Comprehensive Notes on Dental Adhesion

Phosphoric Acid in Dentistry
Buonocore (1955) pioneered the use of phosphoric acid to improve adhesion to teeth, leading to the age of adhesive dentistry.
Traditional mechanical methods are largely replaced by tooth-conserving adhesive methods.
Black's concepts of large preparations are replaced by smaller, more conservative techniques.

Advantages of Adhesive Techniques

Adhesive techniques reduce the need for removal of sound tooth structure.
Adhesion reduces microleakage, preventing oral fluids and bacteria from entering the restoration-tooth interface which reduces:
- Postoperative sensitivity
- Marginal staining
- Recurrent caries
Adhesive restorations better transmit and distribute functional stresses, reinforcing weakened tooth structure.
Traditional metal restorations may act as a wedge, increasing the risk of cuspal fracture.
Adhesive techniques allow for repair and replacement of deteriorating restorations with minimal tooth loss.
Adhesive techniques have expanded possibilities for esthetic restorative dentistry.
Patients pay more attention to esthetics, making tooth-colored materials valuable for restoration and recontouring.
Adhesive technology has enabled dentists to improve facial esthetics in a simple and economic way.

Expanding Indications for Adhesive Dentistry

Resin composites are used for carious and fractured tooth structure, erosion, and abrasion defects.
Modern techniques allow adding restorative material to correct unesthetic shapes, positions, dimensions, or shades.
Resin composite can be used to close diastemas, add length, or mask discoloration.
Due to concerns about mercury toxicity, research focuses on alternatives to amalgam.
Posterior resin composites can be directly or indirectly bonded into Class I and Class 2 preparations.
Adhesive techniques bond anterior and posterior ceramic restorations like veneers, inlays, and onlays.
Adhesives can bond silver amalgam restorations and retain metal frameworks.
Adhesives can be used to cement crowns and fixed partial dentures, bond orthodontic brackets, create periodontal or orthodontic splints, treat dentinal hypersensitivity, and repair fractured porcelain, amalgam, and resin restorations.
Pit and fissure sealants use adhesion as a preventive treatment.
Adhesive materials are often used with core buildup foundations.

Principles of Adhesion

Adhesion is derived from the Latin word "adhaerere," meaning to stick.
Adhesion/bonding is the attachment of one substance to another.
The adherend is the surface or substrate being adhered to.
The adhesive/adherent (bonding agent or adhesive system) joins surfaces, resists separation, and transmits loads across the bond.
Adhesive/bond strength measures the load-bearing capability of the adhesive.
Durability is the time period during which the bond remains effective.
Adhesion refers to forces/energies between atoms/molecules at an interface.
In debonding tests, adhesion is subjected to tensile or shear forces, and the mode of failure is quantified.
Adhesive failure occurs at the interface between substrates.
Cohesive failure occurs within one of the substrates.
Failure is often mixed.

Theories of Adhesion

Mechanical Theories: Solidified adhesive interlocks micromechanically with roughness and irregularities of the adherend surface.
Adsorption Theories: Chemical bonds (primary and secondary) between the adhesive and adherend.
- Primary bonds: Ionic and covalent.
- Secondary bonds: Hydrogen, dipole interaction, and London dispersion.
- London dispersion forces are almost always present, depending solely on the presence of nuclei and electrons.
Diffusion Theories: Adhesion results from bonding between mobile molecules.
- Polymers from each side of an interface cross over and react.
- Eventually, the interface disappears.
Electrostatic Theories: An electrical double layer forms at the interface between a metal and a polymer, contributing to bond strength.

An important requirement for interfacial phenomena is close and intimate contact between materials.
Sufficient wetting of the adhesive occurs only if its surface tension is less than the surface-free energy of the adherend.
Wetting: Characterized by the contact angle of a droplet on the surface.
- Complete wetting: Liquid spreads completely, contact angle of 0 degrees.

