Introduction

The demand for aesthetic adhesive restorations has increased since Buonocore introduced the acid etching technique in 1955¹. This has led to the development of different adhesive strategies²⁾ based on the preservation/integration of the smear layer^3-4: smear layer removal or etch-and-rinse technique^5-8, and self-etch, which integrates the smear layer. These adhesive techniques behave differently over dentin. Although the etch-and-rinse technique is the most widely used system, its effectiveness is questioned because the adhesive monomers do not always manage to cover all the etched surfaces, leaving a hybrid layer with empty spaces and unsupported collagen fibers⁹. Additionally, with the self-etch technique, it is possible to remove minerals from the smear layer surface and replace them simultaneously with resin monomers^10-11. The adhesion process includes a phenomenon called nanofiltration, which occurs due to the presence of nanometric empty spaces that take fluids from the dental pulp to the external surface of dentin¹⁴. These spaces appear because of an uncomplete infiltration of the resinous monomers into the spaces created between the collagen fibers. This creates a demineralized dentin zone under the hybrid layer, that eventually causes degradation of the resin-dentin bond by hydrolysis^11,15. In addition, unprotected collagen fibers (not surrounded by resin or dentin) can be degraded by endogenous proteolytic enzymes found in dentin, called matrix metalloproteinases (MMPs). These enzymes lead to degradation of the extracellular matrix in different physiological processes^11,15. At the hybrid layer, these enzymes are responsible for a decrease in the long-term bond strength of resinous materials to dentin¹⁶.

The use of different agents to counteract the effect of MMPs has been suggested, such as riboflavin¹⁷, natural extracts¹⁷ and chlorhexidine^11,15,17_. Chlorhexidine is a synthetic chemical compound with remarkable antibacterial properties; it is widely used in various medical, surgical and dental procedures^11,15. In restorative dentistry, chlorhexidine was initially introduced as a cavity disinfectant used before applying restorative materials in order to eliminate bacteria that could irritate the dental pulp^11,15. In addition, MMPs -2, -8 and -9 have shown an inhibitory effect against proteolytic activity, thus contributing to increased durability of the resin-dentin bond^11,15.

In order to simplify decision making and the clinical technique implemented, nowadays, the market offers universal adhesives, which are highly versatile (they can be applied as part of the etch-and-rinse or self-etch techniques) and are compatible with multiple substrates (teeth, metals and/or ceramics)^3,5,14.

In addition, adhesive systems with chlorhexidine are available on the market. This could contribute to adhesive stability by decreasing adhesive bond degradation, increasing the durability and success of the adhesive plastic restorations ^(11,15. The recent introduction of this universal adhesive with chlorhexidine into the market raises questions about its effectiveness, advantages and properties for consumers (dentists) as there is not enough evidence proving that adding chlorhexidine does not affect its performance.

In this context, this work aims to compare the dentin-bond strength of a new universal commercial adhesive system with chlorhexidine and to compare the values obtained with a universal commercial adhesive system without chlorhexidine in the etch-and-rinse and self-etch techniques.

The hypotheses proposed are that the adhesive system with chlorhexidine will have similar bond strength values compared to the adhesive system without chlorhexidine. In addition, there will be no statistically significant differences between the etch-and-rinse and the self-etch techniques.

Methodology

Sample preparation for the microtensile test

Each tooth was worn down transversely, using a wet trimmer, removing enamel from the occlusal face, exposing a flat area of coronal dentin without pulp exposure. The coronal dentin was subsequently worn down with silicon carbide sandpaper with 600 grit size by making gentle eight-shaped movements and supplying copious irrigation to standardize the smear layer. The corresponding adhesive treatment was applied on the exposed dentinal surface according to the group that the sample was part of and following the manufacturer’s instructions (Table 1). The selected adhesives were used according to the manufacturer’s instructions. After the adhesive procedures, a 4 mm block of 3M Filtek™ Z250 XT composite resin (3M THICK; ST PAUL, MN, USA) was created, handling the material as directed by the manufacturer. Polymerization was performed using an Optilight Max light-curing unit (Gnatus; Ribeirao Preto, Brazil). The irradiance of the light-curing unit was tested with a Bluephase Meter (Ivoclar Vivadent; Schaan, Linchenstein, Germany), which ensured it was working properly.

Table 1:  Name, composition and application procedure of the materials used in this study 

Once prepared, the samples were stored for 24 hours in distilled water at 37°C. After that period, the teeth were sectioned using a Gellings-Hamco micro trimmer (Hamco Machines INC.; Rochester, NY, USA), to obtain 1 mm² sectioned rods (Fig. 1), (approximately eight rods per tooth) which were half dentin and half composite resin. Only five rods obtained from the central region of each tooth (n=25) were selected for the microtensile test.

Fig. 1: Specimen preparation process 

A device with two sliding stainless steel plates was used for this test. Each rod was glued from both ends with cyanoacrylate to said plates, leaving the tooth-restoration interface free between both plates. In this way, the tensile strength was transmitted to the rod¹⁸.

The microtensile test was performed using an MTS SANS CMT 2000 5KN universal testing machine (MTS Systems Corporation; Shanghai China). The specimens were subjected to microtensile tests, according to ISO 11405 (Fig. 2)¹⁹. Once fractured, the specimens were analyzed under the microscope at a 40x magnification to classify the type of fracture into adhesive, dentin cohesive and resin cohesive.

