Effect of digluconate chlorhexidine on bond strength between dental adhesive systems and dentin: A systematic review.

AIM: This study aimed to systematically review the literature for the effect of digluconate chlorhexidine (CHX) on bond strength between dental adhesive systems and dentin of composite restorations.
MATERIALS AND METHODS: The electronic databases that were searched to identify manuscripts for inclusion were Medline via PubMed and Google search engine. The search strategies were computer search of the database and review of reference lists of the related articles. Search words/terms were as follows: (digluconate chlorhexidine*) AND (dentin* OR adhesive system* OR bond strength*).
RESULTS: Bond strength reduction after CHX treatments varied among the studies, ranging 0-84.9%. In most of the studies, pretreatment CHX exhibited lower bond strength reduction than the control experimental groups. Researchers who previously investigated the effect of CHX on the bond strength of dental adhesive systems on dentin have reported contrary results, which may be attributed to different experimental methods, different designs of the experiments, and different materials investigated.
CONCLUSIONS: Further investigations, in particular clinical studies, would be necessary to clarify the effect of CHX on the longevity of dentin bonds.

INTRODUCTION

Dental composite resins are the most frequently used direct tooth-colored restorative materials. Nevertheless, the longevity and integrity of composite restorations depend on multiple factors, such as polymerization shrinkage and hydrolysis of the hybrid layer, which may lead to postoperative sensitivity, secondary caries formation and, as a result, failure of the restoration.[12] Recently, many investigators have focused their research on the subject of durability of the bond between dental adhesive systems and dentin. It is well known that in composite restorations the hybrid layer gradually degenerates, as a result of the hydrolytic degradation of collagen fibers, even when bacteria and their toxins are not present.[345]

Matrix metalloproteinases (MMPs) are a group of 23 mammalian enzymes capable of degrading all extracellular matrix components. Human dentin matrix contains at least MMP-2 and MMP-9 (gelatinolytic activity), MMP-8 (collagenolytic activity), and enamelysin MMP-20.[678] Breschi et al.[9] indicate that MMP-2 may be the prevalent MMP in dentin. The incomplete infiltration of collagen fibers by the dental adhesive leads to the exposure of the fibers and to the collagenolytic activity of MMPs, which may be activated by low pH values during the etching of dentin.[10] As a result, the hybrid layer degenerates, the bond strength gradually deteriorates, and the composite restoration ends in failure.[5]

Chlorhexidine (CHX) is a cationic bisbiguanide, widely known as an antimicrobial agent in oral health. Moreover, it has been reported that CHX is able to inhibit the MMPs’ collagenolytic activity, improving the longevity of the bond between adhesives and dentin.[1112] In fact, Gendron et al.[13] found that the minimum concentrations that are adequate for this inhibition are 0.001% for MMP-2, 0.02% for MMP-8, and 0.002% for MMP-9.

Therefore, the aim of this study was to systematically review and discuss the results of previous studies regarding the effect of CHX on the bond strength of dental adhesive systems in composite resin restorations. The hypothesis tested was that there is no difference between the bond strength of dental adhesives and dentin when CHX treatment is applied.

MATERIALS AND METHODS

The electronic databases that were searched to identify manuscripts for inclusion were the Medline via PubMed and Google search engine. The search strategies were computer search of the database and review of reference lists of the related articles. No publication year limit was used, and the last search was made in November 2015. Search words/terms were as follows: (digluconate chlorhexidine*) AND (dentin* OR adhesive system* OR bond strength*). In terms of reviewing reference lists, the references of all included articles were manually searched for further relevant studies that could fulfill the inclusion criteria. The inclusion criteria were as follows: In vitro or in vivo studies that evaluated the effect of CHX application on the bond strength of composite restorations during the adhesive procedure after storing of the adhesive interface for at least 6 months (any type of storage medium). Only studies that had a control group for comparison were included. Studies that did not evaluate the immediate and the aged bond strength were also excluded from evaluation. The exclusion criteria of this systematic review are presented in Table 1.

