Adhesion in Dentistry PDF

Dr. Shaimaa nagi Adhesion in Dentistry Operative 5 Definitions Bondodontics: is the science that studies adherence of materials to tooth structure. Adhesion: the force that binds two dissimilar materials together when they are brought into inti...

Dr. Shaimaa nagi Adhesion in Dentistry Operative 5 Definitions Bondodontics: is the science that studies adherence of materials to tooth structure. Adhesion: the force that binds two dissimilar materials together when they are brought into intimate contact. Adherend: Substrate to which the material adhere. Adherent: Adhesive material used to produce adhesion. Adaptation: maximum degree of proximity between two adjacent surfaces. Mechanisms of adhesion involved in dentistry A. Mechanical adhesion. It occurs when the solidified adhesive interlocks micromechanically with the roughness and irregularities of the surface of the adherend (substrate) B. Chemical adhesion It is the bonding of dissimilar materials by the interaction of atoms or molecules at an interface that holds two phases together. C. Electrostatic adhesion Involves electrostaticinteractions that are relatively weak.It may be the only type of adhesion if surfaces are smooth and chemically dissimilar. Requirements for adhesion: To produce good bonding, the adhesive or adherent should be able to flow easily over the adherend to produce good wetting. Wettability is: Spontaneous flow and full spread of a liquid phase (adhesive) over a solid phase (substrate surface),and is measured by the contact angle. 1  Dr. Shaimaa nagi The contact angle is: the angle between the liquid and the solid surface. - 0 or 180 degrees ---> maximum adhesion - Less than 90 degrees ---> moderate adhesion - More than 90 degrees ---> poor adhesion In addition, sufficient wetting of the adhesive will only occur if its surface tension is less than the surface energy of the adherend. Surface energy is: The inward attractive forces of the molecule of a solid. The surface energy of a liquid is known as the surface tension. Thus, the requirements for successful wetting of adhesive are: 1. Intimate molecular contact between adhesive and tooth tissues (adherend). 2. Cleanliness, smoothness and dryness of the tooth tissues. 3. High surface energy of the tooth tissues to be able to attract the atoms of the adhesive. 4. Low-surface tension of the adhesive material (adherent) to be able to properly wet the adherend. 5. Low viscosity of the adhesive to penetrate into the micro-porosities 6. To be able to displace air and moisture during the bonding process. Advantages of adhesive techniques: Bonded restorations have a number of advantages over traditional non- adhesive methods: Ultra conservation of tooth structure: through elimination of macromechanical preparations and excessive removal of tooth structure. 2  Dr. Shaimaa nagi Long term restoration retention: strong bond provides longevity of the restoration and prompted patient acceptance. Adhesive restorations better transmit and distribute functional stresses across the bonding interface to the tooth thus provide better resistance. Reinforcement of the remaining tooth structure: (still a controversy) due to the ability of bonding between the restoration and tooth that participate in its load carrying ability. Decrease microleakage and its sequelae, which are: postoperative hypersensitivity, recurrent caries, cytotoxic pulp reaction, discoloration, fracture or dislodgment. Expanded the range of esthetic restorative dentistry. Indications for adhesive dentistry: Conservative restorations of class I, II, III, IV, V, VI carious or traumatic defects Change shape and color of anterior teeth Fractured tooth reattachment Bond all indirect restorations Provide foundations for crowns Seal pits and fissures Bond orthodontic brackets Periodontal splints Repair of restorations Dentin desensitization 3  Dr. Shaimaa nagi Fundamental principle of adhesion to tooth substrate: The fundamental principle of adhesion to tooth substrate is based upon an exchange process; as the inorganic tooth material is exchanged for synthetic resin. This process involves two phases: a. Removal of calcium and phosphate content to expose microporosities in both enamel and dentin surfaces. b. The second phase (hybridization phase) involves infiltration and subsequent in situ polymerization of resin within the created surface microporosities. This results in micromechanical interlocking that is primarily based upon mechanism of diffusion. Challenges in bonding to dental substrates: - Enamel is formed of 96% inorganic materials and only 4% of organic and water while dentin is formed of 75% inorganic and 25% organic substances and water by weight. -Bonding to enamel (nearly homogemous structure) is a relatively simple process; however; bonding to dentin, on the other hand, presents a much greater challenge. Several factors account for this difference between enamel and dentin bonding: 1. Heterogeneous composition of dentin: The Inorganic and organic constitutes are unevenly distributed in the intertubular dentin and peritubular dentin. 2. Complex histological structure of dentin Peritubular dentin hypermineralized Intertubular dentin high collagen and organic components Intratubular dentin odontoblastic process and fluid. 