bonding systems Flashcards
what is dental adhesive
- provides a high strength bond to tooth tissues
- is an immediate hight strength bond - sticks straight away
- is a durable bond - lasts for a long time
- is impermeable
- easy to use and safe
what’s easier to bond to: enamel or dentine
enamel
why is enamel easier to bond to than dentine
due to the structure of enamel
- heterogenous structure (densely packed prisms)
- highly mineralised (95% organic)
- ‘dry’ (no moisture)
what technique is used to bond
acid etch technique
what happens to the surface of enamel during the acid etch technique
the long enamel prisms are filled with imperfectly packed hydroxyapatite cyrstals. The acid modifies this surface by roughening it creating an etched pattern
what does this roughened surface fo enamel allow
micro-mechanical interlocking of resin filled materials
etching also increases surface energy of the enamel. What does this allow?
the removal of surface contaminants creating higher surface energy leads to better wet ability of the enamel which allows the resin to adapt better the the roughened enamel
why must enamel be dry
must be dry as moisture will prevent the flow fo resin into the etched surface
what type of bonding is enamel
mechanical - pour liquid into the rough surface and then it hardens and interlocks with etched surface so ti doesn’t fall out
what viscosity of resin is applied first
low viscosity resin is applied onto the rough surface then high viscosity on top
what is the concentration of acid normally used for etching
can be done with a variety of organic and inorganic acids but in practice 30-50% aqueous solution of phosphoric acid is mainly used
what is a common low viscosity resin
(phosphorylated) Bis-GMA is applied to the etched and dried enamel surface to penetrate into the roughened surface and light cured
bonding to dentine is?
complicated
composition of dentine
20% organic - mostly collagen
70% inorganic - mostly hydroxyapatite
10% water
what is dentine full of
permeable tubules
why is the surface of dentine often wet
because fluid is pumped up from the pulp to the dentine floor of any cavity through the tubules
what is one difference between dentine and enamel
dentine is an inconsistent material (unlike enamel)
- aged dentine is more mineralised
- dentine near the pulp has more tubules and increased moisture content
does dentine have a low or high surface energy
low
what is a factor that makes bonding to dentine difficult
dentine isa hydrophilic material whereas most simple bonding agents are hydrophobic (oil based)
what layer does dentine have that make sit more difficult to bond to
smear layer
requirements for dentine bonding agents
- ability to flow
- potential for intimate contact with dentine surface
- low viscosity
- adhesion to substrate
what are the 3 types of adhesion to dentine
mechanical, chemical and Van Der Waals
what is mechanical adhesion to dentine
- same as enamel
- achieved by the dentine bonding agent and dentine surface meshing and interlocking with minimum gaps
- smaller holes are important - etched collagen around the tubules gives a strong bond
what is chemical adhesion to dentine
- bonding at a molecular level
- mineralised dentine = ionic bond
- organic dentine = covalent bond
what is Van Der Waals adhesion to dentine
- based on electrostatic or dipole interaction between bonding agent and substrate
- strength of bond depends on contact angle
- best adhesion/bonding is achieved when forces are optimised
what can the contact angle indicate
is an indication of the wettability of a solid by a liquid - angle of < 90 degrees = hydrophilic
what is critical surface energy
surface tension of a liquid that will just spread on the surface of a solid
what surface energy must a liquid have
must have a lower surface energy than the surface it is being placed on for it to flow onto it and stick
why will a low surface energy liquid run onto a high surface energy substrate
because this leads to a lower surface energy of the material as a whole
why does nothing stick to non-stick frying pans
because they have a very low surface energy
what surface energy does wet dentine have
low surface energy, lower than composite filling agents
what must you do to make composite to stick to dentine
must make the dentine have a higher critical energy that the composite
what do dentine bonding agents do
increase the surface energy of dentine and allow composite to flow and stick to the surface
what are surface wetting agents
- change low surface energy substrate to high
- have polar and non-polar groups
- oily end sticks to the wet dentine
how is bonding to dentine carried out
by adhesion through molecular entanglement
how does the adhesive work
- absorbed onto the surface of dentine but can also penetrate into the surface
- due to good wetting of the dentine by the adhesive and appropriate surface energies of the two
what can form a long chain polymer
the component absorbed into the dentine
*this polymer meshes with the substrate (molecular entanglement) leading to high bond strength
what is the smear layer
