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Bonding to enamel and dentin

Bonding to enamel and dentin



For a restorative material, adhesion is the primary requirement so that restorative materials can be bonded to enamel or dentin and without the need of extensive tooth preparation

For a restorative material, adhesion is the primary requirement so that restorative materials can be bonded to enamel or dentin and without the need of extensive tooth preparation



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  • Adhesion involves an “adhesive” that is placed on a “substrate (or adherend)” and which forms an “interface”. The interface is solid under the film and in contact with air above the film. [CLICK] If the adhesive is intended to hold something in place, they one creates a joint (or dental joint) with the adhesive between two substrates and forming two interfaces. This can be seen in the schematic illustration above. <br /> A wide range of substrates or adherends are possible. [CLICK] For example, a substrate (shown on the left) could be any number of dental ones (such as enamel, dentin, liner/base/cement, a post/core, remaining dental amalgam from a previous preparation, or a dental implant). The other substrate (shown to the right) is part of the new restoration (and could include a composite, amalgam, cast inlay/onlay/crown, an all-ceramic inlay/onlay/crown, veneer, Maryland bridge, or orthodontic bracket). The adhesive could be simply a bonding system or might be a two bonding systems coupled with an intervening composite. This is the most complicated case and will be discussed at the end of this presentation. <br /> Adhesion, in almost all circumstances, depends on micromechanical interlocking of the adhesive with the substrate irregularities. To create this interlocking event a set of 5 very important prerequisites must occur. These are explained in the next slide. <br />
  • There are several requisite steps in the formation of good adhesion. [CLICK] In the figure above, the 5 prerequisites are shown in combination with a set of schematic representations of the interface of the adhesive with the substrate. <br /> [CLICK] (1) Clean Adherend (Substrate): The adherend must not contain any surface debris or adsorbed films of oil or dirt that would prevent an adhesive from coming into good contact with the adherend. Surfaces can be cleaned with water, solvent, or acid. <br /> [CLICK] (2) Good Wetting: The adhesive must spread onto the adherend with a low contact angle so that it wets the surface and develops good intimate contact. Unfortunately some adhesives are not well-matched to the surface energies of certain substrates. Hydrophilic materials do not wet hydrophobic surfaces very well, and vice versa. Enamel and dentin are hydrophilic while most composites are hydrophobic. The challenge for dental adhesives is to provide acceptable wetting for both of these materials. <br /> [CLICK] (3) Intimate Adaptation: If good wetting occurs and the material flows adequately, then it can penetrate into all the surface interstices and develop good adaptation. This process should occur with entrapping air or leaving unfilled surface spaces. <br /> [CLICK] (4) Bonding: The adhesive should interact in as many ways as possible with the substrate. It should develop physical bonding, chemical bonidng, and micromechanical bonding. The last is the most important and contributes the most (&gt;90%) to the strength of the joint. <br /> [CLICK] (5) Good Curing: The entire process depends on the adhesive (and restorative filling materials) becoming fully cured. Under-curing allows chemical erosion and/or debonding of the adhesive system. <br />

Bonding to enamel and dentin Bonding to enamel and dentin Presentation Transcript

  • Bonding to Enamel and Dentin Dr. Hakan Çolak DDS. PhD Department of Restorative Dentistry Ishık University
  • Introduction • The traditional “drill and fill” approach is fading now because of numerous advancements taking place in restorative dentistry.
  • Introduction • For a restorative material, adhesion is the primary requirement so that restorative materials can be bonded to enamel or dentin and without the need of extensive tooth preparation.
  • Introduction • Buonocore – developed acid etching of the enamel. – showed that when enamel is treated with a dilute acid for 30 seconds, it results in a microscopically roughened, porous surface into which the resin forms retentive tags
  • Introduction
  • Terminology Adhesion or bonding: The forces or energies between atoms or molecules at an interface that hold two phases together. Adherend: The surface or substrate that is adhered Adhesive/adherent: A material that can join substances together, resist separation and transmit loads across the bond Adhesive failure: The bond that fails at the interface between the two substrates. Cohesive failure: The bond fails within one of the substrates, but not at the interface.