3. Inherent dentin wetness 4  Dr. Shaimaa nagi Dentinal tubules are filled continuously with dentinal fluid by the capillary attraction which renders dentin under constant wetness. In addition, the dentinal fluid is under a slight, but constant, outward pressure from the pulp. The intrapulpal fluid pressure is estimated to be 25 to 30 mmHg. 4. Dentin is considered as Dynamic tissue Mineral content of dentin increases in different situations, including aged dentin, dentin in the vicinity of a carious lesion, and dentin exposed to the oral cavity in noncarious cervical lesions, in which the tubules become obliterated with tricalcium phosphate crystals. The dentin that undergoes these compositional changes is called “sclerotic dentin” and is much more resistant to acid-etching than “normal” dentin. Consequently, the penetration of a dentin adhesive is limited. Though, change in dentin structure requires some clinical considerations during adhesives application. Increase etching time is recommended with sclerotic dentin. Classification of dental adhesives: 1- Chronological “Generational” Classification 2- Scientific Classification (Van Meerbeek et al, 2001) 3- Classification according to the clinical application steps 4- Classification according to smear layer handling mechanism 5  Dr. Shaimaa nagi Development of adhesives: Chronological “Generational” classification: Only adhesives from fourth to eighth generation are currently on the market. The First, second and third Generation adhesives, showed many disadvantages and low bond strength to the tooth structure so they are not available in the market any-more. Fourth-Generation Adhesives: - In the 1990’s, significant advances were made in adhesive dentistry with the development of multistep dentin adhesive systems - That involves the pretreatment of dentin with conditioners and primers that make heterogenous and hydrophilic dentin more receptive to bonding. - Manufacturers used the term conditioner instead of etchant as these conditioners were to be applied to both enamel and dentin following the total-etch approach. - Fourth generation adhesives are referred to as three-step etch and rinse adhesives. - A final step in these adhesives involves the application of unfilled or semi-filled low viscosity adhesive that copolymerizes with the primed dentin and simultaneously offers bonding receptors for copolymerization with the restorative resin composite. 6  Dr. Shaimaa nagi Fifth-Generation Adhesives: - The fifth generation adhesive systems were introduced in attempts to sum up the steps of bonding into two-step application (etch-and-rinse), “Fewer bottles and/or less time” They utilized a separate etch-and-rinse (total-etch) phase followed by the application of a combined primer-adhesive resin solution. Bond strength of these adhesives has been found to be comparable to those of the fourth- generation systems. Sixth-Generation Adhesives: Further demand for simplification has urged manufacturers to develop adhesives with even fewer clinical steps. The sixth-generation consists of self-etch adhesives. They are characterized by the omission of a separate conditioning step and are composed of two solutions. They consist of two types of adhesives: Self-etching primer and a separate adhesive resin (two-step self-etch adhesives) Those combine conditioner, primer, and adhesive resin but require mixing(one-step, two-component self-etch adhesives) Similar to the second-generation, Sixth-generation adhesives use the smear layer on enamel and dentin as bonding substrate. - The main differences between the second and the sixth generation are: The acidity of the primer (Sixth generation =Smear layer dissolving adhesives) 7  Dr. Shaimaa nagi Self-etch adhesives are more hydrophilic. Moreover, in order to ensure etching capability of these monomers, water must be present to act as ionizing medium. Seventh-Generation adhesives: These systems are single-component, one-step self-etch adhesives and combine conditioning, priming, and adhesive resin application without mixing. Disadvantages: Due to the complex nature of the mixed solutions, they are: Prone to phase separation and formation of droplets within their adhesive layers. Act as semipermeable membranes, permitting bidirectional water currents. Lower bond strength than fourth- and fifth-generation adhesives. Eighth –Generation adhesives: They are the nano-filled adhesive products or the self- adhesive resin composite products. Other classifications for the adhesive systems: - Scientific Classification (Van Meerbeek et al, 2001) - Classification according to the clinical application steps - Classification according to smear layer handling mechanism Van Meerbeek et al, 2001; have suggested a scientifically based classification with three main groups of adhesives: Etch and Rinse Approach. Self-Etch Approach. Glass-ionomer Approach. 8  Dr. Shaimaa nagi These classifications are simple and proved to be reliable and consistent. The three classifications could be combined as followed: Etch-and-rinse adhesives 3 or 2 step adhesives Smear layer removing adhesives Self-etch adhesives 2 or 1step adhesives Smear layer dissolving adhesives Glass ionomer based adhesives 1 or 2 step adhesives Smear layer modifying adhesives Scientific Classification (Van Meerbeek et al, 2001) 9  Dr. Shaimaa nagi I. Etch and Rinse Approach Involves either: - Three steps etch and rinse adhesive system: application of conditioner or acid etchant, followed, by the primer & then application of the bonding agent. - Two steps etch and rinse adhesive system that combine the second and the third steps, but still have a separate etch & rinse phase. Clinical steps 3-steps 2-steps 1. Isolation with rubber dam 1. Isolation with rubber 2. Acid Etching with phosphoric dam st 1 acid 37% for 15 seconds 2. Acid Etching with step 3. Washing with water for 10-20 phosphoric acid 37% for 1st seconds 15 seconds step 4. Air Drying 3. Washing with water for 10-20 seconds 2nds 