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Table of Contents   
ORIGINAL ARTICLE  
Year : 2021  |  Volume : 24  |  Issue : 4  |  Page : 393-398
Shear bond strength of the adhesive/dentin interface after different etching protocols


1 Department of Prosthodontics and Dental Materials, School of Dentistry of Ribeirão Preto, University of São Paulo, São Paulo, Brazil
2 Department of Pediatric Clinics, Dental School of Ribeirão Preto, University of São Paulo, São Paulo, Brazil
3 Department of Social and Pediatric Dentistry, Federal University of Bahia, Salvador, Brazil
4 Department of Pediatric Clinics, Dental School of Ribeirão Preto, University of São Paulo, Ribeirão Preto, Brazil

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Date of Submission29-Apr-2021
Date of Decision07-Jul-2021
Date of Acceptance25-Aug-2021
Date of Web Publication13-Jan-2022
 

   Abstract 


Background : Lack of bond stability between the composite resin and dentin remains one of the main reasons for having to replace esthetic restorations.
Aim : This study aimed to evaluate the influence of chlorhexidine (CHX) on the bond strength of a total etching system to dentin tissue after degradation of the adhesive interface.
Materials and Methods : One hundred and eighty fragments of human molars were divided into three groups (n = 15) according to the materials used during the acid-etching step: Group 1 (control), 37% phosphoric acid; Group 2, a combined solution of 37% phosphoric acid and 2% CHX; and Group 3, 37% phosphoric acid followed by 2% CHX. Following the restorative procedures, the groups were divided into four subgroups according to the number of thermal cycles (TC) and to the time of storage in water (SW), and then subjected to a shear strength test, until fracture: (A) 0 TC/24-h SW; (B) 500 TC/1-week SW; (C) 2,000 TC/1-month SW; and (D) 12,000 TC/6-month SW. Two-way analysis of variance and Duncan's complementary test were used to perform multiple comparisons.
Results : After the 24-h and 1-week SW time periods, no statistically significant difference was found among the shear strength values of the control and the experimental groups: 5.48 ± 0.59 MPa, 5.44 ± 0.56 MPa, and 5.65 ± 0.94 MPa for G1, G2, and G3, respectively. However, the shear strength values decreased significantly in all the study groups after 1 month, namely 3.60 ± 0.41 MPa, 3.08 ± 0.65 MPa, and 3.49 ± 0.23 MPa for G1, G2, and G3, respectively. After 6 months, similar results were found for G1 and G3, namely 2.77 ± 0.58 MPa and 1.74 ± 0.52 MPa, respectively, whereas the 0.77 ± 0.26 MPa value found for G2 was significantly lower than those found for the G1 and G3 groups. No differences were found between the groups with respect to fracture-type frequencies.
Conclusion : The use of CHX as an agent to rehydrate the dentin had a negative influence on bond strength.

Keywords: Adhesive systems; chlorhexidine; dentin

How to cite this article:
Galo R, Marinho MT, Silva Telles PD, Borsatto MC. Shear bond strength of the adhesive/dentin interface after different etching protocols. J Conserv Dent 2021;24:393-8

How to cite this URL:
Galo R, Marinho MT, Silva Telles PD, Borsatto MC. Shear bond strength of the adhesive/dentin interface after different etching protocols. J Conserv Dent [serial online] 2021 [cited 2022 Jan 27];24:393-8. Available from: https://www.jcd.org.in/text.asp?2021/24/4/393/335743



   Introduction Top


Dentin is a mineralized conjunctive tissue that contains matrix metalloproteinases (MMPs) in its endogenous matrix.[1] MMPs are a group of neutral enzymes dependent on calcium and zinc, which regulate the physiological and pathological metabolism of tissues composed of collagen fibers.[2]

Recent studies have shown that the hybrid layer formed by a total-etch adhesive system is highly susceptible to degradation.[3] This is because part of the MMPs remain adhered to the dentin when the hydroxyapatite is dissolved by the acid etching,[4] and are activated by the acidic resinous component of the adhesive system.[5] This activation is responsible for the hydrolytic degradation of the exposed collagen fibers in the hybrid layer, and causes decreased bond strength when the restoration is subjected to a degradation process.[6]

Synthetic MMP inhibitors have been studied,[7] and chlorhexidine (CHX) has shown an inhibitory action on certain types of MMPs[8] contained in human dentin.[9] Thus, CHX could potentially inhibit the enzymatic activity of endogenous proteases present in the adhesive interface.[10] Previous studies have observed an enhanced stability of the hybrid layer when CHX was used after acid etching of the substrate.[11] However, there are no studies focusing on the effect of CHX on the bond strength of adhesive restorations, or its effect on this variable after degradation of adhesive restorations.[12]

The null hypothesis tested was that there would be no difference among the acid-etching protocols tested with respect to bond strength to dentin, irrespective of whether the dentin was pretreated with CHX, and irrespective of the number of thermal cycles (TC), and time periods of storage in water (SW) to which the specimens were subjected.


