Bonding systems Flashcards
What are the properties of a dental adhesive
Provide a high bond strength to tooth tissues
Immediate high strength bond
Durable bond
Impermeable bond
Easy to use
Safe
Why is bonding to enamel easy
Heterogeneous structure: densely packed prismatic
Highly mineralised: 95% inorganic
‘Dry’
What percentage aqueous solution of phosphoric acid is normally used in practice for enamel etching
30-50%
What does the acid etch technique do
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
What is the composition of dentine
20% organic (mostly collagen)
70% inorganic (mostly hydroxyapatite)
10% water
What makes dentine an inconsistent material
Aged dentine is more mineralised. Dentine near the pulp has more tubules and increased moisture content
What are the requirements of a dental bonding agent
Ability to flow
Potential for intimate contact with dentine surface
Low viscosity
Adhesion to substrate
-Mechanical
-Chemical
-Van der Waals
-Combination of the above
What bonds are formed within organic dentine
Covalent
What bonds are formed within mineralised dentine
Ionic
When is the best adhesion/bonding within dentine acheived
When Van der Waals forces are optimised (strength depends on contact angle <90% indicates solid surface is hydrophilic)
What is critical surface energy
The surface tension of a liquid that will just spread on the surface of a solid
When will a liquid flow over a surface and stick to it
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.
Why are dental bonding agents required in restoring dentine
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
What is molecular entanglement
Adhesive is absorbed and can penetrate into surface of dentine where it can form a long chain polymer which meshes with the substrate
Why is molecular entanglement desired
Leads to a high bond strength
What is the smear layer
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
What is the thickness of the smear layer
0.5-5 microns
What do we do with the smear layer
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
Why did earlier fillings using (Phosphorylated Bis-GMA : bisphenol-A glycidyl methacrylate (Scotchbond), NPG-GMA: N-phenylglycidyl methacrylate (Crevident)) tend to fail more fresuently
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
What do materials today rely on for bond strength
Total etch
Self etch
To remove and/or modify the smear layer
What generation of DBA are we on currently
8th
What are examples of total etch bonding agents
Scotchbond multipurpose
Clearfil photo bond
Optibond FL
What are the 3 components of a total etch DBA
Dentine conditioner
Primer
Adhesive
What does a dentine conditioner do
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.
What does a DBA primer do
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
What does lack of flexibility of the molecule within primer result in
Lack of flexibility reduces bonding sites and bond strength
What is a suitable solvent for the molecule/group of molecules within DBA primer
Ethanel
Acetone
Water
What does HEMA stand for
Hydroxy ethyl methacrylate
Why is HEMA found in many primers
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
What is a DBA adhesive
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.
What is the purpose of adhesive within DBAs
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.
Breakdown the steps of a DBA
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
What are problems with total etch
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
What occurs if the dentine is over dried
Collapsed dentine-decrease in porosity
Decrease in porosity - poor penetration of
dentine by primer - poor bond
What are some total etch 2-stage bonding agents
Scotchbond One
Prime and Bond
Optibond Solo
i-Bond TE
XP Bond
How do self-etching primers work
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.
What bifunctional monomers do self etching primers contain
Acidic methacrylate monomers such as Methacryloyloxyalkyl acid phosphatase
What does methacryloyloxyalkyl acid phosphatase do to assist bonding to dentine
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
With self etching primers what happens to the smear layer
The smear layer is dissolved but then incorporated into the hybrid layer
Why is strong self etch not always best
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
What is the aim of every self etching material
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
What are the positives with self-etching bonding 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
What are the imperfections with self etching bonding agents
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.