Table 1

Exclusion criteria of the study

The 144 studies titles found were checked for duplicity, and 102 studies were finally included. The abstracts and if necessary the full texts of 102 studies were then assessed, depending on the exclusion and inclusion criteria, to identify 17 relevant studies; 85 studies were excluded.

RESULTS

The study design of the selected studies and the percentage of bond strength reduction during the experimental periods are shown in Table 2. The oldest report was published in 2007 and the most recent in 2015. Bond strength reduction after CHX treatments varied among the studies, ranging 0-84.9%. In most of the studies, pretreatment CHX exhibited lower bond strength reduction than the control experimental groups. Exceptions were reported by Zhou et al.,[14] who found that the reduction of bond strength after treatment with 0.05% CHX was 28.8% compared to 18.7% of the control group after 12 months of storage, and by De Munck et al.,[15] who demonstrated a bond strength reduction of 68.7% after pretreatment with 0.05% CHX compared to 49.7% of the control group after 12 months of storage. In addition, Sabatini[16] reported that there was no reduction of bond strength after storage for 6 months: Neither in CHX treated specimens nor in control specimens.

Table 2

Data from the studies included in the present systematic review

In three studies, CHX was incorporated into the adhesive system, while in other studies CHX was applied as a pretreatment solution. Fourteen studies evaluated bond strength using microtensile bond strength (μTBS) test, one study using tensile bond strength (TBS) test, one study using microshear bond strength (μSBS) test, and one study using shear bond strength (SBS) test. Furthermore, in seven studies the type of storage medium was distilled water, in seven other studies was artificial saliva, in one study was deionized water, and in one study was 0.9% sodium fluoride (NaF) and 0.02% sodium azide solution. In one study, the aging was performed with oral function. The evaluation of bond strength of the specimens was carried out after 6 months of storage in seven studies, after 9 months in one study, after 12 months in four studies, after 6 and 12 months in four studies, and after 18 months in one study. Fifteen studies investigated the effect of 2% CHX on bond strength between dentin and dental adhesive; four studies the effect of 0.2% CHX; two studies the effect of 0.5% CHX; two studies the effect of 0.05% CHX; and one study the effects of 0.002%, 0.004%, 0.02%, 0.1%, 1%, and 4% CHX.

DISCUSSION

There are many factors that influence the bond strength of a restorative material to the dentinal substrate. Mechanical stresses by chewing forces, changes of temperature and pH, water sorption, resin shrinkage, and enzymatic action affect bond integrity to different extents. Moreover, the type and composition of the composite resin and the adhesive system as well as the dentinal substrate are of great importance.[1718]

Previous scientific reports have demonstrated that application of CHX for 60 s immediately after etching with three-step[192021] and two-step etch-and-rinse[2223] dental adhesives results in preservation of bond strength. Additionally, Stanislawczuk et al.[24] found that even when CHX is contained in self-etch adhesives or in the etchant agent, a better stability of hybrid layer may be observed.

It has been found that resin-dentin bonds often fail 30-40% in 6-12 months.[1225] As it was previously mentioned, dentin is known to contain MMPs, which are activated by weak acids released by caries-producing bacteria, and acid-etchants used in adhesive systems.[25] The clinical application of 2% CHX for 60 s to the etched dentin after rinsing off the acid and before applying the dentin-bonding primer and resin minimizes significant in vivo degradation of bond strength from MMPs for at least 14 months.[19] Therefore, many clinicians currently apply 2% CHX for 60 s to acid-etched dentin during resin bonding in an attempt to increase the durability of resin-dentin bonds by inhibiting endogenous MMPs in the dentin matrix.[202326] This method is the only one proven clinically, is easy to adopt, and will likely be the first to gain wider acceptance.

