Bonding Systems Flashcards
what are the properties of dental adhesives
- provide a high bond strength to tooth tissues
- immediate high strength bone
- durable bond
- impermeable bond
- easy to use
- safe
why is enamel bonding easy
structure of enamel - heterogenous, densely packed and prismatic
highly mineralised and dry
describe the acid etch technique in relation to enamel bonding
- long enamel prisms are filled with imperfectly packed hydroxyapatite crystals
- this surface can be modified by application of acid
- the acid roughens the surface of the enamel producing a characteristic etched pattern
why is acid etching used for enamel bonding
- roughened surface allows micro mechanical interlocking of resin filling materials
- etching also increases the surface energy of the enamel surface by removing surface contaminants leading to better wettability of the enamel
- better gettability allows the resin to adapt better to roughened enamel surface
- enamel must be dry for this to work
why does enamel need to be dry for bonding
moisture contamination will prevent the flow of resin into the etched surface
what does wettability mean
ability of a liquid to maintain contact with a solid surface
what is used for enamel acid etching
variety of organic and inorganic acids, in practice a 30-50% aqueous solution of phosphoric acid is normally used
what bonding agent is applied to the etched enamel surfaec
low viscosity bis-GMA resin or any one of the dentine bonding agent resins (often phosphorylated bis-GMA)
how are bonding agents applied to the etched enamel surface
applied to etched and dry enamel surface, penetrate into the rough surface and then be light cured
what % phosphoric acid is usually used to etch enamel
35%
what are the advantages of using phosphoric acid as an etching agent
straight forward bond
strong acid etches well
resin can flow into the roughness
why is bonding to dentine more complicated
the composition of dentine is 20% organic, 70% inorganic and 10% water and is full of permeable tubules
fluid pumps up from the pulp to dentine floor of any cavity which makes the surface wet
aged dentine is more mineralised, dentine near the pulp has more tubules and increases the moisture content
dentine is hydrophilic whereas most simple bonding agents are hydrophobic
low surface energy
what are some requirements for dentine bonding agents
- ability to flow
- potential for intimate contact with the dentine surface
- low viscosity
- adhesion to substrate
what is bonding to dentine actually achieved by - ie what is the mechanism
dentine bonding agent and dentine surface mesh and interlock with minimal gaps
easy to see microscopically
what does inorganic mean
there is no carbon in the molecular structure so it is mostly a mineral
what does it meant to describe enamel as dry
it has a low moisture content
why can composite be described as a resin
when it is applied it goes into the pits and pore to lock in
how does etching affect the surface energy of enamel
It increases the surface area so the resin can adapt better over the surface
why is it better for the viscosity to be low for the resin when bonding to enamel
the runnier it is the more it can get into the nooks and crannies
what are the forces in action when an agent binds to dentine
mechanical
chemical
van der waals
describe the mechanical method of dentine bonding
it is the same as in enamel bonding. achieved by the dentine bonding agent and the dentine surface meshing and interlocking with minimum gaps
easy to see microscopically especially on SEM
which bond type is used for the chemical adhesion of mineralised dentine
ionic
which bond type is used for the chemical adhesion of organic dentine
covalent
describe van der Waals adhesion to dentine
it is based on electrostatic or dipole interaction between bonding agent and substrate
strength of the interaction depends on contact angle, which is a good indication of wettability of a solid by a specific liquid
best bonding is achieved when the van der Waals forces are optimised
what does a contact angle of less than 90 degrees mean for dentine
the solid surface is optimised
what is the critical surface energy
this means the surface tension of a liquid will just spread on the surface of a solid without sticking or absorbing
describe how critical surface energy works
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
describe how critical surface energy relates to dentine
wet dentine has a low surface energy and is lower than the composite filling materials. for composite rain to stick to dentine, you must make the surface of dentine have a higher critical energy than the composite.
