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Principles of bonding and adhesives in dentistry

Dental materials

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What is adhesion?

The force that binds two dissimilar materials together when they are brought into intimate contact

In dentistry, bonding refers to the process of attaching a restorative material to tooth structure by adhesion

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Basic principles in the bonding process

Surface preparation to remove plaque & debris Acid etching with phosphoric acid, to remove

mineral, create porosity, wettability Bonding agent applied and flows to fill the

porosities and create resin tags (micromechanical retention)

Resin applied and bonds chemically to underlying bonding agent (primary bonding)

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Adhesion For proper adhesion to occur, intimate

contact between the adhesive and the substrate is needed. This intimate contact is affected by:

Wettability of the substrate surface The viscosity of adhesive The morphology or surface roughness,

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Factors affecting adhesion

1. Wettability and surface energy

High surface energy low surface energy (solid)

Surface energy: the attraction of atoms to a surface (directed inward). In liquids, it is called surface tension

liquidθ

solid solid

liquidθ

Low surface tension

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2. Viscosity of bonding agent

3. Interpenetration (formation of hybrid zone)

4. Micromechanical interlocking

5. Chemical bonding

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Enamel etching

Introduced by Michael Buonocore in 1950s Etching time: 10-30 seconds (around 15

seconds) Primary teeth and fluoride treated teeth

require more time Etched enamel looks frosty white when dried Etching produces a rough surface (pits) into

which resin flows and forms resin tags = micromechanical retention

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Enamel etching

Resin tags may penetrate to a depth of 10-20 microns in etched enamel

The depth of penetration depends on: Etching time Rinsing time

These two actors determine how effective etching was, and how well debris were removed from enamel surface

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Enamel etching

Liquid or gel (the gel is made by adding colloidal silica to the acid) phosphoric acid 30-50% (usually 37%).

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Procedure

Acid etch is applied, how ? Etchant is applied for 15 seconds, or longer

as mentioned previously Rinsing for 20 seconds then drying.

Appearance of enamel? Enamel should be kept clean and

contaminant free (saliva, blood, etc) If contamination occurs? Re-etch.

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Enamel bonding

In the past, etching and bonding involved only enamel. Currently, total etch technique is done, and bonding agents are applied to both enamel and dentine.

Bonding agents used for enamel bonding were made from resin combined with diluents to lower viscosity. (Bis-GMA + TEGDMA)

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Dentine etching and bonding

What makes dentine a challenge when it comes to adhesive bonding: Dentine is a living tissue (50% HA, 30% collagen,

20% fluid) Tubular nature of dentine (dentinal fluid) Branching patterns in tubules, may enhance

retention Smear layer presence Possible side effects on the pulp

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Dentine etching 1979 etching was done for dentine as well as

enamel using 37% phosphoric acid. Research proved enhanced bonding

Over etching, effects on dentine structure and pulp? Over etching dentine leads to weaker bond and

sensitivity Over drying should be avoided to prevent collapse

of collagen and occluding tubules

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Another study showed how resin tags from bonding agents in dentine infiltrated a surface layer of collagen in demineralized dentine to form the HYBRID LAYER

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Bonding agents

Several years ago, it was believed that bonding to dentine can be achieved by chemical bonding between resin and either collagen or mineral content of dentine. Molecules designed for these purposes had the following presentation: M-R-X: M is a methacrylate group, R is a spacer such as hydrocarbon chain (ensure mobility of M group when X is immobilized), an X is a functional group that can bond to calcium in HA (usually an acidic group)

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Generations of bonding agents First generation (1950s): based on silane

coupling agents model. Based on M-R-X model: M=methacrylate group R= hydrocarbon group X= glycerolphosphoric acid dimethacrylate

Success rate was low, due to high polymerization shrinkage and high CTE in unfilled resins used in those time

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Bonding agents

Second generation ( late 60s early 70s): similar concept to first generation agents. Low success rate. Attempts were made to deal with the smear layer

Third generation agents: same as the previous generation, however attempts were made to modify or remove the smear layer which consists of:

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Bonding agents

Smear layer: it is weakly bonded to dentine Dentine particles Bacteria Salivary constituents.

Procedure in 3rd generation agents: Application of dentine conditioner (HEMA, or 2% nitric acid,

or maleic acid) Application of primer (dentine bonding agent based on M-

R-X) Application of adhesive (unfilled resin) Placement of resin composite

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Bonding agents

Fourth generation: procedure, Total etch technique for enamel and dentine, dentine

conditioned for 15 seconds. Rinse and dry but do not over dry to prevent collapse of

collagen fibers Slightly moisten dentine Absorb excess water with cotton Apply hydrophilic primer (contains resin that polymerizes

within collagen and a solvent that evaporates to ensure drying of tooth surface).

Apply adhesive (bonding resin) then cure Composite applied and cured

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Bonding agents

Fifth generation agents: fewer steps, better results. Rely on micromechanical retention involving: Penetration into partially opened dentinal tubules Formation of hybrid layer (hydrophilic monomer

penetrate and polymerize to form interpenetrating network with collagen fibrils

Chemical interactions involving 1st and 2nd order bonds

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Self-etching primers Acidic groups are added to etch tooth surface No need for rinsing and drying May not be effective on unprepared enamel

Self priming adhesive: most commonly used now

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5th generation

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Sixth generation systems (all-in-one)

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Microleakage

Occurs when the restoration does not completely seal the surrounding margins of the cavity preparation

Possible outcomes of microleakage?

What contributes to microleakage?

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Factors that prevent good bonding

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Measurements of bond strength Tests used:

Shear bond strength Tensile bond strength

Data were variable due to variability of tooth surface, and different testing methods Microtensile and microshear bond strength: less

variability. Current bonding agents shifted the bonding

failure from cohesive to adhesive

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Amalgam bonding

Older amalgam restorations leak less due to corrosion products

Technique: Cavity preparation then isolation Etching of enamel and dentine to remove smear

layer Primer applied and cured Self-cure or dual cure bonding resin applied then

amalgam is applied

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Clinical applications of bonding Porcelain bonding and repair involves:

Sandblasting Special etchant (hydrofluoric acid) Silane applied for 30 seconds then dried to

evaporate solvent (leaving a layer of vinyl that bonds resin to adhesive)

Bonding agent applied Composite applied

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Metal bonding: PFM Resin bonded bridges (Maryland)

Lab. And clinical techniques for bonding: Sandblasting for micromechanical retention Electrochemical etching or placing a layer of tin by

electroplater Surface cleaned and dried, then coated with

bonding resin and cemented

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Metal bonding continue,

For repair of fractured porcelain on a PFM or bridge: Porcelain and metal are prepared as described

previously Bonding resin applied and cured for 20 seconds An opaque masking resin applied, cured 20

seconds Proper shaded composite applied and cured 20

seconds

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Pit and fissure sealants

Filled and unfilled resins GIC Success depends on good wetting, intimate

contact through etching which will also ensure longevity of the sealant.

PRR: minimal cavity preparation, resin composite placement, sealant placement on top.

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Thank you

Reference,

1. Philips science of dental materials,

Chapter 14

2. Dental materials, clinical application for dental assistants and dental hygienists,

Chapter 5