1
Q
6 properties of dental adhesive
A
- Provide a high bond strength to tooth tissues
- Immediate high strength bond
- Durable bond – lasts forever
- Impermeable bond
- Easy to use
- Safe – no harm to patient
2
Q
3 reasons why enamel bonding is easy
A
- Heterogeneous structure: densely packed prismatic
- Highly mineralised: 95% inorganic
- ‘Dry’ no moisture
3
Q
describe the mechanisms of ‘acid-etch’ technique
A
- Long enamel prisms are filled with imperfectly packed hydroxyapatite crystals.
- This surface can be modified by application of acid
- The acid roughens the surface of the enamel producing a characteristic etched pattern.
Surface is broken down
Etching pattern like keyholes
Rough surface on microscopic level
4
Q
what does the roughened surface of enamel allow?
A
the micromechanical interlocking of resin filling materials
and increases surface energy of enamel
5
Q
what does higher surface energy of etched enamel allow?
A
better wettability
resin to adapt better to the enamel
6
Q
what will prevent the flow of resin into etched enamel?
A
moisture contamination
7
Q
what type of bonding is enamel bonding/
A
essentially mechanical
- micromechanical adhesion
- polymerises (goes hard)
8
Q
what acid is commonly used in practice for enamel etching?
A
30-50% phosphoric acid
30-35% common
9
Q
what is applied to etched and dried enamel?
A
dentine bonding agent
10
Q
name a dentine bonding agent
A
low viscosity Bis-GMA resin
phosphorylated Bis-GMA
11
Q
what does a dentine bonding agent do?
A
penetrates into the rough surface and is light cured
12
Q
what is the breakdown of dentine composition?
A
20% organic (mostly collagen)
70% inorganic (most HA)
10% water
13
Q
is dentine wetter than enamel?
A
yes
fluid pumps up from pulp to dentine floor of any cavity - surface wet
14
Q
what is the surface energy of dentine?
A
low
due to wettness
15
Q
is dentine hydrophillic or hydrophobic?
A
hydrophillic
whereas most simple bonding agents are hydrophobic
16
Q
why is dentine hard to bond to?
A
its physical and chemical qualities
17
Q
what are some of the qualities that make dentine hard to bond to?
A
- full of permeable tubules
- wetter/ lower surface energy than enamel
- inconsistent material
- smear layer
18
Q
describe how dentine is inconsistent?
A
- aged dentine is more mineralised
- near the pulp has more tubules and increased moisture content.
- Different sorts of dentine -Secondary and tertiary is harder to bond to
19
Q
what are 4 qualities required of a dentine bonding agent?
A
- ability to flow
- potential for intimate contact with dentine surface
- low viscosity
- adhesion to substrate
20
Q
how is dentine bonding achieved?
A
dentine bonding agent and the dentine surface meshing and interlocking with minimum gaps
- easy to see microscopically and on SEM
- stick to holes in the spaces around the lager holes
21
Q
what type of bond does mineralised component of dentine form?
A
ionic
22
Q
what type of bond does the organic component of dentine form?
A
covalent
23
Q
what is Van der Waals adhesion based on/
A
electrostatic or dipole interaction between bonding agent and substrate
24
Q
what do Van der Waal strength of interaction depend on?
A
contact angle
- contact angle of less than 90 degree means the solid surface is hydrophilic - want lowest angle possible
25
when is the best dentine bonding/adhesion achieved?
when Van der Waal forces are optimised
Availability to flow onto surface is dependent on how polar material is compared to substrate
26
what is critical surface energy?
the surface tension of a liquid will just spread on the surface of a solid
- A liquid must have a lower surface energy than the surface it is being placed on for it to flow onto it and stick.
27
what type of surface energy is required for something to spread?
lower
A low surface energy liquid will spread on a higher surface energy substrate because this leads to a lower surface energy of the material as a whole.
- Flows onto object, lowering surface energy as a whole
28
what do dentine bonding agents do?
increase the surface energy of the dentine
allows composite to flow and stick to dentine
they are SURFACE WETTING AGENTS
o Wet end and oily end
o Wet end sticks to substrate oily end sticks out higher surface energy
o Intermediate chemical
29
what is molecular entanglement adhesion?
- The absorbed component can form a long chain polymer
- This polymer meshes with the substrate - molecular entanglement
- leading to high bond strength
This is due to good wetting of the dentine by the adhesive and appropriate surface energies of the two.
