BDS2 > Bonding > Flashcards

Bonding Flashcards

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1
Q

What properties are required of a dental adhesive?

A

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

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2
Q

Why is enamel bonding relatively easy?

A

due to the structure of enamel

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3
Q

What features of the enamel structure makes it easy to bond to?

A

heterogenous structure (densely packed prisms), highly mineralised (95% inorganic - hydroxyapatite crystals), dry

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4
Q

What technique is used to bond to enamel?

A

acid-etch technique

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5
Q

How does the acid-etch technique allow bonding to enamel?

A

acid roughens the enamel surface which allows micro-mechanical interlocking of resin filling materials. Etching also increases surface energy of enamel which improves wettability

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6
Q

How does etching increase the surface energy of enamel?

A

etching removes surface contaminants which improves wettability of enamel

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7
Q

How does increased wettability of enamel improve its bonding?

A

better wettability allows the resin to adapt better to the roughened enamel surface

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8
Q

Why is composite considered technique sensitive?

A

following acid-etching, the enamel must be dry to allow the flow of resin into the etched surface

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9
Q

Is enamel bonding essentially chemical or mechanical?

A

mechanical - micro-mechanical interlocking of resin filling materials to roughened surface

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10
Q

What chemical is normally used for enamel etching?

A

35% (30-50%) phosphoric acid solution (can be done with a variety of organic/inorganic acids)

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11
Q

What material is applied to the enamel following acid-etching?

A

low viscosity BIS-GMA resin / dentine bonding agent resin - can penetrate into roughened surface and is light cured

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12
Q

What material is applied on top of the low viscosity BIS-GMA resin?

A

higher viscosity composite resin filling material

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13
Q

Outline bonding to enamel

A
  • etched with strong acid
  • dried surface
  • resin flows into roughness
  • polymerised with light cure which leads to a strong bond (>20MPa)
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14
Q

What is the strength of composite bond to enamel?

A

> 20MPa

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15
Q

What is the composition of dentine?

A

70% inorganic (mostly HA), 20% organic (mostly collagen), 10% water

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16
Q

Why is bonding to dentine complicated?

A
  • full of permeable tubules leading to pulp
  • fluid pumps from pulp to dentine floor making surface wet
  • dentine is an inconsistence material
  • has a low surface energy
  • dentine is hydrophilic
  • smear layer
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17
Q

Why is dentine an inconsistent material?

A

aged dentine is more mineralised, dentine near the pulp has more/wider tubules and increased moisture content

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18
Q

Why is it a problem that dentine is hydrophilic (attracts water)?

A

most bonding agents are hydrophobic

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19
Q

What are the requirements of a dentine bonding agent (DBA)?

A

low viscosity (so it flows and has an intimate contact with dentine surface), adhesion to substrate

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20
Q

What methods of adhesion are involved when bonding to dentine?

A
  1. mechanical
  2. chemical
  3. van der waals
  4. molecular entanglement
  5. combination of all above
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21
Q

How is mechanical bonding to dentine achieved?

A

interlocking of dentine bonding agent and the etched dentine surface (same as enamel bonding) - can be seen on SEM

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22
Q

What is chemical adhesion?

A

bonding at a molecular level

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23
Q

How is chemical adhesion achieved when bonding to dentine?

A

mineralised dentine components form ionic bonds, organic components form covalent bonds

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24
Q

Which dentine constituents form ionic bonds?

