Adhesion Chapter 4 PDF

**[Chapter 4]** **[Adhesion]** G.V. Black described the retention of restorations based upon cavity design and undercut dentin. Additional tooth structure needed to be removed to fulfill the requirements for retention of the restorative material, even after the caries removed, because of the limit...

**[Chapter 4]** **[Adhesion]** G.V. Black described the retention of restorations based upon cavity design and undercut dentin. Additional tooth structure needed to be removed to fulfill the requirements for retention of the restorative material, even after the caries removed, because of the limitations of the restorative materials available at the time (as dental amalgam). With the use of adhesive systems, there has been a significant change in the principles of cavity preparation design, from the traditional principles of "extension for prevention" described by G.V. Black to a more carious lesion-centered approach. One of the greatest benefits of this conservative approach is that it allows the clinician to maintain as much tooth structure as possible. Since then, many significant advances in the development of dental adhesives have been accomplished. The adhesives currently available offer reliable adhesion between restorative materials and tooth structure (enamel or dentin). **Enamel and Dentin Bonding: An Historical Perspective** In 1955, initial advancement was made by a pedodontist, Buonocore described a clinical technique that utilized diluted phosphoric acid to etch the enamel surface for 30 seconds, resulted in a microscopically roughened, porous surface and provided for retention of self-cured acrylic resins. The resin mechanically locked to the microscopically roughened enamel surface, forming small "tags" as it flowed into the 10-to-40-micrometer-deep enamel microporosities and then polymerized. **[Adhesion / Bonding:]** Two surfaces are held together by interfacial forces which may consist of valence forces or interlocking forces or both. **[Adhesive / Adherent:]** It is a material, frequently a viscous fluid, which joins two substrates together and solidifies, and therefore is able to transfer a load from one surface to the other. **[Adherend:]** It is **t**he surface or substrate that is adhered. **Diagrammatic representation of dental adhesive system, where Adherend 1 is enamel, dentin or both. Adhesive is bonding agent, Adherend 2 is composite resin** **[Adhesive Strength:]** It is the measure of the load-bearing capacity of an adhesive joint. **[Adhesive Failure:]** The bond that fails at the interface between the two substrates. **[Cohesive Failure:]** The bond that fails within one of the substrates, but not at the interface. **[Abbreviations Commonly Used for Resin Chemicals:]** Bis-GMA Bisphenol glycidyl methacrylate HEMA 2-Hydroxyethyl methacrylate TEGDMA Triethylene glycol dimethacrylate 4-META 4-Methacryloxyethyl trimellitate anhydride UDMA Urethane dimethacrylate Phenyl-P 2-(Methacryloxy) ethyl phenyl hydrogen phosphate 1. Good substrate wetting, a low 'contact angle'. 2. A clean substrate. **[Wetting:]** It is an expression of the attractive forces between molecules of adhesive (adherent) and the adherend. It depends on two factors: **Cleanliness of the adherend:** Cleaner the surface, greater is the adhesion. **Surface energy of the adherend:** More surface energy results in better adhesion. Harder the surface is, higher will be the surface energy. This means adhesive properties of the material will be higher. **[Contact Angle:]** It is the angle formed between the surface of a liquid drop and its adherend surface. Stronger the attraction between adhesive (adherent) and adherend, smaller is the contact angle. (A) \(B) (C) **Lesser is the contact angle, better is the adhesion** **(A) Good wetting; (B) Partial wetting; (C) Non wetting** **A good wetting ensures good adhesion** 1. **[Micromechanical:]** Penetration of resin and formation of resin tags within the tooth surface. 2. **[Adsorption:]** Chemical bonding to the inorganic component (hydroxyapatite) or organic components mainly type I collagen of tooth structure. 3. **[Diffusion:]** Precipitation of substances on the tooth surfaces to which resin monomers can bond mechanically or chemically. 4. **[Combination]**: Of the previous three mechanisms. 1. Conservation of tooth structure. 2. Reinforcement of remaining tooth structure. 3. Restoration retention. 4. Reduction or elimination of marginal microleakage. 5. Expansion the range of esthetic possibilities. 1. Treatment of dentin hypersensitivity. 2. Bonded amalgam restorations. 3. Indirect adhesive restorations. 4. Repair of porcelain or composite. 5. To restore caries or fractured tooth structure 6. To alter the shape and color of anterior teeth 7. To restore pit and fissure lesions 8. To bond orthodontic brackets 9. To repair fractured composite, porcelain, amalgam or ceramometal restorations 10. To desensitize exposed root surfaces 11. To bond prefabricated or cast posts 12. To seal apical restorations placed during endodontic surgery 13. To bond fractured pieces of anterior teeth 14. To bond conservative tooth replacement prosthesis and periodontal splint 1. Bonds composite resin to tooth enamel and dentine 2. Minimizes removal of tooth structure 3. Minimizes chances of microleakage or nanoleakage 4. Beneficial in the management of dental hypersensitivity 5. Helps in cusp reinforcement after tooth preparation 6. Substantially reinforces remaining tooth enamel or dentine making them less susceptible to fracture I. **According to bonding strategy:** 1. Etch-and-rinse adhesive systems (Total-etch adhesive systems) 2. Self-etch adhesive systems (Etch-and-dry adhesive systems) 3. Glass ionomer adhesive systems II. **According to the clinical application steps** 1. Three-step adhesive systems 2. Two-step adhesive systems 3. One-step adhesive systems III. **[According to generations]** 1. First generation 2. Second generation 3. Third generation 4. Fourth generation 5. Fifth generation 6. Sixth generation 7. Seventh generation - Hydrophobic monomers - Acid etching is only for enamel (not dentin). - Self cure bonding agent. - Chemically :- composed of suface active monomers NPG-GMA(N-phenylglycine glycidyle methacrylate)which is theoritically can chelate