Dental Adhesion* Factors that affect the adhesion are : 1) Compositional and structural aspects of enamel and dentin: I said before: The composition and structure of enamel is different than that in dentine, so bonding in enamel is totally different than in dentin 2) Changes in dentinal structure. 3) The smear layer… what does smear layer mean? & how can treat it? 4) Internal and external dentinal wetness or humidity of dentin. ------------------------------------------------------*Enamel:Just to remind you that enamel has high inorganic content-you took this in histology: 88% by volume almost mineral while the water 10%&organic content 2%. Enamel is homogenous in structure and composition irrespective of its depth and location this is mean: the surface of enamel is the same as enamel in DEJ, No change in composition for enamel, but there is difference in minerals. But however mainly the enamel is homogenous: the same structure through all the depth of enamel. *in comparison to dentin: dentin has high organic content &water (Water is very important will affect a lot of adhesion) this is the main difference between enamel &dentin which: water content &Inorganic content. Inorganic content in enamel 88% by volume while in dentine 50% by volume & water in dentin 25%. Dentinal tissue is heterogeneous: heterogeneous means: the dentin in DEJ is different from dentin near to the pulp, when we go toward the pulp the dentinal tubules diameter &size become bigger. ( dentine mainly composed of dentinal tubules & inter tubular space). Dentinal tubules size and number increases from the DeJ to the pulp side, this will affect the adhesion, so bonding of adhesion near the enamel is different from of dentin which near or close to the pulp. Dentin is a vital and dynamic tissue. why it is vital &dynamic tissue?...what is inside the dentinal tubules ?fluid so it is vital &dynamic &this fluid will affect the adhesion it will compete resin composite when it go inside dentinal tubules ,when we do acid etching –we will talk about it later-then we put resin composite so the resin composite go through the micro porosities &water dynamic or motion of water or fluid inside these para tubules will push the resin composition or compete resin composite as result of that: the bonding to dentine is very difficult than the bonding to enamel for this it has special treatment than enamel (we will talk about this later). Dentinal fluid is under a constant outward pressure from the pulp: the dentinal fluid inside dentinal tubules under constant pressure from the pulp outward toward dentinal tubules upward toward enamel so this will compete resin composite from bonding to tooth so bonding to dentin is very difficult from bonding of enamel. *The smear layer: Any debris , calcific in nature, produced by reduction or instrumentation of dentin, enamel or cementum . so this mean that smear layer will produce after preparing the cavity .and it is the result of instrumention or the cutting process with heat ,to any part of the tooth structure enamel, dentin or cementum. The thickness of the smear layer vary from o.5 to 5 micro meter This layer of debris has a great influence on any adhesive bond formed between the cut tooth the restorative material. The smear layer can be removed by: 1)ethyl-enediaminetraacetic acid (EDTA), or by 2) acidic conditioners ,like(citric ,polyacyclic ,lactic and phosphoric acids). *Although smear layer occlude the dentinal tubules, it is porous and allow small amount of dentinal fluids to pass through.so it is not a barrier. Another factor that affect adhesion, it is "internal and external dentinal wetness" We can call it "dentinal wetness" or "dentinal humidity", they are the same .We have two types of dentinal wetness/humidty: 1)external humidity: It is an environmental humidity, or from the oral cavity, it can affect the adherent. or any contamination (like saliva)during the bonding process, for that ,we need an isolation. 2)internal humidity: it is from the fluid inside the dentinal tubule. Remove of smear layer creates wet bonding surface on which dentinal fluids exudes from dentinal tubules. This wet environment affects adhesion ,because water competes by hydrolysis for all the adhesion sites on the hard tissues. Bond strength of several adhesive systems decrease as the depth of the preparation increased because dentinal wetness is greater. External humidity or environmental humidity can also affect the bond strength. *Wetting of the adhesive : it is the attraction of any material to the surface. or the ability to spread the material over the surface. To wet mean to flow at the surface ,and it is depend on the angle between the drop of the material over the surface, Large angle lead to…….. low wettability Small angle lead to…….. high wettability So the adhesive should have high wettability *Adhesive systems must sufficiently wet the tooth surface: Have a viscosity that is low enough to penetrate the micro porosities and be able to displace the moisture and air during the bonding process. HEMA (2-hydroxyethyl methacrylate) is a surface active agent that enhance the wettability of adhesive. Enamel acid etching techniques : We will talk separately about enamel and dentin because we said acid etching and bonding to enamel is different than that in dentine. so we will talk separately about enamel then we talk about dentin. Enamel etching transform the smooth enamel surface to an irregular high surface, when we acid etching for the enamel surface two things that happen :increase micro porosities because the adhesion to enamel and composite resin is micromechanical we need to make the surface of enamel irregular in order for adhesion to come &penetrate through this irregularities so when we do enamel etching 2 things will happen. Acid etching remove 10 micrometer of the enamel surface so acid etching will remove part of enamel surface and creates a micro porous layer from 5 to 50 micrometer deep so it cause 2 things: 1)remove part of enamel surface. 2)cause micro porosity or irregularity. The enamel bonding agent wet the etched enamel surface and pass through the micro porosities ,so when we etched enamel surface, we remove part of enamel surface we put the bonding agent or the adhesive which penetrate thru these micro porosities &cause interlocking into micro porosities. (the light blue is adhesive agent.) When we put the adhesive,& the adhesive will go thru these micro porosities because of it is high viscosity so it will go through we said the adhesive is composed of resin. why we don't use composite alone? because composite contain filler so the viscosity is low it can't go through micro porosities so we use adhesive to go through micro porosities. that form due to acid etching then the bonding. because the bonding agent or adhesive go through the micro porosities then we put the composite over the adhesion. So steps in clinic: 1)acid etching 2) rinsing 3)then we put bonding agent. 