   Materials and Methods Top


Experimental design

The factors addressed in this study are related to dentin surface treatment before application of the adhesive system (3 levels: 37% phosphoric acid in Group 1, a combined solution of 37% phosphoric acid and 2% CHX in Group 2, and 37% phosphoric acid followed by 2% CHX in Group 3), and to the number of TC and time periods of SW [4 levels: 0 TC/24-h SW in subgroup A, 500 TC/1-week SW in subgroup B, 2000 TC/1-month SW in subgroup C, and 12,000 TC/6-month SW in subgroup D; [Table 1]].
Table 1: Experimental groups

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The study sample comprised 180 test specimens, randomly divided into 12 groups (n = 15). The quantitative response variable was the bond strength of the specimens, determined by a shear strength test.

Selection of teeth and materials

Ninety recently extracted human molars (that had been stored for no longer than 7 days after extraction) were obtained from the institutional tooth bank and stored in 0.9% saline with 0.4% sodium azide at 4°C. This study was approved by the institutional research ethics committee (approval no. CAAE-02791312.7.0000.5419).

Test specimen preparation

The tooth crowns were sectioned in the mesiodistal direction using a diamond disc under refrigeration (Minitom, Struers A/S, Copenhagen, DK-2610, Denmark), and the tooth roots were excluded, thus producing 180 sections of the buccal and lingual aspects. These sections were embedded in epoxy resin inside PVC cylinders (2.0 cm × 1.0 cm) and were then abraded with a polishing machine under refrigeration (Struers A/S, Copenhagen, Denmark) using water sandpaper with grit sizes #180 to #600 to produce flat dentin surfaces.[13] Subsequently, the specimens were washed and then stored in distilled water at room temperature for 24 h.

Restorative procedures

The specimens were randomly assigned to the study groups, and the experimental procedures were carried out in each group as follows:

In Group 1, the surfaces of the specimens were etched with 37% phosphoric acid (FGM, Joinville, SC, Brazil) for 15 s and washed for 5 s, after which excess humidity was removed with absorbent paper. Each surface received two layers of the adhesive (Single Bond, 3M Dental Products, St. Paul, MN, USA), applied on the dentin for 5 s. A gentle air jet was applied to each surface to evaporate the solvent, after which photopolymerization was carried out for 20 s with a halogen light-curing unit set to provide a light output of number <450 mW/cm2 (Curing Light XL-3000; 3M Dental Products, St. Paul, MN, USA).

In Group 2, the surfaces of the specimens were etched with a combined solution of 37% phosphoric acid and 2% CHX (Villevie, Joinville, SC, Brazil), following the same protocol performed in Group 1.

In Group 3, the surfaces of the specimens were etched with 37% phosphoric (FGM, Joinville, SC, Brazil) for 15 s, rinsed with a 2% CHX solution (FGM, Joinville, SC, Brazil) for 5 s, and dried using absorbent paper. The adhesive was applied in the same way as in the previous groups.

The specimens were fixed individually in a metallic clamping device, and the test surfaces were pressed against a split polytetrafluoroethylene jig to provide a cylindrical cavity (4 mm high × 3 mm diameter), coincident with the delimited dentin bonding site. The composite resin (Filtek Z250, 3M Dental Products, St. Paul, MN, USA) was inserted into the device in 2mm increments with the appropriate spatula, and then each increment was light cured for 40 s using a halogen light-curing unit (450 mW/cm2; Curing Light XL-3000; 3M Dental Products, St. Paul, MN, USA) until the entire cavity was filled with the composite resin. Subsequently, each test specimen was removed from the metallic device, leaving a cylinder of composite resin adhered to the surface of the specimen.