with calcim present on the tooth surface to generate water resistant chemical bond. - **Disadvantages** - First-generation bonding agents ignored the smear layer. - They had very low bond strength of 2--3 MPa. - The bond strength would decrease over a period of time. - Individual components lacked stability during storage. - Exampels : - Cervident - Cosmic bond  1. **Second generation** - Phosphorous-ester monomers enhanced surface wetting - claimed chemical bond to calcium smear layer predominately intact - fear of etching dentin - Low bond strengths : 2 to 8 MPa - It was phosphate ester material (phynyle-p and HEMA )in ethanol. - Its mechanism of action was based on the polar interaction between negatively charged phosphate group and positively charged calcium ions in the smear layer. - Smear layer was the weakest link in that system because of its relatively loose attachment to underlying dentin surface. - Resin bond were contain a hydrophobic groups that had a large contact angels on the moist surfaces. - It did not wet dentin well nor penetrate the entire depth of smear layer to establish ionic bond Second generation 2. **Third generation** Mechanism of action - modified/altered smear layer - mildly acidic hydrophilic monomer - Removal of smear layer by chelating agent (EDTA) has given a marked improvement in shear bond strength. - in 1984 Kurary co. introduced the(Clearfil new bond) that was Phosphate based material contain HEMA &ten carbon molecules and For the first time long hydrophobic chian with short hydrophilic chian. - **Advantages** - High bond strength of 8--15 MPa. - Reduced chances of microleakage. - Forms strong bond to wet and dry dentine. - **Disadvantages** - Bond strength reduced over a period of time. - Chances of microleakage increase with time. - Examples - Scotchbond II  Third generation bonding agents involved alteration removal of smear layer by conditioning and priming before bonding - Developed in early 1990s - Based on total etch technique and moist bonding concept - Based on concept of hybridization and hybrid layer formation - Three steps application, i.e. Total etching + Application of primer+ Application of bonding agent - High bond strength Mechanism of Bonding - Fourth "generation" is characterized by the process of hybridization at the interface of the dentin and the composite resin. - **Hybridization** is the phenomenon of replacement of the hydroxyapatite and water at the dentin surface by resin. - This resin, in combination with the collagen fibers, forms a hybrid layer. In other words, hybridization is the process of resin interlocking in the demineralized dentin surface (Fig. 16.22). **[Components of Fourth Generation Adhesives:]** 1\. **Conditioner (Etchant):** Commonly used acids are 37 percent phosphoric acid, nitric acid, maleic acid, oxalic acid, pyruvic acid, hydrochloric acid, citric acid or a chelating agent, e.g. EDTA. Use of conditioner/etchant causes removal or modification of the smear layer, demineralizes peritubular and intertubular dentin and exposes collagen fibrils. 2\. **Primer:** Primers consist of monomers like HEMA (2-Hydroxyethyl methacrylate) and 4-META (4-Methacryloxyethyl trimellitate anhydride) dissolved in acetone or ethanol. Thus, they have both hydrophilic as well as hydrophobic ends which have affinity for the exposed collagen and resin respectively. Use of primer increases wettability of the dentin surface, bonding between the dentin and resin, and encourages monomer infiltration of demineralized peritubular and intertubular dentin. 3\. **Adhesive:** The adhesive resin is a low viscosity, semi-filled or unfilled resin which flows easily and matches the composite resin. Adhesive combines with the monomers to form a resin reinforced hybrid layer and resin tags to seal the dentin tubules. - **Examples :** 1. Scotchbond multipurpose (3M) 2. Optibond FL (Kerr) 3. Clearfil liner bond-2 (Kuraray). - **Advantages** - Ability to form a strong bond to both enamel and dentin. - High bond strength to dentin (17--25 MPa) - Ability to bond strongly to moist dentin - Can also be used for bonding to substrates such as porcelain and alloys (including amalgam). - **Disadvantages** - Time consuming - More number of steps - Technique sensitive - Dentinal substrate after acid etching. The smear layer has been removed and both the peritubular and intertubular dentin are demineralized. The collagen fibers are exposed and bathed in water. This substrate is highly hydrophilic and particularly sensitive to dehydration. The blue coloration represents the water content of the structures illustrate  - Dentinal substrate after priming. The water has been replaced by hydrophilic resins (primers) that have impregnated the collagen fibers. The solvent of the primer can be organic (alcohol or acetone) or inorganic (water). Priming with water based primers is a relatively slow process, while organic solvents will displace water more rapidly (convective movement). Evaporation of the solvent will leave the collagen fibers coated and stiffened by the resins. The substrate has changed from hydrophilic to hydrophobic. The red coloration represents the extent of primed dentin - Dentinal substrate after adhesive resin application. The hydrophobic resin diffuses slowly into the dentinal tubules and impregnates the intertubular dentin. If resin penetration is not complete, it will leave non-infiltrated areas of demineralized dentin and non-adherent resin plugs. These defects are responsible for poor sealing of the dentin and rapid degradation of the adhesive interface.  - Dentinal substrate after polymerization of the adhesive resin. The polymerized resin has completely infiltrated the demineralized dentin and offers effective protection to the pulp-dentin complex Fifth-generation DBAs were made available in the mid-1990s. They are also known as "one-bottle" or "one-component" bonding agents. In these agents the primer and adhesive resin are in one bottle. Basic differences between fourth and fifth generation is the number of basic components of bottles. Fourth generation bonding system is available in two bottles, one primer and other adhesive, fifth generation dentin bonding agents are available in one bottle only **Advantages** High bond strength, almost equal to that of fourth generation adhesives, i.e. 20 to 25 MPa Little technique sensitivity Reduced number of steps Bonding agent is applied directly to the prepared tooth surface Reduced postoperative sensitivity. **Disadvantages** Lesser bond strength than fourth generation bonding agents. Examples of fifth generation DBA: -- Prime and Bond (Dentsply) -- Single bond (3M) These were made available in 2000. In fifth-generation, primer and adhesive are available in single bottle, and etchant in separate bottle. In sixth generation etching step is eliminated, because in sixth generation etchant, primer and bonding are available in single solution. Most self-etching primers are moderately acidic with a pH that ranges between 1.8 and 2.5. Because of the presence of an acidic primer, sixth generation bonding agents do not have a long shelf-life and thus have to be refreshed frequently. - **Types of Sixth Generation Bonding Agents** Sixth generation bonding agents are of two types: **i. Self-etching primer and adhesive**: Available in two bottles: -- Primer -- Adhesive Primer is applied prior to the adhesive Water is the solvent in these systems. **ii. Self-etching adhesive** Available in two bottles: -- Primer -- Adhesive A drop from each bottle is taken, mixed and applied to the tooth surface, - example Prompt L-pop Mechanism of Bonding In these agents as soon as the decalcification process starts, infiltration of the empty spaces by the dentin bonding agent is initiated - **Advantages** - Comparable adhesion and bond strengths to enamel and dentin. - Reduces postoperative sensitivity because they etch and prime simultaneously. - It etches the dentin less aggressively than total etch products. - Demineralized dentin is infiltrated by resin during the etching process. - Since they do not remove the smear layer, the tubules remain sealed, resulting in less sensitivity. - They form a relatively thinner hybrid layer than traditional product, which results in complete infiltration of them demineralized dentin by the resin monomers. This results in increased bond strength. - Much faster and simpler technique. - Less technique sensitive as fewer number of steps are involved for the self-etch system. - **Disadvantages** pH is inadequate to etch enamel, hence bond to enamel is weaker as compared to dentin. Bond to dentin is 18 to 23 MPa. Since they consist of an acidic solution, they cannot be stored and have to be refreshed. May require refrigeration. High hydrophilicity due to acidic primers. Promote water sorption. Limited clinical data.  They achieve the same objective as the sixth generation systems except that they simplified multiple sixth generation materials into a single component, single bottle one-step self-etch adhesive, thus avoiding any mistakes in mixing. Seventh generation DBAs have shown very little or no postoperative sensitivity. However, due to complex mixed solution, they are prone to phase separation and formation of droplets within their adhesive layers.  **[Etching:]** It is the process of selective demineralizing the substrate (removal of minerals from enamel/dentin) and creating the spaces that are responsible for micromechanical bonding between tooth and restorative resin. **Primer / Adhesive promoting agents:** They are hydrophilic monomers that are capable of increasing surface wettability. **[Bonding Agents:]** They are hydrophobic monomers that penetrate within the demineralized surface and polymerize forming the micromechanical retention. - **Concept:** This strategy is based on an exchange process, in which minerals are removed from the dental hard tissues (enamel/dentin) forming porosities, then resin monomers are infiltrated, polymerized and micromechanically interlocked in these porosities. - **Classification according to steps of application:** -It is the most conventional form that involves three steps: - Application of the acid etch. - Followed by the primer (adhesion promoting agent). - Then the bonding agent (adhesive resin). **Etch Primer Bonding Agent** -It is the most effective approach to achieve **efficient** and **stable bonding** to **enamel**. -Although the 3-step etch-and-rinse adhesives are considered as the '**gold standard**' but, they are **time consuming**, and **technique sensitive.** - -Simplified 2-step system (**one-bottle system**) involves: - Separate etching step. - The priming step and bonding agent step are become into one single solution. **Etch Primer & Bond Agent** -It is **more simple** and **user friendly technique**. -By blending primer and bonding agent components in a single solution, simplified 2-step adhesives become **hydrophilic** in nature, more prone to **water sorption** and more susceptible to **hydrolytic degradation**.  **ENAMEL** Composition: - It is a highly mineralized homogenous tissue, which is formed of 95% inorganic hydroxyapatite crystals, 4% water and 1% organic substance. - The hydroxyapatite crystals arranged together forming enamel prisms which surrounded by prism sheath. There is interprismatic substance between the prisms which is less in mineralization. **Composition of enamel by weight** **Etching time:** 20 **seconds** **Effect of etching:** - Cleaning enamel surface from contaminants - Removing the outer fluoridated enamel layer - Demineralization (removal of minerals creating microporosities) - Transformation of the smooth enamel surface into an irregular and rough surface - Increasing the enamel surface free energy (about 72 dynes/cm) thus increasing its wettability **The endpoint of etching:** Enamel appears clinically frosty white after rinsing and dryness. **Factors Affecting Etching:** Type of acid used in either gel or liquid form Concentration of acid Time of etching Whether enamel is fluoridated or demineralized \"*Teeth with mild fluorosis may need up to 120 seconds. Severely mottled teeth may require longer than 120 seconds\".* Whether enamel is freshly cut or unprepared \" *Freshly cut enamel etches faster than unprepared enamel\".* Type of dentition, i.e. primary or permanent *\"The average time to etch primary teeth is usually longer than for permanent teeth\".