhance the interaction between composite resin and GIC by promoting mechanical interlocking through increased surface porosity.[20] There are concerns that acid etching might weaken the cohesive strength of the cement. However, Farshidfar et al. discovered that pretreating with 35% phosphoric acid for 15 s prior to universal bonding agents enhanced the microtensile bond strength between GIC and resin composite.[21] Roughening the GIC surface by acid etching increases its surface energy and facilitates a stronger bond with the composite resin. CLSM analysis in this study revealed that acid-etched RMGIC surfaces exhibited more pronounced surface irregularities compared to nonetched controls, indicating that etching effectively promotes a robust RMGIC–composite resin interface.

Er, Cr:YSGG laser operating at 2790 nm wavelength was utilized for conditioning RMGIC in this study. CLSM analysis revealed that laser pretreatment significantly increased adhesive penetration depth compared to both the acid-etched and control groups. The Er, Cr: YSGG laser’s wavelength exhibits a strong affinity for water and a high absorptive capacity for hydroxyl groups, facilitating surface alterations. Given that RMGIC primarily comprises fluoroaluminosilicate glass and water-based polyacrylic acid, it is hypothesized that laser application induces micro-irregularities and increased porosity, thereby enhancing composite adhesion.[22,23]

Accordingly, this study showed that the laser improved the sealing capacity with a deeper adhesive penetration on the RMGIC surface. A research by Ghubaryi et al. reported that conditioning RMGIC with an Er, Cr: YSGG laser resulted in shear bond strength similar to that observed with acid etching. To understand the impact of laser treatment on RMGIC, it is essential to consider its chemical structure. The initial setting of RMGIC occurs through the polymerization matrix formation, followed by a slower chemical reaction that hardens and strengthens the material. The fully set material comprises two interpenetrating matrices: an ionic matrix formed through the acid–base reaction and a polymerization matrix resulting from free-radical reactions. Although the higher resin content in RMGIC typically reduces its permeability to acid after the initial setting, the Er, Cr:YSGG laser can penetrate the ionic matrix. Its hydrokinetic effect interacts with water and hydroxyl groups in the cement’s structure, creating micro-irregularities on the surface. These structural changes significantly enhance the bond strength in laser-treated groups compared to acid-etched counterparts.[8]

This study acknowledges certain limitations inherent to its in vitro design and the specific dye concentration employed. Future research should focus on examining the micromorphological characteristics of pretreated RMGIC surfaces. Investigating the effects of laser pretreatment and air abrasion, along with assessing the depth of adhesive penetration, are essential areas for further exploration. Addressing these areas in future studies will enhance our understanding and application of RMGIC in restorative dentistry.

CONCLUSION

Given the limitations of this study, the results indicate laser pretreatment before placing composite restorations in the sandwich technique may enhance the bond strength by increasing adhesive penetration depth. Conversely, etching of RMGIC with phosphoric acid prior to composite application does not significantly improve the bonding in sandwich restorations. Therefore, laser treatment of RMGIC surfaces before composite placement could be a clinically effective method to improve the bond in sandwich restorations.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.