1
Q
What are the properties of a dental adhesive? (6 points)
A
- Provide a high bond strength to tooth tissues
- Immediate high strength bond
- Durable bond
- Impermeable bond (not allow fluid to pass through)
- Easy to use
- Safe (for the patient)
2
Q
Is bonding to enamel easy or hard?
A
- Easy due to the structure of enamel
3
Q
What is meant by the heterogenous structure of enamel?
A
- Not having a uniform quality throughout
- Densely packed prismatic
4
Q
Enamel is highly mineralised. What percentage of it is organic?
A
95%
5
Q
IS enamel dry or wet?
A
- Dry - there isn’t any moisture coming out of enamel
6
Q
What is the acid etch technique? (5 points)
A
- Long enamel prisms are filled with imperfectly packed hydroxyapatite crystals
- Acid is applied to the surface of enamel
- The enamel surface is then broken down and preferentially you end up with an etching pattern
- It is a very rough surface on a microscopic level
- Whatever resin you stick to it will impregnate the surface, polymerise, set in there and then you can’t pull it out
7
Q
What does the acid etch pattern look like?
A
- Key holes
8
Q
What does the roughened surface of etched enamel allow the composite to do?
A
- Allows micromechanical interlocking of resin filling materials
9
Q
Etching of enamel increases the surface energy of the by removing surface contaminants. What does this lead to?
A
- Better wettability of the enamel
10
Q
What does good wettability of enamel allow?
A
- The resin to adapt better to the roughened enamel surface
- For this to work the enamel must be dry. Moisture contamination will prevent flow of the resin into the etched enamel
11
Q
What is wettability?
A
- The ability of any solid surface to be wetted when in contact with a liquid; that is, the surface tension of the liquid is reduced so that the liquid spreads over the surface
12
Q
Enamel etching can be done with a variety of organic and inorganic acids but in practice what is normally used?
A
- 30%-50% aqueous solution of phosphoric acid
13
Q
When a flowable resin (usually Bis-GMA) is applied to the etched and dried enamel surface to penetrate into the rough surface. When it is light cured and polymerised what is the bond strength?
A
- Roughly 20MPa
14
Q
What is more difficult to bond to: dentine or enamel?
A
- Dentine
15
Q
What is the composition of dentine? (3 points)
A
- 20% organic matric (mainly collagen)
- 70% inorganic matrix (mostly hydroxyapatite)
- 10% water
16
Q
You can dry dentine, but why will it not stay dry?
A
- It will wet from within, this is because it has dentinal tubules which allow moisture to come to the surface so you have a constant fluid pump up
17
Q
What is dentine full of?
A
- Permeable tubules
18
Q
Why is dentine an inconsistent material? (2 points)
A
- Aged dentine is more mineralised
- Dentine near the pulp has more tubules and increased moisture content
19
Q
Because dentine is wet you get a low surface energy, what does this mean?
A
Dentine is hydrophilic whereas most simple bonding agents are hydrophobic - so they do not want to bond
20
Q
What is formed when you cut the dentine surface and acts as a further complicating factor?
A
- The smear layer
21
Q
What are the requirements of a dentine bonding agent? (4 points)
A
- Ability to flow
- Potential for intimate contact with dentine surface
- Low viscosity
- Adhesion to substrate (mechanical, chemical, Van der waals, or a combination of these)
22
Q
What is mechanical adhesion to dentine?
A
- The same as enamel bonding
- Achieved by the dentine bonding agent and the dentine surface meshing and interlocking with minimun gaps.
23
Q
What is Van der Waals adhesion based on?
A
- Electrostatic or dipole interaction between bonding agent and substrate
24
Q
What does the strength of Van der Waals adhesion depend on?
A
- The CONTACT ANGLE, which is an indication of the WETTABILITY of a solid by a specific liquid. A contact angle of <90 degrees means the solid surface is hydrophilic
25
In relation to Van der Waals forces, when in best adhesion/bonding to dentine achieved?
- When Van der Waals forces are optimised
26
What is critical surface energy?
The surface tension of a liquid that will JUST spread on the surface of a solid
27
Does a liquid have to have a lower or higher surface energy than the surface it is being placed on for it to flow onto it and stick?
- Lower surface energy
- 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
28
Wet dentine has a low surface energy, lower then composite filling materials. For composite resin to stick to dentine you must make the surface if the dentine have a higher critical energy than the composite. Dentine bonding agents increase the surface energy of the dentine surface and allow the composite to flow and stick to the surface. What is the name for these dentine bonding agents?
- Surface wetting agents
29
How does a surface wetting agent work?