* **Etching patterns:** **Type I etching pattern:** Involves the dissolution of prism cores without dissolution of prism peripheries. **Type II etching pattern:** (It is the opposite of type I) The peripheral enamel (interprismatic enamel) is dissolved, but the cores are left intact. **Type III etching pattern:** Etching is less distinct than the other two patterns. It includes areas that resemble the other patterns and areas whose topography is not related to enamel prism morphology. **Difference in appearance of etched and unetched enamel rods**  **Unetched enamel Etched enamel** DENTIN Bonding to dentin has been referred to be a **less reliable** and **predictable** technique when compared to enamel bonding. This is because of morphological, histological and compositional differences between the two substrates. **Challenges in Dentin Bonding:** - ***Dentin is a heterogeneous structure:*** - Dentin composed of a network of tubules filled with fluid that connect the pulp with the dentino-enamel junction (DEJ). - A hypermineralized dentin called peritubular dentin lines the tubules. - A less mineralized dentin called intertubular dentin (between the tubules) contains organic material primarily type I collagen fibrils. - Hydroxyapatite crystals are randomly arranged in the organic matrix. **Composition of dentin by weight** - ***Dentin is a hydrophilic structure:*** - Dentin is an intrinsically hydrated tissue, penetrated by tubules filled with fluid. - Movement of this fluid from the pulp to the DEJ is a result of a slight but constant pulpal pressure that has a magnitude of 25-30 mm Hg. - This fluid makes the exposed dentin surface moist and hydrophilic. - ***Type of Dentin:*** - Dentin is a dynamic structure characterized by its changing microstructure due to its physiologic and pathologic alterations (sclerotic dentin, caries affected dentin) where demineralization is more difficult. - Adhesion can be affected by the varying anatomical location of dentin after tooth preparation (superficial, intermediate and deep dentin). - ***Smear layer:*** - **Definition:** Thin porous layer which is composed of hydroxyapatite crystals and altered denatured collagen. It is formed when a tooth surface is prepared using hand or rotary instruments, the cutting debris are adhered on enamel/dentin surface due to the friction and heat forming a **smear layer**. - Debris that fill and occlude the orifices of dentinal tubules are called **smear plug**. - The morphology and the thickness of the smear layer depend on the type of the instrument and on the site of the dentin. - **Advantages of the Smear Layer:** It acts as a barrier -It decreases dentin permeability by nearly 90%. -Resists fluid movement. **Smear layer and smear plugs** **Magnified presentation of smear layer and smear plugs** C:\\Users\\Dr Radwa\\Desktop\\images\\F000051f005-004-9781437708554.jpg **Smear layer and smear plugs** - ***Stresses at Resin/Dentin Interface*** - Resin composites shrink when they polymerize, creating stresses (polymerization stresses) within the composite mass. Unrelieved stresses in the composite lead to internal bond disruption and marginal gaps around the restorations that increase **microleakage.** - Depending on the configuration of the preparation ([**C**-**factor:**] which is defined as the relation between the number of bonded and unbonded surfaces), polymerization stresses within the resin composite are relieved by flow from the unbonded surface. - When a restoration is exposed to wide temperature variations in the oral environment, the restoration undergoes volumetric changes. This occurs because the linear coefficient of thermal expansion of the resin composite is about **four times greater** than that of the tooth structure resulting in **microleakage**. **Etching time of dentin:** 15 **seconds** **Effect of etching on dentin:** - Removing the smear layer and smear plug - Opening of dentinal tubules resulting in dentinal fluid movement so, increasing the wetness and postoperative sensitivity - Demineralization of dentin surface (both intertubular and peritubular dentin) - Exposure of collagen fibers and decreasing dentin surface energy (44.8dynes/cm) due to the high protein content. Wetting of such a low energy surface is difficult. **Unetched dentin Etched dentin** **SEM of etched dentin showing exposed collagen fibers** **Disadvantages of etching on dentin:** Aggressive etching causes demineralization of dentin to a depth that might inaccessible to complete resin impregnation and produces **Nanoleakage**. C:\\Users\\Dr Radwa\\Desktop\\B9781437724189000071\_f007-015-9781437724189.jpg **Microleakage and Nanoleakage** - **Rinsing:** - Copious amount of water for **10-20 second** is used **after etching** of both enamel and dentin so as to remove the etchants byproducts. - **If too little rinsing occurs:** residual acid may over-etch the dentin **or** the byproducts may block the narrow channels around the collagen fibers and prevent resin infiltration. - **Drying:** - Air drying of enamel results in frosty white appearance. - Air drying of dentin leads to decrease in volume of collagen by 65% (collagen collapse), loss of interfibrillar spaces and compromise resin infiltration. - Only blotting with minisponge or cotton pellet is enough for dentin drying.  **ENAMEL** **Effect of priming:** - Acid etched enamel doesn't need a separate primer application to achieve effective bonding. - On the other hand, primers can be applied on acid etched enamel without harming the enamel bonding process. **DENTIN** Primer (adhesion promoting agent) is used to overcome the challenges of dentin surface (hydrophilicity, low surface energy, and the sensitivity of collagen fibers). **Effect of priming:** **Dentin is changed from hydrophilic to hydrophobic state, promoting the infiltration of the bonding agent monomers:** - Solvent (acetone and ethanol) can displace water from dentin surface and the moist collagen network promoting and facilitating the infiltration of bonding agent monomers through the nano-spaces of the exposed collagen network. - The evaporation of the solvent will leave the collagen fibers coated and stiffened by the bonding agent resin monomers (HEMA&TEGDMA). - **Acetone based primers:** -Acetone has a very high vapor pressure. -Acetone based primers are excellent on **wet dentin** which is called \"Wet bonding technique\" where the primer should be applied to displace all remaining