Bond Durability Lecture (AAST) PDF

Bond Durability CCC.AAST College of Dentistry. Prof.Amal Sakr  2 Limitations in Bonding to Dentin and Experimental Strategies to Prevent Bond Degradation J Dent Res 90(8):953-968, 2011 Dentin Bonding (REV.) Development Chronologic Classific...

Bond Durability CCC.AAST College of Dentistry. Prof.Amal Sakr  2 Limitations in Bonding to Dentin and Experimental Strategies to Prevent Bond Degradation J Dent Res 90(8):953-968, 2011 Dentin Bonding (REV.) Development Chronologic Classification Seven generations 4  Fourth Generation Three-step Etch & rinse Fifth Generation Two-step Etch & rinse Currently Available Sixth Generation Generations (REV.) Two-step Self-etch One-step Self-etch mix Seventh Generation One-step Self-etch no mix 5 (REV.) Operative Dentistry, 2003, 28-3, 215-235 6 Classification of total etch adhesives (etch & rinse) (REV.) Post operative 2 steps sensitivity Total etch adhesives 3 steps More durable (REV.) In order to understand the hybrid layer formation using total etch technique and the self etch technique, it is necessary to understand the components of bonding systems that consist of three main components: 1) etchant, 2) primer and 3) bonding resin 8 Etch & Rinse (Three- Step) (REV.) etchant Primer Adhesive resin 9 Etch & Rinse (Two-Step) (REV.) etchant Combined primer and adhesive higher technique sensitivity higher solvent-to-monomer ratio risk of applying too thin apply multiple layers Hashimoto, Oper Dent 2004 10  11 Chemical alteration of surface acids Etchant (REV.) phosphoric, citric, maleic, nitric Removes dentinal smear layer exposes collagen fibrils Simultaneous enamel etch Rinse keep moist (REV.) Etchant: in total-etch technique the etchant used is 35– 12 37% phosphoric acid. It prepares enamel and dentin to receive the primer. It creates microporosities, up to 7.5 microns which helps to create the resin tag formation and thereby results in micro mechanical bonding. The etchant in self-etch bonding agents is typically an acidic monomer that also serves as the primer. High magnification of Dentin etched with 35% phosphoric acid. 13  14 Rinsing &drying of Dentin (REV.) Clinical aspect of moist dentin—a glistening appearance without accumulation of water. (From Rubinstein S, Nidetz A: The art and science of the direct posterior restoration: Recreating form, color, and translucency, Alpha Omegan 100(1):30–35, 2007.) (REV.) Scanning electron micrograph of dentin that was kept moist after rinsing off the etchant. The abundant intertubular porosity serves as a pathway for the penetration of the dentin adhesive. T, dentinal tubule. 15 16 Primer (REV.) Bifunctional monomer Link hydrophilic collagen hydrophobic resin CH3 Example HEMA H2C=C-C-O-CH2-CH2-OH O 17  the primer is composed of hydrophilic monomers (REV.) usually carried in a water-soluble solvent (acetone, ethanol, water) to promote good flow and penetration into hydrophilic dentin, which can influence the resulting bond strength. Self-etch bonding agents utilize primers that are acidic monomers. 18  -Unfilled or lightly-filled monomers equivalent to enamel bonding Bis-GMA, UDMA, Adhesive TEGDMA Resin (REV.) -Stabilize the hybrid layer fills up remaining pores -Resin tags Links primer to composite resin 19  Dentin bonding agent (or Dentin Adhesive): can be defined as a thin layer of (usually unfilled) resin applied between the conditioned dentin and resin matrix of a composite. The adhesive (REV.) promotes bonding between enamel or dentin and resin composite restorative material or resin cement. Adhesives act as a link between the hydrophilic resin primer and the hydrophobic resin composite. Proper curing is required to provide good retention and sealing. Seventh generation bonding agents utilize 20 primer-adhesives that are acidic monomers. Fillers (REV.) recently nanofillers have been added ranging from 0.5% to 21 40% by weight in the 8th generation adhesive systems. Fillers control handling and may improve strength. Fillers may increase film thickness of the adhesive layer. Solvent (REV.) solvents include acetone, ethanol and water. The solvent affects the evaporation rate on the tray and in the mouth. Acetone evaporates quickly and requires the shortest drying time in the mouth. 