BONDING TO ENAMEL AND DENTIN

S.MANEESH AHAMED ROLL NO: 12 FINAL YEAR PART 2

CONTENTS1. Introduction2. Basic concepts of adhesion3. Requirements for good adhesion 4. Applications of Adhesive restorative techniques5. Enamel adhesion6. Mechanism of etching and etch patterns 7. Dentin adhesion8. Challenges in dentin bonding9. Generations of dentin bonding agents10. Conclusion 11.Bibliography

INTRODUCTION• Adhesion is one of the ideal requirements of a restorative

material.

• The obvious advantage of adhesion would be the absence of marginal gap between the restoration and tooth structure

• This minimizes microleakage and the possibility of secondary caries.

• The possibility of adhesion to tooth structure has opened up many new avenues that have radically altered all aspects of restorative dentistry.

BASIC CONCEPTS OF ADHESIONThe state in which two surfaces are held together by interfacial forces which may consist of valance forces or interlocking forces or both.

ADHESION :

(The American Society for Testing and Materials Sp. D 907)

• An adhesive is a material, frequently a viscous fluid, that joins two substrates together and solidifies and is able to transfer load from one surface to the other. (Sturdevant)

ADHESIVE : this refers to the bonding agent that when applied to the surfaces or substances, can join them together, resist separation and transmit loads across the bond.

ADHEREND : this refers to the surface or substrate that is adhered to.

MECHANISMS OF ADHESIONS :

1. Mechanical adhesion : interlocking of the adhesive with irregularities in the surface of the substrate, or adherent.

2. Adsorption adhesion : chemical bonding between the adhesive and adherent

3. Diffusion adhesion : interlocking between mobile molecules, such as the adhesion of two polymers through diffusion of polymer chain ends across an interface,

In dentistry bonding of resins to the tooth structure is a result of four possible mechanisms, as follows :

4. Electrostatic adhesion : an electrical double layer at the interface of a metal with a polymer that is part of the total bonding mechanism

1. Mechanical : 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. A combination of the previous three mechanisms

REQUIREMENTS FOR GOOD ADHESION• The surface of the substrate should be clean

• The adhesive should wet the substrate well, have a low contact angle, and spread onto the surface.

• There should be intimate adaptation between the adhesive and the adherent.

• The bond strength between the adhesive and the adherent should be strong enough to resist debonding.

• The adhesive should be well cured.

Adhesive restorative techniques are currently used to accomplish the following :

1. Restore Class I,II,III,IV,V and VI carious or traumatic defects.

2. Change in shape and the colour of anterior teeth.

3. Improve retention for metallic crowns or for porcelain-fused-to-metal crowns

4. Bond all ceramic restorations

5. Bond indirect resin-based restoration

6. Seal pits and fissures

7. Bond orthodontic brackets

8. Bond periodontal splints and conservative tooth replacement prosthesis

(Sturdevant)

9. Repair existing restorations (composite, amalgam, ceramic, or ceramometal)

10.Provide foundations for crowns

11.Desensitize exposed root surface

12.Seal beneath or bond amalgam restorations to tooth structure

13.Impregnate dentin that has been exposed to the oral fluids, making it less susceptible to caries.

14.Bond fractured fragments of anterior teeth

15.Bond prefabricated and cast posts

16.Reinforce fragile roots internally

17.Seal apical restorations placed during endodontic surgery.

ENAMEL ADHESION• Buonocore in 1955 introduced the acid-etch technique

for the adhesion of resin to the tooth.

• He envisioned the use of acids to etch enamel for sealing pits and fissures.

• Acid etching transforms the smooth enamel into an irregular surface and increases its surface free energy.

• When a fluid resin-based material is applied to the irregular etched surface, the resin penetrates into the surface, aided by capillary action.

• Monomers undergo polymerization and the material becomes interlocked with the enamel surface.

• The formation of resin microtags within the enamel surface is the fundamental mechanism of resin-enamel adhesion

• Originally Buonocore suggested the use of 85% phosphoric acid for 2 minutes on the enamel surface to etch it.

• Silverstone et al found that the application of 30-40% phosphoric acid resulted in retentive enamel surfaces.

• Presently a 37% concentration of phosphoric acid is preferred.

• An etching time of 60 seconds originally was recommended for permanent enamel using 30-40% phosphoric acid.

• But studies using scanning electron microscopy showed that a 15 second etch resulted in a similar surface roughness as provided by a 60 second etch

• Acid may be available as a liquid or gel form.