Adhesion to Enamel vs. Dentin

Adhesion to enamel is easier than adhesion to dentin.
- Enamel: Primarily hydroxyapatite, high surface-free energy.
- Dentin: Hydroxyapatite and collagen, low surface-free energy.
Tooth surface contamination (organic saliva pellicle) impairs wetting, with a low critical surface tension of $(28 dynes/cm)$ .
Instrumentation during cavity preparation produces a smear layer with a low surface-free energy.
The natural tooth surface should be thoroughly cleaned and pretreated before bonding to increase its surface-free energy.
Several mechanisms contribute to bond strength.
Glass-ionomer cement has intrinsic self-adhesive capacity to bond to tooth tissue without pretreatment.
Other materials like resin composites require an intermediate resin.
Adhesion to enamel: Resin bonding agent bonds primarily by micromechanical interlocking with etched substrate irregularities.
Micromechanical bonding: Largely involved in bonding to dentin.
Secondary, weak London-van der Waals forces may play a contributing role in resin-tooth attachment.

Factors Affecting Adhesion to Tooth Tissue

Factors influencing strength and durability of adhesive

Physicochemical properties of the adherend and adhesive.
Structural properties of the adherend, which is heterogeneous.
Formation of surface contaminants during cavity preparation.
Development of external stresses that counteract bonding.
Mechanisms of transmission and distribution of applied loads through the bonded joint.
Oral environment (moisture, physical stresses, temperature and pH changes, dietary components, chewing habits).

Compositional and Structural Aspects of Enamel and Dentin

Adhesion to enamel and dentin differs due to their distinct composition and structure.

Enamel

Inorganic content: $95-98 wt\%$ and $86 vol\%$ , primarily hydroxyapatite.
Water: $4 wt\%$ and $12 vol\%$ .
Organic material: $1-2 wt\%$ and $2 vol\%$ .
Crystallites oriented in three dimensions contribute to the rod or prism.
Natural surface: Smooth, keyhole pattern.
Operatively prepared surfaces: Expose rods in tangential, oblique, and longitudinal planes.
Almost homogeneous, except for aprismatic (prismless) enamel at the outer surface where crystallites run parallel.