  • Interface 1 Adhesive DENTAL ADHESION Adhesive System or Luting Cement Interface 1 Adherend 1 ENAMEL, DENTIN > Liner, Base, Cement > Post and Core > Dental Amalgam > Implant > Interface 2 Adherend 2 DENTAL JOINT < Composite, Amalgam < Cast Inlay, Onlay, or Crown < All-Ceramic Inlay, Onlay, or Crown < Veneers, Maryland Bridges < Orthodontic Brackets
  • Diagrammatic representation of dental adhesive system, where Adherend Diagrammatic representation of dental adhesive system, where Adherend 11is enamel, dentin or both. is enamel, dentin or both. Adhesive is bonding agent, Adhesive is bonding agent, Adherend 22is composite resin Adherend is composite resin
  • Indications for Use of Adhesives • To treat carious and fractured tooth structure • To restore erosion or abrasionp defects in cervical areas • To correct unesthetic contours, positions, dimensions, or shades of teeth • To correct unesthetic contours, positions, dimensions, or shades of teeth • For the repair of fractured porcelain, amalgam and resin restorations
  • Indications for Use of Adhesives (con’t) • • • • • For pit and fissure sealants Bond composite restorations Bond amalgam restorations Lute crowns Bond orthodontic brackets
  • Advantages of Bonding Techniques • Adhesion of composite resin restorations to enamel and dentin • Minimizes removal of sound tooth structure • Management of dentin hypersensitivity • Adhesion reduces microleakage at tooth restoration interface • As a part of resin cements for bonding cast restorations • Adhesion expands the range of esthetic possibilities
  • Advantages of Bonding Techniques • Bonding of porcelain restorations, e.g. porcelain inlays, onlays and veneers • Reinforces weakened tooth structure • Reduction in marginal staining • For repair of porcelain or composite • Bonding amalgam restorations to tooth • Repair of amalgam restorations • To bond orthodontic appliances.
  • Mechanism of Adhesion • Chemical means • Physical means • Mechanical means
  • Physical means of adhesion • Van der Waals interactions: Attraction between opposite charges on ions and dipoles. • Dispersion forces: Interaction of induced dipoles • Hydrogen bond: It is a particularly strong bond and can be included among physical forces
  • Chemical means of adhesion Covalent bond: It involves sharing electrons between two atoms or molecules. It represents strong bonds. Formation of a covalent bond liberates considerable energy. A covalent bond is present in all organic compounds. Ionic bond: It involves an actual transfer of electrons from one atom to another. For example, the ion exchange adhesion mechanism in GICs. Metallic bond: It is the chemical bond characteristics of metals in which mobile electrons are shared among atoms in a usually stable crystalline structure.
  • Mechanical means of adhesion • Bonding occurs because of penetration of one material into another at the microscopic level. • For example, in composite resins the bonding involves the penetration of resin into enamel and dentin and formation of resin tags.
  • Factors Affecting Adhesion • Wetting – Wetting is an expression of the attractive forces between molecules of adhesive and adherent. In other words, it is the process of obtaining molecular attraction • Wetting ability of an adhesive depends upon two factors: – Cleanliness of the adherend : Cleaner surface, greater adhesion. – Surface energy of the adherend: More surface energy, greater adhesion
  • Factors Affecting Adhesion
  • Factors Affecting Adhesion
  • Factors Affecting Adhesion • Contact Angle – refers to the angle formed between the surface of a liquid drop and its adherent surface. – The stronger the attraction of the adhesive for the adherent, the smaller will be the contact angle. The zero contact angle is the best to obtain wetting
  • Factors Affecting Adhesion
  • Factors Affecting Adhesion • Surface Energy – The surface tension of the liquid and the surface energy of the adherend, ultimately determine the degree of wetting that occurs. – Generally, the harder the surface is the higher the surface energy will be, which means the adhesive properties of the material will be higher.