• Gel etchants are preferred due to better control in placement over the enamel surface.

• The acid may be applied by means of syringe or brush. Syringe placement is easy and precise.

MECHANISM OF ETCHING AND ETCH PATTERNS

• Acid etching converts smooth enamel into a very irregular surface with high surface energy.

• It removes 10µm of surface enamel and creates a microporous layer which is 5 to 50µm deep.

• Optimum concentrations of acid produces monocalcium phosphate monohydrate precipitate which can be easily rinsed off.

Microscopically, three types o etch patterns have been described : Type I Dissolution of the prism cores leaving the

prism peripheries intact.

Type II Dissolution of the prism peripheries leaving the prism cores intact.

Type III No prism structures are evident.

• After etching, the enamel surface should be thoroughly rinsed with a continuous stream of water spray for 5-10 seconds so that the acid is completely washed off.

• This should be followed by proper drying which will produce a frosty, white appearance.

ENAMEL BONDING AGENTS

• Originally enamel bonding agent consisted of Bis-GMA or UDMA resins with a diluent like TEGDMA to lower their viscosity.

• These agents flow easily into the microporosities of the enamel surface and when polymerized by light activation form ‘resin tags’ which lock them into the enamel surface.

• Bond strength of composite resins to etched enamel is in the range from 15 to 25 MPa.

DENTIN ADHESION• The classic concepts of operative dentistry were

challenged in the 1980-90s by the introduction of new adhesive techniques to dentin as well as to the enamel.

• Dentin adhesion primarily relies on the penetration of adhesive monomers into the filigree of collagen fibers left exposed by acid etching.

CHALLENGES IN DENTIN BONDING

• Bonding to enamel is a relatively simple process. Adhesion to dentin presents a much greater challenge

• Structurally dentin consist of a substantial proportion of water and organic material as compared to the 90% mineral composition of enamel.

• Type I collagen is the principal organic content of dentin.

• Dentin is an intrinsically hydrated tissue, penetrated by a maze of 1-2.5µm-diameter- fluid filled dentinal tubules.

• Dentinal tubule connects pulp with the DEJ. The constant pressure from the pulp causes the fluid to move towards the DEJ.

• Dentin is a dynamic tissue that shows changes due to ageing, caries, or restorative procedures.

• Diameter of dentinal tubules reduces as we move from the periphery towards the pulpal side. Inversely the relative area occupied by the tubules increase.

• Cut dentinal surface form an unique structure called as the ‘smear layer’. It is composed of debris of hydroxyappatite crystals and denatured collagen.

• The smear layer fills the orifices of dentin tubules, forming ‘smear plugs’ and decreases dentin permeability by 85%.

• Submicron porosity of the smear layer still allows for diffusion of dentinal fluid.

• The removal of smear layer and smear plugs with acidic solutions will result in an increase of fluid flow onto the exposed dentin.

• This fluid can interfere with adhesion because of the hydrophobic nature of resins even if the resin tags are created.

STRESSES AT RESIN-DENTIN INTERFACE

• Composites shrink as they polymerize, creating a stress within the composite mass.

• This is depended on the configuration of the cavity preparation – the ‘configuration factor’(C-factor)

• Stresses within the composite are relieved by flow from the unbonded surface.

• Stress relief within a three- dimensional bonded restoration is limited

DENTIN BONDING AGENTSBEGINNING• During 1950s, it was reported that a resin containing

glycerophosphoric acid dimethacrylate (GPDM) could bond to a hydrochloric acid-etched dentin surface.

• A few years before that report, another researcher had used the same monomer chemically activated with sulfinic acid.

• The bond strength of these primitive adhesion techniques were severely reduced by immersion in water

FIRST GENERATION

• NPG-GMA(N-phenyl glycine glycidyl methacrylate), a surface active comonomer is considered as the first generation dentin bonding system.

• Both Invitro and invivo clinical study results were discouraging.

• Theoretically, the comonomer could chelate with calcium on the tooth surface to generate water-resistant chemical bonds of resin to dentin.

• Based on C-13 nuclear magnetic resonance analysis, It seems that no ionic bonding develops between NPG-GMA and hydroxyapatite.

• Example : Cervident

SECOND GENERATION • They are phosphate ester material (phenyl-P and

hydroxyethyl methacrylate in ethanol)

• Mechanism of action is based on polar interaction between phosphate group and calcium in the smear layer.