Dentin

Water: $12 wt\%$ .
Organic material: $18 wt\%$ , mainly type I collagen.
Hydroxyapatite: $70 wt\%$ .
Organic material ( $25 vol\%$ \$\$) and water (25 vol\% $\$\$) combined equal inorganic material ($ 50 vol\% $.</li><li>Uneven distribution in intertubular and peritubular dentin.</li><li>Numerous dentinal tubules radiate from the pulp, making it permeable.</li><li>Tubules contain odontoblastic processes, forming a direct connection to the vital pulp.</li><li>Diameter of tubules decreases from$ 2.5 \mum $at the pulp side to$ 0.8 \mum $at the dentinoenamel junction.</li><li>Number of tubules decreases from$ 45,000/mm^2 $near the pulp to$ 20,000/mm^2 $near the dentinoenamel junction.</li><li>Average of$ 30,000 tubules/mm^2 $in the middle part of cut human dentin.</li><li>Each tubule is surrounded by hypermineralized peritubular dentin.</li><li>Intertubular dentin is less mineralized and contains more organic collagen fibrils.</li><li>Tubules filled with tissue fluid, organic membrane structure (lamina limitans), and intratubular collagen fibrils.</li><li>Fan-shaped radiation of dentinal tubules:<ul><li>$ 96\% $of superficial dentinal surface is intertubular dentin.</li><li>$ 1\% $occupied by fluid in tubules,$ 3\% $by peritubular dentin.</li><li>Near the pulp, peritubular dentin represents$ 66\% $, intertubular dentin only$ 12\% $, and$ 22\% $is water.</li><li>$ 3\% $of cut surface is tubules in superficial dentin,$ 25\% $in deep dentin.</li></ul></li><li>Mean diameter of tubules ranges from$ 0.63 $to$ 2.37 \mum $, depending on depth.</li><li>Dentinal fluid in tubules is under slight, constant outward pressure from the pulp.</li><li>Intrapulpal fluid pressure: 25 to 30 mm Hg or 34 to 40 cm water.</li></ul><h5 id="b0980c3e-4c35-4832-8d2f-9a8cd2c6c55a" data-toc-id="b0980c3e-4c35-4832-8d2f-9a8cd2c6c55a" collapsed="false" seolevelmigrated="true">Changes in Dentinal Structure</h5><ul><li>Dentin undergoes continuous physiologic and pathologic changes in composition and microstructure.</li><li>Dentin affected by caries, abrasion, or erosion differs from unaffected dentin.</li><li>Physiologic dentinal sclerosis occurs as part of aging.</li><li>Reactive sclerosis occurs in response to mild irritations like abrasion or chemical erosion.</li><li>Tertiary/reparative dentin is produced in the pulp chamber in response to insults.</li><li>Hypermineralization and obstruction of tubules by whitlockite crystalline deposits are responses to caries.</li><li>Sclerotic dentin (transparent, glasslike) has several characteristics in cervical areas of teeth.</li><li>Sclerosis results from obstruction of tubules by apposition of peritubular dentin and precipitation of rhombohedral mineral crystals.</li><li>Refractive index of obstructed tubules is similar to intertubular dentin, resulting in a glasslike appearance.</li><li>Sclerotic dentin usually contains few (if any) patent tubules, has low permeability, and tends to be insensitive to external stimuli.</li><li>Odontoblastic processes associated with sclerotic dentin often exhibit partial atrophy and mineralization.</li><li>Heavily sclerotic dentin has areas of complete hypermineralization without tubule exposure, even when etched with acid.</li><li>Some areas show mineral sclerotic casts extending from tubule orifices.</li><li>Morphologic and structural transformations result in dentin less receptive to adhesive treatments.</li></ul><h5 id="0e8c6b3f-7f11-4202-ac72-f65baa310f45" data-toc-id="0e8c6b3f-7f11-4202-ac72-f65baa310f45" collapsed="false" seolevelmigrated="true">The Smear Layer</h5><ul><li>Cutting debris smeared over enamel and dentinal surfaces during cavity preparation forms the smear layer.</li><li>Defined as debris produced by reduction/instrumentation of dentin, enamel, or cementum or as a "contaminant".</li><li>Iatrogenically produced layer influences adhesive bonds between the cut tooth and restorative material.</li><li>Burnishing action generates frictional heat and shear forces, attaching the smear layer to the underlying surface.</li><li>Ethylenediaminetetraacetic acid (EDTA) is the most potent conditioner for removing the smear layer and opening tubule orifices.</li><li>Acidic conditioners: Citric, polyacrylic, lactic, and phosphoric acids (in order of increasing potential).</li><li>Cavity cleansers: Tubulicid and hydrogen peroxide have only a slight effect.</li><li>Morphologic features, composition, and thickness depend on the instrument used, irrigation, and site of dentin.</li><li>Composition: Pulverized hydroxyapatite and altered collagen, mixed with saliva, bacteria, and other grinding debris.</li><li>Thickness: Varies from$ 0.5 $to$ 5.0 \mum $.</li><li>Smear debris occludes tubules with the formation of smear plugs.</li><li>Smear layer is porous and penetrated by submicron channels allowing small amounts of fluid to pass through.</li><li>Smear layer reduces dentinal permeability by about$ 86\% $. Internal and External Dentinal Wetness</li><li>Dentinal permeability and internal wetness depend on several factors:<ul><li>Diameter and length of the tubule.</li><li>Viscosity of dentinal fluid and the molecular size of substances dissolved in it.</li><li>Pressure gradient.</li><li>Surface area available for diffusion.</li><li>Patency of the tubules.</li><li>Rate of removal of substances by pulpal circulation.</li></ul></li><li>Occlusal dentin is more permeable over pulp horns than at the center of the occlusal surface.</li><li>Proximal dentin is more permeable than occlusal dentin.</li><li>Coronal dentin is more permeable than root dentin. High dentinal permeability facilitates bacterial and toxin penetration.</li><li>Variability in dentinal permeability makes dentin a more difficult substrate for bonding.</li><li>Removal of smear layer creates a wet bonding surface with dentinal fluid exudes.</li><li>Water competes effectively (by hydrolysis) for all adhesion sites on hard tissue.</li><li>Early dentin bonding agents failed because hydrophobic resins were not capable of sufficiently wetting the hydrophilic substrate.</li><li>Bond strengths decrease as preparation depth increases because dentinal wetness is greater.</li><li>No significant difference in bond strengths observed between deep and superficial dentin when the smear layer is left intact.</li><li>More recent adhesive systems that remove the smear layer are less affected by differences in dentinal depth due to better bonding to the wet dentinal surface.</li><li>External dentinal wetness/environmental humidity negatively affects bond strengths to dentin. Wetting of the Adhesive</li><li>An ideal interface simulates the natural attachment of enamel to dentin.</li><li>Intimate molecular contact is a prerequisite for strong adhesive joints.</li><li>The adhesive system must sufficiently wet the solid surface, have low viscosity to penetrate microporosities, and displace air and moisture during the bonding process.</li><li>Primers usually contain hydrophilic monomers like 2-hydroxyethyl methacrylate (HEMA) to enhance the wettability of hydrophobic adhesive resins.</li><li>Solvents in modern primers (ethanol or acetone) ensure adequate removal of air and liquid by rapid evaporation.</li><li>Polarity and solubility determine wetting behavior.</li><li>If an adhesive monomer has similar polarity and solubility to a polymer substrate, it may act as a solvent for the polymer and infiltrate it.</li></ul><h4 id="851f8b34-c8d7-4907-8ab2-f4a3d34e2c51" data-toc-id="851f8b34-c8d7-4907-8ab2-f4a3d34e2c51" collapsed="false" seolevelmigrated="true">Polymerization Contraction of Restorative Resins</h4><ul><li>Dimensional rearrangement of monomers into polymer chains during polymerization leads to volume shrinkage.</li><li>High filler loading reduces polymerization contraction; current resin composites shrink$ 2.9 $to$ 7.1 vol\%. Contraction stresses can be up to 7 MPa.
Curing contraction is restrained by the developing bond to cavity walls which induces polymerization contraction stress.
If the bonding interface with the tooth is the weakest link, the resin-enamel bond may survive, but the weaker resin-dentin interface may not.
No dental resin composite material currently available is free of shrinkage during polymerization; research is underway to develop nonshrinking materials.