30
what is the smear layer?
an adherent layer of organic debris that remains on the dentine surface after the preparation of the dentine during the restoration of a tooth.
- Cut surface of dentine and what’s left over
(Cut with high speed bur)
31
qualities of the smear layer
- It is 0.5 – 5 microns in thickness.
- Variably attached to the dentine surface.
- Generally contaminated with bacteria.
- Originally it was thought of as a protective barrier reducing permeability of the dentine and protecting the pulp.Now it is considered to interfere with adhesion.
Blockage to good adhesion
Newer adhesion materials bond to it
- Difficult to remove – need to use chemicals
32
2 bonding solutions to the smear layer
- Remove it and bond to the ‘clean’ dentine beneath
| - Incorporate it by penetrating it, infiltrating it with the bonding agent and stick it to the dentine below
33
what do modern dentine bonding agents rely on?
total etch
self etch
to remove and/or modify the smear layer
34
examples of dentine bonding agents that use total/self etch
- Scotchbond multipurpose (first one)
- Clearfil photo bond
- Optibond FL
35
3 components of a total etch dentine bonding agent
Dentine conditioner:
- An acid, usually 35% phosphoric.
Primer:
- Really the adhesive part of the agent with a hydrophilic/hydrophobic molecule
- C=C end and hydrophilic end
Adhesive:
- A resin which penetrates into the surface of the dentine attaching to the primers hydrophobic surface.
- Mildly phosphorylated resin
36
what does the dentine conditioner in a dentine bonding agent do?
Removes smear layer
- Opens dentinal tubules by removing smear plugs
Decalcifies the uppermost layer of the dentine
etchant is washed off with water.
The collagen network in this top 10um of the dentine is exposed and subsequently penetrated by the next two components.
- Extensive new surface area
37
what does the primer component of dentine bonding agent do?
really the adhesive element in the process.
A coupling agent.
- Bifunctional molecule: a hydrophilic end to bond to the hydrophilic dentine surface and a hyrdophobic, methacrylate end to bond to the resin.
Molecule must also have a spacer group
- Make it long enough to be flexible when bonding.
- Lack of flexibility reduces bonding sites and bond strength.
Molecule or group of molecules is dissolved in a suitable solvent.
- Ethanol, acetone or water.
38
what is a common coupling agent found in many primers of dentine bonding agents?
HEMA
| - Hydroxy ethyl methacrylate
39
name some chemicals used as dentinal primers in dentine bonding agents
HEMA
- Hydroxy ethyl methacrylate
NTG-GMA
4- META
MDP
GPDM
40
what is the adhesive component of dentine bonding agents?
Mixture of resins.
- Usually Bis-GMA and HEMA.
Predominately hydrophobic.
- May contain some filler particles to make it stronger.
Will contain Camphorquinone to allow it to light cure.
41
what does the adhesive component of dentine bonding agents do?
Penetrates the primed dentine which now has a hydrophobic surface.
- Forms a micromechanical bond within the tubules and exposed dentinal collagen fibres.
MOLECULAR ENTANGLEMENT
Forms the HYBRID LAYER of collagen plus resin.
- Collagen fibrils, hydroxyapatite and resin meshed in one layer
42
what is the hybrid layer of collagen and resin?
Collagen fibrils, hydroxyapatite and resin meshed in one layer
formed by the micro-mechanical bond within dentinal tubules and exposed dentinal collagen fibres
- molecular entanglement due to adhesive
43
3 main steps in bonding to dentine
Etching Dentine --> demineralisation of the outer layer.
- The remaining collagen is hydrophilic and has low surface energy.
- Normal resin will not bond to it.
Primer can penetrate the demineralised dentine due to its
bifunctional molecules. This gives the surface high energy,
making it hydrophobic
- Single molecule fit
Adhesive can now penetrate--> micro mechanical retention
- Resin tags into tubules
(impregnate)
44
2 problems with total etch
- over etching
| - moisture dependent
45
what will happen if you over etch dentine?
collapse of the collagen fibres so no resin can penetrate – can’t stick, decreased porosity (not loofa like)
- too deep an etch and the primer cannot penetrate to the full depth of the etch. – primer doesn’t go deep enough – sensitivity
46
what will happen if dentine is too wet?
primer is diluted so reduced bond strength
47
what will happen if dentine is too dry?
dentine surface will collapse - nothing to stick to
| dull, dry
48
what are the 2 condensed versions of dentine bonding?
