Introduction to Adhesion in Dentistry

Introduction to Adhesion

• Tooth/restoration interface discontinuity represents the weakest point in restoration.
• Michael Buonocore in the 1960s introduced the idea of improving adhesion of tooth-colored materials by etching enamel.
• Buonocore's method showed significant results in minimizing interfacial gaps.
• Fusyama in 1989 advocated using phosphoric acid to etch dentin.

Key definitions

• Adhesion refers to surface attachment involving intermolecular attraction between adhesive and adherend.
• Adhesive is a material or film used to produce adhesion.
• Adherend refers to the substrate to which the material adheres, for example, enamel.

Adhesion Mechanisms

Mechanical Adhesion

• Mechanical Adhesion relies on the old concept of mechanical retention, such as amalgam restorations.
• It occurs when solidified adhesive interlocks micro-mechanically with the adherend surface's roughness, for example, resin composite.

Chemical Adhesion (True Adhesion)

• Chemical Adhesion involves forces/energies between atoms/molecules at an interface, holding two phases together.
• Chemical Adhesion can be achieved through adsorption or diffusion.

Adsorption Mechanism

• Adsorption occurs when chemical bonds develop between the adhesive and the adherend.
• These bonds include primary (ionic and covalent) valence forces

Diffusion Mechanism

• Polymers from each side of an interface cross over and react, allowing the interface to disappear.
• In glass ionomer, the adhesive bond is called "diffusion-based adhesion".
• In glass ionomer, the carboxylate group displaces calcium and phosphate ions to enrich the interface.

Electrostatic Adhesion

• Electrostatic Adhesion, an electrical double layer forms at the interface between a metal and a polymer.

Factors for successful adhesive junction

• High surface energy of tooth tissues is key
• Cleanliness of tooth tissues
• Low surface tension and viscosity of adhesive
• The extent of wetting which is determined by the contact angle between the adhesive and adherend.
• If the adhesive spreads completely, the contact angle is 0, with wetting decreasing as the angle increases.
• Achieve stability of the adhesive joint in the oral environment.

Adhesion to Tooth Structure

• Teeth are not homogenous
• Enamel and dentin vary in organic content, so adhesives must bond to organic and inorganic components.
• Tooth preparation forms a smear layer of microscopic debris, which reduces wetting.
• Saliva and the smear layer are major obstacles to adhesion.
• Dental adhesives should displace or react with water, and bond effectively in an aqueous environment.
• Resin composite fillings can be "glued" to enamel and dentin, enabling less invasive treatments.
• Bonding's most important contribution is that it helps withstand chewing forces and prevent leakage along the margins of restoration
• Clinically, failure of composite fillings occur because of inadequate sealing and discoloration, instead of loss of retention.
• Current adhesives' mechanism is an exchange process, substituting inorganic tooth material with resin monomers that interlock in micro-porosities; diffusion is key for micromechanical retention.
• Chemical bonding between monomers and calcium in hydroxyapatite has been corroborated as a possible additional mechanism.

Adherend Foundation Preparation

• Acid etching makes this possible.

Goals of Acid Etching

• Clean enamel
• Remove the enamel smear layer
• Remove the abhesive layer
• 10U of enamel should be removed to create 5-50 μm deep micropores.
• Achieve microscopic roughness
• Increase surface free energy from 32 dynes/cm to 72 dynes/cm.

Demineralization Methods

• Phosphoric acid 37% for 15 seconds is the most commonly used etchant.
• EDTA is a strong decalcifying agent that prompts low bond strength .
• Nitric acid 2.5%.
• Citric acid 10%.
• Maleic acid 10%.
• Oxalic acid 1.6-3.5%.
• Lasers alter the substrate via thermal transients, boosting inorganic dentin's bondable fraction that decreases organic fraction.
• Applying aluminum oxide particles of different sizes to enamel and dentin describes air abrasion
• Air abrasion removes demineralized and discolored tissue.
• Micro abrasion can be useful in self-etching adhesive systems.

Factors affecting Acid Etching

• Higher acid concentrations lead to acid monophosphate dehydrate formation and block microspores.
• 15 seconds is nearly as good as 60 seconds for etching time.
• Enamel must be dry and clean before etching and kept that way until resin placement.
• Saliva Contamination must be avoided
• If saliva contamination occurs, rinse off the contaminant, dry the tooth and re-etch.

Etching Patterns

• Type I etching is preferential removal of enamel prism cores
• Type II etching is preferential removal of prism peripheries
• Type III etching pattern is not related to prism morphology.

Clinical Features of Etching

• Surface is white frosted (chalky white) after etching
• Conditioning uses glass ionomer restorations and alters the surface, without removing calcium.
• Etching is used with resin restorations which causes preferential demineralization.

Adherent Composition and Infiltration

• Enamel adhesives typically use BisGMA or UDMA resins.
• HEMA & TEGDMA are coupled with hydrophilic monomers of lower viscosity to enhance wetting.

Obstacles to Bonding to Dentin

• Complex histological structure.
• Heterogeneous composition.
• High organic and water content.
• Randomly arranged hydroxyapatite in organic matrix consists mainly of collagen.
• Divergence of dentinal tubules from the pulp.
• Presence of pulpal fluid and odontoblastic processes in tubules.
• Differences in dentin permeability.
• Inherent surface wetness.
• Fluid in tubules is under pressure from pulp (intrapulpal pressure).
• Presence of smear layer on the dentinal surface.
• Bonding to dentin is more difficult than bonding to enamel due to its complexity and hydration.
• Enamel is a uniform substrate with 90 vol% inorganic material. It's structure consists of complex composite with less than 50 vol% inorganic material and water 21 vol%
• Pulpal water flow results in a complex substrate in vital teeth affecting adhesion.
• Tooth type, bonding location, and tooth qualities impact adhesion.