Some researchers who previously investigated the effect of CHX on the bond strength of dental adhesive systems on dentin have reported that the CHX pretreatment did not affect the SBS of dental adhesives.[27282930] Nevertheless, other studies have reported that CHX acting as MMPs’ inhibitor decelerates the rate of resin-dentin bond degradation.[203132] Furthermore, Campos et al.[21] reported that application of 2% CHX was deleterious to bond strength and should be avoided prior to self-etch adhesive systems. This is in agreement with previous reports investigating the effect of 0.05% CHX solution on the bond strength of adhesive systems with dentin.[1415] These contrary results of the previous studies may be attributed to different experimental methods, different design of the experiments, and the different materials investigated.

Chang and Shin[33] evaluated the influence of CHX with different application methods on bond strength to dentin. The application methods used in this study were:

CHX + rinsing + etching;

CHX + etching;

Etching + CHX + rinsing; and

Etching + CHX.

Considering the amount of reduction in bond strength after thermocycling, this study showed improved dentin bond strength with CHX when used after etching. Other studies have postulated that the incorporation of CHX into the adhesive[143435] or the etchant[32] in order to avoid a CHX treatment step can preserve bond strength to dentin.

Another factor that may influence the effectiveness of CHX in bond strength between adhesives and dentin is the type of the applied CHX solution. Ali et al.[36] demonstrated that pretreatment of the dentin with ethanol-based CHX had a negative effect on the bonding of the tested single-step self-etch adhesive; however, water-based CHX showed bond stability under intrapulpal pressure simulation. Moreover, Ekambaram et al.[35] found that the incorporation of 2% CHX into ethanol-wet bonding preserved bond strength to both sound and caries-affected dentin, and reduced interfacial nanoleakage after 12 months, while the incorporation of 2% CHX to water-wet bonding preserved bond strength only to sound dentin after aging. Nevertheless, Simoes et al.,[37] who evaluated the effect of CHX and/or ethanol application on the bond strength of an etch-and-rinse, hydrophobic adhesive system under either in vitro aging or in situ cariogenic challenge, showed that CHX, ethanol, or CHX + ethanol did not affect the μTBS after 6 months.

It has been reported that the association between the concentration of CHX and the bond strength is not linear.[38] CHX has been shown to directly inhibit MMP-2, -8, and -9 activities at extremely low concentrations (0.0001-0.02%). However, with low CHX concentrations (0.2-0.45%), there is no interaction with synthetic hydroxyapatites, while with higher concentrations (1-4.5%), CHX was progressively retained with time.[39] On the other side, Loguercio et al.[40] found that the use of 0.002% CHX for 15 s seems to be sufficient to preserve resin-dentin interfaces over a 6-month period. In addition, Breschi et al.[9] and Lin et al.[41] reported that treating acid-etched dentin with 0.2% CHX reslted in the formation of hybrid layers that are as stable as those formed with 2% CHX. Stanislawczuk et al.[42] claimed that the addition of CHX in concentrations until 0.2% in the simplified etch-and-rinse adhesive systems may be an alternative to increase the long-term stability of resin-dentin interfaces, without jeopardizing the adhesives’ evaluated mechanical properties. Moreover, Zhou et al.,[14] who investigated the incorporation of CHX into a two-step self-etch adhesive in different concentrations, found that CHX can preserve the dentin bond as long as the concentration is higher than or equal to 0.1%. In another research study, it has been demonstrated that the addition of 2% CHX or, especially, 5% CHX in experimental adhesives produced significant reductions in both the μTBS and the Fourier transform infrared spectroscopy (FT-IR) percent conversion.[43]

The dentinal substrate may also affect CHX action on bond strength. Specifically, Komori et al.[44] evaluated the effect of 2% CHX on the long-term bond strengths of two etch-and-rinse adhesive systems to normal and caries-affected dentin and found that CHX significantly lowered the loss of bond strength after 6 months in normal dentin specimens, but did not alter the bond strength of caries-affected dentin specimens.[29] In another, similar study, it has been postulated that 2% CHX has an interaction effect on the preservation of bond durability to both sound and caries-affected dentin after 12 months.[35] On the contrary, other studies reported that application of a self-etch adhesive on CHX-treated sound dentin or artificially caries-affected dentin did not change the bond strengths.[284546] Generally, the presence of carious dentin decreases bond strengths[47] and the higher the level of caries progression, the lower the bond strengths of adhesives to carious dentin.[48] Moreover, Francisconi et al.,[49] who focused on the effect of 2% CHX on the bond strength of adhesive systems to sound and artificially eroded dentin, found that although there was a significantly lower μTBS in artificially eroded dentin compared to sound dentin, the influence of CHX on bond strength conservation was not found to be persistent. In another investigation, Manfro et al.[50] assessed the effect of 0.5% CHX and 2% CHX on bond strength to primary tooth dentin and reported that CHX pretreatment may prevent the degradation of adhesive interface in primary teeth.

