Principles of bonding and adhesives in dentistry Dental

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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

• 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

Low surface tension
liquid
liquid
?
?
solid
solid
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• Viscosity of bonding agent
• Interpenetration (formation of hybrid zone)
• Micromechanical interlocking
• 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
• Mmethacrylate 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,
• Philips science of dental materials,
• Chapter 14
• Dental materials, clinical application for dental
  assistants and dental hygienists,
• Chapter 5