Fig. 2: Universal testing machine (MTS SANS CMT 2000 5KN) 

Results

The values (mean and standard deviation) of microtensile bond strength in dentin are shown in Figure 3. According to the chart, Peak Universal Bond (Ultradent) system applied with the self-etch technique yielded the highest bond strength values, while the Single Bond Universal (3M ESPE) self-etch system yielded the lowest bond strength values on average. However, no statistically significant differences were observed among any of the four groups studied after performing a two-way ANOVA test (material factor p=0.537, mode factor p=0.864, factor interaction p=0.27).

Regarding the types of failure, all groups had mainly adhesive failures, and to a lesser extent, cohesive failures in dentin. However, the group where Peak Universal Bond adhesive system was used and the self-etch technique was applied exhibited more cohesive failures in dentin (Fig. 4). No mixed failures were observed. The variability in the frequencies of failure types when comparing different groups was not statistically significant (p=0.121).

Fig. 3: Chart: Microtensile bond strength according to adhesive system (Single Bond Universal - SBU or Peak Bond Universal - PBU) and mode of use (etch-and-rinse - TE or self-etch - SE). The same capital letters indicate no difference between the adhesive systems used in the etch-and-rinse mode. Table: Arithmetic mean andstandard deviation of microtensile bond strength  

Fig. 4: Analysis of the failure mode of the groups evaluated 

Discussion

This study found no statistically significant differences in the bond strength values between the two adhesive systems used, as well as no statistically significant differences between the etch-and-rinse and self-etch techniques. Therefore, our working hypothesis can be accepted.

These results are consistent with various studies that show that nowadays, thanks to technological and scientific advances, statistically similar values are achieved with both etch-and-rinse and self-etch adhesive systems. Traditionally, some papers defended the superiority of etch-and-rinse systems over self-etch in dentin^4-5,13. A few current studies show that the dentin self-etch technique allows forhigher adhesion values than the traditional etch-and-rinse technique¹².

It has been shown that dentin bond strength values obtained through microtensile testing using two-step self-etch and total-etch adhesive systems presented statistically similar values, with the exception of one-step self-etch, which deviates from the values⁸. It is reported in the literature that one-step self-etch systems do not have good results in bond strength tests, possibly due to the combination of hydrophilic and hydrophobic acids and monomers in a single solution²².

One study concluded that, regardless of the substrate on which the systems are applied (superficial or deep dentin), bond strength was higher in the two-step self-etch system than in the one-step or conventional total-etch system²². In addition, self-etch adhesive systems are less reliable regarding bond strength and nanofiltration compared to three-step etch etch-and-rinse systems, which remain the gold standard for comparative studies¹. Among self-etch systems, two-step systems have better clinical and laboratory performance than one-step systems. Additionally, self-etch adhesive systems perform better in dentin adhesion than in enamel²³.

The use of universal adhesives is indicated for the total-etch and self-etch techniques, and according to the manufacturer, they are equally effective in any of the modes used. Recent studies have shown that during the acid-etch technique, matrix metalloproteinase enzymes (MMPs) are released from the dentin and can degrade the hybrid layer. In order to control the effect of MMPs, the use of different agents, including chlorhexidine, has been studied. This compound has antibacterial properties that inhibit the proteolytic activity of MMPs at very low concentrations, thus inhibiting hybrid layer degradation and achieving greater resin-dentin bond durability^11,15.

This study compared the adhesive efficacy of two universal adhesive systems, one of which contains chlorhexidine in its composition. The results showed that the use of chlorhexidine does not influencebond strength²⁴. These results suggest that adhesive systems containing 0.2% chlorhexidine can be used. Despite this result, it must be noted that the group that used the Peak Universal Bond as an adhesive system in self-etch mode showed a greater number of cohesive failures in dentin, which indicates a possible greater interaction of this adhesive system with the tooth substrate compared to that of other groups studied.

Although the use of chlorhexidine does not directly affect immediate bond strength, long-term outcomes may differ given its role as an MMP inhibitor. Another review studies including chlorhexidine in the first of some two-step self-etch adhesives. The authors found that during the adhesion protocol, it inhibits the action of MMPs to a certain extent and has no adverse effects on the immediate bond strength to the dental substrate ²³.

In summary, both the aforementioned reviews and this study agree that chlorhexidine plays a major role in inhibiting MMPs without having adverse effects on the bond strength regardless of the adhesive system used, as the technique selected, etch-and-rinse or self-etch, shows no difference in bond strength. In order to expand the knowledge already created with this in vitro study, the authors recommend further in vitro studies in the medium and long term to monitor the presence of chlorhexidine in the adhesive interface and its effect on bond strength. Furthermore, once they are completed, we recommend conducting randomized clinical trials that can confirm the clinical performance of these adhesive systems to verify restoration durability and the bond interface obtained with the various systems.

It should be noted that this study has limitations in terms of the number of samples studied, since a greater n could decrease the standard deviation, which might allow us to detect a statistically significant difference. In addition, we must consider that bond strength, as the only factor taken into account does not provide comprehensive knowledge about the performance of an adhesive system; there are other in vitro methodologies such as microfiltration, bond strength tests after aging, and electron microscopy analysis of adhesive interfaces, among others, that would make it possible to have a complete notion of the laboratory performance of an adhesive system. Additionally, the strengths of this study are its methodological compliance with current ISO regulations, the standardization of samples and protocols, and the use of specific equipment, as well as data verification, as they are consistent with the existing literature, supporting the reliability of the published data.

Conclusions

Despite the limitations of this study, the performance of universal adhesive systems does not depend on how they are used in dentin. In addition, including chlorhexidine in a universal adhesive system did not significantly affect its performance.

Considering that chlorhexidine plays a major role in inhibiting MMPs, the use of such adhesives may prevent degradation of the hybrid layer and, therefore, increase the durability of the resin-dentin bond. Further studies should be conducted to confirm this.