22 Ethanol evaporates more slowly and requires moderate drying time. Water evaporates very slowly and requires longest drying time. Bonding agents should be dispensed immediately before use to prevent premature evaporation of the solvent. The complete evaporation of solvents is nearly impossible. Hybrid Layer (REV.) Etchant demineralizes dentin Inter diffused with low-viscosity monomer displaces water-Bi functional Resin mechanically interlocks collagen 23 (REV.) Bonding of resin to dentin using an etch-and-rinse technique 24 Lecture (1) Bond Durability Composition of mineralized vs. demineralized dentin vs. hybrid layers Mineralized Etched/rinsed Primed/infiltrated Aged/degradation dentin dentin dentin Mineral 50 0 0 0 Collagen 30 30 30 0-30b Water 20 70 0a 0-30b Resin 0 0 70a 20-50c Theoretical composition of demineralized dentin before and after bonding procedures. a Perfect hybrid layer; b water replaces destroyed collagen fibrils; c loss of collagen fibrils often leads to loss of interfibrillar resin. Factors affecting STABILITY OF hybrid layer 1.Time: The bonding strength decrease over the time Factors affecting STABILITY OF hybrid layerSTABILITY OF hybrid layer 2.Collagen degradation: About half of the hybrid layer volume would not take up heavy metal stains that collagen fibrils typically bind. This was the first indication that collagen fibrils in hybrid layers could degrade. Factors affecting STABILITY OF hybrid 3.Peripheral enamel seal : -Good peripheral enamel acid-etched resin seal will lead to good durability -Resin-enamel bonds are more durable than resin-dentin bonds. Because of : -Absence of collagen in enamel -Enamel dryness relative to dentin Enamel can be dried with an air stream unlike dentin, where such procedures usually draw additional water from underlying tubules to the dentin surface. STABILITY OF hybrid layer 4.functional stress: Teeth under function undergo flexing and compression stresses which accelerate degradation of resin-dentin hybrid layers.  Factors that Compromise the Durability of Resin-Dentin Bonds Hydrolytic Degradation by Water Sorption. Incomplete Infiltration of Resin Monomers. Collagenolysis by Endogenous MMPs Etch-and-Rinse vs. Self-etch Adhesives. 31 Factors that Compromise the Durability of Resin- Dentin 1 Bonds Hydrolytic Degradation by Water Sorption The water “wet bonding” technique was introduced in the early 1990s to prevent the problem of collagen collapse after acid etching. Two hydroxyethyl methacrylate (HEMA) was incorporated into many dentin adhesives to serve as a solvent for non-water-compatible resin monomers, TO: 1-Reduce phase separation of those monomers after evaporation of the volatile solvent. 2-Enhance the wetting properties of those adhesives on acid-etched dentin 3- has affinity to demineralized collagen Hydrolytic Degradation by Water Sorption The presence of water within hybrid layer may cause hydrolysis which is (a chemical process that breaks covalent bonds between polymers by addition of water on ester bonds, resulting in resin degradation compromising bond strength in latter periods of time). Incorporation of hydrophilic and acidic resin monomers in adhesive blends with decreased longevity of resin-dentin bonds results in : Water sorption Decreases the mechanical Plasticization of properties of polymers, resins 35 Nano-phase separation: 1. Is a nanoscopic phenomenon that occurs with the inclusion of water in adhesive components. 2. Appear as nanoscopic worm-like structures. 3. Worm-like features were found on the surface between the polymerized hydrophilic and hydrophobic resin phases (HEMA/BisGMA) cured in the presence of water. 36 37 How does nano-phase separation affect adhesives ? Decreasing the dynamic mechanical properties Increasing susceptibility to esterase- catalyzed hydrolysis. WATER TREES All-in-one adhesives are very acidic and very hydrophilic. They attract and absorb water, leading to leaching of unpolymerized monomers or hydrolytic degradation products through water-filled channels called water trees. It lowers the bond strength and makes it less durable. 39 40  Factors that Compromise the 2 Durability of Resin-Dentin Bonds Incomplete Infiltration of Resin Monomers 2- Incomplete Infiltration of Resin Monomers The main goal of dentin bonding is complete infiltration of resin monomers into demineralized collagen fibrils exposed by acid-etching or self- etch adhesives 2- Incomplete Infiltration of Resin Monomers To date, complete replacement of lost apatite's by resin within the intrafibrillar spaces has never been demonstrated. Due to large molecular weight of BisGMA. 