- Separate etchant with primer and sealer combined. (2 separate)
- Self-etching primer and sealer all combined and applied together as a single solution. (all combined)
49
what is the separate etchant with primer and sealer combined dentine bonding technique?
Total Etchant is still Phosphoric Acid.
Primer and sealer are combined in a single bottle and applied to the etched, washed dentine and light cured.
- Should allow the wetting and sealing of the dentine to occur simultaneously.
The chemicals in the single bottle are the same as in the primer bottles (e.g. HEMA, GPDM, MDP, 4-META) but are combined with a resin Bis-GMA and a solvent such as alcohol or acetone
- and camphorquinone to allow light curing.
50
what is the most commonly used dentine bonding technique?
separate etchant with primer and sealer combined
51
how does self-etching dentine boning work?
Do not attempt to remove the smear layer. (no phosphoric acid)
- infiltrate it and incorporated themselves into it. (phosphoric monomers?)
They are not washed off.
- This removes the problem of how dry to make the dentine.
- No risk of over or under drying
Not as technique sensitive but bond doesn’t seem to be as strong to all tooth tissues.
52
what are the molecules in self-etch like?
contain acidic methacrylate monomers such as Methacryloyloxyalkyl acid phosphatase.
- Acidic monomer
These very complex Bifunctional monomers etch and infiltrate the dentine surface with their hydrophilic end while polymerising like a simpler bis-GMA material at the hydrophobic end.
- Complicated long chain molecules, do everything at once
The acidic groups react with Ca2+ ions in the tooth bonding to the surface and also create an amorphous Calcium chelate on the surface.
53
what happens to the smear layer in self-etching dentine bonding?
smear layer is dissolved but then incorporated into the hybrid layer
- only penetrates about 2 um into the surface but can give good bond strengths to dentine.
54
what is an advantage of self-etching dentine bonding
evidence for reduced sensitivity.
- No drying stage so no chance of collapse of dentine architecture preventing resin penetration and hybrid layer formation
55
what us the fundamental mechanism of all bonding?
mineral exchange
minerals removed from the dental hard tissue are replaced by resin which once mineralized mechanically interlock in these porosities.
- Molecular Entanglement
o Take mineral out of HA
o Matrix with them and resin
56
what is the interaction if these molecules with HA based tissue described as?
Adhesion-Decalcification concept ‘AD-concept’
57
what happens in the adhesion- decalcification concept?
Initially all acid monomers bond to the calcium in HA ionically.
Whether they stay bonded depends on the stability of the HA-monomer bond.
- Monomers with lower pKa (not necessarily pH) do not form a stable bond as they continue to dissolve HA
leads to a hybrid layer with unstable calcium phosphates incorporated
58
what are the problems with strong self-etch to dentine?
Etching by-products are not washed away as in total etch
- These are soluble and weaken the integrity of the bond
- Acid stays there as is not washed away, causes weakening of bone – never want an extreme environment
stability
If too much HA is dissolved away the exposed collagen is vulnerable to breakdown and the bond will fail
- Hydrolytic enzymes in mouth dissolve bond between the dentine and resin
59
what does strong self-etch bond better too?
bonds better to enamel than dentine
60
what does mild self-etch do to dentine?
only partially demineralises the dentine (e.g. Scotchbond Universal)
- HA crystals remain around the collagen
- Protective against hydrolytic breakdown
- Remaining Ca ions allow more ionic bonding
61
which chemicals are better for dentine bonds?
MDP
4-META
better than HEMA (more acidic and adsorb more water --> less durable bond)
62
2 advantages of self-etching bonding agents
Less technique sensitive
- No rinsing
- No excessive drying
- No dentine collapse leading to low bond strength
Simultaneous demineralisation and resin infiltration
- Less chance of post-op sensitivity
63
what form of etching is best for self-etching DBAs?
selective etching of enamel
| good bond to dentine already
64
what is a problem with the smear layer?
If thick then ‘Mild’ self-etch adhesives may not penetrate through it.
- Not penetrated enough
3-4 microns into surface etching
5 microns smear layer
65
what is HA role in bonding?
- required for strong durable bonding
| - protects the dentine from hydrolytic breakdown
BDS2 Dental Materials Sciences
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