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BDS2 - operative dentistry > bonding systems > Flashcards

bonding systems Flashcards

1
Q

what is dental adhesive

A
  • provides a high strength bond to tooth tissues
  • is an immediate hight strength bond - sticks straight away
  • is a durable bond - lasts for a long time
  • is impermeable
  • easy to use and safe
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2
Q

what’s easier to bond to: enamel or dentine

A

enamel

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3
Q

why is enamel easier to bond to than dentine

A

due to the structure of enamel

  • heterogenous structure (densely packed prisms)
  • highly mineralised (95% organic)
  • ‘dry’ (no moisture)
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4
Q

what technique is used to bond

A

acid etch technique

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5
Q

what happens to the surface of enamel during the acid etch technique

A

the long enamel prisms are filled with imperfectly packed hydroxyapatite cyrstals. The acid modifies this surface by roughening it creating an etched pattern

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6
Q

what does this roughened surface fo enamel allow

A

micro-mechanical interlocking of resin filled materials

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7
Q

etching also increases surface energy of the enamel. What does this allow?

A

the removal of surface contaminants creating higher surface energy leads to better wet ability of the enamel which allows the resin to adapt better the the roughened enamel

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8
Q

why must enamel be dry

A

must be dry as moisture will prevent the flow fo resin into the etched surface

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9
Q

what type of bonding is enamel

A

mechanical - pour liquid into the rough surface and then it hardens and interlocks with etched surface so ti doesn’t fall out

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10
Q

what viscosity of resin is applied first

A

low viscosity resin is applied onto the rough surface then high viscosity on top

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11
Q

what is the concentration of acid normally used for etching

A

can be done with a variety of organic and inorganic acids but in practice 30-50% aqueous solution of phosphoric acid is mainly used

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12
Q

what is a common low viscosity resin

A

(phosphorylated) Bis-GMA is applied to the etched and dried enamel surface to penetrate into the roughened surface and light cured

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13
Q

bonding to dentine is?

A

complicated

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14
Q

composition of dentine

A

20% organic - mostly collagen
70% inorganic - mostly hydroxyapatite
10% water

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15
Q

what is dentine full of

A

permeable tubules

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16
Q

why is the surface of dentine often wet

A

because fluid is pumped up from the pulp to the dentine floor of any cavity through the tubules

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17
Q

what is one difference between dentine and enamel

A

dentine is an inconsistent material (unlike enamel)

  • aged dentine is more mineralised
  • dentine near the pulp has more tubules and increased moisture content
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18
Q

does dentine have a low or high surface energy

A

low

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19
Q

what is a factor that makes bonding to dentine difficult

A

dentine isa hydrophilic material whereas most simple bonding agents are hydrophobic (oil based)

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20
Q

what layer does dentine have that make sit more difficult to bond to

A

smear layer

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21
Q

requirements for dentine bonding agents

A
  • ability to flow
  • potential for intimate contact with dentine surface
  • low viscosity
  • adhesion to substrate
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22
Q

what are the 3 types of adhesion to dentine

A

mechanical, chemical and Van Der Waals

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23
Q

what is mechanical adhesion to dentine

A
  • same as enamel
  • achieved by the dentine bonding agent and dentine surface meshing and interlocking with minimum gaps
  • smaller holes are important - etched collagen around the tubules gives a strong bond
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24
Q

what is chemical adhesion to dentine

A
  • bonding at a molecular level
  • mineralised dentine = ionic bond
  • organic dentine = covalent bond
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25
Q

what is Van Der Waals adhesion to dentine

A
  • based on electrostatic or dipole interaction between bonding agent and substrate
  • strength of bond depends on contact angle
  • best adhesion/bonding is achieved when forces are optimised
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26
Q

what can the contact angle indicate

A

is an indication of the wettability of a solid by a liquid - angle of < 90 degrees = hydrophilic

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27
Q

what is critical surface energy

A

surface tension of a liquid that will just spread on the surface of a solid

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28
Q

what surface energy must a liquid have

A

must have a lower surface energy than the surface it is being placed on for it to flow onto it and stick

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29
Q

why will a low surface energy liquid run onto a high surface energy substrate

A

because this leads to a lower surface energy of the material as a whole

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30
Q

why does nothing stick to non-stick frying pans

A

because they have a very low surface energy

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31
Q

what surface energy does wet dentine have

A

low surface energy, lower than composite filling agents

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32
Q

what must you do to make composite to stick to dentine

A

must make the dentine have a higher critical energy that the composite

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33
Q

what do dentine bonding agents do

A

increase the surface energy of dentine and allow composite to flow and stick to the surface

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34
Q

what are surface wetting agents

A
  • change low surface energy substrate to high
  • have polar and non-polar groups
  • oily end sticks to the wet dentine
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35
Q

how is bonding to dentine carried out

A

by adhesion through molecular entanglement

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36
Q

how does the adhesive work

A
  • absorbed onto the surface of dentine but can also penetrate into the surface
  • due to good wetting of the dentine by the adhesive and appropriate surface energies of the two
37
Q

what can form a long chain polymer

A

the component absorbed into the dentine

*this polymer meshes with the substrate (molecular entanglement) leading to high bond strength