43 44 Factors that Compromise the Durability of Resin- Dentin 3 Bonds Collagenolysis by Endogenous MMPs  Phosphoric acid used in the etch-and-rinse Factors technique is highly acidic and activated Matrix metalloproteinase (MMPs )due to its low acidity. related to Matrix metalloproteinase contributes to the etchant that breakdown of collagen of resin-infiltrated hybrid decreases layers. Type of MMPs that are involved in the durability degradation process are MMP-2 and MMP-9 For both etch and-rinse and self-etch adhesives. 46 Application etch-and- rinse adhesives Activation of MMPs Collagenolytic and Gelatinolytic activity Collagen degradation Hybrid layer disruption Loss of bond strength that affects the longevity of the restoration 47 Factors that Compromise the Durability of Resin- 4 Dentin Bonds ETCH-AND-RINSE VS. SELF-ETCH ADHESIVES  49 4- Etch-and-Rinse vs. Self-etch Adhesives The annual failure rates of Class V bonded restorations are: 1- Step 2-step 2-step 3-step 2-step self- self-etch etch-and- self-etch etch-and- etch(mild (aggressive) rinse &moderate  The 2-step self-etch adhesives has better durability because of the use of a separate, relatively more hydrophobic resin layer on top of the hydrophilic self-etch primer.  It makes the interface less susceptible to water sorption, and partial dissolution of the apatite minerals which will lead to a protective effect on collagen degradation.  51 Strategies to Enhance bond durability Increasing the Degree of conversion 52 and Esterase resistance of hydrophilic Adhesives  The degrees of conversion of hydrophilic adhesive components may be improved by the use of hydrophilic photoinitiators.  Use of novel bulky/branched esterase- resistant hydrophilic urethane-modified resin monomers  Use of water-soluble photoinitiators.  to improve the polymerization of hydrophilic adhesives in the presence of water  53  chemically grafting CHX to resin monomers to create CHX- methacrylates and incorporating these resin monomers with anti- MMP potentials into dentin adhesives. Ethanol Wet-bonding with 54 Hydrophobic Resins When hybrid layers created with commercially available etch and- rinse adhesive using water wet-bonding or ethanol wet bonding were compared, significantly less nanoleakage (micropermeability of the fluorescent tracer could be seen in hybrid layers created with ethanol wet- bonding) (Sauro et al., 2009a). The results suggest that ethanol wet-bonding is capable of increasing resin uptake and producing better sealing of the collagen matrix. Inhibitors of Collagenolytic Enzymes 55  Thus, application of MMP inhibitors to the demineralized collagen matrix prior to the application of dentin adhesives appears to be a rational approach for extending the longevity of resin-dentin bonds  56 Chlorhexidine (CHX), a biguanide antimicrobial agent, has been successfully applied to inhibit the activities of MMP-2, MMP-8, and MMP-9. CHX as a non-specific MMP-inhibitor during adhesive application may be applied to the demineralized dentin directly or incorporated into an acid conditioner prior to the application of adhesives.  57  Another non-specific MMP inhibitor, GM 6001 (galardin), is often used. Galardin has been used as an experimental primer on dentin prior to the application of an etch-and-rinse adhesive and prevented degradation of the hybrid layer after 12 months.  58  Chemically modified tetracyclines (i.e., tetracyclines that lack antibiotic activities but retain their anti-MMP activities) are effective non- specific MMP inhibitors.  59  Another group of antimicrobial agents, the quaternary ammonium salts, also possesses anti- MMP activities.  60  Encapsulating MMPs with hydrophobic resins represents another innovative way of immobilizing the catalytic domains of these enzymes to inactivate their functional activities. MMP Silencing via the 61 Use of Cross-linking Agents Over the last few years, the experimental use of cross- linking agents to increase the longevity of resin-dentin bonds has taken on a life of its own, with various attempts to use agents such as glutaraldehyde for long time periods (generally > 1 hr) to introduce additional cross-links to acid- demineralized dentin collagen. (A) Tooth structure. EN, enamel; D, dentin; PD, predentin; P, pulp. (B) Magnified section shows that protein aggregates proteoglycans and endogenous distribution in coronal dentin, dentin enamel junction and mainly on predentin region. (C) Dentinal tubule represent collagen fibrils. (D) Collagen network hold by collagen micro-fibrils. (E) Collagen cross-link provide tensile properties.