an adherent layer of organic debris that remains on the dentine surface after the preparation of the dent during the restoration of a tooth
- its what is left after you cut the surface of dentine with high speed power
how thick is the smear layer
around 0.5-5 microns thick
what is the smear layer generally contaminated with
bacteria
what is the smear layer thought to do
- originally thought it was a protective barrier to the pulp
- now thought it interferes with adhesion
how do you remove the smear layer
with acid etch
- is very difficult to remove
what do you 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
what generation of dental bonding agents now incorporate the smear layer into them
8th generation
what were the early dental bonding agents
phosphorylated esters of unfilled resins
what is phosphorylated Bis-GMA
a dental bonding agent
- formed of bisphenol-A glycidyl methacrylate
- also known as Scotch Bond
what is NPG-GMA
dental bonding agent
- formed of N-phenylglycidal methacrylate
- also known as Crevident
what is the bonding mechanism of these DBA’s
thought to be an ionic bond to the calcium in the dentine by the chlorophosphate or hydroxyl group
what were early DBA’s applied to
an untreated dentine surface
-bond was only to the top of the dentine layer
why were early DBA;s not very effective
- they were weak bonds as the smear layer is not firmly attached to the underlying dentine and they only bonded to this smear layer
- the phosphate to Ca bond was not strong and was affected by moisture (saliva or dentinal fluid caused hydrolysis of the bond)
- overall, early DBA’s had low bond strength and not great success
what do materials now rely on
- total etch or,
* self etch to remove the smear layer
which of total etch or self etch DBA’s were developed first
total etch
what are total etch bonding agents
- these are the 4th generation DBA and involve the complete removal of the smear layer
- still the ‘Gold Standard’ of DBA’s
what are examples of total etch DBA’s
- Scotchbond multipurpose
- Clearfil photo bond
- Optibond FL
what are the components of total etch DBA’s
- dentine conditioner (acid, usually 35% phosphoric acid)
- primer (adhesive part of the agent with a hydrophilic/phobic molecule
- adhesive (mildy phosphorylated resin which penetrates into the surface of the dentine attaching to the primers hydrophobic surface
what is the dentine conditioner
- generally phosphoric acid, but can also be EDTA or nitric acid
- removes smear layer
- opens dentinal tubules by removing smear plugs
- decalcifies the uppermost layer of the dentine
- etchant is washed off with water
- the collagen network in the top 10 microns of the dentine is exposed and subsequently penetrated by the next 2 components
what is the primer
- adhesive element (a coupling agent)
- single molecule thick
- bifunctional molecule with hydrophobic end to bond to the resin and a hydrophilic end to bond to the hydrophilic dentine surface
- must also have a spacer group to make it long enough to be flexible when bonding (lack of flexibility reduces bond strength)
- molecule is dissolved in a suitable solvent (ethanol, acetone or water)
- coupling agent found in many primers is HEMA
how does the primer work
- coupling agent found in many primers is HEMA
- hydrpxyl ethyl methacrylate
- C=C bond opens and forms a strong covalent bond with the next resin layer
- the hydroxyl group can combine with similar polar groups on the hydroxyapatite and can react with amine groups on the collagen protein (may be a true chemical bond instead of just an attraction)
- still gives dentine a hydrophobic surface for resin to flow onto
what are some examples of primers used
- HEMA - in scotch bond, optioned, clearfil
- NTG-GMA - all bond
- MDP - clearfil
- 4-META - hybrid bond, G-bond
- GPDM - optibond
what is the adhesive
- mixture of resins (usually Bis-GMA and HEMA)
- predominantly hydrophobic
- may contain some filler particles to make it stronger
- will contain camphorquinone to allow it to light cure
- penetrates the primed dentine
- forms a micro mechanical bond within the tubules and exposed dentinal collagen fibres (MOLECULAR ENTANGLEMENT)
- forms a hybrid layer or collagen plus resin
how does etching dentine work
- demineralisation of the outer layer
- 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 (gives surface high energy, making it hydrophobic)
- adhesive can now penetrate (micro-mechanical retention)
what are some problems with total etch
- over etching - can collapse collagen fibres then no resin can penetrate, too deep an etch and then the primer can not penetrate to the full depth (can then get sensitivity)
- moisture dependant - too dry and dentine surface collapses, too wet and primer is diluted (reduced strength)
- if tooth is over dried - collapsed dentine (decrease in porosity leads to poor penetration of dentine by primer leads to poor bond), moist dentine gives a good bond (opposite of above)
bonding to dentine is what
clinically very technique sensitive
- not easy to do well
how has the bonding process been made to be easier