A

mineralised dentine constituents

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25
Which dentine constituents form covalent bonds?
organic dentine constituents
26
What is van der waals adhesion?
intermolecular forces between bonding agent and substrate based on electrostatic or dipole interactions
27
What does the strength of van der waals interaction depend on?
contact angle
28
How can wettability of a solid by a specific liquid be determined?
by the contact angle between the liquid and solid
29
What contact angle indicates that the solid surface is hydrophilic?
contact angle <90 degrees (liquid spreads on solid)
30
What is the critical surface energy?
how well a liquid will spread on a surface (the surface tension of a liquid that will just spread on the surface of a solid)
31
How do the surface energies compare if a liquid flows onto and sticks to the surface it is placed on?
liquid has a lower surface energy than the surface for it to flow onto it and stick (low surface energy liquid will spread on a higher surface energy substrate as this leads to a lower surface energy of the material as a whole
32
Example: what are the relative surface energies of rain drops on leaves?
rain drops have a higher surface energy than leaves - therefore drops do not spread or stick
33
What is the surface energy of a PTEE pan?
very low surface energy (nothing sticks to it)
34
What is the surface energy of wet dentine?
low surface energy - lower than composite filling materials
35
How do you make composite resin stick to dentine if dentine has a lower surface energy?
apply dentine bonding agents (DBA) to increase the surface energy of dentine so that it is higher than the critical surface energy of composite. This allows composite to flow and stick to the surface
36
What are dentine bonding agents (DBA) that increase the surface energy of dentine referred to as?
surface wetting agents (improve wettability so resin adapts better to surface)
37
How does adhesion through molecular entanglement occur when bonding to dentine?
adhesive/DBA is absorbed onto the surface and can penetrate into the dentine due to good wetting of dentine and appropriate surface energies. The absorbed component can form a long chain polymer which meshes with the substrate (collagen in dentine) - molecular entanglement - high bond strength
38
What is the smear layer?
adherent layer of organic debris that remains on the dentine surface after the preparation of dentine during the restoration of a tooth
39
How thick is the smear layer?
0.5-5 microns thick
40
Why is the smear layer a problem?
blocks the dentinal tubules, hence interfering with adhesion
41
What are the 2 options of addressing the smear layer?
1. remove it and bond to 'clean' dentine beneath 2. incorporate it by penetrating, infiltrating and sticking it to dentine below using the bonding agent
42
How many generations of dentine bonding agents are there currently?
8 generations
43
What were the earliest dentine bonding agents?
mainly phosphorylated esters of unfilled resins
44
Examples of phosphorylated esters of unfilled resins that were the earliest of dentine bonding agents
1. phosphorylated BIS-GMA (Scotchbond) 2. NPG-GMA (Crevident)
45
What was the bonding mechanism of the earliest bonding agents (phosphorylated esters of unfilled resins)?
ionic bond to calcium in dentine by the chlorophosphate / hydroxyl group
46
How did the earliest dentine bonding agents (e.g. phosphylates BIS-GMA / NPG-GMA) address the smear layer?
earliest DBA were applied to an untreated dentine surface (did not address smear layer)
47
How successful where the earliest dentine bonding agents (phosphorylated esters of unfilled resins e.g. phosphorylated BIS-GMA, NPG-GMA)?
not very successful - low bond strength (no longer used)
48
What factors resulted in the low bond strength / unsuccess of the earliest dentine bonding agents?
- bond was only to the top of the smear layer (weak - smear layer not firmly attached to underlying dentine) - phosphate to calcium bond was weak and affected by moisture - saliva / dentinal fluid hydrolysed the bond resulting in leaking of filling
49
Which dentine bonding agents were developed first - total etch or self etch?
total etch bonding agents were developed first (4th generation of dental bonding agents (DBAs))
50
What 2 systems are used to bond to dentine nowadays?
total etch or self etch (remove and/or modify smear layer)
51
How do total etch bonding agents address the smear layer?
complete removal of smear layer
52
What is the gold standard dentine bonding agent?
total etch
53
Examples of total etch DBAs
Scotchbond multipurpose, Clearfil photo bond, Optibond FL
54
What are the components of a total etch dentine bonding agent?
1. dentine conditioner (etchant) 2. primer 3. adhesive
55
What is the dentine conditioner (etchant)?
an acid, usually 35% phosphoric acid
56
What is the function of the dentine conditioner (etchant)?