  • Factors Affecting Adhesion • Surface Contamination – The substrate surface should be clean as contamination prevents the adhesion. Adhesive should be able to fill their regularities making the surface smooth allowing proper or intimate contact
  • Factors Affecting Adhesion • Water – The higher the water content, the poorer is the adhesion. – Water can react with both materials by the high polar group and hydrogen bond which can hamper the adhesion
  • ENAMEL BONDING • Enamel – the hardest tissue in the human body – consists of 95 percent mineralized inorganic substance – hydroxyapatite arranged in a dense crystalline structure and a small amount of protein and water
  • Steps in Forming Good Adhesion (1) Clean adherend (2) Good wetting (3) Intimate adaptation (4) Bonding (5) Good curing – + chemical physical mechanical bonding bonding bonding
  • ENAMEL BONDING • To bond to enamel, it is very important to focus on the mineral component (hydroxyapatite) of enamel. • Buonocore, 1955 – was the first to reveal the adhesion of acrylic resin to acid etched enamel. – used 85 percent phosphoric acid for etching • Silverstone revealed that the optimum concentration of phosphoric acid should range between 30 to 40 percent to get a satisfactory adhesion to the enamel.
  • ENAMEL BONDING • If the concentration is greater than 50 percent – monocalcium phosphate monohydrate may get precipitated • concentrations lower than 30 percent, – dicalcium phosphate monohydrate is precipitated which interferes with adhesion.
  • ENAMEL BONDING • Percentage of etchants used: – %35 • Use of lower concentrations of phosphoric acid and reduced etching time has shown to give an adequate etch of the enamel while avoiding excessive demineralization of the dentin
  • • acid application time: ideally 10 seconds – studies show that enamel should not be etched for more than 15 to 20 seconds
  • Steps for Enamel Bonding • Clean and wash the teeth with water. Isolate to prevent any contamination from saliva or gingival crevicular fluid • Apply acid etchant in the form of liquid or gel for 10 to 15 seconds. Deciduous teeth require longer time for etching than permanent teeth because of the presence of aprismatic enamel in deciduous teeth
  • Steps for Enamel Bonding • Wash the etchant continuously for 10 to 15 seconds • Note the appearance of a properly etched surface. It should give a frosty white appearance on drying • If any sort of contamination occurs, repeat the procedure
  • Steps for Enamel Bonding • Now apply bonding agent and low viscosity monomers over the etched enamel surface. Generally, enamel bonding agents contain BisGMA or UDMA with TEGDMA added to lower the viscosity of the bonding agent. The bonding agents due to their low viscosity, rapidly wet and penetrate the clean, dried, conditioned enamel into the microspaces forming resin tags. The resin tags which form between enamel prisms are known as Macrotags
  • • The finer network of numerous small tags are formed across the end of each rod where individual hydroxyapatite crystals were dissolved and are known as microtags. • These microtags are more important due to their larger number and greater surface area of contact. The formation of resin micro and macro tags within the enamel surface constitute the fundamental mechanism of enamel-resin adhesion.
  • Dentin Bonding
  • Dentin Bonding • Bonding to dentin has been proven more difficult and less reliable and predictable than to enamel. This is basically because of difference in morphologic, histologic and compositional differences between enamel and dentin
  • Enamel vs Dentin • In enamel, it is 95 percent inorganic hydroxyapatite by volume, in dentin it is 50 percent. Dentin contains more water than does enamel
  • Enamel vs Dentin • Hydroxyapatite crystals – have a regular pattern in enamel whereas in dentin, hydroxyapatite crystals are randomly arranged in an organic matrix • The presence of the smear layer – makes wetting of the dentin by the adhesive more difficult • Dynamic structure of dentin – shows changes due to aging, caries or operative procedures.
  • Enamel vs Dentin • Fluid present in dentinal tubules constantly flows outwards which reduces the adhesion of the composite resin. • Dentin contains dentinal tubules which contain vital processes of the pulp, odontoblasts. – This makes the dentin a sensitive structure
  • Conditioning of Dentin • For removal or modification of the smear layer, many acids or/and calcium chelators are used
  • Conditioning of Dentin - Acids • Commonly used 37 percent phosphoric acid • Not only removes the smear layer but also exposes the microporous collagen network into which resin monomer penetrates • Usually, it forms exposed collagen fibrils which are covered with an amorphous layer, a combination of denatured collagen fibers and the collapsed residual collagen layer.