5. Application of primer for 20 4. Air Drying tep seconds 6. Air drying 5. Application of primer & the bonding agent in one 2nds tep 7. Application of the bonding agent step for 20 seconds 3rd for 20 seconds 6. Air thinning step 8. Air thinning 7. Light curing for 20 9. Light curing of the bonding agent seconds for 20 seconds 8. Application of resin 10. Application of resin composite composite 10  Dr. Shaimaa nagi N.B : Conditioners: Any material that could lead to alteration of tooth surface. Etching: Selective removal/demineralization, i.e. stripping of calcium from its phosphorus. A- Conditioning Enamel: Objectives: 1. Removal of organic pellicle and prismless enamel 2. Removal of the smear layer 3. Creation of enamel microporosities (5-50μm) 4. Increase Enamel surface energy from 32 to 72 dynes/cm2 Methods: 1. Chemical conditioners: a. Phosphoric acid 37% for 15 seconds: the most widely used etchant. b. Nitric acid 2.5%. c. Citric acid 10%. d. Maleic acid.10%. e. Oxalic acid.1.6-3.5% f. EDTA: it’s a strong decalcifying agent (Chelator) but prompted low bond strength to enamel as it does not etch preferentially. g. Hcl acid 15% may be used for deeper enamel etching, followed by resin infiltration (Icon) to modify the refractive spectral of enamel white lesions. 2. Physical conditioners (LASER): - The substrate surface is altered by microscopic explosions caused by thermal transients increasing the bondable fraction of inorganic dentin. - This will lead to a decrease in the organic fraction of the substrate. - Laser leads also to a desensitized dentin due to occlusion of dentinal tubules. 11  Dr. Shaimaa nagi 3. Mechanical conditioners (Air Abrasion): - It is based on applying aluminum oxide particles of different sizes and velocities to the enamel and dentin surfaces. - It has been suggested that air abrasion could be useful in self-etch adhesive systems. Factors affecting successful enamel acid etching: 1- Type of acid: phosphoric acid is the most commonly used. 2- Acid concentration; higher concentration will lead to formation of acid monophosphate dehydrate, which precipitate on the surface thus, no micropores will form. 37% concentration 15 seconds 70% concentration time 15% concentration time 3- Time of etching; 15 seconds was found to be as efficient as 60 seconds. 4- Form of etchant: gel form etchants are preferred in upper teeth and they are easy applied. Liquid etchant is used in deep grooves and fissure. 5- Rinsing time: at least 10 seconds to remove acid remnants and dissolved calcium phosphates. 6- Method of activation of etching (rubbing, agitation, and/or repeated application of fresh acid) 7- Instrumented or non -instrumented enamel 8- Chemical composition and condition of enamel: Increase of fluoride content of enamel needs increase period of acid application. 12  Dr. Shaimaa nagi 9- Primary or permanent teeth: Primary teeth have thicker layer of prismless enamel than permanent teeth. On the other hand permanent teeth have higher mineral content 10- Enamel is prism-structured or prismless. Patterns of etching: Three enamel-etching patterns have been described: - Type I etching pattern: preferential removal of enamel prism cores. - Type II etching pattern: preferential removal of prism peripheries. - Type III etching pattern: not related to prism morphology. Or simply enamel etching patterns could be in the form of interprismatic (10-20μm) and intraprismatic pattern (2-5μm) of etching. Clinical features: etched enamel appears white frosted (chalky white). Type I Type II Type III 13  Dr. Shaimaa nagi B- Dentin Conditioning: Objectives: 1. Total removal of the smear layer and smear plugs 2. Demineralize the superficial dentin surface 3. Exposes a microporous layer of organic collagen fibers thus increasing the microporosity of the intertubular dentin (0.05-0.1μm) 4. Demineralizes the peritubular dentin producing more funneling of dentinal tubules (1-3μm) Rinsing: Using copious amount of water for 10-20 seconds to remove acid remnants and dissolved calcium phosphates. Air Drying or Blotting: - Mineralized dentin contains 50% minerals, 30% collagen and 20% water by volume, whereas demineralized dentin is 30% collagen and 70% water. With the removal of the mineral phase, the collagen fibers are suspended in water. - If there is a substantial zone of demineralization and the water supporting the collagen network is removed, either by air drying or the action of an air syringe, the collagen will collapse. - Thus drying of dentin could be done by using air syringe or minisponge or cotton pellet. Excessive dryness should be avoided to avoid collagen collapse. Air-drying leads to decrease in volume of collagen by 65% that could be regained by rewetting. 14  Dr. Shaimaa nagi Collapsed collagen fibers Problems encountered dentin after etching: 1. Decrease surface free-energy (44.8 dynes/cm2) due to high proteins (collagen) exposed. 2. Increased permeability and wetness of dentin hinder adhesion. Thus, To overcome these obstacles (problems), an adhesion-promoting agent (Primer) has to be used. C- Primers: They contain hydrophilic monomers (HEMA) and hydrophobic part dissolved in solvents such as acetone, ethanol, and/or water. Because the volatile characteristics of solvents, they displace water from dentin surface and moist collagen network, promoting the infiltration of monomers through the nanospaces of exposed collagen network. Effective primers contain: Hydrophilic monomers having a high affinity for the exposed collagen fibril arrangement Hydrophobic properties for copolymerization with the adhesive resin. 