dentine bonding agents increase the surface energy of the dentine surface and allow composite to flow and stick to the surface
how does adhesion to dentine work
through molecular entanglement - adhesive is absorbed onto the surface but can also penetrate into the surface of the dentine
this is due to good wetting of the dentine by the adhesive and appropriate surface energies of the two
absorbed component forms a long chain polymer and this polymer meshes with the substrate through molecular entanglement and this leads to high bond strength
what is the smear layer
this is the adherent layer of organic debris that remains on the dentine surface after preparation of the dentine during restoration of the tooth
it is variably attached to the dentine surface and is contaminated with bacteria
originally thought of as a protective barrier reducing the permeability but is now considered to interfere with adhesion
how thick is the smear layer
0.5-5 microns
what are some theories on what should be done 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
what was the bonding mechanism of early dentine bonding agents such as phosphorylated bis-BMA and NPG GMA
ionic bond to the calcium in dentine by chlorophosphate or hydroxyl group
what is bis GMA the shortened version of
bisphenol-A glycidyl methacrylate
what is scotchbond
phosphorylated bis-GMA
describe why the early dentine bonding techniques were not successful
they were applied to untreated dentine surfaces
the bonds were only to the top of the smear layer, which is inherently weak as the smear layer is not firmly attached to the underlying dentine
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
overall means there were low bond strengths and not a great success
what do modern materials for dentinal bonding rely on to remove the smear layer
total etch
self etch
describe total etch bonding agents
4th generation of dentine bonding agents that involve the complete removal of the smear layer and is the gold standard of bonding agent
what are some examples of total etch bonding agents
scotch bond multipurpose
clearfil photo bond
optioned FL
what are the main components of total etch dentine bonding agents
dentine conditions: usually 35% phosphoric acid
primer: the adhesive part of the agent, hydrophilic/hydrophobic molecule
adhesive: a resin which penetrates into the surface of the dentine attaching to primers hydrophobic surface
what is an adhesive
a resin which penetrates into the surface of the dentine attaching to the primers hydrophobic surface
describe dentine conditioners and their function
generally these are phosphoric acid, can be nitric in older systems. functions are:
- remove the smear layer
- open dentinal tubules by removing smear plugs
- decalcifies the uppermost layer of the dentine
- etchant is washed off with water
- collagen network in top 10 micrograms of dentine is exposed and subsequently penetrated by the next two components (prier and adhesive)
what are the three components of total etch dentine bonding agents
conditioner
primer
adhesive
describe the primer component of total etch dentine bonding agents
primer is the adhesive element in the process and acts as a coupling agent
bifunctional molecule with a hydrophilic end to bond to the hydrophilic dentine surface and a hydrophobic, methacrylate end to bond to the resin
molecule must also have a spacer group to make it long enough to be flexible when bonding. lack of flexibility reduces bonding sites and bond strength
molecule or group of molecule is dissolved in a suitable solvent ie ethanol, acetone or water
the C=C bond opens and forms a strong covalent bond with the next resin layer
hydroxyl COOC2H4 group can combine with similar polar groups on the hydroxyapatite and can react with amine groups on the collagen protein
what is the coupling agent found in many primers
hydroxy ethyl methacrylate (HEMA)
which chemical primer is food in scotch bond, gluma, optibond, clear fit and all bond
HEMA
which chemical primer is found only in all bond
NTG-GMA
which chemical primer is found in I bond, hybrid bond, and g bond
4-META
which chemical primer is found in clearfil only
MDP
which chemical primer is found in optibond only
GPDM
describe the adhesive component of total etch dentine bonding agents
mixture of resins, usually bis-GMA and HEMA
predominantly hydrophobic
contains some filler particles to make it stronger
will contain camphorquinone to allow it to light cure
what does the adhesive component of total etch dentine bonding agents contain to allow it to light cure
camphorquinone
how does the adhesive component of total etch dentine bonding agents work
penetrates the primed dentine which now has a hydrophobic surface
forms a micro mechanical bond within the tubules and exposed dentinal collagen fibres
forms the hybrid layer of collagen plus resin
summarise dentine bonding using total etch dental bonding agents
dentine is etched meaning there is demineralisation of the outer layer. the remaining collagen is hydrophilic and has low surface energy. normal resin will not bond
the primer can penetrate the demineralised dentine due to its bifunctional molecules, giving the surface high energy, making it hydrophobic
adhesive can now penetrate, leading to micro mechanical retention
what are the problems with the total etch technique for dental bonding
over etching can lead to collapse of the collagen fibres so no resin can penetrate
if the etch is too deep the primer cannot penetrate to the full depth of the etch
