bonding agents Flashcards by Becky Wall

properties of a dental adhesive

high bond strength to tooth tissues
immediate strong bond
durable bond
impermeable bond
easy to use
safe

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why is it easy to bond to enamel?

structure

heterogeneous (prismatic)
highly mineralised (95% inorganic)
‘dry’

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bonding to enamel

acid etch technique
37% H3PO4
micromechanical interlocking of resin filling materials
etching increases surface area and removes contaminants
then low viscosity resin
mechanical bonding

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requirements of a DBA

ability to flow
potential for intimate contact with dentine surface
low viscosity
adhesion to substrate
 - mechanical
 - chemical
 - VdW
 - or combination

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smear layer

adherent layer of organic debris on dentine surface after preparation of dentine
variably attached to dentine surface
usually contaminated with bacteria
interferes with adhesion

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smear layer thickness

0.5-5 microns thick

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options for smear layer

1 - remove it

2 - incorporate it - penetrate and infiltrate it with bonding agent and stick it to dentine below

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difficulties of bonding to dentine

composition - 20% organic, 70% inorganic, 10% H2O
structure
- permeable tubules
- wet (fluid up from pulp)
- inconsistent
- aged more mineralised
- nearer pulp more tubules and more moisture
- low SE
- hydrophilic but most simple BAs hydrophobic
- not homogeneous
- smear layer

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mechanical chemistry of bonding to dentine

DBA and D meshing and interlocking

etching not as important

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chemical chemistry of bonding to dentine

chemical adhesion

mineralised - ionic
organic - covalent

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how do DBAs work on dentine?

wet dentine low SE
DBA increases surface energy of dentine to allow composite to flow and stick
surface wetting agents

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molecular entanglement

adhesive absorbed and penetrates

absorbed component can form a long chain polymer - polymer meshes with substrate - molecular entanglement

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VdW adhesion

based on electrostatic/dipole interaction between bonding agent and substrate
contact angle <90 degrees - solid surface is hydrophillic
- indicates wettability

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surface energy

intermolecular forces at the interface between 2 surfaces

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wettability

ability of a liquid to maintain contact with a solid surface

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contact angle

angle where a liquid meets a solid surface - measured through the liquid

critical surface energy

surface tension of a liquid that will just spread on the surface of a solid
a liquid needs a lower SE than surface it is being placed on for it to flow onto it and stick
- because this leads to a lower SE of material as a whole

low SE e.g. waxy - things won’t stick to it

early DBAs

mostly phosphorylated esters of unfilled resins
applied to an untreated surface
but bond was only to top of smear layer - inherently weak as smear layer not firmly attached
phosphate-Ca bond not strong - affected by moisture

hybrid layer

collagen and resin

sequence of bonding to dentine

etch - demineralise outer layer. remaining collagen hydrophillic and low SE. normal resin won’t bond to it

primer can penetrate demineralised dentine - bifunctional molecules. Gives surface high energy - hydrophobic

adhesive can penetrate - micro mechanical retention

total etch DBAs

gold standard
4th gen
completely remove smear layer
1 - dentine conditioner
2 - primer
3 - adhesive

total etch DBAs - dentine conditioner

35% phosphoric acid (EDTA/nitric)
removes smear layer
opens dentinal tubules
decalcifies upper layer
"loofah"

total etch DBAs - primer

adhesive element - coupling agent

bifunctional - hydrophilic end bonds to D and hydrophobic methacrylate end to bond to resin

spacer group so long enough for flexibility when bonding

dissolved in solvent e.g. ethanol, acetone, H2O

e.g. HEMA

total etch DBAs - adhesive

mix of resins, filler particles, camphorquinone
hydrophobic
molecular entanglement - forms micro mechanical bond within tubules and exposed dentinal collagen fibres

HEMA

coupling agents in lots of primers | hydroxyl group can combine with similar polar groups on HA and can react with amine groups on collagen protein

problems with total etch

overetching | moisture dependent

problems with total etch - overetching

collagen fibres collapse so resin can't penetrate | if etch is too deep primer can't penetrate to full depth of etch

problems with total etch - moisture dependent

``` too dry - collapsed dentine - decrease in porosity - poor penetration by primer too wet - primer diluted - reduced strength ```

self-etching primers

don't remove smear layer - infiltrate it and incorporate themselves into it not washed off not as technique sensitive but bond doesn't seem as strong to all tooth tissues one bottle evidence they can etch dentine and form hybrid layer no drying so no chance of collapse of D

self-etching primers chemistry

contain acidic methacrylate monomers - bifunctional - etch and infiltrate D with their hydrophilic end while polymerising like a simpler Bis-GMA at hydrophobic end acidic groups react with Ca2+ in tooth bonding to surface and also create an amorphous calcium chelate on the surface smear layer dissolved but then incorporated into the hybrid layer only penetrates approx 2um but can give good bond to D

separate etchant with primer and sealer combined

``` reduces clinical time comparable bond strength still total etch what usually used 1 - etch - phosphoric acid 2 - primer and sealer in single bottle - allow wetting and sealing of dentine simultaneously - HEMA, resin, solvent, camphorquinone ```

mineral exchange

minerals removed from hard tissue are replaced by resin - once mineralised mechanically interlock in porosities molecular entanglement

adhesion-decalcification concept

interaction of the molecules with hydroxyapatite based tissue initially all acid monomers bond to Ca in HA ionically - stability of HA-monomer bond determines whether they stay bonded monomers with lower pKa (not necessarily pH) don't form a stable bond - continue to dissolve HA = hybrid layer with unstable co-phosphates incorporated

mild self-etch

only partially demineralises dentine HA crystals remain around collagen - protect against hydrolytic breakdown - remaining Ca ions allow ionic bonding

problems with smear layer and bonding

if thick - 'mild' self-etch adhesives may not penetrate through it want to penetrate smear layer without decalcifying tooth surface too much and removing HA - need for strong durable bonding - protects dentine from hydrolytic breakdown

pros of self-etch

less technique sensitive simultaneous demineralisation and resin infiltration - less chance of post-op sensitivity with self-etch - good to etch E with H3PO4 to obtain best bond waiting for more long-term clinical evaluations

cons of self-etch

etching by-products aren't washed away - soluble - weaken bond integrity if too much HA dissolved away - exposed collagen vulnerable to breakdown and bond will fail strong self-etch bonds well to E but not as well to D lots of variation between products of initial pH of solution materials with lower initial pH may not etch E efficiently little evidence of stronger bond to dentine than with total etch