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BDS2 Dental materials science > Bonding systems > Flashcards

Bonding systems Flashcards

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1
Q

Name properties of a dental adhesive

A 
Provide a high bond strength to tooth tissues
Immediate high strength bond
Durable bond
Impermable bone
Easy to use
Safe
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2
Q

Why is bonding to enamel easy?

A

heterogenous structure: densely packed prismatic
highly mineralised: 95% inorganic
‘Dry’

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3
Q

Describe the acid etch technique

A
  • Long enamel prisms are filled with imperfectly packed hydroxyapatite crystals.
  • This surface can be modified by application of acid (first recorded by Buonocore in 1955)
  • The acid roughens the surface of the enamel producing a characteristic etched pattern.
  • This roughened surface allows micromechanical interlocking of resin filling materials
  • The etching also increases the surface energy of the enamel surface by removing surface contaminants leading to better wettability of the enamel.
  • Better wettability allows the resin to adapt better to roughened enamel surface.
  • For this to work the enamel must be dry. Moisture contamination will prevent flow of the resin into the etched surface.
  • Enamel bonding is essentially mechanical
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4
Q

What is used to etch enamel

A

30-50% phosphoric acid

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5
Q

what is applied to the etched and dried enamel surfacee to penetrate into the rough surface and light cured?

A

a low viscosity Bis-GMA resin (or another dentine bonding agent resin)

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6
Q

Describe dentine composition and why it’s hard to bind to

A
  • 20% organic (mostly collagen)
  • 70% inorganic (mostly hydroxyapatite)
  • 10% water
  • Full of permeable tubules
  • Fluid pumps up from pulp to dentine floor of any cavity making the surface wet.
  • Inconsistent material. Aged dentine is more mineralized. Dentine near the pulp has more tubules and increased moisture content.
  • Low surface energy
  • Dentine is hydrophilic whereas most simple bonding agents are hydrophobic

A further complicating factor when bonding to cut Dentine is the Smear Layer

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7
Q

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

A
  • Ability to flow
  • Potential for intimate contact with dentine surface
  • Low viscosity
  • Adhesion to substrate via Mechanical/ Chemical/ Van der Waals/ Combination of all
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8
Q

How does mechanical bonding to dentine work?

A

Same as in enamel bonding. Achieved by the dentine bonding agent and the dentine surface meshing and interlocking with minimum gaps. Easy to see microscopically especially on SEM

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9
Q

How does chemical bonding to dentine work?

A

Dentine constituent Bond type
mineralized ionic
organic covalent

Van der Waals Adhesion
• Based on electrostatic or dipole interaction between bonding agent and substrate
• Strength of interaction depends on CONTACT ANGLE, which is a good indication of WETABILITY of a solid by a specific liquid. A contact angle of <90o means the solid surface is hydrophilic
• Best adhesion/bonding is achieved when Van der Waals forces are optimized

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10
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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11
Q

Why is it important that dentine bonding agents are surface wetting agents?

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.
Wet dentine has a low surface energy, lower than composite filling materials.
For composite resin to stick to dentine you must make the surface of the dentine have a higher critical energy than the composite.
Dentine bonding agents increase the surface energy of the dentine surface and allow composite to flow and stick to the surface.

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12
Q

Describe how adhesion of DBAs happens

A

molecular entanglement

  • 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
  • This polymer meshes with the substrate - molecular entanglement – leading to high bond strength
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13
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.
• It is 0.5 – 5 microns in thickness.
• It is variably attached to the dentine surface.
• It is generally contaminated with bacteria.
• Originally it was thought of as a protective barrier reducing permeability of the dentine and protecting the pulp.
• Now it is considered to interfere with adhesion.

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14
Q

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

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15
Q

What are the two types of DBAs that we use today to remove/modify the smear layer?

A

Total etch

Self etch

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16
Q

What does total etch do to the smear layer?

A

completely remove it

17
Q

3 examples of total etch

A
  • Scotchbond multipurpose
  • Clearfil photo bond
  • Optibond FL
18
Q

What are the components of a total etch DBA?

A
  • Dentine conditioner: An acid, usually 35% phosphoric.
  • Primer: Really the adhesive part of the agent with a hydrophilic/hydrophobic molecule
  • Adhesive: A resin which penetrates into the surface of the dentine attaching to the primers hydrophobic surface.
19
Q

What does 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.
20
Q

What does the 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.
  • 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.
  • This molecule or group of molecules is dissolved in a suitable solvent. Ethanol, acetone or water
21
Q

What is the coupling agent found in lots of primers?

A

A coupling agent found in many primers is HEMA (Hydroxy ethyl methacrylate)

22
Q

How does HELA work as a coupling agent

A

The C=C bond opens and forms a strong covalent bond with the next resin layer.
The hydroxyl COOC2H4_OH group can combine with similar polar groups on the hydroxyapatite and can react with amine groups on the collagen protein.
• This may just be an attraction of molecule to molecule rather than a true chemical bond (Van Der Waal) but it will nonetheless give the dentine a hydrophobic surface into which the next layer of resin will flow.