the 3 stage process has been reduced to 2
- reduces clinical time but don’t necessarily mean better bonding
how has the steps been reduced for bonding
by combining the components of the bonding system
- can be primer and sealer combined but separate etchant
- can be self etching primer and sealer all combined and applied all together
what type of etch technique is separate etchant with primer and sealer combined
still total etch technique
how does the process of bonding work with separate etchant with primer and sealer combined
- etchant is still phosphoric acid
- primer and sealer are combined in a single bottle and applied to the etched, washed dentine and then light cured
- should allow the wetting and sealing of the dentine to occur simultaneously
what are the chemicals in the single bottle of primer and sealer
the same as is in the primer bottles
- HEMA, GPDM, MDP and 4-META
- but are combined with a resin Bis-GMA and a solvent such as alcohol or acetone and camphorquinone to allow light curing
examples of total etch 2-stage bonding agents
- Scotchbond one
- Prime and bond
- Optibond solo
- I-bond TE
- XP bond
- clinical handling of these is very similar to that of the 3-part total etch materials and bond strengths are similar
what are self etching primers
- work in a different way from all the other materials
- do not remove the smear layer - instead have molecules in it which infiltrate and incorporate it into themselves
- these are not washed off - removes problem of having to dry dentine
- most are combined with the bonding resin to make a one bottle system - etches, primes and bonds all in one
what is good about the self etching primers
- not as technique sensitive
* are simple to use and there is evidence of their ability to etch dentine and form a hybrid layer
what is not good about self etching primers
the bond is not as strong to all the tooth tissues
what do self etching primers contain to allow them to etch the dentine
acidic methacrylate monomers such as methacryloyloxyalkyl acid phosphatase
how do self etching primers work
- these very complex bifunctional monomers etch and infiltrate the dentine surface with heir hydrophilic end while polymerising like a simpler Bis-GMA material at the hydrophobic end
- the acidic group react with calcium ions in the tooth bonding to the surface and also create and amorphous calcium chelate not eh surface
- the smear layer is dissolved and incorporated into eh hybrid layer
how far does the self etching primers penetrate into eh surface of dentine
about 2microns but can give a good bond to the dentine
how is their reduced sensitivity using self etching primers
there is no drying stage so no chance of collapse of dentine architecture preventing resin penetration and hybrid layer formation
what is the fundamental mechanism of all bonding
mineral exchange where minerals are removed from the dental hard tissue and are replaced by resin which once mineralised, mechanically interlocks in these porosities
-MOLECULAR ENTANGLEMENT
what is the adhesion-decalcification concept
- initially all acid monomers bond to the Ca in HA ionically
- whether they sat bonded depends not eh stability of the HA-monomer bond
- monomers with a low pKa (not necessarily low pH) do not form a stable bone as they continue to dissolve HA
- leads to hybrid layer with unstable calcium phosphates incorporated
why don’t you want a strong acid for bonding
because it causes weaker bonds
why is self etch not better than total etch
- etching by-products are not washed away as in total etch
- these by-products are soluble and weaken the integrity of the bond
- if too much HA is dissolved away the exposed collagen is vulnerable to breakdown and the bond will fail
- strong self etch bonds well the enamel but not as well dentine
how does mild self etch work
- only partially demineralises the dentine (Scotchbond is example)
- HA crystals remain around the collagen - protective against hydrolytic breakdown, and the remaining Ca ions allow ionic bonding
why are MDP and 4-META better than HEMA containing materials
HEMA materials are more acidic and absorb more water so form a less durable bond
what are the problems with the smear layer
- if thick then the mild self etch adhesives may not penetrate through it
- need to penetrate the smear layer without decalcifying the tooth tissue too much and removing the HA (HA needed for strong bonds)
- if thick then the self etch may not be able to penetrate as deep as the bottom of the smear layer (smear layer may be 5 microns but etch can only go 3-4microns)
what’s good about the 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 bad about self etching bonding agents
- great variability between products with regard to initial pH of the solution
- difference in pH results in different etch and different penetration of resin - materials with lower pH may not etch enamel efficiently
- there is little evidence to show that is has stronger bond to dentine than with total etch
what do bonding agents work as
surface wetting agents
bonding to enamel is?
easy
bonding to dentine is?
difficult
