Theconcept of adhesion was introduced into the field of dentistry by Buonocore in1955 (1). Adhesive dentistry rapidly expanded treatment possibilities andrevolutionized the way direct and indirect restorations were traditionallyperformed. Paralleling the growing demand for adhesive restorations, dentinebonding systems too have undergone an evolution to improve their bond strengthsas well as to reduce their technique sensitivity.
Dentin bonding agents have evolved from the gold standard – etch andrinse fifth generation adhesives to the present universal adhesives.The different generations of dentin bonding agents have witnessed a change inchemistry, mechanism of action, procedural steps and a varying degree ofclinical efficiency(2). A recent innovation in the one bottle adhesive systemsis their expansion to a more universal bond with 10-Methacryloyloxydecyldihydrogen phosphate (MDP) as the active ingredient. These universal bondingagents can be used in all etch modes for both direct and indirect restorations.Single Bond Universal(SBU),marketed as Scotchbond Universal in USA, was the first commercialuniversal adhesive and is popularly used by the clinicians worldwide (3,4,5,6). SBUapart from MDP, also has methacrylate-modifiedpolyalkenoic acid copolymer (PAAC) in its composition.( Table/Figure 1) Mitra and co-workers have reported that PAACbonds chemically to calcium in hydroxyapatite showing excellent long-termclinical performance thereby further improving the bond strength (7).
Tetric N Bond Universal (TNBU)is a relatively new Universal adhesive which has its matrix based on a combination of monomers of hydrophilic, hydrophobicand intermediate nature allowing it to reliably bridge the gap between thehydrophilic tooth substrate and the hydrophobic resin restorative.( Table/Figure 1) However studies using this bonding agent are scarce (8,9,10).Oneof the major problems associated with the use of adhesive systems is thedifficulty in obtaining a moisture-free clean tooth surface for adequatebonding (11). Moisture control in the working field is particularly difficultin situations such as equigingival or subgingival cavity margins, seating ofindirect restorations, newly erupted molars or when patients have limited mouthopening (12). Contamination during the bonding process from sources such asgingival crevicular fluid, hand piece oil, blood and saliva, can adverselyaffect the quality of the bond predisposing it to microleakage at thetooth-restoration interface.
As a consequence, loss of the restoration,recurrent caries, postoperative sensitivity and discoloration may occur (13).Studies in the past have shown that salivary contamination has adeleterious effect on bonding (14,15,16,17,18). But manufacturers are claiming that universal bonding agents areresistant to salivary contamination. In accordance to this, study bySantschi and colleagues concluded that saliva contamination did not affect thebond strength of SBU (19). Inthe event of contamination, use of an appropriate decontaminating agent torestore bond strengths has been advocated (20,21). Workby Yoo et al.
and Santschi et al. has shown that for all-in-one adhesives, washing, dryingand adhesive reapplication was the most effective decontamination protocol (12,19).However,to date, studies which have investigated the effect of salivary contaminationon the universal bonding agents are scant and conflicting(19, 22). Hence the aim of this study was to evaluate the influence ofsalivary contamination and water rinsing as a decontamination method on theshear bond strength of universal bonding agents. Materials and MethodType of study- Originalresearch (in- vitro study)Name and place of theinstitute- Y.M.T.
Dental College and Hospital, Navi Mumbai, Maharashtra.Time duration- April 2017-September, 2017Sample selectionNinetyfreshly extracted intact, caries free human premolars were selected for thestudy as per statistician’s recommendation (attached atthe end of the document). All the collected teeth were cleared of bloodand saliva and cleaned under tap water with a scaler andstored in buffered isotonic saline solution.
Teeth with cracks, restorations orany anatomical deformities were excluded from the study. Sample preparation and mounting ofspecimensTeeth were mounted in self-cure acrylicresin (Dental Products India Ltd.). The occlusal surfaces of the teeth were sectioned off with a double facediamond disc under water cooling to prepare flat dentin surfaces at a depth of1.5 mm from the cuspal tip of the tooth. The dentin surface to be bondedwas ground with #600 SiC paper under running water to produce a standardizedsmear layer.
Saliva collectionTo achieve standardised salivarycontamination, unstimulated human saliva was collected from a single individualat least one hour after any consumption of food or drink in a sterile beakerand was used immediately.Divisionof samplesSampleswere randomly divided into two groups of forty five samples each according tothe universal bonding agent used as follows;Group I- SingleBond Universal (3M ESPE)Group II- Tetric®N-Bond Universal (Ivoclar Vivadent)Theforty five premolars in each adhesive group were further divided among threeexperimental subgroups (n =15) as follows:Subgroup 1- Control group:The premolars in this group were not subjected to any contamination. Theadhesive was applied according to manufacturer’s instructions and light curedfor 10 seconds using Bluephase N® LED unit (Ivoclar Vivadent).Subgroup 2- Contamination group: Theadhesive was applied according to manufacturer’s instructions. The specimens were covered with freshwhole saliva for 20 seconds using a disposable brush. A gentle stream of airwas then applied for 2 seconds to dry the surface followed by light curing asin subgroup 1.