Degradation of the adhesive interface

The specimens (n = 45) were kept in distilled water at 37°C and then separated into four subgroups (n = 15) according to the number of TC and to the SW time. The water was changed every day. Each cycle of the thermocycling machine consisted of water baths at two different temperatures (5°C and 55°C), with each bath lasting 30 s and a transfer time of 5 s. After each cycle, the specimens were once again stored in water at 37°C. In Group B, the degradation process consisted of 500 TC/1-week SW; in Group C, 2000 TC/1-month SW; and in Group D, 12,000 TC/6-month SW.

Shear strength test

The specimens were subjected to a shear strength test in a universal testing machine (MEM 2000; EMIC Ltda, São José dos Pinhais, PR, Brazil) set to operate with a 50 Kgf load cell at a 0.5 mm/min speed until fracture. After the tests, the specimens were assessed to determine the observed fracture type (adhesive, cohesive, or mixed).

Data analysis

The distribution of the shear strength values (in MPa) was analyzed and was found to be homogeneous. Therefore, two-way analysis of variance and Duncan's complementary test were used to analyze the data (SPSS version 20.0, Chicago, IL, USA).


   Results Top


The shear strength means and standard errors found for the study groups are presented in [Table 2]. The statistical tests revealed that the shear strength values were significantly affected by the application of CHX (P = 0.018) and by the thermal cycling regimen, after the 6month period of SW (P = 0.002). No significant two-factor interaction was observed (P = 0.546). After the first 24 h, there was no significant difference among the groups with or without CHX with respect to the observed shear strength values and the same occurred after 1 week of SW. However, after both the 1- and 6-month time periods, the shear strength values were significantly reduced compared to the no-cycling and the 1-week groups. In addition, although the shear strength values decreased after the 1-month time period, there was no significant difference among the groups with or without CHX. In contrast, after 6 months, the shear strength values for Group 3 – where the acid was applied separately from the CHX solution – were significantly lower than those found for the other groups.
Table 2: Mean and standard error values of the shear bond strength (MPa) found for the study groups

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No statistically significant difference was found among the groups with respect to fracture type frequency [P = 0.45; [Figure 1]]. An analysis of failure mode in relation to thermocycling revealed that type-3 fractures were the most common [Figure 1]. In contrast, after 2000 TC, cohesive fractures were the most common (>70%), followed by adhesive fractures, for all of the dentin surface treatment protocols. For specimens not subjected to thermocycling, the mixed fracture type was more common than all the other types in Group 3, although not significantly.
Figure 1: Different types of fractures for groups

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   Discussion Top


The aim of this study was to evaluate the influence of CHX on the shear bond strength of a composite resin to dentin after degradation of the adhesive/dentin interface by thermocycling. The null hypothesis considered was that there would be no significant difference among the acid-etching protocols tested with respect to the bond strength of the resin to dentin, irrespective of whether the dentin was pretreated with CHX. Based on the results, this hypothesis was rejected because the shear bond strength values of specimens subjected to aging for long periods of time were lower when the dentin surface was treated with CHX than otherwise.

The use of CHX did not affect the bond strength of the resin to dentin after 1 day of SW at 37°C, and the strength values obtained with CHX after 1 day were close to those obtained without it (5.48 MPa),[14] except for Group 3 (phosphoric acid followed by CHX). However, in Group 2, where a combined solution of phosphoric acid and CHX was applied, and in Group 3, where CHX was applied after the acid etching procedure, the bond strength values were unaffected after both 24 h (0 TC) and 1 week of SW (500 TC).

Thermocycling simulates the changes caused in the oral cavity by food and beverage consumption, and even by breathing habits. It is expected that this aging method together with temperature variations will promote volumetric changes and fatigue of the adhesive interface.[15] The fact that the application of 12000 TC (6 months) significantly reduced the shear bond strength in the present study indicates that the time period during which the adhesive interface was subjected to degradation led to fracture propagation in some groups.[16]

According to Suma et al.,[17] the use of CHX as a metalloproteinase inhibitor reduces shear bond strength significantly, confirming the results of the present study where CHX proved ineffective in maintaining the bond strength values. Gürgan et al.[18] also found that the use of the same metalloproteinase inhibitor, either before or after acid etching and without rinsing, reduced the bond strength to dentin. This can be explained by the ability of these inhibitors to resist acid etching, thus inhibiting the ability of resin to impregnate the dentinal surface.

