bonding agents

Question 1

properties of a dental adhesive

Answer 1

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

Question 2

why is it easy to bond to enamel?

Answer 2

structure

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

Question 3

bonding to enamel

Answer 3

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

Question 4

requirements of a DBA

Answer 4

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

Question 5

smear layer

Answer 5

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

Question 6

smear layer thickness

Answer 6

0.5-5 microns thick

Question 7

options for smear layer

Answer 7

1 - remove it

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

Question 8

difficulties of bonding to dentine

Answer 8

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

Question 9

mechanical chemistry of bonding to dentine

Answer 9

DBA and D meshing and interlocking

etching not as important

Question 10

chemical chemistry of bonding to dentine

Answer 10

chemical adhesion

• mineralised - ionic
• organic - covalent

Question 11

how do DBAs work on dentine?

Answer 11

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

Question 12

molecular entanglement

Answer 12

adhesive absorbed and penetrates

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

Question 13

VdW adhesion

Answer 13

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

Question 14

surface energy

Answer 14

intermolecular forces at the interface between 2 surfaces

Question 15

wettability

Answer 15

ability of a liquid to maintain contact with a solid surface

Question 16

contact angle

Answer 16

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

Question 17

critical surface energy

Answer 17

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

Question 18

early DBAs

Answer 18

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

Question 19

hybrid layer

Answer 19

collagen and resin

Question 20

sequence of bonding to dentine

Answer 20

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

Question 21

total etch DBAs

Answer 21

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

Question 22

total etch DBAs - dentine conditioner

Answer 22

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

Question 23

total etch DBAs - primer

Answer 23

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

Question 24

total etch DBAs - adhesive

Answer 24

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

Question 25

HEMA

Answer 25

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

Question 26

problems with total etch

Answer 26

overetching

moisture dependent

Question 27

problems with total etch - overetching

Answer 27

collagen fibres collapse so resin can’t penetrate

if etch is too deep primer can’t penetrate to full depth of etch

Question 28

problems with total etch - moisture dependent

Answer 28

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

Question 29

self-etching primers

Answer 29

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

Question 30

self-etching primers chemistry

Answer 30

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

Question 31

separate etchant with primer and sealer combined

Answer 31

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

Question 32

mineral exchange

Answer 32

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

Question 33

adhesion-decalcification concept

Answer 33

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

Question 34

mild self-etch

Answer 34

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

Question 35

problems with smear layer and bonding

Answer 35

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

Question 36

pros of self-etch

Answer 36

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

Question 37

cons of self-etch

Answer 37

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