The type and composition of adhesive systems may affect their bond strength. There are many studies that have investigated the SBS of etch-and-rinse and self-etch adhesive systems. Some of them found that there were no statistically significant differences between etch-and-rinse and self-etch adhesives,[5152] but other reports have claimed that etch-and-rinse adhesives exhibit higher SBS than self-etch adhesive systems[5354] due to the better hybridization or infiltration of resin within the exposed collagen fibrils of the dentinal surface.[17] A recent study[55] has investigated the effect of 2% CHX on the μTBS of an etch-and-rinse and a self-etch adhesive after 9 months of aging, and the authors concluded that CHX prevented the decrease in μTBS upon aging of the etch-and-rinse but not of the self-etch adhesive. This finding is supported by Nishitani et al.,[56] who concluded that self-etch adhesives may activate MMPs and increase their adverse activity, contributing to the degradation of the dentin bond over time.

The presence of solvents in resins is absolutely necessary in the chemical composition of adhesives so that they can bond to dentin adequately. The wet nature of dentin permits good wetting only when hydrophilic bonding is used.[57] Water is a poor solvent for organic compounds, such as monomers, which are rather hydrophobic, while acetone and ethanol can combine hydrophobic and hydrophilic components. Additionally, the absence of solvents in the chemical compositions of some new dental adhesives results in a thicker adhesive layer. Some authors have assumed that this increases hydrophilicity and that as a result the adhesive behaves as a semipermeable membrane, allowing more fluids to pass through, leading to lower bond strength.[58] It has been found that solvent-free self-etch adhesives, when used for bonding composite resins to dentin, underperform as compared to conventional adhesives.[5960]

Intermediary strong (pH ~1.5) self-etch dental adhesives have a hybrid layer of about 1-2 μm, with some hydroxyapatite preserved at the bottom part. Ultramild (pH >2.5) self-etch adhesives do not eliminate the smear layer, and interact with the smear layer-covered dentin only up to a few hundredths of nm. Previous reports have shown that mild self-etch adhesives create lower dentin bonds than the other adhesives.[1761]

In many studies, scanning electron microscopy (SEM) analysis has been used to reveal the failure mode distribution of the specimens demonstrating adhesive failures. Failure is considered to be a) adhesive, if it occurred at the dentin-adhesive interface; b) cohesive, if it occurred in the material or in the substrate; and c) mixed, if it involved both the interface and the material. Quantitative SEM analysis improves the accuracy of the failure mode distribution analysis and allows for discrimination between the failures occurring in the top and the bottom of the hybrid layer to understand the mechanisms leading to the reduction in dentin bond strengths with time.[20] Carrilho et al.[20] found significantly lower percent failure mode in the hybrid layer, especially in the bottom part, after 6 months with CHX pretreatment and indicated that the higher bond strengths observed in this group reflected the preservation of hybrid layer collagenous matrix, especially in the bottom zone, where partially exposed collagen fibrils are most prone to initial enzymatic degradation.

CONCLUSIONS

Researchers who previously investigated the effect of CHX on the bond strength of dental adhesive systems with dentin have reported contrary results. Although there is evidence that CHX is able to inhibit MMPs’ collagenolytic action, it is not clear if this ability has a clinical significance in composite restorations. Due to the many factors that influence the bond strength of a composite material to the dentinal substrate, further investigations, in particular clinical studies would be necessary to clarify the effect of CHX on the longevity of dentin bonds.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.