15  Dr. Shaimaa nagi Objectives To transform the hydrophilic dentin surface into a hydrophobic and spongy state that allows the adhesive resin to wet and penetrate the exposed collagen network. To induce denaturation and precipitation of proteins from dentinal fluid and decrease dentinal permeability N.B.: After conditioning the demineralized collagen network is susceptible to collapse when water was removed by drying. Depending on the primer, two techniques (wet and dry bonding) have been proposed Wet versus Dry Bonding: - Surface moisture is an important factor in optimal bonding. - On enamel, a dry condition is theoretically preferred. - On dentin, a certain amount of moisture is needed to avoid collapse of exposed collagen, which insures best penetration of adhesive monomers. Consequently, in the treatment of enamel and dentin, it is difficult to achieve the optimal environment of substrates. ü One way to achieve this goal is to keep the substrate dry field (dry bonding technique) and use adhesive systems with water based primers to rehydrate and thus re-expand, the collapsed collagen network, enabling the resin monomer to interdiffuse efficiently. ü A balance should be present between the amount of water needed to allow collagen re-expansion and the amount of water in the primer. Low water concentration leads to stiffness of the collagen network and lesser degree of re-expansion. On the contrary, too much water would rapidly expand the collagen network but also dilute the monomer concentration leading to decrease in the formed bond strength. 16  Dr. Shaimaa nagi ü The alternative is to keep the acid-etched dentin moist (wet bonding technique) and to rely on the water-chasing capacity of alcohol based primers. This clinical technique referred to as Wet Bonding introduced by Kanca 1992. The role of acetone and ethanol could be summarized as follows: 1. When applied to the substrate surface, acetone or ethanol diffuses into the moist dentin, while water diffuses into acetone or ethanol. 2. They occupy the spaces previously filled by water, then evaporate rapidly leaving behind sufficient room for the coming infiltrating resins. 3. The chemical dehydration increase the modulus of elasticity of collagen Therefore the wet bond technique is considered a successful method. 4. Acetone and ethanol are also characterized by having higher vapour pressure then water and also generate less surface tension forces in collagen fibrils with ultimate increase in bond strength Disadvantages of wet bonding technique: 1. Over-wet condition: if water inside the collagen network is not completely displaced, the polymerization of resin inside the hybrid layer may be affected or the remaining water may compete for space with resin inside the demineralized dentin. This condition has been well documented with adhesives that provide water-free acetone based primers. In such condition, excessive incompletely removed water during priming appeared to cause phase separation of hydrophobic and hydrophilic monomer components, resulting in blister and globule formation at resin dentin interface. 17  Dr. Shaimaa nagi 2. Acetone quickly evaporates from the primer bottle so that after the primer solution is dispensed in a dish, the primer bottle should be closed at once and the primer solution should be immediately applied to the etched surface 3. Keeping the cavity walls wet after conditioning will not allow white frosted appearance of properly etched enamel to be detected For successful priming: 1. Dentin surface must not be over dried or over wet. 2. Avoid excess water as it may dilute the primer renders it less effective. 3. In dry bonding, air dryness for 5-10 seconds is required while for wet bonding, moisture should be removed by short air blast (gentle air drying for 2 seconds) or blotting or wiped off with a dry sponge or small tissue paper. 4. In dry bonding technique, air drying of demineralized dentin reduces its volume by 65% but the original dimensions could be regained by rewetting or re-moisten with water or antibacterial solution such as chlorhexidine. 5. Apply multiple coats: this insures total covering of the substrate and allow for more penetration of the hydrophilic monomer. 6. Primer application time should be at least 15 seconds to allow monomers to inter-diffuse to complete depth of surface demineralization and evaporation of the solvent. 7. The primer should be actively rubbed into the dentin surface with a disposable brushes or sponge applicators to improve the inter-diffusion process. 8. Enamel and dentin must appear glossy, shiny and hydrated with visibly moist surface after priming. 18  Dr. Shaimaa nagi 9. Acid-etched enamel does not need a separate primer application to achieve effective bonding to air dried enamel 10. In wet bonding technique, primers should always be applied on acid- etched enamel to displace residual moisture. 11. Short and gentle air-drying after primer application should be done. C- Adhesive Resin: The bonding agent consists primarily of hydrophobic monomers as Bis-GMA or UDMA. It also contains some hydrophilic monomers such as TEGDMA to regulate viscosity and HEMA as a wetting agent. Adhesive resin could be chemically or photo cured. For the photo cured, resin polymerization is done prior to packing of the composite resin. The main goal is to totally seal all the micro pores created previously by resins and achieve a biologic tissue that is formed of resins encapsulating and hydroxyapatite crystals and collagen totally to form an elastic intermediary joint to link resin composite to tooth tissues. Resin/enamel interface: Adhesive resin absorbed by capillary attraction within the created etched enamel surface enveloping individually exposed hydroxyapatite crystals followed by in situ polymerization of resin to create two types of resin tags: Macrotags fill up the space surrounding the enamel prisms Microtags result from resin infiltration/polymerization within the tiny etch pits and at the cores of etched enamel prisms. They are thought to be the major contributors to enamel retention. 