38
Q

what is the smear layer

A

an adherent layer of organic debris that remains on the dentine surface after the preparation of the dent during the restoration of a tooth
- its what is left after you cut the surface of dentine with high speed power

39
Q

how thick is the smear layer

A

around 0.5-5 microns thick

40
Q

what is the smear layer generally contaminated with

A

bacteria

41
Q

what is the smear layer thought to do

A
  • originally thought it was a protective barrier to the pulp
  • now thought it interferes with adhesion
42
Q

how do you remove the smear layer

A

with acid etch

- is very difficult to remove

43
Q

what do you do with the smear layer

A
  • remove it and bond to the clean dentine beneath

* incorporate it by penetrating it, infiltrating it with the bonding agent

44
Q

what generation of dental bonding agents now incorporate the smear layer into them

A

8th generation

45
Q

what were the early dental bonding agents

A

phosphorylated esters of unfilled resins

46
Q

what is phosphorylated Bis-GMA

A

a dental bonding agent

  • formed of bisphenol-A glycidyl methacrylate
  • also known as Scotch Bond
47
Q

what is NPG-GMA

A

dental bonding agent

  • formed of N-phenylglycidal methacrylate
  • also known as Crevident
48
Q

what is the bonding mechanism of these DBA’s

A

thought to be an ionic bond to the calcium in the dentine by the chlorophosphate or hydroxyl group

49
Q

what were early DBA’s applied to

A

an untreated dentine surface

-bond was only to the top of the dentine layer

50
Q

why were early DBA;s not very effective

A
  • they were weak bonds as the smear layer is not firmly attached to the underlying dentine and they only bonded to this smear layer
  • the phosphate to Ca bond was not strong and was affected by moisture (saliva or dentinal fluid caused hydrolysis of the bond)
  • overall, early DBA’s had low bond strength and not great success
51
Q

what do materials now rely on

A
  • total etch or,

* self etch to remove the smear layer

52
Q

which of total etch or self etch DBA’s were developed first

A

total etch

53
Q

what are total etch bonding agents

A
  • these are the 4th generation DBA and involve the complete removal of the smear layer
  • still the ‘Gold Standard’ of DBA’s
54
Q

what are examples of total etch DBA’s

A
  • Scotchbond multipurpose
  • Clearfil photo bond
  • Optibond FL
55
Q

what are the components of total etch DBA’s

A
  • dentine conditioner (acid, usually 35% phosphoric acid)
  • primer (adhesive part of the agent with a hydrophilic/phobic molecule
  • adhesive (mildy phosphorylated resin which penetrates into the surface of the dentine attaching to the primers hydrophobic surface
56
Q

what is the dentine conditioner

A
  • generally phosphoric acid, but can also be EDTA or nitric acid
  • 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 the top 10 microns of the dentine is exposed and subsequently penetrated by the next 2 components
57
Q

what is the primer

A
  • adhesive element (a coupling agent)
  • single molecule thick
  • bifunctional molecule with hydrophobic end to bond to the resin and a hydrophilic end to bond to the hydrophilic dentine surface
  • must also have a spacer group to make it long enough to be flexible when bonding (lack of flexibility reduces bond strength)
  • molecule is dissolved in a suitable solvent (ethanol, acetone or water)
  • coupling agent found in many primers is HEMA
58
Q

how does the primer work

A
  • coupling agent found in many primers is HEMA
  • hydrpxyl ethyl methacrylate
  • C=C bond opens and forms a strong covalent bond with the next resin layer
  • the hydroxyl group can combine with similar polar groups on the hydroxyapatite and can react with amine groups on the collagen protein (may be a true chemical bond instead of just an attraction)
  • still gives dentine a hydrophobic surface for resin to flow onto
59
Q

what are some examples of primers used

A
  • HEMA - in scotch bond, optioned, clearfil
  • NTG-GMA - all bond
  • MDP - clearfil
  • 4-META - hybrid bond, G-bond
  • GPDM - optibond
60
Q

what is the adhesive

A
  • 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
  • penetrates the primed dentine
  • forms a micro mechanical bond within the tubules and exposed dentinal collagen fibres (MOLECULAR ENTANGLEMENT)
  • forms a hybrid layer or collagen plus resin
61
Q

how does etching dentine work

A
  • demineralisation of the outer layer
  • 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 (gives surface high energy, making it hydrophobic)
  • adhesive can now penetrate (micro-mechanical retention)
62
Q

what are some problems with total etch

A
  • over etching - can collapse collagen fibres then no resin can penetrate, too deep an etch and then the primer can not penetrate to the full depth (can then get sensitivity)
  • moisture dependant - too dry and dentine surface collapses, too wet and primer is diluted (reduced strength)
  • if tooth is over dried - collapsed dentine (decrease in porosity leads to poor penetration of dentine by primer leads to poor bond), moist dentine gives a good bond (opposite of above)
63
Q

bonding to dentine is what

A

clinically very technique sensitive

- not easy to do well

64
Q

how has the bonding process been made to be easier

A

the 3 stage process has been reduced to 2

- reduces clinical time but don’t necessarily mean better bonding

65
Q

how has the steps been reduced for bonding

A

by combining the components of the bonding system

  • can be primer and sealer combined but separate etchant
  • can be self etching primer and sealer all combined and applied all together
66
Q

what type of etch technique is separate etchant with primer and sealer combined

A

still total etch technique

67
Q

how does the process of bonding work with separate etchant with primer and sealer combined