Subgroup 3- Decontamination (Water rinse and reapplication): Theadhesive was applied according to manufacturer’s instructions. After saliva contaminationas in subgroup 2, the contaminated surface was rinsed for 60 seconds with astream of water from an air-water syringe. A gentle stream of air was thenapplied for 2 seconds to dry the surface and adhesive was reapplied as a part of decontamination protocol andlight cured as in subgroup 1.Composite placementATeflon tube of 3 mm inner diameter and 4 mm length was placed on the surfaces.The Teflon tube was filled with composite resin (Filtek TM Z350,shade A2, 3M ESPE) in two horizontal increments wherein each increment wastightly compressed and light cured for 20 seconds using Bluephase N® LED unit(Ivoclar Vivadent). The Teflon tubewas removed and the resin cylinder additionally cured.
Preparation of samples for shear bondstrength analysisTheprepared specimens were stored in distilled water at 37°C for 24 hours andshear bond strength test was carried out using a Universal Testing Machine(UNITEST 10, Acme Engineers, India) at a crosshead speed of 0.5 mm/min.Two examiners evaluated the debonded surfaces at 10X magnification by using a stereomicroscope (CromaSystems) to identify the mode of bond failure (adhesive, cohesive or mixed). Table/Figure 2StatisticalanalysisThedata obtained in the present study was subjected to statistical analysis usingOne-way ANOVA test.
The intra-group and inter-group comparison was subjected tostatistical analysis using Tukeys HSD test (p<0.05). Statistical Package for the Social Sciences (SPSS) software version 17 wasused for statistical analysis.ResultsShearbond strength (SBS) values obtained for different test groups with Single BondUniversal (SBU) and Tetric® N-Bond Universal (TNBU) are presented in Table/Figure3.
A drop in mean SBS was seen after salivary contamination for both thegroups. As compared to the contamination group there was an increase in meanSBS in water rinsing group. The intergroup comparison showed that TNBU groupshowed significantly better results as comparedto SBU group (p=0.
000). The mode of failure inall groups was mainly adhesive Table/Figure 4.Discussion The present study was conducted to ascertain both the effect ofsalivary contamination and decontamination method on shear bond strength of twouniversal bonding agents- Single Bond Universal and Tetric N Bond Universal.Theshear bond strength (SBS) of dentin was adverselyaffected by salivary contamination for both the adhesives.
Further, statisticalanalysis revealed that the decontamination protocol had a significant increase in SBS of both the adhesives. In laboratory tests, the efficacy of dentine adhesion isoften evaluated by its SBS. It is useful for a relative comparison of differentadhesive systems and for screening new materials (23).The condition of the substrate that is the tooth structureand the chemical composition of the adhesive system influences the bondstrength (24). As a result, enhancing the efficacy of adhesive restorativematerials has been an area of active research.Pleffken and colleagues and Loguercio and colleagues suggested thatactive application of the adhesive on dentin improved the bonding performanceas well as reduced the degradation rate of the adhesive systems (25,26). Hencein this study, adhesive was applied to the tooth surface in scrubbing action asinstructed by the manufacturer to maximize the bond strength.During the study on bond strengths of universalbonding agents, Muñoz and coworkers observed that the self etchapproach led to more stable bonds even after long-term water storage as againstthe etch and rinse approach which seemed to be ultra-structurally moresusceptible to biodegradation over time(27).
Hence in this study, the adhesivewas used in self-etch mode.In the current study, naturalhuman saliva was used as the contaminant. Using artificial saliva or salivasubstitutes could have diminished the clinical significance of the study.Moreover, work from several researchers has deemed whole human saliva as anacceptable contaminant (16,19,28,29,37). Unstimulated saliva collected fromsingle, healthy individual was used to reduce variability in pH of the salivaand electrolyte, enzyme, or protein content.In the present study, SBU has consistently shown lower bond strengthvalues as compared to TNBU. This can be a result of PAAC in SBU competing withMDP by binding to the calcium present in hydroxyapatite.