  • Priming of Dentin • Primers are agents which contain monomers having a hydrophilic end with affinity for exposed collagen fibrils and a hydrophobic end with affinity for adhesive resin • Commonly used primers have HEMA and 4META monomers, dissolved in organic solvents.
  • Priming of Dentin • Primers are used to increase the diffusion of resin into moist and demineralized dentin and thus optimal micromechanical bonding. For optimal penetration of primer into demineralized dentin, it should be applied in multiple coats. • It is preferred to keep the dentin surface moist, otherwise collagen fibers get collapsed in dry condition resisting the entry for primer and adhesive resin
  • Moist vs Dry Dentin • By etching dentin, the smear layer and minerals from it are removed, exposing the collagen fibers • Areas from where minerals are removed are filled with water. This water acts as a plasticizer for collagen, keeping it in an expanded soft state. Thus, spaces for resin infiltration are also preserved. But these collagen fibers collapse when
  • Moist vs Dry Dentin • But these collagen fibers collapse when dry and if the organic matrix is denatured. This obstructs the resin from reaching the dentin surface and forming a hybrid layer
  • Moist vs Dry Dentin
  • Moist vs Dry Dentin • The desired effect of acid etching, which is increased permeability • For this reason, presence of moist/wet dentin is needed to achieve successful dentin bonding. When primer is applied to wet/moist dentin, water diffuses from the primer to the organic solvent and the solvent diffuses along with the polymers into the demineralized dentinal matrix and tubules.
  • Wet Bonding • primers consisting of hydrophilic resin monomers dissolved in water miscible organic solvents like ethanol and acetone are used • Acetone-based primers are dependent on a moist dentin surface for hybridization because the acetone displaces water present in the interfibrillar spaces of the collagen network and carry hydrophilic resin along with it for hybridization
  • Dry Bonding • water-based primers are used. • Water-based primers are not dependent on moist dentin because of their ability to selfwet a dried dentin surface and thus separating the collapsed collagen fibers.
  • Dry Bonding • Studies show that moist dentin is friendly with all primer types, it is advisable to have moist dentin for resin-dentin bonding. • To get moist dentin after etching, do not dry the dentin with compressed air after rinsing away etchant. • Instead use high-volume evacuation to remove excess water and then blot the remaining water present on the dentin surface using gauze or cotton to leave dentin optimally moist.
  • DENTIN BONDING AGENT • Mechanism of Bonding M R X • Where, M is the double bond of methacrylate which copolymerizes with composite resin. • R is the spacer which makes the molecule large. • X is a functional group for bonding which bonds to inorganic or organic portion of dentin
  • Mechanism of Bonding • Ideally a dentin bonding agent should have both hydrophilic and hydrophobic ends. The hydrophilic end displaces the dentinal fluid, to wet the surface. The hydrophobic end bonds to the composite resin.
  • Mechanism of Bonding • Bonding to the inorganic part of dentin involves ionic interaction among the negatively charged group on X – for example, phosphates, amino acids and amino alcohols, or dicarboxylates) and the positively charged calcium ions. • Commonly used bonding systems employ use of phosphates.
  • Mechanism of Bonding • Bonding to the organic part of dentin – Interaction with • • • • Amino (–NH), Hydroxyl (–OH), Carboxylate (–COOH), Amide (–CONH) groups present in dentinal collagen. present in dentinal collagen. • Dentin bonding agents have – – – – – isocyanates, aldehydes, carboxylic acid anhydrides and carboxylic acid chlorides which extract hydrogen from the which extract hydrogen from the above mentioned groups and bond above mentioned groups and bond chemically. chemically.
  • Classification of Dental Adhesives • Based on generations – First generation bonding agent – Second generation bonding agent – Third generation bonding agent – Fourth generation bonding agent – Fifth generation bonding agent – Sixth generation bonding agent – Seventh generation bonding agent
  • Classification of Dental Adhesives • Based on smear layer treatment – Smear layer modifying agents – Smear layer removing agents – Smear layer dissolving agents.
  • Classification of Dental Adhesives • Based on number of steps – Three step – Two step – Single step.
  • Evolution of Dentin Bonding Agents
  • First Generation • Developed in 1960s • Relied on adhesion to smear layer • No. of steps involved were two; etching of enamel + application of adhesive • Did not recommend dentin etch • Low bond strength (2–3 MPa).