- You have a molecule that has a wet end and an oil end, the wet end has a lower surface energy than the substrate, so it will stick to the substrate and the oily bit sticks up at the end and so changes the substrate from having a low surface energy to a higher surface energy by the use of the intermediate molecule
- The dentine bonding agent is just like a surface wetting agent
30
How is adhesion between a material and dentine achieved?
- Through molecular entanglement
31
The adhesive is absorbed onto the surface but can also penetrate into the surface of the dentine. This is due to good wetting of the dentine by the adhesive and appropriate surface energies of the two. The absorbed component can form a long chain polymer. What happens to this polymer?
- it meshes with the substrate - molecular entanglement - leading to high bond strength
32
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
33
How thick is the smear layer?
- 0.5-5 microns in thickness
34
Where is the smear layer attached to?
- Variably attached to the dentine surface
35
What is the smear layer generally contaminated with?
- Bacteria
36
What was originally thought of the smear layer and what is thought now?
- Originally: thought of as a protective barrier reducing permeability if the dentine and protecting the pulp
- Now : considered to interfere with adhesion
37
Why is the smear layer difficult to remove?
- You cannot mechanically scrape it off, you will just produce more smear layer
- If you are going to take it off you need to take it off chemically in some way - we use acid etch to do this
38
What 2 things can you do with 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
39
What were the earliest dentine bonding agents made from and give examples of these? (3 points)
- Mainly phosphorylated esters of unfilled resins
- Phosphorylated Bis-GAMA
- NPG-GMA
- The bonding mechanism was thought to be an ionic bond to the calcium in the dentine by the chlorophosphate group or hydroxyl group
- These were generally applied to an untreated dentine surface
40
What did the earlies t dentine bonding agents bond to?
- The top of the smear layer
41
Why was the bond between earliest dentine bonding agents and the smear layer inherently weak?
- Because the smear layer is not firmly attached to the underlying dentine
- The phosphate to calcium bond was not strong and was affected by moisture
42
What 2 things caused hydrolysis of the bond between the earliest dentine bonding agents and the smear layer, so breaking it down resulting in leakage of the filling?
- Saliva and dentinal fluid
| - This led to low bond strengths and not a great success
43
The materials in the earliest dentine bonding agents are now no longer used. What do todays materials mainly rely on to remove and/or modify the smear layer? (2 points)
- Total etch
| - Self etch
44
What was developed first: self etch or total etch bonding agents ?
- Total etch
45
Total etch bonding agents are the 4th generation of DBA and involve the complete removal of the smear layer. What are 3 examples of these?
- Scotchbond multipurpose
- Clearfil photo bond
- Optibond FL
46
What are the different components of a total etch dentine bonding agent? (3 points)
- Dentine conditioner
- Primer
- Adhesive
47
What is the dentine conditioner found in total etch dentine bonding agents?
- An acid, usually 35% phosphoric
48
What is the primer found in total etch dentine bonding agents?
- Really the adhesive part of the agent with a hydrophilic/hydrophobic molecule
49
What is the adhesive found in total etch dentine bonding agents?
- A resin which penetrates into the surface of the dentine attaching to the primers hydrophobic surface
50
The dentine conditioner in total etch dentine bonding agents is usually phosphoric acid but what else can it be in older systems? (2 points)
- EDTA
| - Nitric acid
51
What does the dental conditioner in total etch dentine bonding agents do? (5 points)
- Removes smear layer
- Opens dentinal tubules by removing smear plugs
- Decalcifies the uppermost layer of the dentine
- The etchant is washed off with water
- The collagen network is the uppermost 10um of the dentine is exposed and subsequently penetrated by the next 2 components
52
What does the primer in total etch dentine bonding agents do and how? (3 points)
- The primer is really the adhesive element in the process. A coupling agent
- It has a bifunctional molecule with a hydrophilic end to bond to the hydrophilic dentine surface and a hydrophobic, methacrylate end to bond to the resin
The molecule must also have a spacer group to make it long enough to be flexible when bonding. Lack of flexibility reduces bonding sites and bond strength
53
The primer in total etch is dissolved in a suitable solvent. What are examples of these? (3 points)
- Ethanol
- Acetone
- Water
54
What coupling agent is commonly found in primers?