19  Dr. Shaimaa nagi Resin/enamel interface Dentin Hybridization and Resin Tags formation: ü The part of the resin that impregnates into the intertubular dentin is called hybrid layer while the part that enters the dentinal tubules is called resin tags. Hybridization: is the formation of resin interlocking in the demineralized tooth surface; providing micromechanical retention. Requirements for an ideal dentin bonding agent: 1. It should be hydrophilic in order to bond to wet dentin. 2. It should contain hydrophobic part to copolymerize with the subsequently applied resin. 3. It should have low viscosity for better diffusion. 4. It should be biocompatible 5 It should possess minimum film thickness for better wettability. 6. It should possess high bond strength to both enamel and dentin immediately after placement. 7. It should minimize microleakage to nanoleakage. 8. It should be easy to be applied. 9. It should have good shelf life. 20  Dr. Shaimaa nagi N.B: Concerns have been raised that aggressiveness etching of dentin may cause demineralization to a depth that might be inaccessible to complete resin impregnation. Incomplete resin penetration (also called hybridoid layer), causes a microporous dentinal zone at the base of the hybrid layer; thought to be a pathways for nanoleakage fluids, causing hydrolysis of collagen. Etch and rinse approach Diagram illustrating the concepts of microleakage (right side) and Nano-leakage (left side) when bonding dental resins (R) to dentin (D) 21  Dr. Shaimaa nagi II Self-Etch approach (Smear layer dissolving) 2 steps 1 step 1st Application of self-etching Application of the self step primer for 20 seconds etching primer together Air drying with the bonding agent in Application of the bonding one step 2nd step agent for 20 seconds Air thining Air thining Light curing Light curing for 20 seconds Application of resin composite Self-etching primer is applied for 15-20 seconds etch through the dentinal smear layers and into the underlying dentin, creating acceptable dentin bond strengths. The degree of etching of enamel seems to be minimal. Self-etching primers contains acidic monomers, which are not rinsed off, simplifying the procedure. On enamel: The self-etching primer should be applied for at least 30 seconds and actively applied by rubbing with repeated application of fresh material. Or, separate conventional etchant can be applied before application of the self-etching primer (selective etching to enamel). 22  Dr. Shaimaa nagi Advantages of self-etch adhesives 1- Reduced the clinical application time, 2- Reduces the possibility of over-wetting or over-drying, 3- Reduces the risk of errors during application and manipulation (less technique sensitive). 4- Less postoperative sensitivity. 5- They are less likely to result discrepancies between the depth of demineralization and depth of resin infiltration as both occur simultaneously. Depending on the etching (added acid groups) aggressiveness, they can be divided into strong (Ph≤1), intermediary strong (pH=1.5)and mild(pH≥2) self-etch adhesives. Self-etch approach 23  Dr. Shaimaa nagi III Glass-ionomer approach One-step or two-step Glass ionomers are the only materials that are self-adhesive to tooth tissues - One step or direct application of the material - Two steps application involving pre-treatment with a weak polyalkenoic acid conditioner. - The application of Polyalkenoic-acid conditioner for 10-to-20 seconds increases bonding efficiency due to: 1. Cleaning effect 2. Partial demineralization by which the surface area is increased and micro porosities for micromechanical interlocking are exposed. 3. Chemical interaction of polyalkenoic acid with residual hydroxyapatite. The auto-adhesion of glass ionomer has recently been determined to be due to: 1. Micromechanical interlocking. 2. True chemical bonding occurs between the carboxyl groups of the polyalkenoic acid and calcium of hydroxyapatite. 24  Dr. Shaimaa nagi Factors affecting the interface durability. I. Dentin related factors II. Adhesive system related factors III. Micro mechanics of adhesive interface IV. Extrinsic and intrinsic water sorption V. role of saliva in bond degradation VI. Miscellaneous factors I. Dentin related factors; Heterogeneous composition and complex histological structure (as discussed before) Dentin smear layer: A layer of 2-5μm of calcific debris (hydroxyapatite crystals, collagen, saliva and blood and microorganisms) produced by reduction or instrumentation of dentin, enamel or cementum or as a contaminant. This tenaciously adherent layer is burnished on the cut surface by the effect of heat and pressure of cutting and cannot be removed by rinsing. Advantages of smear layer: 1- It reduces dentin permeability by about 86% through plugging the dentinal tubules thus provides a drier surface for adhesion. 2- Minimizes post-operative hypersensitivity and preventing the ingress of irritants from the restorative material to the tubules. Disadvantages of smear layer: It incorporates microorganism and is loosely attached to the underlying dentin providing for a weak joint between the tooth tissues and the restorative material. 