Compensation for Polymerization Contraction

Flow

Plastic deformation/flow of the resin composite occurs and may partially compensate for shrinkage stress.
Irreversible plastic deformation takes place during the early setting stages.
Contraction and flow gradually decrease as stiffness increases.
Fast-setting light-curing resin composites exhibit less flow-related stress relief.
Self-/autocuring resin composites give the developing adhesive bond to dentin more time to survive.
The combination of a slow curing rate and rapid formation of an adhesive bond is considered favorable for preserving marginal integrity.
The presence of air bubbles also contribute to the amount of free surface and eventually increase the flow capacity of the resin composite.

Configuration

Restriction of flow is affected by the configuration of the restoration, known as the C-factor.
The C-factor is the ratio of bonded (flow-inactive) to unbonded/free (flow-active) surfaces.
An increase in the number of bonded surfaces results in a higher C-factor and greater contraction stress on the adhesive bond.
Only the free surface of a resin restoration can act as a reservoir for plastic deformation in the initial stage of polymerization.
The higher the ratio of bonded to free resin surfaces, the less flow may compensate.
To improve marginal integrity of resin composite in a Class 5 restoration, a flatter and more wedge-shaped cavity design would be preferred.
The use of a base material (glass-ionomer cement) within the cavity preparation (sandwich restoration) decreases the resin composite volume.

