05. Bonding to Teeth

Question 1

L5: What are the required properties of a dental bond?

Answer 1

• Provide a high bond strength to tissues;
• Immediate high strength bond;
• Durable bond;
• Impermeable bond;
• Easy to use;
• Safe.

Question 2

L5: How easy is it to bond to enamel and why?

Answer 2

Easy, due to its structure

Question 3

L5: What structural properties of enamel make it easy to bond to?

Answer 3

• Heterogeneous, densely packed prisms;
• Highly mineralised, 95% inorganic;
• ‘Dry’.

Question 4

L5: What is the purpose of applying acid etch?

Answer 4

To roughen the surface of enamel and produce a characteristic etched pattern

Question 5

L5: Etching enamel provides a roughened surface to bond to, how do materials bond?

Answer 5

• Microchemical interlocking of resin filling materials;
• Increased surface energy of the enamel, due to removed surface contaminants i.e. better wettability;
• Better wettability allows the resin to adapt better to the enamel surface;
• Bonding is essentially mechanical.

Question 6

L5: When bonding to enamel, what environmental factor is necessary?

Answer 6

Enamel should be dry

Question 7

L5: In practice, what solution is usually used to etch?

Answer 7

A 30-50% aqueous solution of phosphoric acid

Question 8

L5: What type of bond is typically applied to teeth, after etching?

Answer 8

A low viscosity Bis-GMA resin (plastic)

Question 9

L5: When bond (low viscosity resin) is applied it flows into the roughness of enamel surface, then what happens?

Answer 9

It polymerises and forms a strong bond, >20 MPa

Question 10

L5: What is the strength of a bond between enamel and dental bond?

Answer 10

> 20 MPa

Question 11

L5: How easy is it to bond to dentine and why?

Answer 11

• Difficult;
• Presence of permeable tubules;
• Fluid pumped from pulp to dentine floor making surface wet;
• Inconsistent material depending on age and proximity to pulp;
• Low surface energy;
• Dentine is hydrophilic (most bonding agents are hydrophobic).

Question 12

L5: What is the general composition of dentine and how does this change with age?

Answer 12

• 20% organic (collagen), 70% inorganic (hydroxyapatite), 10% water;
• Becomes more mineralised with age.

Question 13

L5: What are the further requirements to bond to dentine?

Answer 13

• Ability to flow;
• Low viscosity;
• Adhesion via mechanical, chemical and Van Der Waals forces.

Question 14

L5: What is the primary form of bonding to dentine?

Answer 14

Mechanical (interlocking between resin and meshed surface)

Question 15

L5: At a molecular level, how does DBA bond to mineralised dentine content?

Answer 15

Ionic bonding

Question 16

L5: At a molecular level, how does DBA bond to organic dentine content?

Answer 16

Covalent bonding

Question 17

L5: What is Van der Waals adhesion?

Answer 17

• Based on electrostatic or dipole interaction between DBA and substrate;
• Strength of interaction depends on bond angle (which indicates wettability of a solid, by a liquid).

Question 18

L5: What contact angle, with VDW forces, means that a surface is hydrophilic?

Answer 18

<90 degrees

Question 19

L5: For a liquid to stick to a surface it is placed on to, how must the surface energies differ?

Answer 19

Liquid must have a lower surface energy than the surface it is applied to

(remember PTFE has a very low surface energy, which is why nothing sticks to it)

Question 20

L5: What should have a higher critical surface energy to form a bond, dentine or composite resins?

Answer 20

Dentine

if wet, it is lower than most resins and they therefore won’t stick!

Question 21

L5: Some dental adhesive is absorbed into the dentine surface, where it can then… ?

Answer 21

Polymerise and entangle at a molecular level, leading to high bond strength

Question 22

L5: What is the smear layer?

Answer 22

An adherent layer of organic debris that remains on the dentine surface, after preparation of dentine during restoration of a tooth. This is generally contaminated with bacteria.

Question 23

L5: How thick is the smear layer?

Answer 23

0.5-5 microns thick

Question 24

L5: How does the smear layer affect adhesion to dentine?

Answer 24

It interferes with it as the smear layer is now firmly attached to the dentine

Question 25

L5: How can the smear layer be dealt with (2 options)?

Answer 25

- Remove and bond to the 'clean' dentine beneath; | - Incorporate it into the DBA and stick it to the dentine below.

Question 26

L5: Previous etch generations were developed to bond to the smear layer however this proved ineffective, what two types of etch are now used?

Answer 26

- Total etch; - Self etch. (to remove and/ or modify the smear layer)

Question 27

L5: What are total etch bonding agents and how do they work?

Answer 27

- 4th generation DBA; | - Involve complete removal of smear layer.

Question 28

L5: Give three examples of total etches:

Answer 28

- Scotchbond multipurpose; - Clearfil photo bond; - Optibond FL.

Question 29

L5: What are the components of a total etch DBA and what do they do?