Another possibilitycould be the prevention of monomer infiltration during polymerization due toits high molecular weight (30). However, Mohamed Moustafa Awad on comparingthe same adhesives concluded that, when applied in self-etch mode, both caninfiltrate into dentin producing high quality interfacial morphology (8).Likewise, a study by Jayasheel andcoworkers comparing shear bond strength of universal adhesivesinferred that the bond strength values of the TNBU regardless ofapplication mode were comparable to SBU making them reliable for working underdifferent clinical conditions(9).
Nevertheless, both the above studies wereconducted under ideal conditions without taking salivary contamination intoconsideration.Saliva is composed mostly of water (99%) with immunoglobulins,polysaccharides, proteins, enzymes and a variety of electrolytes (31). Researchershave implicated proteins in saliva to be the main factors responsible forreduction in bond strength (14-18,32).
It has been proposed that salivamacromolecules, especially glycoproteins, adsorbed on the enamel surface act asa barrier preventing complete wetting of resin, in turn, inhibiting themonomers from penetrating the collagen network of dentine (33). Moreover,salivary proteins compete with hydrophilic monomers during the hybridizationprocess, preventing complete polymerization of the adhesive, thereby, furtherreducing bond strength (34,35). Furthermore, dilution of theadhesive by excess saliva produces a weak hybrid layer.Vitrebond™ copolymer, thepatented product present in SBU is claimed to be moisture tolerant. But despitethat we found that the SBS has reduced after salivary contamination. This maybe due to adsorption of the biofilm and competition of the monomer during hybridization(36). Also degradation of Bis- GMA due to the hydrolytic enzymes of saliva hasbeen reported which can further compromise bonding (37).
Stage of saliva contamination is also critical towards its effect onbonding (34,38). In this study specimens were contaminated with saliva afterapplication of bonding agent before light curing. Hence it evaluated the effectof salivary contamination on the uncured bonding agent as this would directlyhamper the formation of hybrid layer. Salivary contamination beforepolymerization is particularly significant as Taneja and coworkers demonstratedgreater decrease in bond strength by contamination at this stage (16).Moreover, Santschi has suggested that as the bonding agent is highly watersoluble and most liable to dilution before polymerization (19). Water rinsing is an easy choice to combatsaliva contamination of a prepared tooth surface. In a study by Sattabanasukand colleagues showedthat simply rinsing saliva-contaminated enamel surfaces with water restores thebond strength (32).
On the other hand, studies have demonstrated thatconventional washing protocols do not completely remove the coating of salivaryproteins on the enamel surface and a subsequent reapplication of the adhesiveafter water rinsing and air-drying restores bond-strength value (39). Thiscould be attributed to increased rein- dentin interaction due to multiplecoatings of adhesive (40,41,42). Erickson et al. and Cobanoglu et al. after evaluatingseveral saliva decontamination procedures, proposed application of adhesiveafter rinsing and drying to be more reliable than just drying, rinsing (31,38).They suggested thatwashing and drying should remove the adhesive layer providing a demineralizedsurface non-infiltrated by monomers.
Hence, water decontamination followed byreapplication of adhesive was the method of choice used for decontamination inthis study.The type of dentin substrate used could alter also bondstrength, as there could be inter-tooth discrepancy and dentinal tubulediameter variation with age and degree of mineralization (43,44). Thesevariable factors were overcome by the use of teeth from patients whose agesranged from 15 to 25 years and within six months of extraction.Stereoscopic microscopy helped us evaluate thenature of failure and further gave us an insight in the probable cause forfailure.
Cohesive mode of failure is persistent if bond strength is more than20 Mpa (45). As most of the samples in the study had bond strength less than 20Mpa, adhesive failure was common in this study. Thereare limitations in simulating the oral environment in vitro indicating that the excellent physical properties of theadhesive resin that were obtained in vitro are not alwaysattained in vivo. Lower bond strength and failure ofadhesives in vivo can be attributedto exposure to oral environment including moisture contact, intraoraltemperature, tooth flexure, higher C factor and bacterial enzymes (46). Further long term in vitro and in vivostudies are recommended to improve the understanding of the interaction effectof saliva with various bonding systems which have different chemistry andacidity. Bond durability and sealingability of the samples decontaminated by the protocol as mentioned in the studyafter salivary contamination should be investigated.
Ongoing research should bedirected towards exploration for a novel dentin bonding agent that would beresistant to contamination.ConclusionWithin the limitations of this in vitro study,it can be concluded that salivary contamination reduces the shear bond strengthof universal adhesives to dentin. Reapplication of the adhesive after waterrinsing and drying after salivary contamination improves the bond strengthsignificantly. However, long-term in vivostudies are necessary to substantiate the clinical performance of thistechnique.