Additional Methods

Placement of glass or ceramic blocks into soft resin composite before light curing results in reduced microleakage.
Prepolymerized resin composite inserts may also be used to help offset polymerization contraction.
Resin composite inlays cemented with a luting resin avoid the direct adverse effect of polymerization contraction on the developing resin-tooth bond.
Slowing curing allows more flow to compensate for shrinkage stress (soft-start or ramped light-curing technique).
Curing lights produce low-intensity light initially, then increase intensity.

High-Powered Light-curing

Laser and xenon high-powered light-curing technology aims to reduce curing times without decreasing material properties.
Advocates recommend placement of small resin composite increments to ensure sufficient polymerization.
Evolution of curing technology is expected to continue.
The recent development of long-lasting, high-intensity light-emitting diodes (LEDs) may become useful.

Hygroscopic Expansion

Fluid absorption causes resin composite to swell, somewhat tempering the effect of polymerization shrinkage.
The configuration of the cavity determines the effectiveness of this compensation mechanism.
Microfilled resin composites absorb more water than macrofilled materials due to the greater resin volume.
Hygroscopic expansion occurs during the days/weeks following resin composite placement, after the dentin bonding may have already failed.
Hygroscopic expansion may force a Class 5 resin composite restoration to expand beyond the margin of the preparation.

Implications

In dental adhesive technology, the collagen phase of dentin is a polymer.
Both primer and adhesive resin contain monomers that penetrate the exposed collagen layer to form a micromechanical bond.
If a conditioner conveys a specific polarity/solubility to the dentinal surface, the primer must match.
The same is true for the adhesive resin applied to the primed dentinal surface.

Transmission of Stress Across the Restoration-Tooth Interface

The adhesive bond between a restorative material and tooth has a biomechanical role in distributing functional stress.
The use of glass-ionomer cements may help compensate for contraction stress.
The elastic modulus of the resin composite itself may offer some means to compensate for contraction stress.
To improve marginal integrity of resin composite in a Class 5 restoration, a flatter and more wedge-shaped cavity design would be preferred.

Elasticity

If the resin-tooth bond remains intact, the resin composite's final stiffness/rigidity may compensate for remaining polymerization contraction stress.
Stiffness is quantified by Young's modulus of elasticity.
The lower the Young's modulus, the greater the flexibility, and the more capacity it has to reduce remaining contraction stress.
Resin composites with high filler content have a higher Young's modulus and higher remaining contraction stress.
Viscous adhesive resins produce a thick resin bonding layer between the stiff dentinal cavity wall and the shrinking restorative resin composite.
Stretching this intermediate layer (with a low Young's modulus) provides sufficient elasticity to relieve polymerization contraction stresses.
A bonding layer thickness of 125 \mum reduces shrinkage stresses below dentin bond strengths, preserving the bond.
A flexible intermediate resin layer may better transmit/distribute stresses induced by thermal changes, water absorption, and occlusal forces across the interface.
A thick adhesive resin layer permits limited inhibition of polymerization by oxygen without impairing the resin-dentin bond.

Cervical Sealing

Sealing of cervical marginal gaps with unfilled low-viscosity resin, after curing the restorative resin, overcomes negative effects of polymerization shrinkage.
Use of a restorative resin with high elasticity and low curing contraction in combination with a low-viscosity resin provides sufficient strain relief.
This technique is laborious and prone to failure in the event of contamination.

Initial Polymerization

Initiating polymerization at the resin-tooth interface, directing the shrinking resin material toward the cavity wall, is advantageous.
Contraction has been claimed to occur toward the light source in light-curing resin composites, whereas initial setting has been said to occur in the center of the bulk of material in self-curing resin composites.
For both light-curing and self-curing systems, tensile stresses operate across the resin composite-dentin interface, pulling the material away from the cavity walls.
The direction of polymerization shrinkage is not significantly affected by the orientation of the incoming curing light and the cavity shape and bond quality determine the polymerization vectors.

