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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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Continue,
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