A
  • etchant is still phosphoric acid
  • primer and sealer are combined in a single bottle and applied to the etched, washed dentine and then light cured
  • should allow the wetting and sealing of the dentine to occur simultaneously
68
Q

what are the chemicals in the single bottle of primer and sealer

A

the same as is in the primer bottles

  • HEMA, GPDM, MDP and 4-META
  • but are combined with a resin Bis-GMA and a solvent such as alcohol or acetone and camphorquinone to allow light curing
69
Q

examples of total etch 2-stage bonding agents

A
  • Scotchbond one
  • Prime and bond
  • Optibond solo
  • I-bond TE
  • XP bond
  • clinical handling of these is very similar to that of the 3-part total etch materials and bond strengths are similar
70
Q

what are self etching primers

A
  • work in a different way from all the other materials
  • do not remove the smear layer - instead have molecules in it which infiltrate and incorporate it into themselves
  • these are not washed off - removes problem of having to dry dentine
  • most are combined with the bonding resin to make a one bottle system - etches, primes and bonds all in one
71
Q

what is good about the self etching primers

A
  • not as technique sensitive

* are simple to use and there is evidence of their ability to etch dentine and form a hybrid layer

72
Q

what is not good about self etching primers

A

the bond is not as strong to all the tooth tissues

73
Q

what do self etching primers contain to allow them to etch the dentine

A

acidic methacrylate monomers such as methacryloyloxyalkyl acid phosphatase

74
Q

how do self etching primers work

A
  • these very complex bifunctional monomers etch and infiltrate the dentine surface with heir hydrophilic end while polymerising like a simpler Bis-GMA material at the hydrophobic end
  • the acidic group react with calcium ions in the tooth bonding to the surface and also create and amorphous calcium chelate not eh surface
  • the smear layer is dissolved and incorporated into eh hybrid layer
75
Q

how far does the self etching primers penetrate into eh surface of dentine

A

about 2microns but can give a good bond to the dentine

76
Q

how is their reduced sensitivity using self etching primers

A

there is no drying stage so no chance of collapse of dentine architecture preventing resin penetration and hybrid layer formation

77
Q

what is the fundamental mechanism of all bonding

A

mineral exchange where minerals are removed from the dental hard tissue and are replaced by resin which once mineralised, mechanically interlocks in these porosities
-MOLECULAR ENTANGLEMENT

78
Q

what is the adhesion-decalcification concept

A
  • initially all acid monomers bond to the Ca in HA ionically
  • whether they sat bonded depends not eh stability of the HA-monomer bond
  • monomers with a low pKa (not necessarily low pH) do not form a stable bone as they continue to dissolve HA
  • leads to hybrid layer with unstable calcium phosphates incorporated
79
Q

why don’t you want a strong acid for bonding

A

because it causes weaker bonds

80
Q

why is self etch not better than total etch

A
  • etching by-products are not washed away as in total etch
  • these by-products 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 self etch bonds well the enamel but not as well dentine
81
Q

how does mild self etch work

A
  • only partially demineralises the dentine (Scotchbond is example)
  • HA crystals remain around the collagen - protective against hydrolytic breakdown, and the remaining Ca ions allow ionic bonding
82
Q

why are MDP and 4-META better than HEMA containing materials

A

HEMA materials are more acidic and absorb more water so form a less durable bond

83
Q

what are the problems with the smear layer

A
  • if thick then the mild self etch adhesives may not penetrate through it
  • need to penetrate the smear layer without decalcifying the tooth tissue too much and removing the HA (HA needed for strong bonds)
  • if thick then the self etch may not be able to penetrate as deep as the bottom of the smear layer (smear layer may be 5 microns but etch can only go 3-4microns)
84
Q

what’s good about the self-etching bonding agents

A
  • 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
85
Q

what bad about self etching bonding agents

A
  • great variability between products with regard to initial pH of the solution
  • difference in pH results in different etch and different penetration of resin - materials with lower pH may not etch enamel efficiently
  • there is little evidence to show that is has stronger bond to dentine than with total etch
86
Q

what do bonding agents work as

A

surface wetting agents

87
Q

bonding to enamel is?

A

easy

88
Q

bonding to dentine is?

A

difficult

BDS2 - operative dentistry (5 decks)

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