it is moisture dependent, meaning if it is too dry then the dentine surface collapses but if it is too wet and the primer dilutes then the strength is reduced
what happens if the tooth is over dried during bonding
the dentine will collapse, decreasing porosity. decreased porosity leads to poor penetration of dentine by the primer which leads to a poor bond overall
why does moist dentine give a better bond than dry
expanded dentine surface is porous and can absorb more primer which leads to a better bond
how has the number of bonding steps been reduced from three to one to save time
by combining the components of the bonding system through two different ways:
- separate etchant with primer and sealer combined
or
- self etching primer and sealer all combined and applied together as a single solution
describe the method of using a separate etchant with primer and sealer combined to reduce dentine bonding time
- total etch technique
- etchant is phosphoric acid
- primer and sealer combined in a single bottle and applied to the etched, washed dentine and light cured
- should allow the wetting and sealing of the dentine to occur simultaneously
- chemicals in the single bottle are the same s in the primer bottles, but are combined with resin bis-GMA and a solvent such as alcohol or acetone and camphorquinone to allow light curing
what are some examples of total etch two stage bonding agents
scotch bond one
prime and bond
optioned solo
I - bond te
xp bond
describe the use of self etching primers to reduce dentine bonding time
these materials work in a different way from all the previous materials
they do not attempt to remove the smear layer, instead they infiltrate it and incorporate themselves into it and are not washed off
this removes the issue of how dry to make the dentine
it also is not as technique sensitive but the bond doesn’t seem to be as strong to all tooth tissues
describe how dentine bonding agents can become a one bottle system
self etching primers can be combined with the bonding resin to make the one bottle system that can etch, prime and bond all together from one application.
explain the chemistry of how materials can etch dentine, prime it and then bond to it.
- they contain acidic methacrylate monomers such as methacryloyloxyalkyl acid phosphatase
- 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
- acidic groups react with the calcium ions in the tooth bonding to the surface, and also create an amorphous calcium chelate on the surface
- the smear layer is dissolved but then incorporated into the hybrid layer, penetrating only 2 micrometers into the surface but providing good bond strengths to dentine
why is the self etching primer potentially reducing the sensitivity of the tooth
there is no drying stage so there is no chance of collapse of dentine architecture preventing resin penetration and hybrid layer formation
what is molecular entanglement
the fundamental mechanism of all bonding is mineral exchange where minerals are removed from the dentine hard tissue and are replaced by resin which once mineralised can mechanically interlock in these porosities
what is the AD concept
the adhesion decalcification concept describing the interaction of bonding molecules with hydroxyapatite based tissue
initially, all acid monomers bond to the calcium in hydroxyapatite ionically
whether they stay bonded depends on the stability of the hydroxyapatite monomer bond
monomers with lower pKa do not form a stable bond as they continue to dissolve hydroxyapatite
this leads to a hybrid layer with unstable calcium phosphates incorporated
why is strong self etch not ideal
etching by products are not washed away as seen in total etch techniques, and are more 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
strong self etch bonds well to enamel but less well to dentine
why is mild self etch better than strong
it only partially demineralises the dentine
so the hydroxyapatite crystals remain around the collagen and protect against hydrolytic breakdown whilst the remaining calcium ions allow ionic bonding
why are MDP and 4-META agents better than HEMA containing materials
HEMA materials are more acidic and absorb more water, so this creates a less durable bond
what is an example of a mild etching material
scotchbond
what is the issue with the smear layer when using self etch
if the layer is thick then the mild self catch adhesives may not penetrate through it
the challenge in these materials is to penetrate the smear layer without decalcifying the tooth surface too much and removing the hydroxyapatite
why is it important that not too much hydroxyapatite is removed when etching dentine
it is required for strong durable bonding and protects the dentine from the hydrolytic breakdown
what is the benefit of self etching bonding agents
they are less technique sensitive, require no rinsing or excessive drying and there is no dentine collapse leading to low bond strength
there is less chance of post op sensitivity because of the simultaneous demineralisation and resin infiltration
what are the negatives of using a self etching bonding agent
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 and there is little evidence of stronger bonds to dentine than with total etch
what do bonding agents work as
wetting agents
what is recommended to do alongside the use of self etching dental bonding agents
etch the enamel with phosphoric acid to obtain the best bond