surface moisture through evaporation of the solvent. -Acetone based primers do not work on dried dentin; they are unable to rewet dried dentin surface nor to re-expand and infiltrate a collapsed collagen network. -**Disadvantages:** acetone can evaporate from the package, changing the concentration and also the efficacy of the bonding systems. To reduce this problem special delivery systems (single dose) have been developed. - **Water based primers:** -Water based primers must be used on **dry dentin** which is called \"Dry bonding technique\" and the surface is further air dried to evaporate as much water as possible. -Water as a solvent has excellent rewetting capacity and is able to infiltrate and re-expand the collapsed collagen network. - **Ethanol (Alcohol) based primers:** -The properties of the alcohol as a solvent are located somewhere between acetone and water. It has some capacity to work on **dry dentin** by increasing time of application, and on **moist dentin** by multiple layers application. **Types of solvents in the primer**  **ENAMEL** Infiltration and polymerization of hydrophobic bonding agent monomers into the pores created from enamel etching. **Forms of resin infiltration:** - Macro-tags fill the spaces surrounding the enamel prisms. - Micro-tags result from resin infiltration within the tiny pores at the cores of the etched enamel prisms. **Formation of microtags and macrotags when bonding agent is applied to etched enamel surface** **DENTIN** **Forms of resin infiltration:** - Infiltration and polymerization of hydrophobic bonding agent monomers into the interfibrilar spaces of the collagen network forming the **hybrid layer**. - Infiltration and polymerization of hydrophobic bonding agent monomers within the dentinal tubules forming **resin tags**. **Diagrammatic representation of hybrid layer and resin tags** C:\\Users\\Dr Radwa\\Desktop\\1-s2.0-S188276161000013X-gr7.jpg **Hybrid layer**  **Hybrid layer** - **Technique Sensitivity (problems) of Etch-and-Rinse adhesives:** ***1. [\"Postoperative hypersensitivity\":]*** -Etching of dentin results in complete removal of smear layer and smear plugs, opening of dentinal tubules and movement of dentinal fluid which restrict the inward diffusion of adhesive. ***2. [\"Sensitivity of dentin to over wetting or over dryness\":]*** -It is related to the etching step itself and to the role of water in the bonding protocol. -The demineralized collagen network must be kept loosely arranged during adhesive procedures in order to allow proper resin monomer infiltration. -Water acts as a plasticizer for collagen, keeping it in an expanded soft state. -Certain amount of water (**moist dentin**) is needed to prevent the collagen network from collapsing and preserve the interfibrillar spaces. So only blotting with minisponge or cotton pellet is enough. ***3. [\"]** [ **Risk of dentin overetching, leading to incomplete resin infiltration within the demineralized microporous collagen network resulting in nanoleakage and hydrolytic degradation\".**]* **Incomplete resin infiltration** - **Concept:** -Self-etch adhesives were introduced to overcome the problems of the etch-and-rinse adhesives. -Self-etch adhesives **do not** require a separate etching step so: - **Elimination of acid etching:** - No risk for dentin over etching and exposure of collagen fibers that will not be infiltrated with adhesive. - No risk for removal of smear layer or smear plug. - Elimination of opening the dentinal tubules so, no risk for postoperative sensitivity and no risk for moisture contamination of the adhesive. - **Elimination of rinsing and dryness:** - No risk for the possibility of over wet or over dryness and collapse of collagen fibers. - **Advantages of SE over TE:** - More faster, simple and user-friendly - less technique-sensitive - Spontaneously demineralization and resin infiltration - Reduce postoperative sensitivity - **Classification according to the acidity or etching aggressiveness of the acidic monomers:** -This high acidity results in deep demineralization in both enamel and dentin. -The interfacial features produced by these adhesives on dental substrates resemble those of etch-and-rinse systems, despite the fact that the products originated from demineralization are not rinsed away. -They produce hybrid layer with completely demineralized top and partially mineralized base. -In dentin, they cause superficial demineralization leaving residual hydroxyapatite crystals attached to the collagen fibrils within the hybrid layer, which acts as a receptor for **chemical bonding** with carboxylic acid based monomers such as 4-MET or phosphate based monomers such as Phenyle-P and 10 MDP. -This **two fold of bonding mechanism** (i.e. micromechanical and chemical adhesion) is believed to be advantageous in bonding effectiveness and durability. -In enamel, they result in poor demineralization and poor bonding. -Self-etch adhesives with low acidity and reduced the ability to dissolve the smear layer and demineralize the underlying dentin surface. -They can only very superficially expose collagen on dentin, creating a characteristic nanometer-sized hybrid layer, which has been termed a nano-interdiffusion zone. -They have the ability to **chemically bond** to the mineral content of the partially demineralized dentin. **Aggressiveness of Self-Etch adhesives** - **Classification according to the steps of application:** **Form:** Two bottles one is self-etch primer (SEP) and the other is bonding agent. **Composition:** - **SEP:** It is **hydrophilic acidic primer** that includes a phosphonated resin monomers and performs two functions **simultaneously etching** and **priming** of enamel and dentin. - **Bonding agent: (hydrophobic monomers)**  **Application:** -**SEP** is applied by a micro-brush for **20 seconds** in rubbing motion every 10 seconds to remove the buffered byproducts. -Then, gentle air for solvent removal. -After that, application of bonding agent by a micro-brush for **10 seconds** in rubbing motion. -Then, gentle air for solvent removal and finally **light curing**. **Advantages:** 1. Separation between hydrophilic (SEP) and hydrophobic (bonding agent) components so: - Increase shelf life - Decrease liability of phase separation 2. Two folds of adhesion (Micromechanical & chemical) so: - Reliable and high bond strength to dentin **Disadvantages:** - Low bond strength to enamel. To overcome this problem, **selective etching** is required. **It is called:** \"**all-in-one**\" adhesives. **Form:** One bottle (responsible for **simultaneously etching**, **priming and bonding).