  • First Generation
  • Second Generation • • • • Developed in 1970s Did not recommend dentin etching Low bond strength (4–6 MPa) Relied on adhesion to smear layer but some of them employed • Use of mild cleansing agent • No. of steps involved were two; etching of enamel + application • of adhesive
  • Second Generation • Problems with second generation bonding agents: – Low bond strength • Unstableinterface between dentin and resin because of the insufficient knowledge about the smear layer.
  • Third Generation Dentin Bonding Systems • Employed the concept of conditioning and priming before application of bonding agent • Involved removal or modification of smear layer • Three steps application, i.e. – Etching of enamel + Application of primer + Bonding agent application • High bond strength and reduced microleakage.
  • Third Generation Dentin Bonding Systems
  • Fourth Generation Dentin Bonding Agents • They were made available in the mid 1990s. Fourth generation bonding agents represent significant improvements in the field of adhesive dentistry. These agents are based on total etch technique and moist bonding concept.
  • Fourth Generation Dentin Bonding Agents • Mechanism of bonding: The 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 • This concept wasgiven by Nakabayashi in 1982.
  • Fourth Generation Dentin Bonding Agents
  • Fourth Generation Dentin Bonding Agents
  • Fourth Generation Dentin Bonding Agents
  • Components of Fourth Generation Dentin Bonding Agents • The fourth generation adhesives consist of: 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 – Use of conditioner/etchant causes removal or modification of the smear layer, demineralizes peritubular and intertubular dentin and exposes collagen fibrils
  • Components of Fourth Generation Dentin Bonding Agents • Primer: consist of monomers like HEMA (2Hydroxyethyl methacrylate) and 4-META (4Methacryloxyethyl trimellitate anhydride) dissolved in acetone or ethanol – they have both hydrophilic as well as hydrophobic ends which have affinity for the exposed collagen and resin respectively • primer increases wettability of the dentin surface, bonding between the dentin and resin, and encourages monomer infiltration of demineralized peritubular and intertubular dentin.
  • Components of Fourth Generation Dentin Bonding Agents • 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
  • Components of Fourth Generation Dentin Bonding Agents
  • Fifth Generation Dentin Bonding Agents • Developed in mid 1990s • Uses two steps, i.e. Total etching + Application of primer and bonding agent • Primer + Bonding agent are available in single bottle • High bond strength • Easy to use • Reduced postoperative sensitivity
  • Fifth Generation Dentin Bonding Agents • The basic differences between the fourth and fifth generation dentin bonding agents is the number of basic components of bottles. – The fourth generation bonding system is available in two bottles, one consisting of the primer and the other the adhesive, the fifth generation dentin bonding agents are available in one bottle only. – This makes the fifth generation bonding agents simpler and faster than the fourth generation systems
  • Sixth Generation Dentin Bonding Agents • 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. Self etching adhesive Available in two bottles: •  Primer • Adhesive – A drop from each bottle is taken, mixed and applied to the tooth surface, for example, Prompt L-pop.
  • Sixth Generation Dentin Bonding Agents • 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 of Self Etching Primers • Comparable adhesion and bond strengths to enamel and dentin • Reduces postoperative sensitivity because they etch and prime simultaneously • They etch the dentin less aggressively than total etch products • The demineralized dentin is infiltrated by resin during the etching process
  • Advantages of Self Etching Primers • Since they do not remove the smear layer, the tubules remain sealed, resulting in less sensitivity • They form a relatively thinner hybridlayer than traditional product which results in complete infiltration of the demineralized dentin by the resin monomers. This results in increased bond strength
  • Advantages of Self Etching Primers • Much faster and simpler technique • Less technique sensitive as fewer number of steps are involved for the self etch system
  • Seventh Generation Bonding Agents • They achieve the same objective as the sixth generation systems except that they simplified the multiple sixth generation materials into a single component, single bottle one-step self-etch adhesive, thus avoiding any mistakes in mixing.
  • Seventh Generation Bonding Agents • Developed in late 2000s • All in one concept, i.e. components available as single component • Uses self etch primer • Good bond strength No postoperative sensitivity.