Other studies have reported that CHX can adversely affect the ability of hydrophilic monomers to infiltrate dentinal tubules, which, over time, can lead to decreased bond strength and increased microleakage.[12] Acid etching before using conventional adhesive systems promotes a more thorough infiltration into dentin.[19] As a result, CHX could probably infiltrate the dentinal tubules more easily and thus interfere with the bonding process by preventing the formation of a uniform layer and reducing the amount of hybrid resin tags.[20]

The results of this study showed that the use of CHX changed the shear bond strength values. During thermocycling, the specimens are subjected to mechanical stresses generated by different thermal conductivities. However, temperature variations occurring inside the specimens, as a result of water temperature, also have a significant impact.[21] The assessment of adhesive strength has revealed important information about the adhesive interface, and the most widely used aging technique has been SW,[22] which reduces the effectiveness of the bond through degradation of the adhesive interface.[19] Furthermore, CHX is soluble in water, and its long-term effectiveness can be compromised.[23]

Thus, the aging promoted by thermocycling in the present study negatively affected the bond strength of the tested adhesive system and produced values significantly lower than those found for the groups without thermocycling [Table 2], presumably due to increased microleakage and water penetration. There are reports that the self-etching adhesive systems when associated with the dentin reduces the degree of nanoinfiltration, thus promoting better seal and reduced presence of water, hence reduced hydrolytic degradation of the hybrid layer, and potentially increased longevity of restorations.[24] This is because the presence of moisture in the dentin, and its interaction with the hydrophobic components of the adhesive systems, plays an important role in obtaining an effective force of adhesion.[25]

In the study conducted by Sheikh Hasani et al.,[26] where CHX was applied after acid etching, the shear bond strength values were significantly higher when the specimens were subjected to 500 TC (2.34–6.35 MPa), compared to when they were subjected to 6000 TC (0.00–3.60 MPa). These results confirm those of the present study where the specimens subjected to the longest thermocycling time saw a significant decrease in their shear strength values (0.77 MPa) compared with those subjected to a shorter thermocycling time (3.49 MPa).

In this study, the hot water caused by the temperature rising from 5°C to 55°C likely accelerated the hydrolysis of the interface components, and the resulting water absorption led to their extraction from the poorly polymerized and degraded resin.[27] Temperature variation causes contraction and expansion of the restorative material, which has different physical properties than the dental structure, thus causing stress to the adhesive interface.[19] In addition, water can infiltrate and reduce the mechanical properties of the polymeric matrix by causing expansion, and by reducing the forces between the polymeric chains.[27] Nevertheless, the results found in the present study do not corroborate those found by other authors that assessed the rehybridization of the dentin with 2% CHX after acid etching.[28]

The choice of CHX at a concentration of 2% was based on results found in the related literature,[29] considering the inhibitory action of this concentration on enzymatic degradation (MMPs). The hypothesis considered was that rehydration of specimens with CHX could lead to higher adhesive strength values in comparison with rehydration by water, which was not observed in this study. However, the related literature has shown that the presence of hydrophilic monomers, like those found in single bond materials, also has the property of inhibiting the degradation of collagen by the absorption of MMPs, thus favoring the inhibitory action of CHX in the demineralized matrix.[30] In the present study, CHX was associated with lower bond strength values. Based on these results, rehydrating prepared cavities with CHX, or treating them with a combined solution of CHX and phosphoric acid, were found to be unwarranted procedures.


   Conclusion Top


Considering the methodology used and the results obtained in this study, it was concluded that the use of CHX as an agent to inhibit the degradation of the hybrid layer negatively affected the shear bond strength of the tested adhesive to dentin, in which the observed bond strength values decreased significantly after the test specimens were subjected to thermocycling.

Acknowledgments

The authors would like to thank the State of São Paulo Research Foundation (FAPESP – no. 2011/21375-7) for financial support of this research.

Financial support and sponsorship

This study was financially supported by the State of São Paulo Research Foundation (FAPESP – no. 2011/21375-7).

Conflicts of interest

There are no conflicts of interest.



 
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Correspondence Address:
Dr. Rodrigo Galo
Department of Prosthodontics and Dental Materials, Dental School of Ribeirão Preto, University of São Paulo, Avenida do Café, S/N, Monte Alegre. CEP: 14040-904, Ribeirão Preto, São Paulo
Brazil
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcd.jcd_223_21

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