Bonding to Tooth 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 (doesn’t allow fluid to pass through)
Easy to use
Safe
what are the factors that govern the ability of the adhesive to make intimate contact with substrate
The wettability of the substrate by the adhesive
The viscosity of the adhesive
The morphology or surface roughness of the substrate
describe the structure of enamel
heterogenous structure
densely packed prisms
95% mineralized
made up of tiny crystals of hydroxyapatite which are packed together to form prisms, held together by organic matrix
describe the structure of the prisms in enamel
long, rod like shapes having a key hole cross section - aligned perpendicular to the tooth surface
describe the structure of hydroxyapatite crystals
flattened hexagonals
spaces left between crystals is occupied by water and organic material
enamel does have water in it, so why does it remain dry?
much of the water is bound within enamel structure and not easily removed on drying
What is the acid etch technique
surface modification through the application of acid
what does the acid etch do to the enamel
roughens the surface of the enamel
this allows micro mechanical interlocking of resin filling material
what does the acid etch do to enamel in terms of surface energy
increases the surface energy of the enamel surface
increases wettability of the enamel
what does an increased wettability allow
wettability allows resin to adapt better to roughened enamel surface
why must enamel be dry after acid etch
Moisture contamination will prevent flow of the resin into the etched surface.
what is enamel bonding essentially
mechanical
what is usually used for enamel etching
35% phosphoric acid
what happens after the enamel is etched
Generally a low viscosity bis-GMA resin or any one of a number of dentine bonding agent resins (often phosphorylated Bis-GMA) is applied to the etched and dried enamel surface to penetrate into the rough surface and light cured
The higher viscosity composite resin filling material is then applied on top
what happens to the resin after it flows into the ‘roughness’
It polymerises which creates a strong bond greater than 20MPa which is more than enough to keep it in the mouth when chewing
describe the structure of dentine
permeable tubules
70% inorganic - mostly HA
20% organic - mostly collagen
what is the issue with the dentinal tubules being permeable
Fluid pumps up from the pulp to the dentine floor of any cavity making the surface wet
describe how dentine is an inconsistent material
o Dentine near the pulp has more tubules and increased moisture content
o Aged dentine is more mineralized
why does dentine have a low surface energy
because it is wet
dentine is hydrophilic but most bonding agents are hydrophobic
what is the smear layer
cadherent layer of organic debris that remains on the dentine surface after the preparation of the dentine during the restoration of a tooth
generally contaminated with bacteria
variably attached to dentine surface
0.5-5 microns thick
what are the requirements of a dentine bonding agent
ability to flow
Potential for intimate contact with dentine surface
low viscosity
adhesion to substrate mechanically, chemically and through van der waals
describe mechanical bonding in dentine
achieved by dentine bonding agent and the dentine surface meshing and interlocking with minimum gaps
describe what the etch does to the dentine surface
demineralizes the dentine surface so exposing the network of collagen in the dentine matrix and resin infiltrates the tubules
describe chemical adhesion in dentine
bonding at a molecular level
what type of bond does mineralized dentine have
ionic bond type
what type of bond does organic dentine have
covalent bond type
describe van der waals adhesion
based on electrostatic or dipole interaction between bonding agent and substrate
what does the strength of van der waals adhesion depend on
contact angle which is a good indication of wettability of a solid by a specific liquid
what is the contact angle which means the solid surface is hydrophilic
90 degrees
what is the critical surface tension
The critical surface tension of a liquid is the surface energy that will just spread on the surface of a solid
what is the rule for critical surface energy
Whatever you put onto a substrate should have a lower surface energy than the substrate
why will a low energy liquid spread on a higher surface energy substrate
this leads to a lower surface energy of the material as a whole
what needs to be done to dentine for composite resin to stick in terms of energy
Dentine bonding agents increase the surface energy of the dentine surface an allow composite to flow and stick to the surface
they are called surface wetting agents and have a non polar and polar group
what is molecular entanglement
Adhesive is absorbed onto the surface but can also penetrate into the surface of dentine
This is due to good wetting of the dentine by the adhesive and appropriate surface energies of the two
The absorbed component can form a long chain polymer
This polymer meshes with the substrate in whats known as molecular entanglement leading to a high bond strength
what do we do with the smear layer?