  • Seventh Generation Bonding Agents • Seventh generation bonding agents also have disinfecting and desensitizing properties. They have attained consistently lower bond strengths than the fourth and fifth-generation adhesives
  • Seventh Generation Bonding Agents • Both the sixth and seventh generation adhesives are self etching, self priming adhesives which are minimum technique sensitive. The seventh generation DBAs have shown very little or no postoperative sensitivity.
  • Seventh Generation Bonding Agents
  • HYBRID LAYER AND HYBRIDIZATION • A dentin bonding agent is a low viscosity unfilled or semifilled resin for easy penetration and formation of a hybrid layer. • When a bonding agent is applied, part of it penetrates into the collagen network, known as intertubular penetration and the rest of it penetrates into dentinal tubules called intratubular penetration. • In intertubular penetration, it polymerises with primer monomers forming a hybrid layer/resin reinforced layer
  • Hybridization (Given by Nakabayachi in 1982) • Hybridization is the process of formation of a hybrid layer. • The hybrid layer is the phenomenon of formation of a resin interlocking in the demineralized dentin surface. The hybrid layer is responsible for micromechanical bonding between tooth and resin
  • Hybridization (Given by Nakabayachi in 1982) Diagrammatic representation of hybrid layer Diagrammatic representation of hybrid layer
  • Hybridization (Given by Nakabayachi in 1982) • When dentin is treated with a conditioner, it exposes the collagen fibril network with interfibrillar microporosities. These spaces are filled with low viscosity monomers when primer is applied. • This layer formed by demineralization of dentin and infilteration of monomer and subsequent polymerization is called the hybrid layer. • Hybridoid layeris that area of demineralized dentin into which resin fail to penetrate
  • Zones of the Hybrid Layer 1. Top layer: Consists of loosely arranged collagen fibrils and interfibrillar spaces filled with resin. 2. Middle layer: Consists of interfibrillar spaces in which hydroxyapatite crystals have been replaced by resin monomer because of the hybridization process. 3. Bottom layer: Consists of almost unaffected dentin with a partly demineralized zone of dentin
  • Diagrammatic presentation of different zones of hybrid layer Diagrammatic presentation of different zones of hybrid layer
  • SMEAR LAYER • Basically, when a tooth surface is altered using hand or rotary instruments, cutting debris are smeared on the enamel and dentin surface, this layer is called the smear layer
  • Structure • smear layer has an amorphous, irregular and granular appearance. • Cameron (1983) and Mader (1984) described that smear layer consists of two separate parts: – 1. One superficial and loosely attached to the underlying dentin – 2. The other consisting of plugs of dentinal debris in the orifices of dentinal tubules
  • Depth • The smear layer has an average depth of 1 to 5 µm but in the dentinal tubules, it may go up to 40 µm. The depth of the smear layer depends on following factors: • Dry or wet-cutting of the dentin • Type of instrument used • Chemical composition of irrigating solution when doing root canal treatment.
  • Components of the Smear Layer • The inorganic material in the smear layer is made-up of tooth structure and some nonspecific inorganic contaminants • The organic components may consist of heated coagulated proteins (gelatin formed by the deterioration of collagen heated by cutting temperature), necrotic or viable pulp tissue and odontoblastic processes, saliva, blood cells and microorganisms
  • Role of the Smear Layer • The smear layer is apparently responsible for: • Acting as a physical barrier for bacteria and bacterial products • Restricting the surface area available for diffusion of both small and large molecules • Resistance to fluid movement
  • Disadvantages of the Smear Layer • Bonding to the smear layer forms a weak union becausethe smear layer can be torn away from the underlying matrix. • Since this layer is nonhomogeneous and a weakly adherent structure, it may slowly disintegrate, dissolving around a leaking filling material, thus creating a void
  • Disadvantages of the Smear Layer • Smear layer on root canal walls acts as an intermediate physical barrier and may interfere with adhesion and penetration of sealers into dentinal tubules • The prescence of a smear layer causes possibility for leakage of microorganisms and a source of substrate for bacterial growth • Presence of viable bacteria which may remain in the dentinal tubules and make use of the smear layer for sustained growth and activity