** **Application:** -Involves a single step, combining etching, priming and bonding into one solution. -It is applied by a micro-brush for **20 seconds** in rubbing motion every 10 seconds to remove the buffered byproducts. -Then, gentle air for solvent removal. -Finally **light curing**. **Advantages:** - More simple and user friendly - Reduction of the number of application steps - Reduction of manipulation time - Reduction of technique sensitivity **Disadvantages:** 1. Because these adhesives must be acidic enough to be able to demineralize enamel and penetrate dentin smear layers, the **hydrophilicity** of their resin monomers (phosphates and carboxylates) also is **high** which makes them liable to water degradation. 2. Consists of both **hydrophobic** and **hydrophilic** components, together so: - More prone to phase separation and entrapment of water droplets in the adhesive layer - Reduced shelf-life because of hydrolysis of monomers 3. **Poor bonding to enamel**. To overcome this problem adding a preceding etching step (**[Selective etching]**) is beneficial for bond strength to enamel. **Technique Sensitivity of 1**-**Step Self**-**Etch adhesives:** - ***\"Low bond strength\":*** due to their high hydrophilicity, they act as a semipermeable membrane permitting water movement across the adhesive layer leading to **Nanoleakage** or **\"water trees\"** causing accelerated degradation.  **Total etch vs self-etch systems. Total etch technique involves complete removal of smear layer by simultaneous acid etching of enamel and dentin. After total etching, primer and adhesive resin are applied separately or together. Acid removes the dentin smear layer, raises surface energy and modifies the dentin substrate so that it can be infiltrated by subsequently placed primers and resins. In self etch system, self etching primer is applied on prepared tooth surface. Then demineralized dentin and smear layer is infilterated by resin during etching process. In this smear layer is not removed and there is formation of continuous layer incorporating smear plugs into resin tags** **Universal Adhesives (UA)** **Definition:** -Single-bottle, no-mix adhesive system. -It is called \"**Multi-mode**\" or \"**Multi-purpose**\" adhesives. **Function:** -They can be used in total-etch, self-etch, or selective-etch mode.  **Composition:** 1. **Multifunctional cross-linking monomers (10-MDP):** - They must be capable of reacting with a number of different substrates (enamel/dentin). - They must be able to copolymerize with chemically compatible resin-based restoratives and cements. - They must have some **[hydrophilic]** character in order to properly "wet" dentin that has a significant water content, yet at the same time be as **[hydrophobic]** as possible once polymerized to discourage hydrolysis and water sorption over time. 2. **Water:** is required for dissociation (ionization) of the acidic functional monomers. - Enhances resin wetting and infiltration of tooth tissues. - Aids in water removal and evaporation during the air-drying step. **10-MDP (methacryloyloxy-decyl-dihydrogen phosphate):** -**It is a functional monomer** - hydrophobic methacrylate group on one end (capable of chemical bonding to methacrylate-based restoratives and cements) - hydrophilic phosphate group on the other end (capable of chemical bonding to tooth tissues) **-It is Hydrophobic** - It has long carbon chain backbone - It discourages water sorption and hydrolytic breakdown of the adhesive interface - Increase shelf-life. -**It bonds chemically with Ca in hydroxyapatite (HAP)** Stable, non-soluble MDP-calcium salts are deposited in self-assembled nano-layers -**It is acidic monomer** To etch and demineralize tooth tissues C:\\Users\\Dr Radwa\\Desktop\\images\\mdp.png **10-MDP** - Glass-ionomers are still considered the only materials that **self-adhere** to tooth tissue. - A polyalkenoic acid pre-treatment cleans the tooth surface; it removes the smear layer and exposes collagen fibrils up to about 0.5-1 µm deep (conditioning). - Glass-ionomer components inter-diffuse and establish a micro-mechanical bond following the principle of hybridization. - Chemical bonding is obtained by ionic interaction of the carboxyl groups of the polyalkenoic acid with calcium ions of hydroxyapatite that remained attached to the collagen fibrils. - This additional chemical adhesion may be beneficial in terms of resistance to hydrolytic degradation. Consequently, a two-fold bonding mechanism is established, similar to that mentioned above for mild self-etch adhesives. ***FACTORS AFFECTING ADHESION TO TOOTH STRUCTURE*** **I) Factors related to the tooth structure (adherend)** ***1. [Physiological Effects:]*** ***a) Surface energy**:* 1. The relative surface energy of human teeth is between 30-40 dynes/cm. 2. This could be altered by heritage, diet, oral hygiene practices and specific etching protocols applied during demineralization procedure. 3. Upon etching enamel, its surface energy increases while etching dentin results in limited increase in its surface energy. 4. For an adhesive to flow easily over a treated surface, this latter should possess a surface energy higher than the critical surface tension intercept of the adhesive. ***b) Capillary attraction:*** 1. This governs the adhesive mechanism greatly due to the tubular nature of dentin. 2. Its amount will be according to the size of the tubules, their number and their distribution. 3. The communication between the tubuli and the oral environment will create the movement of the dentinal fluid inside the dentinal tubules. ***c) Osmotic pressure**:* 1. This plays a major role since diffusion or convection mechanisms are allowed to occur in the presence of different concentrations. ***2. [Physico-chemical characteristics of enamel:]*** 2. Enamel is composed of 95 -- 98 % by weight hydroxyapatite crystals (inorganic contents) and the reminder is water and organic material. 