25  Dr. Shaimaa nagi So acids with a high reactivity rate with the mineral component of dentin have been considered a good reagent for removing the smear layer. Bonding to deep versus superficial dentin In superficial dentin, there are few dentinal tubules and large amounts of intertubular dentin are available. Thus bonding the mechanism is through hybrid layer formation. On the other hand, in deep dentin, the dentinal tubules are abundant with limited amounts of intertubular dentin, so the resin tags bonding is achieved. Bonding to the gingival marginal dentin - Clinical studies showed that the gingival margin in class II composite restorations is the most common location for bonding failures. - This was explained by the difference in the demineralization depth between dentin at the gingival margin which is less mineralized than dentin at the proximal walls. Thus, acids are expected to etch dentin at the gingival margin faster and deeper than dentin at the proximal wall. - In addition, the density and size of dentinal tubules at the gingival margin are greater than that at the proximal wall; hence acid etching is faster in dentin at the gingival margin. - There was also a distinct difference in adhesive infiltration with considerably less adhesive penetration at the gingival margin as compared to the proximal wall. That was related to the increase water content at the gingival dentin, which makes it difficult bonding substrate. This is not only because of the water already present within the demineralized dentin matrix, but also because patent tubules contribute to the contamination of the prepared surface with a great amount of dentinal fluids. 26  Dr. Shaimaa nagi The cumulative effect of the increased water led to reduced adhesive infiltration and low monomer/polymer conversion of the adhesive at the gingival margin as compared to the proximal wall. Bonding to altered dentin: In most of the clinical situations clinicians usually bond adhesives to caries affected dentin or abraded sclerotic dentin. Authors have reported nearly a 30%-40% drop in the bond strength with both affected dentin substrates. - The hybrid layer formed on caries affected dentin was thicker than those formed on healthy dentin. The thick hybrid layer may be due to the fact that caries affected dentin is partially demineralized and offers a more porous substrate for acid etching than healthy dentin. - Moreover there was also a dramatic reduction in monomer/polymer conversion when the adhesive was used on carries affected dentin. It was postulated that the lack of adhesive penetration in caries affected dentin is due to a phase transition in the collagen that has been disordered by caries. Since caries affected dentin may be characterized by zones of disorganized collagen, acid etching could promote phase transition of this disorganized collagen to a gel. The gel could inhibit adhesive infiltration. II. Adhesive related factors Etch and rinse adhesives Acid conditioners are used to demineralize the layer that is smeared and the underlying intact dentin to create a microporous surface with a rich collagen network providing room for resin infiltration. The 27  Dr. Shaimaa nagi complete filling of the exposed space by subsequent application of resin monomer is unattainable within the short time available, and it is possible that an exposed demineralized dentin zone remains within the bond structure. The exposed collagen fibrils here may be structurally unstable due to hydrolysis, reducing long term bond strength. Moreover it has been hypothesized that the in vivo degradation of the hybrid layer follows a cascade of events that begins when the dentin is acid-etched. Disruption of the tooth structure by drilling stimulates proteolytic enzymes such as matrix metalloproteinases ( MMPs), which can degrade the exposed collagen component of the hybrid layer. This type of degradation is expected to be most important acutely in the period following adhesive application. The process proceeds with hydrolysis and consequent extraction of the adhesive resins that have infiltrated the demineralized dentin matrix Self-etch adhesives: The market driven simplification of adhesive systems of self-etching primers that combines that conditioning and priming steps are thought to overcome the shortcomings of the formation of an exposed collagen network within the bonds of the total-etching adhesives. Later similar morphological evidence of degradation was reported by long term studies. Regions of incomplete resin infiltration or incomplete resin polymerization within the hybrid layers or bonding resin may represent pathways for fluids, a phenomenon termed nanoleakage. All in one adhesive system: - Water sorption of adhesive resin is proportional to its hydrophilic characteristics compared to the hydrophobic based adhesives. 28  Dr. Shaimaa nagi - The self-etching ability of one bottle adhesives is commonly achieved by incorporation of water in resin monomers that enables ionization of acidic monomers. - In addition to the water in the compounds, the ionizable moieties of acidic monomers are also hydropholic. The presence of such a more hydrophilic layer that induce water sorption and water uptake, in turn, endangering the stability of the polymer network. III. Micro-mechanics of the adhesive interface: Under clinical function dentin adhesives are subjected to both chemical and mechanical stresses. The interplay between the two forms of stress is expected to result in an alteration of the properties of the adhesive with time. The mechanical property change results from a variety of mechanisms including proliferation of surface and subsurface flaws due to combined effects of mechanical loads and exposure to salivary esterase and changes the chemical nature of the polymer. Therefore, the change in the mechanical property of the adhesive with time can result in a gradual loss of mechanical integrity. IV. Extrinsic and intrinsic water sorption. Adsorption of extrinsic water leads to plasticization of the adhesive and loss of interfacial adhesive dentin bond strength as a result of water attack. This increases water sorption after polymerization and/ or extraction of water soluble unreacted monomers. 