- HEMA
| - Hydroxy ethyl methacrylate
55
HEMA is a common primer, what is the full name for this and what products is it found in? (6 points)
- Hydroxyethylmethacrylate
Found in:
- Scotchbond
- Gluma
- Optibond
- Clearfil
- All bond
56
4-META is a common primer, what is the full name for this and what products is it found in? (4 points)
- 4-methacryloxyethyl trimellitate anhydride
Found in:
- I-bond
- Hybrid bond
- G-bond
57
MDP is a common primer, what is the dull name for this and what products can it be found in? (2 points)
- 10-Methacryloyldeacamethylene phosphate
Found in:
- Clearfil
58
What is contained within the adhesive in total etch? (4 point s)
- Mixture of resins: usually Bis-GMA and HEMA
- Predominantly hydrophobic
- May contain some filler particles to make it stronger
- Will contain Camphorquinone to allow it to light cure
59
What does the adhesive in total etch do? (3 points)
- Penetrates the primed dentine which now has a hydrophobic surface (by using wetting agent)
- Forma a micromechanical bond within the tubules and exposed dentinal collagen fibres (MOLECULAR ENTANGLEMENT)
- Forms the hybrid layer of collagen plus resin
60
What are the problems with over etching with total etch? (2 points)
- Leads to collapse of the collagen fibres so no resin can penetrate
- Too deep an etch and the primer cannot penetrate to the full depth of the etch
61
What are the moisture considerations of total etch that can cause problems? (2 points)
Moisture dependent:
- To dry and the dentine surface collapses (collagen fibres collapse and there is nothing to stick to)
- To wet and the primer is diluted -> reduced strength
62
What happens to the dentine composition and total etch if the tooth is over dried? (4 points)
- Collapsed dentine -> decrease in porosity -> poor penetration of dentine by primer -> poor bond
63
What happens to the composition of dentine and the total etch if the tooth is moist? (3 points)
- Expanded dentine surface is porous and will absorb primer -> good bond
64
The number of bonding steps has been reduced by combining components of the bonding system for bonding to dentine. How can they be combined? (2 points)
- Separate etchant with primer and sealer combined
| - Self etchant primer and sealer all combined and applied together as a single solution
65
What are examples of total etch two-stage bonding agents? (5 points)
- Scotchbond one
- Prime and bond
- Optibond solo
- I-pond TE
- XP Bond
66
How do self etching primers work? (3 points)
- The do not attempt to remove the smear layer. They infiltrate it and incorporate themselves into it.
- They are not washed off
- This removes the problem of how dry to make the dentine
67
What is more technique sensitive and bonds more strongly to all tooth tissues: Total etch or self etching primers?
- Total etch
68
In self etch primers, to allow them to etch the dentine they contain acidic methacrylate monomers such as methacryloyloxyalkyl acid phosphatase. How do these work?
- These very complex bifunctional monomers etch and infiltrate the dentine surface with their hydrophilic end while polymerising like a simpler bis-GMA material at their hydrophobic end
69
What do the acidic groups in self etch primers do?
- They react with calcium ions in the tooth bonding to the surface and also create an amorphous calcium chelate on the surface
70
What happens to the smear layer when using a self etch primer?
- The smear layer is dissolved but then incorporated into the hybrid layer
71
Self etch primers can give good bond strengths to dentine, despite not penetrating the surface very deep. How deep does it penetrate the surface?
- Only about 2um into the surface
72
Why is there evidence for reduced sensitivity when using a self etch primer compared to total etch?
- There is no drying stage so no chance of collapse of dentine architecture preventing resin penetration and hybrid layer formation
73
What is molecular entanglement between dentine and primers?
- The fundamental mechanism of all bonding is mineral e xchange where minerals removed from the dental hard tissue are replaced by resin which once mineralized mechanically interlock in these porosities
74
You would think a strong self etch was the better option. What are the negatives of these? (4 points)
- Etching by-products are now washed away as in total etch
- These are soluble and weaken the integrity of the bond
- If too much HA is dissolved away the exposed collagen is vulnerable to breakdown and the bond will fail
- Strong elf etch bonds well to enamel but less well to dentine
75
What is a 'mild' self-etch and what is an example of this ?
- It only partially demineralises the dentine
| - Scotchbond universal is an example of this
76
What is the problem with 'mild' self etchants of the smear layer is deep?
- If smear layer is thick then 'mild' self etch adhesives may not penetrate through it -> so will not gett as good a bond as you think you will
77
What is the challenge with self etchants in relation to acid?
- Want to have enough acid to etch enough to get into the smear layer effectively so that you are bonding below the smear layer but at the same time not have so much acid that you end up with a poor bond as y ou have continued acid and have a weak matrix produced
- Need to be careful that you don't decalcify the tooth surface too much and remove the HA:
- The HA is required for a strong durable bond
- The HA protects the dentine from hydrolytic breakdown
78
What are the good aspects of self etching primers? (2 points)
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
79
What are the drawbacks of self etching primers? (3 points)
- There is greater variability between products with regard to initial pH of the solution
- The difference on pH results in different etch and different penetration of resin
- Those materials with a lower initial pH may not etch enamel effectively
80
Which has a stronger bond to dentine: total etch or self etch?
- Similar bond strength
81
With self etch DBA's what would it be advantageous to do prior to applying it?
- Etch the enamel with phosphoric acid to obtain the best bond
BDS2 - DMS
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