- removes smear layer - opens dentinal tubules - decalcified uppermost layer of dentine - which exposes the collagen network in the top 10um of dentine (to be penetrated by primer and adhesive)
57
How is dentine conditioner (etchant) applied and removed?
applied onto enamel 20s and dentine for 10s (lower inorganic content) then washed off with water
58
What is the role of the primer?
adhesive - bonds to hydrophilic dentine surface and bonds to resin. Gives the dentine a hydrophobic surface for the next layer of resin to flow
59
What is the primer also referred to as?
coupling agent
60
What type of molecule do primers consist of?
bifunctional molecule with a hydrophilic end to bond to hydrophilic dentine surface and a hydrophobic, methacrylate end to bond to resin. Molecule must also have a spacer
61
What is the role of the spacer group in the coupling agent (primer molecule)?
allows flexibility when bonding (otherwise lack of flexibility reduces bonding sites and strength)
62
What is the coupling agent / molecule in the primer dissolved in?
a suitable solvent - ethanol, acetone, water
63
What does the hydrophilic end of the coupling agent (primer) bond to?
hydrophilic dentine surface
64
What does the hydrophobic (methacrylate end) of the coupling agent (primer) bond to?
resin
65
What coupling agent is found in many primers?
HEMA - hydroxy ethyl methacrylate
66
How does the primer bond to the next resin layer?
C=C bond opens and forms a strong covalent bond with the next resin layer
67
How does the primer bond to the dentine?
hydroxyl group (hydrophilic) combines with similar polar groups on hydroxyapatite and can react with amine groups on collagen protein
68
Why does the dentine require a primer for bonding?
dentine is a hydrophilic surface with low surface energy whereas resin is hydrophobic - primer bonds to dentine to make its surface hydrophobic / high surface energy for the resin to flow on top
69
Examples of different primers
HEMA, 4-META, MDP
70
What does the adhesive (3rd part to total etch system) consist of?
mixture of resins (BIS-GMA and HEMA), some filler particles, camphorquinone
71
What is the purpose of the filler particles in the adhesive?
increases strength
72
What is the purpose of the camphorquinone in the adhesive?
allows light curing
73
Is the adhesive hydrophilic or hydrophobic?
predominantly hydrophobic
74
What is the purpose of the adhesive?
- penetrates primed dentine (now a hydrophobic surface) - molecular entanglement (forms a micromechanical bond within tubules and exposed collagen)
75
What is the name of the layer formed by the adhesive penetrating the primed dentine and forming a micromechanical bond?
hybrid layer (of resin and collagen)
76
What type of retention does the adhesive have to the primed dentine?
micromechanical retention
77
What are potential problems that can arise with total etch technique?
- over etching - moisture dependent (too wet / dry)
78
Why is over etching (total etch) an issue?
- can result in collapse of collagen fibres preventing resin penetrating - too deep of an etch means the primer cannot penetrate to its full depth
79
What happens if the dentine surface is too dry during etch / bonding?
dentine surface collapses
80
What happens if the dentine surface is too wet during bonding?
dilutes the primer resulting in reduced bond strength
81
Why is it undesirable for the dentine to be too dry resulting in the surface collapsing?
collapsed dentine decreases the porosity which results in poor penetration of the dentine by the primer and therefore a poor bond
82
What should the dentine surface look like before primer / adhesive (bond)?
moist shiny surface (not dull and dry)
83
What does a moist shiny dentine surface prior to priming indicate?
expanded dentine surface is porous and will absorb the primer resulting in a good bond
84
Why is bonding to dentine difficult to do well?
very technique sensitive
85
How has the process of bonding to dentine been simplified?
the 3 stage technique has been reduced to 2 and ultimately 1 stage
86
What is the advantage of a reduced number of bonding steps?
reduces clinical time (not necessarily better bonding though)
87
How have the components of the bonding system been combined to reduce the stages?
- 2 stages: separate etchant, primer + adhesive combined (still total etch) - 1 stage: self etching primer and adhesive all combined
88
In the 2 stage technique how are the primer and adhesive (sealer) combined?
combined in a single bottle and applied to the etched washed dentine, the light cured - allows simultaneous wetting and sealing of dentine
89
What are the contents of the single bottle of combined primer and sealer (adhesive)?
same as primer (e.g. HEMA, GPDM, MDP, 4-META) but combined with a resin BIS-GMA, solvent (alcohol, acetone) and camphorquinone
90
Examples of total etch 2-stage bonding agents
- Scotchbond one - Prime and bond (used in GDH) - Optibond solo - I-bond TE - XP bond
91
How does the clinical handling and bond strength of total etch 2-stage bonding agents compare to the 3 part total etch materials?
very similar and comparable
