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Dental material science > Bonding systems > Flashcards

Bonding systems Flashcards

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1
Q

What are the properties of a dental adhesive

A

Provide 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 bonding to enamel easy

A

Heterogeneous structure: densely packed prismatic​

Highly mineralised: 95% inorganic​

‘Dry’

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3
Q

What percentage aqueous solution of phosphoric acid is normally used in practice for enamel etching

A

30-50%

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4
Q

What does the acid etch technique do

A

Long enamel prisms filled with imperfectly packed hydroxyapatite crystals
This surface can be modified by applying acid
This roughens the surface producing an etched pattern
This allows micromechanical interlocking ofresin filling materials
The etching also allows for greater wettability of the enamel by removing contaminants
This allows the resin to adapt better to roughened enamel surface

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5
Q

What is the composition of dentine

A

20% organic (mostly collagen)​

70% inorganic (mostly hydroxyapatite)​

10% water

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6
Q

What makes dentine an inconsistent material

A

Aged dentine is more mineralised. Dentine near the pulp has more tubules and increased moisture content

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7
Q

What are the requirements of a dental bonding agent

A

Ability to flow​
Potential for intimate contact with dentine surface
Low viscosity
Adhesion to substrate​
-Mechanical​
-Chemical​
-Van der Waals​
-Combination of the above​

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8
Q

What bonds are formed within organic dentine

A

Covalent

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9
Q

What bonds are formed within mineralised dentine

A

Ionic

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10
Q

When is the best adhesion/bonding within dentine acheived

A

When Van der Waals forces are optimised (strength depends on contact angle <90% indicates solid surface is hydrophilic)

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11
Q

What is critical surface energy

A

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

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12
Q

When will a liquid flow over a surface and stick to it

A

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

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.​

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13
Q

Why are dental bonding agents required in restoring dentine

A

Dentine bonding agents increase the surface energy of the dentine surface and allow composite to flow and stick to the surface.​

They are SURFACE WETTING AGENTS​

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14
Q

What is molecular entanglement

A

Adhesive is absorbed and can penetrate into surface of dentine where it can form a long chain polymer which meshes with the substrate

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15
Q

Why is molecular entanglement desired

A

Leads to a high bond strength

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16
Q

What is the smear layer

A

The smear layer is an adherent layer of organic debris that remains on the dentine surface after the preparation of the dentine during the restoration of a tooth

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17
Q

What is the thickness of the smear layer

A

0.5-5 microns

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18
Q

What do we 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​

19
Q

Why did earlier fillings using (Phosphorylated Bis-GMA : bisphenol-A glycidyl methacrylate (Scotchbond), NPG-GMA: N-phenylglycidyl methacrylate (Crevident)) tend to fail more fresuently

A

The bond was only to the top of the smear layer
The phosphate to calcium bond was not strong and was affected by moisture.
Saliva or dentinal fluid caused hydrolysis of the bond and it would break down resulting in leakage of the filling

20
Q

What do materials today rely on for bond strength

A

Total etch​

Self etch​

To remove and/or modify the smear layer​

21
Q

What generation of DBA are we on currently

A

8th

22
Q

What are examples of total etch bonding agents

A

Scotchbond multipurpose​

Clearfil photo bond​

Optibond FL

23
Q

What are the 3 components of a total etch DBA

A

Dentine conditioner
Primer
Adhesive

24
Q

What does a dentine conditioner do

A

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 in this top 10um of the dentine is exposed and subsequently penetrated by the next two components.​

25
Q

What does a DBA primer do

A

The primer is really the adhesive element in the process. A coupling agent.​

It has a bifunctional molecue with a hydrophilic end to bond to the hydrophilic dentine surface and a hyrdophobic, methacrylate end to bond to the resin

26
Q

What does lack of flexibility of the molecule within primer result in

A

Lack of flexibility reduces bonding sites and bond strength

27
Q

What is a suitable solvent for the molecule/group of molecules within DBA primer

A

Ethanel
Acetone
Water

28
Q

What does HEMA stand for

A

Hydroxy ethyl methacrylate

29
Q

Why is HEMA found in many primers

A

C=C bond forms a strong covalent bond with the next resin layer
The hydroxyl group can form ionic bonds with polar groups on the hydroxyapatite and react with amine groups on the collagen protein

30
Q

What is a DBA adhesive

A

This is a mixture of resins. Usually Bis-GMA and HEMA.​

It is predominantly hydrophobic.​

It may contain some filler particles to make it stronger.​

It will contain Camphorquinone to allow it to light cure.​

31
Q

What is the purpose of adhesive within DBAs

A

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.​

32
Q

Breakdown the steps of a DBA

A

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

Adhesive can now penetrate –> micro mechanical retention

33
Q

What are problems with total etch

A

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

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

Moisture dependent:​
-Too dry and the dentine surface collapses​
-Too wet and the primer is diluted –> reduced strength​

​

34
Q

What occurs if the dentine is over dried

A

Collapsed dentine-decrease in porosity​

Decrease in porosity - poor penetration of ​

dentine by primer - poor bond

35
Q

What are some total etch 2-stage bonding agents

A

Scotchbond One​

Prime and Bond​

Optibond Solo​

i-Bond TE​

XP Bond

36
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.​

This removes the problem of how dry to make the dentine.​

37
Q

What bifunctional monomers do self etching primers contain

A

Acidic methacrylate monomers such as Methacryloyloxyalkyl acid phosphatase

38
Q

What does methacryloyloxyalkyl acid phosphatase do to assist bonding to dentine

A

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

39
Q

With self etching primers what happens to the smear layer

A

The smear layer is dissolved but then incorporated into the hybrid layer​

40
Q

Why is strong self etch not always best

A

Strong self etch bonds well to enamel but less well to dentine

Etching by-products are not washed away as in total etch​

These are soluble and weaken the integrity of the bond​

If too much hydroxyapatite is dissolved away the exposed collagen is vulnerable to breakdown and the bond will fail​

41
Q

What is the aim of every self etching material

A

The challenge in these materials is to penetrate the smear layer without decalcifying the tooth surface too much and removing the HA​
-The HA is required for strong durable bonding​
-The HA protects the dentine from hydrolytic breakdown​

42
Q

What are the positives with 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​

43
Q

What are the imperfections with self etching bonding agents

A

There is great variability between products with regard to initial pH of the solution.​

The difference in pH results in different etch and different penetration of resin.​

Those materials with a lower initial pH may not etch enamel efficiently.​

There is little evidence of stronger bond to dentine than with total etch.

Dental material science (5 decks)

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