bonding agents Flashcards
properties of a dental adhesive
high bond strength to tooth tissues immediate strong bond durable bond impermeable bond easy to use safe
why is it easy to bond to enamel?
structure
- heterogeneous (prismatic)
- highly mineralised (95% inorganic)
- ‘dry’
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
requirements of a DBA
ability to flow potential for intimate contact with dentine surface low viscosity adhesion to substrate - mechanical - chemical - VdW - or combination
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
smear layer thickness
0.5-5 microns thick
options for smear layer
1 - remove it
2 - incorporate it - penetrate and infiltrate it with bonding agent and stick it to dentine below
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
mechanical chemistry of bonding to dentine
DBA and D meshing and interlocking
etching not as important
chemical chemistry of bonding to dentine
chemical adhesion
- mineralised - ionic
- organic - covalent
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
molecular entanglement
adhesive absorbed and penetrates
absorbed component can form a long chain polymer - polymer meshes with substrate - molecular entanglement
VdW adhesion
based on electrostatic/dipole interaction between bonding agent and substrate
contact angle <90 degrees - solid surface is hydrophillic
- indicates wettability
surface energy
intermolecular forces at the interface between 2 surfaces
wettability
ability of a liquid to maintain contact with a solid surface
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, camphorquinonemineral 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