29  Dr. Shaimaa nagi Water uptake in the adhesive layer increases the percent of elongation of the adhesive from 20 % to 220%. V. Role of saliva in bond degradation: Human saliva contains a variety of enzymes which may participate in the degradation of the adhesive as well as the composite. Human saliva samples have been shown to contain cholesterol esterase and pseudo-cholinesterase activity in sufficient quantity to degrade composite resin. VI. Miscellaneous factors: - Clinical related factors. 1- Position of the tooth Adhesive restorations in Premolars showed better performance than those in molars 2- Cavity size 3- Cavity type 4- The number of restored surfaces C-factor or Configuration factor: Is the ratio of bonded to free unbounded cavity walls. - Resin restorations shrink as they polymerize, creating stresses of up to 7MPa within the resin composite mass depending on the configuration of the preparation. - When the composite is bonded to one surface only, stresses within the composite are relieved by flow from the unbonded surface. - However, stress relief within a three-dimensional bonded restoration is limited. 30  Dr. Shaimaa nagi - Unrelieved stresses in the composite may cause internal bond disruption as well as marginal gaps around restorations. - Immediate bond strengths of approximately17 MPa may be necessary to resist the contraction stresses that develop in the composite during polymerization to prevent marginal debonding. The higher the ratio of bonded to free resin surface, the less flow may compensate for contraction stress. - Operator related factors. The operator qualification and employment status and experience affect the longevity of the restorations. The often mentioned problems of leakage related to posterior composite restorations may be related to adhesive procedures having not been done properly due to the inappropriate selection of adhesive materials and techniques or the application of materials not in accordance with the manufacturers' instructions. 1. Isolation - Before of any bonding procedure begun, adequate isolation and moisture controlof the substrate to be bonded must be achieved. -Salivary contaminationis detrimental because saliva contains proteins that may block adequate resin infiltration in enamel and dentin created microporosities. - Consistent use of a rubber dam remains the most effective method of moisture control. 31  Dr. Shaimaa nagi 2. Dentin and pulp protection -The use of nonadhesive liners and bases beneath adhesive restorations is not recommended. -Adhesive materials such as glass-ionomer cements can be used (Sandwich technique), but in most cases the simple application of an appropriate adhesive is effective. -In deep cavity with remaining dentin thickness less than 0.5mm; calcium hydroxide remains the material of choice however; it rapidly dissolves if the cavity is not adequately sealed. Therefore, when calcium hydroxide is used, it should be covered by less soluble materials as resin-modified glass ionomer cement. 3. Temporization Eugenol-containing temporary restorations must be avoided as eugenol affects the polymerization of the resin material. 4. Post-operative and post restorative care. High level of good oral hygiene is required in order not to allow plaque accumulation with subsequent bond degradation. - Patient related factors. The type of patient and the oral environment play an important role in the survival of dental restorations. The caries risk of patients has been shown to significantly influence the durability of restorations. 32  Dr. Shaimaa nagi - Socio-economic factors. As dental caries is strongly associated with social determinants experienced during the life course, it is possible that social determinants can also influence the longevity of restorations by the same pathway. - Material related factors. Difference in Flexural and compressive strength, Elastic modulus, Fracture strength, toughness, hardness and wear resistance have been shown to be significantly different among materials when laboratory techniques were used to compare the restorations. 33  Dr. Shaimaa nagi Mechanism of dentin-adhesive interface degradation and methods of prevention: Degradation of the collagen Degradation of resin monomers Degradation mechanism Degradation mechanism - Hydrolysis by residual water in - Hydrolysis and plasticization the composition of the adhesive by residual water and by dentin intrinsic humidity - Hydrolysis by residual water - Enzymatic hydrolysis not removed after rinsing the etchant - Exposed collagen by elution of -Decreasing gradient of monomer resin from hydrolytically impregnation of exposed collagen unstable polymers in the hybrid fibers (hybrid layer) with depth layer - Denuded collagen fibers have -Phase separation from increased susceptibility to residual water in self-etch mechanical fatigue and adhesives denaturation after function - Dentin and saliva - Self-etching primers behave collagenolytic enzymes as semi-permeable (MMPs, cathepsins) digest membranes—water blisters unprotected collagen fibers result in mechanical disruption of the bond between the adhesive and the composite