Bonding to Altered Dentin

• Affected dentin
• Infected dentin
• Hypo mineralized dentin
• Hyper mineralized dentin
• Fluorosis
• The bonding location
• Tooth qualities

Requirements for Ideal Dentin Bonding Agent

• Hydrophilic properties for bonding to wet dentin.
• Contains a hydrophobic part to copolymerize with applied resin.
• Low viscosity for good diffusion.
• Possesses minimum film thickness for better wettability.
• Biocompatibility
• High bond strength to enamel and dentin immediately and minimizing microleakage to nanoleakage.
• Easy to be applied and good shelf life

Factors Affecting Bonding to Tooth Structure

• Tooth-related factors
• Material-related factors
• Prepared cavity-related factors
• Technique of restoration and operator's skills
• Oral environmental factors

Physiological Effects on Bonding

• Adhesives needs to have high surface energy
• Critical surface tension allows adherent to spread across adherend. Or, surface tension of a liquid allows a drop to spread across the tooth surface.
• Surface energy can be controlled using surface pretreatment.
• Enamel's critical surface energy increases from 32 to 72 dynes/cm² after acid etching.
• Capillary attraction governs adhesive mechanisms in dentin.
• The adhesive amount depends on the tubules, their number and their distribution.

Osmotic Pressure on Bonding

• Osmotic pressure plays a role since diffusion mechanisms are allowed to occur in different concentrations.

Enamel and Dentin Composition

• Enamel: 96% inorganic, 4% organic/water.
• Dentin: 50% inorganic, 50% organic/water.
• Water percentage is a challenge
• One material must bond to three different tissues.

Smear Layer

• Formed by denaturing dentin during cutting.
• 1-5 microns thick, with debris, dentin chips, saliva, blood, and bacteria.

Contaminants

• Impaired bonding with lower bond strength from blood, saliva or moisture from handpiece.
• Oil as well
• Resin bonding's properties greatly determine bond quality.
• Indestructibility in oral fluids.
• Thermal coefficient of expansion.
• Dimensional stability.
• Modulus of elasticity and stress transfer at the interface.
• Viscosity during insertion.
• Adhesiveness, wetting, and polarity towards tooth structure.
• Initial polymerization site affects outcomes

Cavity Preparation Steps Impact Bonding

• Proper adhesive cavity designs
• Estimated resistance and retention
• Remove carious tissue to avoid pulp exposure.
• Finish the walls of the cavity to provide smoothness and adequate debridement for applied resins.

Technique and Operator Skill

Rubber Dam

• Rubber dams prevent moisture contamination.

Liners and Bases

• Liners and basis affect surface area available for bonding.
• MTA/Biodentin liners at limited sites with enough dentin thickness.

Temporary Restorations

• Eugenol-containing temporary restorations affect surfaces substrate.

C-factor

• C-factor represents the ratio of bonded to free unbonded restoration.
• The higher the ratio of bonded to free resin surface, the less flow may compensate for contraction stress.

Post-Operative Care

• Good oral hygiene prevents plaque and subsequent bond degradation.

Oral Environment Factors

• Cyclic occlusal loads.
• Chemical degradation potentials.
• pH and thermal fluctuations.
• Oral microorganisms.
• Humidity.
• Chewing habits.

Clinical Results of Bonding

• Tooth structure's conservation, and long-term restoration retention promotes patient acceptance.
• Microleakage's reduction decreases postoperative hypersensitivity and recurrent caries etc.
• Bonding does have limitations due to the dimensional shift, but can decrease it.
• Reinforcement remaining tooth structure, is still a controversy.
• Applications of tooth-colored restoratives widens.
• Improves the resistance to marginal chipping of resin composite restorations.

Dentin Adhesive System

• Dentin adhesive system classifications: Van Meerbeek Scientific Classification
• Based on number of steps it can be either Etch and rinse, self-etching or glass ionomer adhesive.
• Self-etching and acid etching recent bonding systems.

Approaches

• Etch and Rinse approach uses three steps or two steps
• Self-Etch approach uses two or one step procedure and incorporates remnants.
• The advantages in Self-Etch is Lessening time and reduces technique sensitivity and minimizes the risk of errors.

Smear Layer Treatment

• There can be 3 different ways: Smear layer modifying, Smear layer dissolving or Smear layer removing.
• Etching is a selective demineralization preparing tooth-s ubstrate.

Objectives of Etching

• Removal of the smear layer to allow bonding to the underlying
• Demineralization of the superficial matrix of dentin
• Uncover peritubular and intertubular dentin
• Etching clean any biofilms

Selective Demineralization

• Do not denature dentin
• A space should be maintained between the demineralized collage
• Remove and avoid contamination when demineralizing and finally rinse and dry

Priming

• Enhances surface wettability due to hydrophilic part.
• It also consist of promoting monomers composed of hydrophilic and hydrophobic groups of ethanol or acetone solvents.
• Seal space and have surface glossy when applying resin such as BIS-GMA.
• Resin seals all micro porous and creates hydroxyapatite

Applying the resin material

• Do it uniformaly
• Thinning is advocated
• 30-40 second lapse is required
• Light curring is required and follow the time from manufacturer