• The smear layer was the weakest link in the system because of its loose attachment with dentin.

• Bond strength was also poor (<10MPa). In addition to this, the resins were relatively devoid of hydrophilic groups.

• They show less wettability and penetration into the dentin crossing the smear layer.

• Example :Clearfil Bond, Scotchbond, Bondlite

THIRD GENERATION • The third generation dentinal adhesives attempted to

deal smear layer in two ways.

• Either by modification of the smear layer to improve its properties or by the removal of the smear layer by keeping the smear plugs intact.

• Fusayama et al in 1979 introduced the concept of phosphoric acid etching of dentin prior to the use of phosphate-ester type bonding agent.

• Materials like phenyl-P or PENTA was used to achieve smear layer modification by penetration of acidic monomers.

• Treatment of smear layer using acid primers were also attempted. 2.5% maleic acid, 55% HEMA, and traces of methacrylic acid were used for this.

• The removal of smear layer using chelating agents such as EDTA was also tried (GLUMA system).

• Example : Clearfil New Bond, Scotchbond 2, GLUMA system

FOURTH GENERATION • They are; three step, total etch adhesive systems.

• Although smear layer acts as a ‘diffusion barrier’ that reduces the permeability of dentin, it also can be considered as an obstacle to the bonding.

• Based on this this consideration a fourth generation dentin adhesives was introduced for use on acid etched dentin.

• This method is commonly known as the total-etch technique or the etch and rinse technique.

• The acid will result in complete or partial removal of smear layer with demineralization of underlying dentin. They also exposes the collagen.

• The fourth generation dentin bonding system consist of three essential components.

1. Phosphoric acid etching gel that is rinsed off2. A primer containing reactive hydrophilic

monomers in ethanol, acetone, or water3. An unfilled or filled resin bonding agent

• Acid treatment not only alters the mineral content of the dentin substrate but also alters the surface free energy.

• When primer and bonding resins are applied to etched dentin, they penetrate the intertubular dentin, forming a resin-dentin interdiffusion zone, or ‘hybrid layer’.

• Examples : Scotch Bond Multi Purpose(3M), All Bond 2, Panavia 21

FIFTH GENERATION

• These adhesives are a simplified version of the fourth generation adhesives. Also known as ‘one bottle’ system.

• The primer and adhesive is combined in one bottle. A separate etching step is still required.

• Though they require fewer steps to achieve dentin bonding, these agents are inferior to the fourth generation bonding agents in terms of bond strength.

• Example : Single Bond(3M), One-Step, Gluma Comfort Bond.

SIXTH GENERATION • Sixth generation dentin boding systems try to further

simplify the process of dentin adhesion by minimizing the clinical steps.

• Acids of lower concentration are generally used :10% Phosphoric, 2.5%nitric, 10% citric, 10% maleic acids.

• They are also known as SEP – Self Etching Primers.

• Commercially they are available in two forms :1. Self etching primers : etchant and

primer is in one bottle while adhesive is in a separate bottle. First the etchant and primer are applied on the tooth surface which is then followed by application of adhesive agent.

example : Clearfil SE bond, Xeno

2. Self etching adhesives : in these, the etchant, primer and adhesive are all in one package but require mixing before application on the tooth surface

example : Prompt-L-Pop(3M)

SEVENTH GENERATION• Also known as All-in-one adhesive system.

• These are the most recent generation of dentin bonding agents.

• They combine etchant, primer, and adhesive in one bottle. They do not require any mixing prior to application.

• Primarily these agents are intricate mixes of hydrophilic and hydrophobic components in one bottle.

• Their bond strength is less than fourth and fifth generation adhesives.

• Example : Cleaefil S3 Bond, G-Bond, Xeno IV

CONCLUSIONAdhesive restorative dentistry originated with the work of Buonocore in 1995 who developed the concept of acid etching of enamel. Since then, adhesive materials and techniques have developed at a rapid rate. Dentin bonding agents have advanced greatly over the last two decades and can now provide predictable bonding to enamel and dentin. The mechanism of bonding is primarily micromechanical by formation of a hybrid layer. The various advances in adhesive technology have expanded the applications of dentin bonding agents to include bonding of composite, ceramic as well as metallic restorations successfully to tooth structure.

BIBLIOGRAPHY Sturdevant’s Art and Science of Operative Dentistry –

Fifth edition Clinical Operative Dentistry Principles and Practice –

Remya Raghu