Most dentine bonding agents take off the smear layer and bond to the clean dentine beneath
Some incorporate it by penetrating it, infiltrating it with the bonding agent and stick it to the dentine below
what are the two ways we remove or modify the smear layer
total etch
self etch
what are examples of total etch
o Scotchbond multipurpose
o Clearfil photo bond
o Optibond FL
what are the components of a total etch DBA
dentine conditioner
primer
adhesive
what is a dentine conditioner
an acid, usually 35% phosphoric
what is a primer
really adhesive part of the agent with a hydrophilic/hydrophobic molecule – it is bifunctional and it changes the surface from a low energy to a high energy one and it allows a hydrophobic material to stick to a hydrophilic one
what is an adhesive
a resin which penetrates into the surface of the dentine attaching to the primers hydrophobic surface – it attaches to the bipolar molecule and the molecular entanglement occurs
what does the dentine conditioner do
o Removes the smear layer
o Opens dentinal tubules by removing smear plugs
o Decalcifies the uppermost layer of dentine
o The etchant is washed off with water
o The collagen network in this top ten nanometers of dentine is exposed and subsequently penetrated by the next 2 components
what does the primer do
the coupling agent
the C=C bond opens and forms a strong covalent bond within the next resin layer
the hydroxyl group can combine with similar polar groups on the HA and can react with amine groups on the collagen proteins
it gives dentine a hydrophobic surface into which the next layer of resin will follow
why must the molecule or group of molecules be dissolved in a solvent in the primer
allows the primer to be carried into the moist collagen fibrillar network and displace the water molecules from the collagen permitting the primer to enter into the micro and nano spaces created around the collagen fibrils after the etching process
give two examples of chemicals used as primers
HEMA (scotch bond, gluma, optibond)
4-META (hybrid bond, G bond)
what is the adhesive
mixture of resin
usually bis-Gm or HEMA
hydrophobic
may contain filler particles to make it stronger
contains camphorquinone to allow it to light cure
penetrates primed dentine
forms micro mechanical bond within the tubules and exposed dentinal collagen fibres
molecular entanglement forms the hybrid layer
what are problems with total etch
overetching causes collapse of collagen fibres so no penetration can occur
over etching causes a deep etch so primer cannot penetrate to full depth of the etch and can result in sensitivity
process is moisture dependent
what happens if the tooth is overdried
collapsed dentine
decreased porosity
poor penetration by primer
poor bond
what does moist dentine allow
a good bone
expanded dentine surface is porous and will absorb the primer and give a good bond
what are the two reduced stage systems
separate etchant with primer and sealer combined
describe the separate etchant with primer and sealer combined
still total etch technique
etchant still phosphoric acid
primer and sealer combined in a single bottle and applied to etched, washed dentine and light cured
what are examples of total etch two stage bonding agents
scotch bond One Prime and bond optibond solo I-bond TE XP bond
what is the general procedure for total etch two stage bonding agents
- Application of etchant and washed off to create demineralized dentine layer
- Application of primer and sealer in a single solution
why are self etching primers different
They do not attempt to remove the smear layer. They infiltrate it and incorporate themselves into it
The acid monomers etch and incorporate themselves into the surface of the smear layer and dentine at the same time
It is not washed off this removes the problem of how dry to make the dentine however it is still possible to contaminate from saliva
what is present in self etching primers that allows them to work the way they do
contain acidic methacrylate monomers
how do the acid monomers work in self etching primers
very complex bifunctional monomers etch and infiltrate the dentine surface with their hydrophilic end while polymerizing like a simpler bis-GMA material at the hydrophobic end.
what does the acid group in the acid monomer react with
calcium ions in the tooth bonding to the surface and also creates an amorphous calcium chelate on the surface
what is the fundamental mechanism of all bonding
Fundamental mechanism of all bonding is mineral exchange where minerals removed from the dental hard tissues are replaced by resin which once mineralized mechanically interlock in these porosities – MOLECULAR ENTANGLEMENT
what does the self etch primer remaining bonded depend on
stability of the HA monomer bond
monomers with a lower pKa do not form a stable bond as they continue to dissolve HA which leads to a hybrid layer with unstable calcium phosphates incorporated
why is a strong self etch not best for dentine
o Etching by-products are not washed away as in total etch
o These are soluble and weaken the integrity of the bond
o If too much hydroxyapatite is dissolved away the exposed collagen is vulnerable to breakdown and the bond will fail
why is a mild self etch better for dentine
only partially demineralizes dentine
o The HA crystals remain around the collagen and are protective against hydrolytic breakdown and the remaining calcium ions allow ionic bonding
o MDP and 4-META agent are better than HEMA containing materials
o HEMA materials are more aci ic and absorb more water resulting in a less durable bond
what are the advantages of self-etching bonding agents
Less technique sensitive as there is no rinsing, no excessive drying, no dentine collapse leading to low bond strength
Simultaneous demineralization and resin infiltration meaning there is less chance of post operative sensitivity
what are the disadvantages of self etching bonding agents
There is great variability between products with regard to the initial pH of the solution
The different 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 a stronger bond to dentine than with total etch