92
What are self etching primers?
1 stage bonding (etch, primer and adhesive all in one)
93
How do self etching primers work differently from previous bonding systems?
- incorporate smear layer instead of trying to remove it (total etch dissolves smear layer and is washed off) - not washed off
94
What is the advantage of using self etching primers?
- less technique sensitive - not washed off so removes the problem of how dry to make the dentine - evidence for reduced sensitivity of restoration
95
What is the disadvantage of using self etching primers?
bond is not as strong as total etch technique
96
What field of dentistry may self etching primers still have a use?
orthodontics - bonding brackets to teeth doesn't have to be a very strong bond
97
Is a hybrid layer still formed by self etching primers?
yes
98
What molecules do self etching primers contain to allow them to etch the dentine?
acid methacrylate monomers (e.g. methacryloyloxyalkyl acid phosphatase)
99
What is the action of acid methacrylate monomers in self etching primers?
bifunctional - hydrophilic end etches and infiltrates dentine surface, hydrophobic end polymerises like a simpler BIS-GMA material. Acidic groups react with Ca2+ in tooth to bond and create an amorphous calcium chelate on the surface.
100
What happens to the smear layer with a self etching primer?
smear layer is dissolved and incorporated into hybrid layer
101
How far does the hybrid layer (resin and collagen) penetrate with a self etching primer?
2um (compared to 10um in total etch technique)
102
What is the fundamental mechanism for all bonding?
molecular entanglement
103
Outline the process of molecular entanglement
mineral exchange - minerals removed from dental hard tissue are replaced by resin which mechanically interlock in these pores once mineralised
104
What is the interaction of self etching primer molecules (acid methacrylate monomers) described as?
adhesion - decalcification concept (AD concept)
105
Outline the adhesion-decalcification concept (interaction of self etch primer with HA)
- initially acid monomers bond ionically to calcium in HA - stability of HA-monomer bond determines longevity of bond - monomers with lower pKA continue to dissolve HA and so do not form a stable bond - results in a hybrid later with unstable calcium phosphates incorporated
106
What is the effect of a self etch primer monomer with a lower pKa?
does not form a stable bond to calcium in HA because it continues to dissolve the HA
107
What are the disadvantages of a strong self etch?
- etching by-products not washed away - they are soluble and hence weaken bond integrity - if too much HA dissolved, exposed collagen is vulnerable to breakdown and bond will fail
108
Does self etch bond better to enamel or dentine?
strong self etch bonds well to enamel but less well to dentine (more organic e.g. collagen)
109
What is an alternative to using a strong self etch?
use a mild self etch
110
What is the advantage of using a mild self etch?
only partially demineralises the dentine - stronger, more durable bond
111
How does a mild self etch have a stronger, more durable bond compared to a strong self etch?
- mild self etch only partially demineralises dentine - HA remain around collagen so they are protective against hydrolytic breakdown and ionic bonding with remain Ca ions can occur
112
Which primer / coupling agents are better?
MDP and 4-META > HEMA containing materials
113
Why are HEMA containing bonding agents not as good?
HEMA materials are more acidic and absorb more water resulting in a less durable bond (more HA dissolved)
114
What is an example of a mild self-etching material?
Scotchbond universal
115
What is a potential disadvantage that mild self-etch adhesives may encounter?
may not be able to penetrate a thick smear layer
116
What is the challenging balance that self-etching materials must display?
must penetrate the smear layer without decalcifying the tooth surface too much and removing the HA
117
Why is it important that not too much HA is removed by self-etching adhesives?
- HA required for strong durable bonding - HA protects dentine from hydrolytic breakdown - contains Ca ions for ionic bonding
118
Why is there a reduced chance of post-op sensitivity using self-etching bonding agents?
demineralisation and resin infiltration occur simultaneously
119
How do different products of self etching bonding agents vary?
great variability wrt initial pH of solution
120
What is the effect of the initial pH on self etching bonding agents?
different pH results in different etch and different resin penetration. Materials with lower initial pH may not etch enamel efficiently
121
How can the best bond to enamel using self etching DBAs be achieved?
etch enamel first (35% phosphoric acid)
122
What is the advantage of self etching DBAs being less technique sensitive?
no rinsing (time saving), no excessive drying, no dentine collapse leading to low bond strength, no diluted primer
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