Bonding Systems Flashcards
What makes enamel good for bonding and how does bonding occur?
What type of bonding?
highly mineralised, dry prisms
acid etching with phosphoric acid creates microporosities on enamel surface
this roughened etched pattern allows micromechanical interlocking of resin materials
etching also increases surface energy for better wetting by resin
bonding to etched, dry enamel is essentially a micromechanical process
What is the surface energy of a material?
A measure of the disrupted intermolecular bonds that exist at the surface of a material. Surfaces with high surface energy attract other materials to reduce this energy.
Why do impurities reduce the surface energy?
surface atoms become attracted to impurities and bond
What is the wetability of a material?
ability of a liquid to spread out and maintain contact with a solid surface.
Why must enamel be dry for bonding?
Moisture contamination will prevent flow of the resin into the etched surface.
What is the bond strength of enamel to composite ?
> 20MPa
usually between 20-30
What is the composition of dentine?
What are the characteristics and why does this affect bonding?
20% organic (mostly collagen) 70% inorganic (HA) 10% water
Full of permeable tubules (increased water contimation)
Less mineralisation
Low surface energy (due to water and weaker forces due to collagen
What water quality are most bonding agents and what water quality is dentine?
Hydrophobic (plastic is oil)
What is an added complicating factor when bonding dentine?
Why is this not present in enamel?
smear layer - an adherent layer of organic debris that remains on the dentine surface after the preparation of the dentine during the restoration of a tooth
does not occur in enamel as enamel is more mineralised (more resistance than collagen) and debris can penetrate less due to lack of tubules
What bonds does the adhesion of dentine include?
Mechanical
Chemical
Van der waals
How does mechanical bonding in dentine occur?
Same as enamel bonding. DBA and dentine surface mesh together and interlock with minimum gaps.
At the molecular level, what bonds do the mineralised and organic components of dentine make?
Mineralised = ionic
Organic = covalent
What does a contact angle of less than 90 mean?
Solid surface is hydrophilic and has good wetability (better adhesion to surface)
smaller contact angle = better wetting
What is a critical surface energy?
the surface tension of the liquid needed to just spread on solid (in that case the contact angle of solid is zero)
For a liquid to flow onto a surface and stick, what must happen?
Liquid must have a lower surface energy than the surface it is being placed on
What is the surface energy of wet dentine?
Low surface energy - lower than composite filling materials
For composite resin to stick to dentine, what must be done?
The surface energy of dentine must be increased higher than the composite so that the composite is able to flow over and adhere
What agents increase the surface energy of dentine?
Dentine bonding agents (surface wetting agents)
What is molecular entanglement?
Is it the same as mechanical bonding
Absorbed adhesive forms a long chain polymer which meshes with substrate (dentine) leading to high strength bond after there is good wetting of the dentine by adhesive and appropriate surface energies of the two
mineral exchange where some minerals from the dental hard tissue are removed and are replaced by the resin which once mineralised can mechanically interlock in these porosities.
It is specific to polymers unlike mechanical bond which is general
What are the options of what to 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 is it not a good idea to incorporate the smear layer?
the smear layer is not tightly bound to dentine and was affected by moisture
therefore saliva or dentinal fluid caused hydrolysis of the bond and it would break down resulting in leakage of the filling
What do todays materials rely on that previous generations of DBAs didn’t?
Total etch or self etch
To modify/ remove the smear layer
What is the gold standard DBA?
Total etch bonding agents
What are examples of total etch DBAs?
Scotchbond multipurpose
Clearfil photo bond
Optibond FL
What are the components of a total etch dentine bonding agent?
Dentine conditioner (etch) - acid usually 35% phosphoric acid
Primer - really the true adhesive part of the agent with a hydrophobic/hydrophilic molecule
Adhesive - resin which penetrates into the surface of the dentine attaching to the primer’s adhesive surface
What does the dentine conditioner do in DBAs?
Removes smear layer
Opens dentinal tubules by removing smear plugs
Decalcifies the uppermost layer of dentine
Increases surface area
Exposes collagen network and subsequently penetrated by the next two components (primer + bond)
What does the primer do?
Coupling agent
Bifunctional molecule with a hydrophilic end to bond to the dentine surface and a hydrophobic methacrylate end to bond to the resin
maintains expanded collagen network while removing residual water off dentine, helps bond infiltrate into collagen network
What must the primer have to ensure flexibility?
The molecule must have a spacer group to make it long enough to be flexible when bonding. Lack of flexibility reduces bonding sites and bond strength
What solvent dissolve primer?
water
acetone
ethanol
What is a common coupling agent found in many primers?
HEMA (water absorbing plastic)
What does the C=C bond in HEMA form?
Strong covalent bond with next resin layer
What does the hydrophilic hydroxyl COOC2H4-OH group combine/react with?
What does this create?
Similar polar heads in dentine HA and reacts with amine groups on the collagen protein
gives the dentine a hydrophobic high energy surface that the bond will flow into
What is the adhesive usually a mix of and what do adhesives have that improve bond strength?
Mix of resins. Usually Bis-GMA and HEMA
May contain some filler particles to make it stronger
What does the adhesive have that allows it to light cure?
camphorquinone
How does the adhesive bond?
Penetrates the primed dentine which now has a hydrophobic surface
Forms a micro-mechanical bond within the tubules and exposed dentinal collagen fibres
Carries out molecular entanglement
Forms the hybrid layer of collagen and resin
What are the problems with total etch?
not a simple technique
Over etching leads to collapse of the collagen fibres so no resin can penetrate
Over etching creates too deep an etch and the primer cannot penetrate to the full depth of the etch.
Too dry and the dentine surface collapses
Too wet and the primer is diluted leads to reduced strength
How have the bonding steps been combined?
Separate etchant with primer and sealer combined.
Self etching primer and sealer all combined and applied together as a single solution.
What type of technique is the separate etchant with primer and sealer combined?
Total etch technique
What are examples of total etch two-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.
1 bottle system
Advantages VS Disadvantages of self etch
Advantages
Less technique sensitive
No rinsing and no excessive drying
Simultaneous demineralisation and resin infiltration
Disadvantages
Difference in pH results in different etch and different penetration of resin.
Those materials with a lower initial pH may not etch enamel efficiently.
Why does pH affect self-etch?
acid monomers with a lower pKa do not form a stable bond as they continue to dissolve HA
What allows self etch DBAS to etch the dentine?
What else do they contain?
Bifunctional complex acidic methacrylate monomers, which infiltrate the dentine surface with their hydrophilic end and then they polymerise a simpler bis-GMA material at the hydrophobic end.
contain same chemicals as total and bonds via molecular entanglement and mechanical bonding
What reacts with Ca+ ions at the tooth surface in self-etch?
What type of bond
acidic groups to form amphorous chelate
ionic
Strong self etch VS Mild self etch
Strong
Soluble byproducts are formed that weaken the integrity of the bond and are not washed away
If too much hydroxyapatite is dissolved away the exposed collagen is vulnerable to breakdown and the bond will fail
Mild
Partially demineralises the dentine - HA crystals remain around the collagen
Protective against hydrolytic breakdown
Remaining Ca ions allow ionic bonding
Why must HA not be completely demineralised?
The HA is required for strong durable bonding
The HA protects the dentine from hydrolytic breakdown
Which direct restorative material relies on chelation between carboxyl groups and the calcium in the tooth as a major component of the bonding mechanism?
Glass ionomer cements