Answer 29

- Dentine conditioner (an acid); - Primer (adhesive part, hydrophilic/ hydrophobic molecule); - Adhesive (a resin which penetrates into the surface).

Question 30

L5: What does the dentine conditioner (etch) component of a DBA do?

Answer 30

Usually phosphoric (35%) acid but can be EDTA/ nitric acid. - Removes smear layer; - Opens dentinal tubules by removing smear plugs; - Decalcifies uppermost layer of the dentine; - Collagen network exposed for penetration by next two components.

Question 31

L5: What does the primer component of a DBA contain and do?

Answer 31

- Adhesive; - Coupling agent, bifunctional molecule; - One hydrophilic end (to bond to hydrophilic dentine surface) and; - One hydrophobic end (to bond to resin); - Spacer group to increase flexibility for bonds (therefore will be stronger, with more bonding sites); - These molecules are dissolved in a suitable solvent (ethanol/ acetone/ water).

Question 32

L5: Name a typical coupling agent in primers, of DBAs:

Answer 32

Hydroxy ethyl methacrylate (HEMA)

Question 33

L5: What does the adhesive component of a DBA contain and do?

Answer 33

- Mixture of resins; - Usually Bis-GMA and HEMA; - Mainly hydrophobic; - May contain some filler particles to make it stronger; - Contains camphorquinone for light curing; - Penetrates the primed dentine surface (now hydrophobic); - Forms micro chemical bond within the tubules; - Molecular entanglement; - Forms the hybrid layer of collagen + resin.

Question 34

L5: Summarise bonding to dentine, by the three components of a total etch:

Answer 34

Etch: - Demineralisation of the outer layer; - Remaining collagen is hydrophilic and has low surface energy (resin will not bond). Primer: - Penetrates the demineralised dentine, due to its bifunctional molecules; - Gives the surface high energy, making it hydrophobic. Adhesive: - Can now penetrate; - Micro-mechanical retention.

Question 35

L5: What are the problems with overreaching when using a total etch?

Answer 35

- Collapse of collagen fibres, so no resin can penetrate; | - Etching too deep so the primer cannot penetrate fully.

Question 36

L5: How do moisture parameters effect the performance of total etch?

Answer 36

- Too dry: dentine surface collapses; - Too wet: primer dilutes and strength is reduced; - Not simple to get right.

Question 37

L5: When bonding to dentine, what environmental factor is necessary?

Answer 37

Dentine should be moist

Question 38

L5: How many clinical stages are there to total etches?

Answer 38

1 or 2, depending on whether the etch is separate or also combined into the primer/adhesive mix

Question 39

L5: How do the bond strengths of two-stage bonding agents compare to that of three part materials?

Answer 39

Comparable

Question 40

L5: What are self etch primers and how do they work?

Answer 40

- 5th generation; | - Involve incorporation of smear layer into themselves.

Question 41

L5: How are self etching primers clinically less sensitive than other etches?

Answer 41

- Do not require washing off; | - Removes the problem of how dry to make the dentine.

Question 42

L5: How does the bond strength of self etches compare to other etches?

Answer 42

Lower bond strength to tooth

Question 43

L5: What happens to the smear layer when using a set etch?

Answer 43

It is dissolved and incorporated into the hybrid layer, penetrating about 2um into the surface

Question 44

L5: How is the 'no-drying' stage of self etches beneficial to the patient?

Answer 44

- Reduced chance of collapsed dentine architecture; - Prevents resin penetration; - Reduced sensitivity.

Question 45

L5: What is the difference in composition between a total etch and self etch?

Answer 45

- Self etches contain an acidic monomer, that incorporate themselves into the surface; - Total etches (usually) contain phosphoric acid which removes the smear layer.

Question 46

L5: What is the fundamental mechanism of all dental bonding?

Answer 46

- Mineral exchange; - Minerals removed from hard tissue; - Replaces by resin; - These mineralise and mechanically interlock in porosities; - i.e. molecular entanglement.

Question 47

L5: What is the most challenging aspect of developing a self etch?

Answer 47

- Not removed by washing away; - Remaining etch can dissolve hydroxyapatite away; - This weakens the bond; - Must design an etch that penetrates the smear layer enough to get through it, but without decalcifying the tooth (too much) and removing the HA which is required for a strong bond; - HA also protects the dentine from hydrolytic breakdown.

Question 48

L5: What are the advantages of self etching bonding agents?

Answer 48

- Less technique sensitive; - Simultaneous demineralisation and resin infiltration (less chance of post-op sensitivity); - Easier.

Question 49

L5: Why are self etching bonding agents less technique sensitive?

Answer 49

- No rinsing (therefore less problems with moisture control); - No excessive drying (therefore prevents collapse of dentine architecture); - No dentine collapse (leading to low bond strength).

Question 50

L5: What are the disadvantages of self etching bond agents?

Answer 50

- Great variability between products (initial pH); - Different pHs mean different etch penetration; - Lower pH materials may not etch efficiently; - Little evidence they form stronger bonds than total etches.