23
Q

What do adhesives do?

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.
  • 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.
24
Q

What does etching dentine achieve?

A

demineralisation of the outer layer

25
Q

What does the primer achieve?

A

It can penetrate the demineralised dentine due to its bifunctional molecules. This gives the surface high energy making it hydrophobic

26
Q

What does the adhesive achieve?

A

Micro mechanical retention

27
Q

Why is total etch not a simple technique?

A

Over etching leads to:

  1. collapse of the collagen fibres so no resin can penetrate
  2. too deep an etch and the primer cannot penetrate to the full depth of the etch.

It’s moisture dependent:
• Too dry and the dentine surface collapses
• Too wet and the primer is diluted which gives reduced strength

  • Clinically this is very technique sensitive.
  • Not easy to do well.
28
Q

What happens if the tooth is over dried?

A

collapsed dentine leads to decreased porosity leads to poor penetration of dentine by primer leads to poor bond

29
Q

What happens if the tooth is over moist?

A

expanded dentine is porous and will absorb primer - gives good bond

30
Q

How has the 3 step total etch been simplified?

A

By combining as follows:
• Separate etchant with primer and sealer combined.
• Self etching primer and sealer all combined and applied together as a single solution

31
Q

How does Separate etchant with primer and sealer combined differ from the 3 step total etch

A
  • This should allow the wetting and sealing of the dentine to occur simultaneously.
  • The chemicals in the single bottle are the same as in the primer bottles e.g. HEMA, GPDM, MDP, 4-META but are combined with a resin Bis-GMA and a solvent such as alcohol or acetone and camphorquinone to allow light curing.
32
Q

Examples of total etch 2 stage bonding agents?

A
  • Scotchbond One
  • Prime and Bond
  • Optibond Solo
  • i-Bond TE
  • XP Bond
33
Q

What are the positives of self etching primers?

A

• These materials work in a different way from all the previous materials.
• 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.
• Not as technique sensitive but bond doesn’t seem to be as strong to all tooth tissues.
Most self etching primers are combined with the bonding resin to make a one bottle system. This etches, primes and bonds all together from one application at the same time.
These materials are simple to use and there is evidence of their ability to etch dentine and form a hybrid layer.

34
Q

How do self etching primers work?

A
  • To make these materials work and allow them to etch the dentine they contain acidic methacrylate monomers such as Methacryloyloxyalkyl acid phosphatase.
  • 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.
  • The acidic groups react with Ca2+ ions in the tooth bonding to the surface and also create an amorphous Calcium chelate on the surface.
  • The smear layer is dissolved but then incorporated into the hybrid layer
  • This only penetrates about 2 um into the surface but can give good bond strengths to dentine.
  • There is evidence for reduced sensitivity.
  • There is no drying stage so no chance of collapse of dentine architecture preventing resin penetration and hybrid layer formation
  • Not just an acidic monomer
  • These materials also contain the same chemicals to promote bonding as total etch materials
  • Different in different materials (Eg. Scotchbond Universal contains: MDP monomer, HEMA, Vitrebond Copolymer)
  • Fundamental mechanism of all bonding is mineral exchange where minerals removed from the dental hard tissue are replaced by resin which once mineralized mechanically interlock (MOLECULAR ENTANGLEMENT) in these porosities.
  • The interaction of these molecules with Hydroxyapatite based tissue is described as the Adhesion-Decalcification concept ‘AD-concept’
  • Initially all acid monomers bond to the calcium in HA ionically.
  • Whether they stay bonded depends on the stability of the HA-monomer bond.
  • Monomers with lower pKa (not necessarily pH) do not form a stable bond as they continue to dissolve HA
  • This leads to a hybrid layer with unstable calcium phosphates incorporated
35
Q

What are the negatives of self etch primers

A
  • 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
  • Strong self etch bonds well to enamel but less well to dentine
  • Mild self etch only partially demineralises the dentine
  • HA crystals remain around the collagen
  • Protective against hydrolytic breakdown
  • Remaining Ca ions allow ionic bonding
  • MDP and 4-META agent are better than HEMA containing materials
  • HEMA materials are more acidic and absorb more water  less durable bond
  • Scotchbond Universal is an example of a ‘mild’ etching material

Problems with the smear layer
• If the smear layer is thick then ‘Mild’ self-etch adhesives may not penetrate through it.
• 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
• Confocal microscopic image of dentine showing penetration of self etching primer into the dentine surface

36
Q

Summarise the good and bad things about self etching primers

A

What’s good about self etching agents?
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

Not perfect
• 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.
• With self etching DBAs it is probably sensible to etch the enamel with phosphoric acid to obtain the best bond. (Peumans et al. 2010)
• The bond strength to dentine is good without this so a form of selective etching probably gives the best long term results.
• We wait for more long term clinical evaluations

BDS2 Dental materials science (10 decks)

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