05. Bonding to Teeth Flashcards
L5: What are the required properties of a dental bond?
- Provide a high bond strength to tissues;
- Immediate high strength bond;
- Durable bond;
- Impermeable bond;
- Easy to use;
- Safe.
L5: How easy is it to bond to enamel and why?
Easy, due to its structure
L5: What structural properties of enamel make it easy to bond to?
- Heterogeneous, densely packed prisms;
- Highly mineralised, 95% inorganic;
- ‘Dry’.
L5: What is the purpose of applying acid etch?
To roughen the surface of enamel and produce a characteristic etched pattern
L5: Etching enamel provides a roughened surface to bond to, how do materials bond?
- 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.
L5: When bonding to enamel, what environmental factor is necessary?
Enamel should be dry
L5: In practice, what solution is usually used to etch?
A 30-50% aqueous solution of phosphoric acid
L5: What type of bond is typically applied to teeth, after etching?
A low viscosity Bis-GMA resin (plastic)
L5: When bond (low viscosity resin) is applied it flows into the roughness of enamel surface, then what happens?
It polymerises and forms a strong bond, >20 MPa
L5: What is the strength of a bond between enamel and dental bond?
> 20 MPa
L5: How easy is it to bond to dentine and why?
- 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).
L5: What is the general composition of dentine and how does this change with age?
- 20% organic (collagen), 70% inorganic (hydroxyapatite), 10% water;
- Becomes more mineralised with age.
L5: What are the further requirements to bond to dentine?
- Ability to flow;
- Low viscosity;
- Adhesion via mechanical, chemical and Van Der Waals forces.
L5: What is the primary form of bonding to dentine?
Mechanical (interlocking between resin and meshed surface)
L5: At a molecular level, how does DBA bond to mineralised dentine content?
Ionic bonding
L5: At a molecular level, how does DBA bond to organic dentine content?
Covalent bonding
L5: What is Van der Waals adhesion?
- 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).
L5: What contact angle, with VDW forces, means that a surface is hydrophilic?
<90 degrees
L5: For a liquid to stick to a surface it is placed on to, how must the surface energies differ?
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)
L5: What should have a higher critical surface energy to form a bond, dentine or composite resins?
Dentine
if wet, it is lower than most resins and they therefore won’t stick!
L5: Some dental adhesive is absorbed into the dentine surface, where it can then… ?
Polymerise and entangle at a molecular level, leading to high bond strength
L5: What is the smear layer?
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.
L5: How thick is the smear layer?
0.5-5 microns thick
L5: How does the smear layer affect adhesion to dentine?
It interferes with it as the smear layer is now firmly attached to the dentine
L5: How can the smear layer be dealt with (2 options)?
- Remove and bond to the ‘clean’ dentine beneath;
- Incorporate it into the DBA and stick it to the dentine below.
L5: Previous etch generations were developed to bond to the smear layer however this proved ineffective, what two types of etch are now used?
- Total etch;
- Self etch.
(to remove and/ or modify the smear layer)
L5: What are total etch bonding agents and how do they work?
- 4th generation DBA;
- Involve complete removal of smear layer.
L5: Give three examples of total etches:
- Scotchbond multipurpose;
- Clearfil photo bond;
- Optibond FL.
L5: What are the components of a total etch DBA and what do they do?
- Dentine conditioner (an acid);
- Primer (adhesive part, hydrophilic/ hydrophobic molecule);
- Adhesive (a resin which penetrates into the surface).
L5: What does the dentine conditioner (etch) component of a DBA do?
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.
L5: What does the primer component of a DBA contain and do?
- 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).
L5: Name a typical coupling agent in primers, of DBAs:
Hydroxy ethyl methacrylate (HEMA)
L5: What does the adhesive component of a DBA contain and do?
- 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.
L5: Summarise bonding to dentine, by the three components of a total etch:
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.
L5: What are the problems with overreaching when using a total etch?
- Collapse of collagen fibres, so no resin can penetrate;
- Etching too deep so the primer cannot penetrate fully.
L5: How do moisture parameters effect the performance of total etch?
- Too dry: dentine surface collapses;
- Too wet: primer dilutes and strength is reduced;
- Not simple to get right.
L5: When bonding to dentine, what environmental factor is necessary?
Dentine should be moist
L5: How many clinical stages are there to total etches?
1 or 2, depending on whether the etch is separate or also combined into the primer/adhesive mix
L5: How do the bond strengths of two-stage bonding agents compare to that of three part materials?
Comparable
L5: What are self etch primers and how do they work?
- 5th generation;
- Involve incorporation of smear layer into themselves.
L5: How are self etching primers clinically less sensitive than other etches?
- Do not require washing off;
- Removes the problem of how dry to make the dentine.
L5: How does the bond strength of self etches compare to other etches?
Lower bond strength to tooth
L5: What happens to the smear layer when using a set etch?
It is dissolved and incorporated into the hybrid layer, penetrating about 2um into the surface
L5: How is the ‘no-drying’ stage of self etches beneficial to the patient?
- Reduced chance of collapsed dentine architecture;
- Prevents resin penetration;
- Reduced sensitivity.
L5: What is the difference in composition between a total etch and self etch?
- Self etches contain an acidic monomer, that incorporate themselves into the surface;
- Total etches (usually) contain phosphoric acid which removes the smear layer.
L5: What is the fundamental mechanism of all dental bonding?
- Mineral exchange;
- Minerals removed from hard tissue;
- Replaces by resin;
- These mineralise and mechanically interlock in porosities;
- i.e. molecular entanglement.
L5: What is the most challenging aspect of developing a self etch?
- 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.
L5: What are the advantages of self etching bonding agents?
- Less technique sensitive;
- Simultaneous demineralisation and resin infiltration (less chance of post-op sensitivity);
- Easier.
L5: Why are self etching bonding agents less technique sensitive?
- 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).
L5: What are the disadvantages of self etching bond agents?
- 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.