resin -Continuing etching of dentin underneath the interface caused by uncured acidic monomers in self- etch adhesives 34  Dr. Shaimaa nagi v Potential prevention solutions for bond degradations: 1. Material dependent attempts: a. Modifying the methacrylate side chains: - Modifying the methacrylate side chains with functional monomers in “mild” SE adhesives, such as 10 methacryloyloxydecyl dihydrogen phosphate (10-MDP), 4 methacryloxyethyl trimellitic acid (4-MET) and 2-(methacryloyloxyethyl) phenyl hydrogen- phosphate (phenyl-P) lead to a chemical bond to calcium ions of the hydroxyapatite crystals. - “Mild” SE adhesives interact only superficially with dentin, forming a thin (even sub-micrometer) hybrid layer. The use of “mild” SE adhesives is believed to minimize nano-leakage, leave a substantial amount of hydroxyapatite around the collagen fibrils to mask the collagen cleavage site and keep the enzymes “fossilized”. Thus, the collagen could not be degraded. 63 b. Photo-initiator incorporation: To address the In-adequate monomer/ polymer conversion, but it should be compatible with the hydrophilic components. c. Crystallization by Resin Adhesive, Glass-Ionomer Cement, or Bio-glass: - Numerous previous reports have shown a caries-preventive effect of glass-ionomer cement by enhancing remineralization of dental hard tissue. A further reason for this caries-preventive ability is attributed to fluoridation in the tissue bordering the restoration. A resin adhesive with glass-filler particles (i.e., a surface reaction type pre-reacted fluoro-boroalumino-silicate glass) have recently been developed and used for antibacterial effects or 35  Dr. Shaimaa nagi remineralization of the hybrid layer by gradual release of various ions such as Na, B, Al, Si, or F. - This type of adhesive offers the potential to act chemically like glass-ionomer cement releasing ions and acting as an adhesive with high bond strength by functional monomers. - The ability to grow crystals between restorative compounds and dental hard tissue may lead to protection of tooth surfaces in interfacial gaps and may contribute to the long-term stability of restoration. d. Addition of protease inhibitors: - Chlorhexidine (CHX) has proven biocompatibility and good anti- bacterial action both in vivo and in vitro. - Application of 2% CHX to the acid etched dentin has been found to have MMP inhibitor and anti-enzyme properties even at this low concentration. CHX inhibits the MMPs in the hybrid layer from its Zn2+ cation-chelating property. 64 ADHESION TO DENTAL TOOTH TISSUES e. Collagen cross-linking agents: It is proposed to enhance mechanical properties of dentin matrix, reduce biodegradation rates of collagen, increase the properties of the dentin- resin bonds, and extend the life of adhesive restorations. - Proanthocyanidins (PA) are natural biocompatible collagen cross- linker broadly distributed in the plant kingdom. - Among all health benefits related to PA, particular attention has been given to their ability to bind to proline rich proteins, such as collagen, and facilitate the enzyme proline hydroxylase activity, essential for collagen biosynthesis. 36  Dr. Shaimaa nagi 65 f. Biomimetic remineralization of resin-dentin bonds: - In dentin bonding, the mineral phase of dentin is intentionally removed by acids, chelating agents, or acidic resin monomers to expose the collagen for creating micromechanical retention of resins. Unfortunately, it seems that contemporary etch- and- rinse and self-etch adhesives are incapable of completely replacing water from the extrafibrillar and intrafibrillar collagen compartments with resin monomers. -Biomimetic mineralization is a proof-of-concept strategy that utilizes nanotechnology principles to mimic what occurs in biomineralization. -This strategy replaces water from resin-sparse regions of the hybrid layer with apatite crystallites that are small enough to occupy the extrafibrillar and intrafibrillar compartments of the collagen matrix, and has been adopted for remineralization of resin dentin bonds. 37  Dr. Shaimaa nagi Resin Bonding to Dentine after Casein Phosphopeptide-Amorphous Calcium Phosphate (CPP-ACP) Treatments 2. Technique dependent attempts: a. Ethanol-wet bonding: - The problem of water hydrolysis of ester-bonds in adhesive polymers and peptide bonds in collagen might be eliminated if water could be excluded from the bonded interface. - This has been the aim in ethanol wet bonding, where ethanol is used to chemically dehydrate acid-etched demineralized dentin matrices to reduce collagen hydrophilicity and facilitate the infiltration of more hydrophobic monomers to dentin. - Ethanol wet bonding coaxes hydrophobic monomers into a demineralized collagen matrix with limited matrix shrinkage. - Infiltration of hydrophobic monomers decreases water sorption/solubility and resin plasticization, but it has been suggested that the elimination of residual water also reduces or eliminates enzyme-catalyzed hydrolytic collagen degradation, which would also contribute to improved durability of the resin bonds. - One problem with this method is that in the clinical setting, this solvent may be diluted because of repeated exposure of the 38  Dr. Shaimaa nagi material to the atmosphere or concentrated, because of separation of the bonding liquids into layers within the bottle. c. Rubber dam isolation to limit the impact of water or saliva contamination, careful attention to handling, management d. Extended adhesive application time with rubbing action, allowing proper resin penetration and solvent evaporation e. Warm air dryness to accelerate solvent evaporation 67 39