| Resin-dentin adhesive restorations have gained satisfactory immediate bondingstrength with the rapid development of adhesive bonding technology and updated adhesivematerials. However, bonding durability to dentin is still questionable and has become animportant scientific issue to be improved of the dental adhesive technology. Formed atthe bottom of mixed layer, the collagen exposed zone due to the mismatching of dentindemineralization depth and adhesive resin penetration depth is the weak area of theadhesive interface and the key structural factor that affect the adhesion durability.Currently, the application of matrix metalloproteinase inhibitors, ethanol-wetting bondingtechniques, low shrinking resin and antimicrobial adhesive agent, etc. can reinforceresin-dentin bonding restorations to some extent, reduce external stimuli and slow downthe degradation of collagen partly. However, exposed collagen within the mixed layer is still not closed and protected and the risk of adhesive failure is inevitable. Thereforeremineralization of the weak areas in bonding interface has been considered to be theultimate measures to improve the life-span of adhesive restorations. Scholars haveachieved biomimetic remineralization of collagen fibers in hybrid layer in in vitroexperiments. However, it takes a long time and collagen has the risk of degradation beforeit has been mineralized. The combination of a variety of measures to improve the bonddurability might be a feasible direction to solve this problem in the future.In recent years, it is confirmed that cross-linking agent can enhance the strength ofdentin matrix and improve the ability of resin-dentin bonding to resist degradation in thestudy of demineralized dentin modification. Among them, the natural crosslinking agent ofproanthocyanidins (PA) attracted much more concern due to the good biocompatibility,moderate reaction rate, extensive sources, and a variety of biological activity. Our researchgroup has studied the application of PA extracted from grape seed in etch&rinse system,and explored clinically feasible PA bonding pretreatment methods. It has been confirmedthat the modification of demineralized dentin with PA in this way can protect theresin-dentin bonding interface, slow down the degradation of the exposed collagen at thebottom of hybrid layer, improve bonding durability, and has good prospects for clinicalapplication. However, the problem of exposed collagen fibers in the hybrid layer are stillremains.PA may affect the demineralization-remineralization balance and inhibit thedevelopment of root caries according to the latest research report. Inspired by this, byusing the bonding pretreatment way, the effects of PA on the demineralization resistanceand remineralization of demineralized dentin were studied in the present study. The effectsof PA pretreatment alone or in combination with the classical remineralization activesubstances casein phosphopetides–amorphic calcium phosphate (CPP-ACP) on theremineralization of demineralized dentin and the balance of demineralization andremineralization in hybrid layer was also observed to provide references for the stability ofresin-dentin bonding against pH cycling and to expand the potential clinical application of PA in improving bonding durability.1. Objectives1.1To determine the total phenolic content of the experimental samples PA and the effectof solution pH on PA stability.1.2To explore whether the PA has a role in inhibiting dentin demineralization, as well asthe effect of solution pH and concentrations on the PA role were considered.1.3To study the effect of PA on the remineralization of demineralized dentin with differentconcentrations and pH, then observe the combination effect of PA pretreatment andCPP-ACP on dentin remineralization.1.4To evaluate the effects of pretreating demineralized dentin with PA ethanol solutionbefore bonding independently and combined with CPP-ACP or PA water solution onthe demineralization-remineralization balance of resin-dentin bonding interfaceduring pH cycling.2. Methods2.1Using Folin-Ciocalteu assay to determine total phenol in experimental PA. UV–VISspectra were used to analyze the stabilities of PA ethanol solution and water solution atdifferent pH.2.2Dentin specimens were prepared, and specimens in experimental groups werepH-cycled following the procedure. In bief, after immersion of the specimens in one ofPA solutions, the blocks were placed in an acidic buffer, and then in a neutral buffer.The PA solution in the negative and positive control groups was taken place by thedeionized distilled water and0.1%sodium fluoride solution respectively. After6-daypH cycling, inductively coupled plasma atomic emission spectroscopy and microhardness tester was used to measure the calcium depletion rate and determine thehardness differences before and after demineralization, and laser scanning confocalmicroscopy was used to analysis the extent of dentin demineralization. 2.3Demineralized dentin specimens after PA pretreatment were immersed in artificialbody fluid for15days, and part of the samples were remineralized by the CPP-ACP orCPP-ACP containing polyacrylic acid every day. The demineralized dentin specimenswithout PA pretreatment and remineralized by0.1%sodium fluoride were set as thecontrol. The surface microhardness of dentin was measured after demineralization,pretreatment and remineralized different time. The surface morphology and sedimentimages on dentin surface were observed and analyzed by field emission scanningelectron microscope (FE-SEM) and X-ray diffraction.2.4The bonding specimens pretreating with PA were aged through water storage and pHcycling. Specimens were treated with deionized distilled water, CPP-ACP and PAwater solution respectively during pH cycling, and bonding specimens without PApretreatment as a control. Micro-tensile bonding strength was then measured. Fracturemodes, the morphology and nanoleakage of adhesive interface were observed using afield emission scanning electron microscope. The ratio of calcium and phosphorus, andthe percent content of Ag atom in hybrid layer were determined using FE-SEMcombined with energy-dispersive X-ray spectrometry. The extent of dentindemineralization on the dentin side of interface was observed using laser scanningconfocal microscopy.3. Results and conclusions3.1The total phenols contained in1g PA was approximately equivalent that in (447±3.67)mg standard gallic acid. The polyphenol structure of PA in neutral or alkaline solutionis more easily oxidized to form quinones, either ethanol or water as a solvent. It shouldnot adjust the pH of PA solution but maintain its original weak acid, and PA solutionshould be used immediately once been prepared.3.2PA pretreatment can inhibit the releasing of calcium in dentin and reduction of dentinsurface microhardness, and reduce the depth and range of demineralization. The effectwas concentration-dependent and15%PA solution presented the best effects as sodium fluoride did. Adjusting the pH to neutral will not significantly affect theinhibiting effect of PA on the demineralization of dentin.3.3PA pretreatment promoted mineral deposits on the dentin surface, the deposits weremainly hydroxylapatite. The effect of PA affected by the pH and solutionconcentrations.15%PA stock solution had the best promoting effects.3.4Pretreatment with15%PA stock solution and application of CPP-ACP can promote theremineralization effect of demineralized dentin significantly, and mineral depositswere mainly hydroxylapatite. The remineralization promoting effect of CPP-ACP hasbeen weakened, when PA pretreatment of demineralized dentin and application ofCPP-ACP on tooth surface were combined. This reduction effect may relieve whenadding polyacrylic acid to CPP-ACP.3.5PA pretreatment before bonding, local application of CPP-ACP on the surface ofadhesive interface alone or combination with each other can improve the stability ofresin-dentin bonding interface against pH cycling, and the combined effect is betterthan application alone. PA used for bonding pretreatment, storing adhesive specimensalone or combination with each other can improve the stability of resin-dentin bondsagainst pH cycling, but the combination method was not superior to application alone,and the mineral deposition promoting effect of PA pretreatment within the bondinginterface was not obvious. |
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