4)light cure the bonding agent and 5) put composite. So bonding agent go through micro porosities that form due to acid etching then composite will bond to bonding agent. So the bonding between the bonding agent and the enamel is micromechanical due to micro porosities , while the bond between the bonding agent and resin composite because both of them resin is chemical. Enamel Etching patterns: Three enamel etching patterns: *type 1 :predominant dissolution of the prism cores *type 2: predominant dissolution of the prisms peripheries. * type 3: no prism structure are evident. * there is types of resin tag: when we do acid etching of the enamel surface there is 2 types of irregularities so the bonding agent will bond the 2 types . These 2 types: 1)macrotags. 2) microtags. What do you expected the bon strength come from macrotags or microtag? The bond strength of enamel come from microtags that form at the core of enamel prism because high in number( the number of micro more than macro),so the bond strength come more from microtags not macrotags that is formed around the enamel prism. *Enamel bonding Agent: Bonding agent composition based on Bis-GMA or urethane dimethacrylate (UDMA) which composite, composite the same composition so the resin of composite either based on bis-GMA or UDMA if you remember you take about composite composition so the adhesion mainly composite but unfilled resin composite so it is form of the resin composite based on either GMA or UDMA. Diluted with a lower viscosity and higher hydrophilicity monomers such as (TEG-DMA)and (HEMA),HEMA increase affinity & increase the viscosity of the surface so we add HEMA to increase viscosity of dentine to get better penetration of adhesive into micro porosities.(this is the composition you take it already in DM but this to remind you). Etching with phosphoric acid : Etching with phosphoric acid 30% to 40% we usually use 35% for etching enamel surface. Etching time is not less than 15 seconds (usually 15 sec for enamel) for dentin should not be more than 15 sec & for enamel should not be less than 15 sec because if we ecth enamel more than 15 sec enamel will not be affected any more so etching of enamel for 15 sec is the same as etching enamel for 20 sec for example. But if we etch enamel less than 15 sec it will affect the bonding, so enamel etching shouldn't be less than 15 sec but if we etch dentine more than 15 sec it will be affected ,so always the etching for enamel is not les than 15 sec so for that we do total etching (etch enamel &dentine together for 15 sec) o what usually use in total acid technique we etch enamel &dentin for 15 sec because we etch them together we don’t etch them separately but it is properly to start with enamel because if enamel etch more than 15 sec it will not be affected but if we etch dentin more than 15 sec it will be affected so usually using total technique or concept we etch enamel &dentin then we wash the acid because in this way the acid etching of enamel maybe etched more than 15 sec &it is ok. Washing time 5 to 10 seconds. Acid gel is preferred over liquid why acid etching is preferred to be gel rather than liquid?? To easier handling &liquid will miss maybe enter oral cavity while the gel it is consistency is better because you can control gel better o it is prefer to use gel rather than liquid.by this we finish the adhesion to enamel. *development of resin Dentin adhesives: which comes in generations if you remember them and on your book we have from first to sixth generation. *adhesion strategies :A scientific classification of modern adhesives: 1)smear layer-modifying Adhesives. 2)smear layer-Removing Adhesives. 3)smear layer-dissolving Adhesives. *Conditioning of dentin : -Conditioners are applied to enamel and dentin (total-etch technique). - the demineralization process exposing the collagen fibrils affect the surface-free energy of dentin. -any chemical alteration of the dentinal surface by acids with the objective of removing the smear layer and simultancously deminerlizing the dentinal surface. -the demineralization process expose the collagen fibrils and thus increase the micro porosities of the inter tubular dentin. -dentine primers: -Primer have hydrophilic properties that have affinity to the collagen fibrils and hydrophobic properties to copolymerize with the adhesive resin. - many modern adhesives have HEMA as a primer and contain other monomers. -one-bottle adhesives: combine priming and bonding function. -primers are adhesion promoting agents. - contain hydrophilic monomers dissolved in organic solvents such as acetone or ethanol. -those solvents are volatile and so can displace water from the dentinal surface and collagen network. This promote infiltration of the monomer of the adhesive in the Nano spaces of the exposed collagen network. Hybridization : Hybrid layer is the area or zone of the adhesive system which micromechanically interlocks with the dentinal collagen. Isolation : Isolation and moisture control is very critical for the bonding procedure. bonding to acid etched enamel require a dry surface to allow the bonding resin to flow into the micro porosities created by acid etching. accidental contamination of the substrate with external fluid prevent effective contact between the adhesive and the bonding substrate. Salivary contamination is detrimental because salivary proteins block the microtentive porosities on the acid etched enamel and dentin and may block infiltration of the resin. Critical steps in adhesion: Isolation . So the critical steps in Adhesion : *Dentin and pulp protection *Enamel and dentin conditioning *Wet vs.dry bonding *Primer application *Adhesive resin application Restorative procedure. * Wet vs.dry bonding : *After acid etching enamel surface should be dried. *On dentin a certain amount of moisture is needed "we bonding" *Dehydration of the acid-etched dentin surface through air drying can result in collapsing and shrinkage of the exposed collagen fibril network which can prevent resin infiltration. *On the other hand the acid-etched dentin surface should not be kept too wet. *Excess water should be removed with a short air blast or with a dry sponge. By: Asal juma'a shalash. 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