3. Operatively prepared surface expose enamel rods in tangential, oblique and longitudinal planes. 4. Irrespective of its depth and location, it is homogenous in structure except for the prismless enamel on the outer surface. 5. It is covered by an organic pellicle with low surface energy of 30 dynes/cm, thus requires its removal to allow: → 1. Creation of a high surface energy after etching which may reach up to 72 dynes/cm. 2. Increase the bonding area. 3. Increase the surface roughness. 1. This is accomplished using different etchants as mentioned previously. ***3. [Physico-chemical characteristics of dentin:]*** 1. Dentin is composed of 70 -wt % hydroxyapatite crystals, 18-wt % organic material and 12-wt % water. 2. Dentinal tissue is heterogeneous in nature as the inorganic and organic constituents are unevenly distributed in inter and peritubular dentin. 3. Numerous tubules radiate from the pulp throughout the entire thickness of dentin making it highly permeable. 4. The diameter of the tubules decreases from 2.5 μm at the pulpal side to 0.8 μm at the DEJ and the number of the tubules decreases from 45.000/mm^2^ near the pulp to about 20.000/mm^2^ near the DEJ with an average of 30.000/mm^2^ in the middle. 5. Another important consideration is that 96% of superficial dentin is composed of intertubular dentin while only 12% are present near the pulp (deep dentin). 6. On the other hand, peritubular dentin represents 66% near the pulp associated with 22% of the surface (area near the pulp) occupied by water. 7. Dentinal fluid or intrapulpal fluid pressure is estimated to be 25 -- 30 mm Hg. 8. The high protein content is responsible for the low surface energy of dentin, which accounts for 44.8 dynes/cm. 9. This is required to be changed as mentioned previously to render it receptive for adhesive application and bonding. 10. There are also other different causes which furtherly complicate bonding to dentin: → a. Dentin type. b. Dentin permeability. c. Fluoride contents of dentin. d. Dehydration. e. Hypermineralization. ***a) Dentin type:*** 1. The amount of minerals will greatly influence the bonding mechanism. 2. This could be reflected by: 1. Primary dentin. 2. Secondary dentin. 3. Tertiary dentin →, which is produced in the pulp chamber at the lesion site in response to an insult and its degree of mineralization, will be different from other types. 4. Sclerotic dentin which is the result of the obliteration of the tubules by mineral casts and apposition of peritubular dentin. It represents a difficult substrate to which an adhesive could be applied. 1. Dentin type is thus governed by a number of factors related to the dentinal tubules characteristics which are:→ 1. Length. 2. Width. 3. Location. 4. Direction in relation to external surface. 5. Peritubular dentin. 6. Intertubular dentin. 7. Age of patient. ***b) Dentin permeability:*** 2. It depends on several factors:→ 1. Diameter and length of the tubules. 2. Viscosity of dentinal fluid. 3. Molecular size of substances dissolved in it. 4. Pressure gradient. 5. Surface area available for diffusion. 6. Patency of tubules. 7. Rate of removal of substances by pulpal circulation. 3. Regional differences result in different dentinal permeability. 4. Smear layer removal increases permeability and water competes for all adhesive sites. 5. Controversy is present concerning the bond strength of adhesive resins observed in deep and superficial dentin when the smear layer is removed as deep dentin is more permeable than superficial dentin. ***c) Fluoride content of dentin:*** 1. Fluoride lowers the adhesive potentials thus its presence in dentinal substrate by a certain saturation could affect the bonding mechanism due to lowered surface energy. ***d) Water Content:*** 2. The vitality of dentinal substrate and hydration will allow provision of sufficient internal wetness required to accomplish bonding between adhesive resins and dentin. ***e) Hypermineralization**:* 3. Hypermineralization will complicate the adhesive mechanism as it implies more aggressive etching and surface alteration to provide a generously porous structure into which resins could be impregnated. ***4. [Presence of smear layer:]*** 4. The smear layer is defined as any calcific debris produced by instrumentation of dentin, enamel or cementum. 5. The burnishing action of the cutting instrument generates considerable amounts of frictional heat locally and shears forces so that the smear layer becomes adequately attached to the underlying tissues and can not be rinsed off. 6. The morphology, composition and thickness of the smear layer are largely determined by the type of instrument used, the method of rinsing and the site of dentin at which it\'s performed. 7. Its presence jeopardizes bonding and hence it was recommended to remove it in recent adhesive systems to ameliorate the bond strength. ***5. [Contaminants:]*** ***a) Blood and/or saliva:*** 1. Blood and saliva can create an environment that is detrimental for bonding, thus the uses of a rubber dam or other dry-field aids are necessary for bonding. ***b) Moisture from hand piece or air / water syringes:*** 2. Water from air rotor hand piece or air-water syringes could result from:→ 1. Lack of drying devices on air-lines coming from the compressor. 2. Condensation of water in air lines. 3. Leakage of water at the dental cart unit. 3. This moisture could alter bonding and complicate the mechanism. *c) Oil from hand piece or air / water syringes:* 4. Oil comes from air- compressors, which are not well maintained. 5. Oil filters are mandatory to avoid such leakage to occur. 6. They are placed after the air compressor and before the air syringe or hand piece. 7. Observation of oil could be done by blowing air from the hand piece or syringe into the surface of a rubber glove. 