Biologic Considerations

Unreacted monomers that diffuse from set resin-based materials may be toxic to pulp cells and other tissues.
The biocompatibility of adhesive restorative materials must be considered.
Monomers with a molecular weight of less than 300 daltons are likely to penetrate the dentinal tubules, whereas those with a molecular weight of greater than 450 daltons are not.
High water solubility has commonly been interpreted as an indication of high cytotoxicity.
Components of resin-based materials may initiate allergic reactions.
Monomer-containing materials can cause contact allergies on skin
The short-term risk of estrogenic effects from bisphenol A-based resins is low, but long-term effects require more investigation.
A "three-finger" syndrome at the fingertips has been described.
A "noncontact" handling of diverse monomer-based materials is strongly advised. Evaluation of dentin adhesive systems has varied conclusions so inferences about the influence of chemical irritants on postoperative sensitivity must be considered premature.
Traditionally acids on vital dentin were avoided because of fear of pulpal irritation. Acids on dentin can have adverse effects when the remaining dentin is less than $1.0 mm$ thick.
Diffusion of penetrating acid is largely limited to 10 \mum, because of the blocking action of odontoblast processes in the tubules of vital teeth and intertubular crystals.

Adhesion to Enamel and Dentin

Concepts in restorative dentistry have been changing; adhesive technology has become more important.
Clinicians face a continuous turnover in adhesive materials.
The trend toward adhesive dentistry began in the mid-1960s with the first commercial restorative resin composites, followed by the acid-etch technique in the early 1970s.
Since then, refined restorative composites have been developed, and adhesive technology has continuously improved bonding agents.
Early one-step dentin bonding agents became multistep systems for more demanding application procedures.
Today, universal/all-purpose adhesive systems bond to enamel, dentin, amalgam, metal, and porcelain.
Adhesion to tooth tissue can also be achieved directly with glass-ionomer cements.
A recent trend combines glass-ionomer and resin composite technology.

Enamel Acid-Etching Technique

Adhesion to enamel is achieved through acid etching, substantially enlarging its surface area for bonding.
Buonocore invented the acid-etching technique in 1955, showing a 100-fold increase in retention of polymethylmethacrylate to incisors when etched with 85% phosphoric acid for 2 minutes.
Taglike resin extensions were formed and micromechanically interlocked due to the microporosity.
Enamel bonding agents commonly based on bis-GMA or urethane dimethacrylate (UDMA).
Bond between enamel and the restorative material is established by polymerization of monomers inside the microporosities and by copolymerization of C-C double bonds with the matrix phase.
Acid etching removes about 10 \mum of the enamel surface and creates a microporous layer from $5$ to 50 \mum deep.
Three enamel-etching patterns have been described:
- Type 1: Predominant dissolution of prism cores.
- Type II: Predominant dissolution of prism peripheries.
- Type III: No prism structures are evident.
Two types of resin tags:
- Macrotags: Formed circularly between enamel prism peripheries.
- Microtags: Formed at the cores of enamel prisms.

Acid Etching Factors

The kind of acid used.
The acid concentration.
The etching time.
The form of the etchant (gel, semigel, or aqueous solution).
The rinse time.
The way in which etching is activated (rubbing, agitation, repeated application of fresh acid).
Whether enamel is instrumented before etching.
The chemical composition and condition of enamel.
Whether enamel is in primary or permanent teeth.
Whether enamel is prism-structured or prismless.
Whether enamel is fluoridated, demineralized, or stained.
An acid gel is generally preferred over a liquid because its application is easier to control.
Resin composite to phosphoric acid-etched enamel = $20 MPa$ .\
If the preparation is completely bordered by enamel, etching reduces microleakage at the cavosurface interface.
Complete removal of the etchant and dissolved calcium phosphates, and preservation of the clean etched field without contamination, are crucial to the longevity of the resin-enamel bond.

Adhesion to Dentin

Successful bonding to enamel achieved more easily than to dentin.
Predictable bonding to dentin more problematic as dentin adhesive systems still approach laboratory results of enamel and a predictable level of clinical success.
Methods for classifying dentin bonding systems: Generational classification system and a classification system based on bonding mechanism.