bonding systems Flashcards
what are the properties of dental adhesive?
high bond strength to tooth tissues immediate high strength bond durable bond impermeable bond easy to use safe
why is bonding to enamel easy?
the structure of enamel heterogeneous structure -densely packed prismatic highly mineralised (95% organic) -more like rock/stone than bio tissue dry
describe the acid etch technique
enamel prisms filled with imperfectly packed hydroxyapatite crystals
surface modified by application of acid
acid roughens surface producing etched pattern
roughened surface allows micromechanical interlocking of resin filling materials
etched surface increases surface energy of enamel surface by removing surface contaminants leading to better wettability of enamel
better wettability allows resin to adapt better to roughened enamel surface
what is the requirement for enamel etch technique and why?
enamel must be dry
moisture contamination will prevent flow of resin into etched surface
what is the acid normally used in etching?
30-50% phosphoric acid
what are the 2 types of resin put on top of enamel etch?
low viscosity bis-GMA to penetrate into rough surface and light cured
higher viscosity composite resin filling material on top
describe the composition of dentine
20% organic 70% inorganic (HA) 10% water permeable tubules fluid pumps from pulp floor of cavity making surface wet inconsistent material -aged more mineralised low surface energy hydrophilic, most bonding agent hydrophobic
what are the requirements of a DBA?
ability to flow potential for intimate contact w dentine surface low viscosity adhesion to substrace -mechanical -chemical -van der waals
describe mechanical dentine bonding
same as enamel
dentine bonding agent and dentine surface meshing and interlocking with minimum gaps
what type of bond does mineralised dentine have?
ionic
what type of bond does organic dentine have?
covalent
what is van der waals adhesion based on?
electrostatic or dipole interaction between bonding agent and substrate
what does the strength of a van der waals interaction depend on?
contact angle
-indicates wettability of a solid by a specific liquid
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 compared to the surface it is being placed on for it to flow and stick?
lower surface energy
why will a low SA spread on a higher SA substrate?
leads to a lower SA of the material as a whole
what SA does wet dentine have?
lower than composite filling materials
how do you get composite resin to stick to dentine?
make the surface of the dentine have a higher critical energy than the composite
how do dentine bonding agents work?
they increase the SA of the dentine surface and allow composite to flow and stick
-surface wetting agents
what are the stages of bonding to dentine?
etching
wetting agent
molecular entanglement
adhesive
describe adhesion through molecular entanglement?
adhesive is absorbed onto surface but can also penetrate into the surface of the dentine
due to good wetting of dentine by adhesive and appropriate SA’s
absorbed component can form long chain polymer
polymer meshes with substrate=molecular entanglement->high bond strength
what is the smear layer?
an adherent layer of organic debris that remains on the dentine surface after the prep of the dentine during the restoration of a tooth
attached to dentine surface
contaminated with bacteria
interferes with adhesion
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 and stick to the dentine below
what are total etch bonding agents?
involve complete removal of the smear layer eg. scotchbond multipurpose clearfil photo bond optibond FL
what are the components of a total etch DBA?
dentine conditioner
-35% phosphoric acid
primer
-adhesive part with hydrophilic/hydrophobic molecule
adhesive
-resin which penetrates into surface of dentine attaching to the primers hydrophobic surface
what does the dentine conditioner do?
removes smear layer opens dentinal by removing smear plugs decalcifies uppermost layer of dentine etchant washed off with water collagen network exposed so penetrated by next 2 components
describe the primer
adhesive element
coupling agent
bifunctional molecule with a hydrophilic end to bond to the hydrophilic dentine surface and a hydrophobic, methacrylate end to bond to resin
also has spacer group to make it long enough to be flexible when bonding
dissolved in suitable solvent
what coupling agent is commonly found in primers?
HEMA
how does bonding to dentine occur?
C=C bond opens and forms strong covalent bond with next resin layer
hydroxyl group can combine w/ similar polar groups on the HA and react with amine groups on the collagen protein
give another example of a chemical used as a primer
MDP -clearfil
describe the adhesive
mixture of resins -bis-GMA & HEMA predominantly hydrophobic filler particles make it stronger camphorquinone makes it light cure
describe how the adhesive works
penetrates primed dentine which now has a hydrophobic surface
forms micromechanical bond within tubules and exposed dentinal collagen fibres
molecular entanglement
forms hybrid layer of collagen plus resin
what problems can occur with total etch?
technique sensitive
over etching-collapse of collagen fibres 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 surface collapse
-too wet primer diluted
how does self etch work?
dont remove smear layer
infiltrate and incorporate themselves into it
not washed off
creates hybrid layer
advantages/disadvantage of self etch
not as technique sensitive
bond not as strong
what component allows self etch to work?
acidic methacrylate monomers eg methacryloyloxyalkyl acid phosphatase
how do the bifunctional monomers work?
etch and infiltrate the dentine surface with their hydrophilic end while polymerising like a simpler bis-GMA material
acidic groups react with Ca2+ ions bonding to the surface and creating an amorphous ca chelate
smear layer dissolved then incorporated in hybrid layer
penetrates 2um into surface but gives good bond strength
no evidence for reduced sensitivity
what is the chemistry of molecular entanglement?
minerals removed from dental hard tissue replaced by resin which once mineralized mechanically interlock in these porosites
all acid monomers bond to ca in HA
whether they stay bonded depends on stability of HA monomer bond
hybrid layer with unstable CaPO incorporated
where is strong self etch best used?
enamel
less bond to dentine
why does self etch not work well with dentine?
etching by products not washed away
they are soluble so weaken integrity of bond
if too much HA dissolved exposed collagen is vulnerable to breakdown and bond will fail
describe mild self etch
only patrially demineralises dentine
HA crystals remain around collagen
MDP and 4-META agent are better than HEMA
-more acidic absorb more water=less durable
what are the problems with the smear layer?
if its too thick mild self etch may not penetrate
challenge=penetrate without decalcifying tooth surface and removing HA
HA required for durable bond and protects dentine from hydrolytic breakdown
advantages of self etch
less technique sensitive
-no rinsing
-no excessive drying
-no dentine collapse leading to low bond strength
simultaneous demineralisation and resin infiltration
less change of post-op sensitivity
disadvantages of self etch
great variability between products initial pH
dif in pH results in dif etch and dif penetration of resin
materials with lower pH may not etch efficiently
little evidence of stronger bond to dentine than total etch
how is the best bond achieved with self etch?
etch the enamel with phosphoric acid