8. This contamination influences the adhesion procedure and thus, should be avoided. **II. Factors related to the material used (adherent)** 1. There are a series of physical properties that are important in the adherents provided:→ ***1. [Indestructibility:]*** 2. The adhesive material should be stable hydrolytically and chemically to provide strong bonding potentials. ***2. [Thermal coefficient of expansion:]*** 3. It should approach as much as possible that of tooth structure in order not to create marginal gaps. ***3. [Dimensional stability:]*** a. During setting:→ it\'s imperative as expansion or contraction yields different results than those, which are required. b. After setting:→ hygroscopic expansion occurs and may contribute for the relaxation of contraction stress. → This occurs during the days and weeks following resinous placement that is after the dentin bonding may have already failed. ***4. [Modulus of elasticity and transfer of stress at the interface:]*** 4. The more the elastic capacity of the adhesive material the more the stresses will be reduced at the interface. 5. This explains why a thick layer is used to act as a shock absorber at the interface, and overcome the polymerization contraction stresses of the resinous restoratives. 6. The adhesive resin plays a biomechanical role in the distribution of functional stress throughout the whole tooth. 7. Bonded restorations may strengthen weakened teeth. ***5. [Viscosity at the time of insertion:]*** 1. The more the fluidity of the adhesive resin the better will be the diffusion and hence the creation of the bonding mechanism. ***6. [Adhesiveness, wetting and polarity towards tooth structure:]*** 2. Intimate molecular contact between the two parts (adherend and adherent) is a prerequisite to develop strong adhesive joints. 3. The adhesive should possess a low contact angle to provide adequate wetting. 4. The collagen phase of dentin is expected to allow primer and adhesive resin to infiltrate it, so if a conditioner is used to provide a specific polarity to the dentin, the primer must match this polarity to achieve penetration. 5. The same is true for the adhesive when applied to the primed surface. ***7. [Initial polymerization site:]*** 1. Initiation of polymerization at the resin / tooth interface directing the shrinking material towards the cavity wall rather than away from it is advantageous. 2. Initial setting in photocured resins occurs towards the light source and consequently shrinkage will be directed towards it. 3. On the other hand, in the autocured resins initial setting occurs in the bulk of the material, which means again away from the cavity walls. It was anticipated by Fusayama that initial setting of autocuring resins starts at the dentinal wall, because of the locally higher temperature of body heat, pulling the shrinking resin towards, rather than away from the cavity wall. 4. The theory was known as directed shrinkage theory but was not substantiated. **III. Factors related to the cavity preparation performed** 1. All steps of cavity preparation influence greatly the bonding equation: 1. Adhesive cavity designs should be adopted. 2. The resistance and retention required should be estimated and built in the preparation. 3. All carious tissue should be removed as bonding to diseased tissue disgraces the bond strength greatly. 4. The walls should be adequately finished to provide smoothness. 5. Adequate debridment and toilet of the cavity accomplishment is required to prepare the dentinal and enamel surfaces for the subsequently applied resins. **IV. Technique of restoration and skill of the operator** ***1. [Use of rubber dam:]*** 2. It is an essential and mandatory requirement to avoid moisture contamination during the bonding procedure. ***2. [The use of liners and bases:]*** 3. The presence of liners and bases will affect greatly the provided surface area for bonding hence it was debated that if the use of calcium hydroxide liner and other bases were profitable. 4. In the state of the art, no application of liners is advocated and a direct application of the adhesive resin is recommended to allow total sealability of the opened tubules. 5. Very limited sites imply the use of calcium hydroxide liners in situations in great proximity to the pulp (remaining dentin thickness less than 0.5 mm). 6. The presence of such lining or basing material will not allow bonding to occur freely and will create several phases and interfaces with a maintained opened dentinal tubules structure not sealed by adhesive resins. 7. This also reduces the C-factor, which is the ratio of bonded to unbonded free restoration essential in the bonding mechanism as its increase is beneficial. ***3. [Constituents of temporary restorations:]*** 1. It was thought that contacting enamel or dentin with eugenol-containing temporary restorations affected the substrate surfaces and provided different bonding characteristics to resin than did virgin tooth structure. 2. Lately, researches revealed that using the recent adhesive systems subsequent to eugenol or non-eugenol containing temporary restoratives yielded results approximating those in case of bonding directly to tooth structure without temporization. 3. Fresh eugenol placed on dentin or enamel just before bonding could be a negative factor. 4. Further investigations are required to answer a lot of questions arising from these observations. ***4. [C-factor:]*** 1. Which is the ratio of bonded to free unbonded restoration. 2. The higher the ratio of bonded to free resin surface, the less flow may compensate for contraction stress, which is importantly applied to enamel and dentin. 3. The skill of the operator is a prime requisite and should be ordered high-leveled skill operator due to the multi-factorial technique sensitive adhesive procedures. 4. A conceivable number of steps that should be attempted sequentially are to be faithfully followed to provide clinically successful results. ***5. [Post-restorative care:]*** 1. High level of good oral hygiene is required in order not to allow plaque accumulation with subsequent bond degradation. 2. Adequate home care measures should be instructed in conjunction with prophylactic visits for patients to maintain their restorations serviceable. 3. Use of non-alcohol containing mouthwashes. **V. Factors related to the oral environment** - The different environmental factors which include the cyclic occlusal loads, the chemical degradation potentials, the pH and thermal fluctuations, the oral microbes, humidity and the chewing habits are all highly contributing factors that deprive us of a long term durable bond. - The complexity of mechanisms should be faced by a resinous material that could comply readily with their aggressiveness and resist their deteriorat