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2 - Dental Material Science > Bonding Systems > Flashcards

Flashcards in Bonding Systems Deck (47)
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1
Q

What makes enamel good for bonding and how does bonding occur?

What type of bonding?

A

highly mineralised, dry prisms

acid etching with phosphoric acid creates microporosities on enamel surface

this roughened etched pattern allows micromechanical interlocking of resin materials

etching also increases surface energy for better wetting by resin

bonding to etched, dry enamel is essentially a micromechanical process

2
Q

What is the surface energy of a material?

A

A measure of the disrupted intermolecular bonds that exist at the surface of a material. Surfaces with high surface energy attract other materials to reduce this energy.

3
Q

Why do impurities reduce the surface energy?

A

surface atoms become attracted to impurities and bond

4
Q

What is the wetability of a material?

A

ability of a liquid to spread out and maintain contact with a solid surface.

5
Q

Why must enamel be dry for bonding?

A

Moisture contamination will prevent flow of the resin into the etched surface.

6
Q

What is the bond strength of enamel to composite ?

A

> 20MPa
usually between 20-30

7
Q

What is the composition of dentine?

What are the characteristics and why does this affect bonding?

A

20% organic (mostly collagen) 70% inorganic (HA) 10% water
Full of permeable tubules (increased water contimation)
Less mineralisation
Low surface energy (due to water and weaker forces due to collagen

8
Q

What water quality are most bonding agents and what water quality is dentine?

A

Hydrophobic (plastic is oil)

9
Q

What is an added complicating factor when bonding dentine?

Why is this not present in enamel?

A

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

does not occur in enamel as enamel is more mineralised (more resistance than collagen) and debris can penetrate less due to lack of tubules

10
Q

What bonds does the adhesion of dentine include?

A

Mechanical
Chemical
Van der waals

11
Q

How does mechanical bonding in dentine occur?

A

Same as enamel bonding. DBA and dentine surface mesh together and interlock with minimum gaps.

12
Q

At the molecular level, what bonds do the mineralised and organic components of dentine make?

A

Mineralised = ionic
Organic = covalent

13
Q

What does a contact angle of less than 90 mean?

A

Solid surface is hydrophilic and has good wetability (better adhesion to surface)

smaller contact angle = better wetting

14
Q

What is a critical surface energy?

A

the surface tension of the liquid needed to just spread on solid (in that case the contact angle of solid is zero)

15
Q

For a liquid to flow onto a surface and stick, what must happen?

A

Liquid must have a lower surface energy than the surface it is being placed on

16
Q

What is the surface energy of wet dentine?

A

Low surface energy - lower than composite filling materials

17
Q

For composite resin to stick to dentine, what must be done?

A

The surface energy of dentine must be increased higher than the composite so that the composite is able to flow over and adhere

18
Q

What agents increase the surface energy of dentine?

A

Dentine bonding agents (surface wetting agents)

19
Q

What is molecular entanglement?

Is it the same as mechanical bonding

A

Absorbed adhesive forms a long chain polymer which meshes with substrate (dentine) leading to high strength bond after there is good wetting of the dentine by adhesive and appropriate surface energies of the two

mineral exchange where some minerals from the dental hard tissue are removed and are replaced by the resin which once mineralised can mechanically interlock in these porosities.

It is specific to polymers unlike mechanical bond which is general

20
Q

What are the options of what to 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 and stick it to the dentine below

21
Q

Why is it not a good idea to incorporate the smear layer?

A

the smear layer is not tightly bound to dentine and was affected by moisture
therefore saliva or dentinal fluid caused hydrolysis of the bond and it would break down resulting in leakage of the filling

22
Q

What do todays materials rely on that previous generations of DBAs didn’t?

A

Total etch or self etch
To modify/ remove the smear layer

23
Q

What is the gold standard DBA?

A

Total etch bonding agents

24
Q

What are examples of total etch DBAs?

A

Scotchbond multipurpose
Clearfil photo bond
Optibond FL

25
Q

What are the components of a total etch dentine bonding agent?

A

Dentine conditioner (etch) - acid usually 35% phosphoric acid
Primer - really the true adhesive part of the agent with a hydrophobic/hydrophilic molecule
Adhesive - resin which penetrates into the surface of the dentine attaching to the primer’s adhesive surface

26
Q

What does the dentine conditioner do in DBAs?

A

Removes smear layer
Opens dentinal tubules by removing smear plugs
Decalcifies the uppermost layer of dentine
Increases surface area
Exposes collagen network and subsequently penetrated by the next two components (primer + bond)

27
Q

What does the primer do?

A

Coupling agent
Bifunctional molecule with a hydrophilic end to bond to the dentine surface and a hydrophobic methacrylate end to bond to the resin

maintains expanded collagen network while removing residual water off dentine, helps bond infiltrate into collagen network

28
Q

What must the primer have to ensure flexibility?

A

The molecule must have a spacer group to make it long enough to be flexible when bonding. Lack of flexibility reduces bonding sites and bond strength

29
Q

What solvent dissolve primer?

A

water
acetone
ethanol

30
Q

What is a common coupling agent found in many primers?

A

HEMA (water absorbing plastic)

31
Q

What does the C=C bond in HEMA form?

A

Strong covalent bond with next resin layer

32
Q

What does the hydrophilic hydroxyl COOC2H4-OH group combine/react with?

What does this create?

A

Similar polar heads in dentine HA and reacts with amine groups on the collagen protein

gives the dentine a hydrophobic high energy surface that the bond will flow into

33
Q

What is the adhesive usually a mix of and what do adhesives have that improve bond strength?

A

Mix of resins. Usually Bis-GMA and HEMA
May contain some filler particles to make it stronger

34
Q

What does the adhesive have that allows it to light cure?

A

camphorquinone

35
Q

How does the adhesive bond?

A

Penetrates the primed dentine which now has a hydrophobic surface
Forms a micro-mechanical bond within the tubules and exposed dentinal collagen fibres
Carries out molecular entanglement
Forms the hybrid layer of collagen and resin

36
Q

What are the problems with total etch?

not a simple technique

A

Over etching leads to collapse of the collagen fibres so no resin can penetrate

Over etching creates too deep an etch and the primer cannot penetrate to the full depth of the etch.

Too dry and the dentine surface collapses

Too wet and the primer is diluted leads to reduced strength

37
Q

How have the bonding steps been combined?

A

Separate etchant with primer and sealer combined.

Self etching primer and sealer all combined and applied together as a single solution.

38
Q

What type of technique is the separate etchant with primer and sealer combined?

A

Total etch technique

39
Q

What are examples of total etch two-stage bonding agents?

A

Scotchbond One
Prime and Bond
Optibond Solo
i-Bond TE
XP Bond

40
Q

How do self-etching primers work?

A

They do not attempt to remove the smear layer. They infiltrate it and incorporated themselves into it.
They are not washed off.
1 bottle system

41
Q

Advantages VS Disadvantages of self etch

A

Advantages
Less technique sensitive
No rinsing and no excessive drying
Simultaneous demineralisation and resin infiltration

Disadvantages
Difference in pH results in different etch and different penetration of resin.
Those materials with a lower initial pH may not etch enamel efficiently.

42
Q

Why does pH affect self-etch?

A

acid monomers with a lower pKa do not form a stable bond as they continue to dissolve HA

43
Q

What allows self etch DBAS to etch the dentine?

What else do they contain?

A

Bifunctional complex acidic methacrylate monomers, which infiltrate the dentine surface with their hydrophilic end and then they polymerise a simpler bis-GMA material at the hydrophobic end.

contain same chemicals as total and bonds via molecular entanglement and mechanical bonding

44
Q

What reacts with Ca+ ions at the tooth surface in self-etch?

What type of bond

A

acidic groups to form amphorous chelate

ionic

45
Q

Strong self etch VS Mild self etch

A

Strong
Soluble byproducts are formed that weaken the integrity of the bond and are not washed away
If too much hydroxyapatite is dissolved away the exposed collagen is vulnerable to breakdown and the bond will fail

Mild
Partially demineralises the dentine - HA crystals remain around the collagen
Protective against hydrolytic breakdown
Remaining Ca ions allow ionic bonding

46
Q

Why must HA not be completely demineralised?

A

The HA is required for strong durable bonding

The HA protects the dentine from hydrolytic breakdown

47
Q

Which direct restorative material relies on chelation between carboxyl groups and the calcium in the tooth as a major component of the bonding mechanism?

A

Glass ionomer cements