Bonding systems Flashcards
what are the properties of a dental bond?
6
provide a high bond strength to tooth tissue
immediate high bond strength
durable bond
impermeable bond
easy to use
safe
why is bonding to enamel easier than bonding to dentine?
due to structure of enamel - densely packed prism, highly mineralised, dry
what acid is used for acid etch?
phosphoric acid 35%
how does acid etch change the surface of enamel?
roughens enamel surface
allows micromechanical interlocking of resins to enamel pits
increases surface energy by removing surface contaminants
leads to better wettability of enamel
what is used after an etch?
dentine bonding agents - phosphorylated BIS-GMA
or low viscosity Bis-GMA
describe the properties and composition of dentine
5
- 20% oroganic, 70% inorganic, 10% water
- full of permeable tubules - fluid pumped from pulp to dentine, making dentine wet
- secondary and tertiary dentine are irregular and more mineralised
- low surface energy
- hydrophilic
what are the requirements of a dentine bonding agent?
4
ability to flow
potential for intimate contact with dentine surface
low viscosity
adhesion to substrate
what is mechanical adhesion?
dentine bonding agent and the dentine surface meshing and interlocking with minimum gaps
what is chemical adhesion? how does mineralised and organic dentine adhere?
3
bonding at a molecular level
mineralised: ionic bonding
organic: covalent bonding
what is Van der Waals adhesion? and what does the strength of interaction depend on?
2
electrostatic or dipole interaction between bonding agent and substrate
strength depends on contact angle and is a good indication of wettability of a solid
what is the difference between high and low surface energy with a low surface energy liquid?
low surface energy substance with low surface energy liquid = does not spread
high surface energy substance with low surface energy liquid = will spread
does wet dentine have a high or low surface energy?
low surface energy so liquids/low viscosity resins do not spread
what do dentine bonding agents do to dentine?
increase the surface energy of dentine and allow the composite resin to flow and stick
what is molecular entanglement?
adhesive is absorbed onto the surface and penetrates dentine due to wetting by adhesive and surface energies
absorbed component forms long polymer that meshes with the substrate
what is the smear layer?
layer of organic debris that remains on the dentine surface after the preparation
what can you do with the smear layer in a restoration?
2
- remove it and bond to “clean” dentine
- incorporate it by penetrating it, infiltrating it with the bonding agent and stick it to the dentine below
what are the components of total etch?
3
conditioner
primer
adhesive
what does the conditioner in total etch do?
3
35% phosphoric acid
* removes smear layer and opens dentinal tubules
* decalcifies uppermost layer
* collagen network in top of dentine is exposed
what does the primer in total etch do?
3
- hydrophilic end bonds to hydrophilic dentine and hydrophobic end bonds to resin
- has a spacer group to make it long enough to be flexible when bonding
- gives dentine high surface energy, making it hydrophobic
what does the adhesive in total etch do?
4
mixture of resins - Bis-GMA, HEMA
* contains camphorquinone to enable light cure
* penetrates the primed dentine which is now hydrophobic
* forms a micromechanical bond within the tubules and collagen fibres - molecular entanglement
what are the problems with total etch?
3
over-etching collapses the collagen fibres so resin cannot penetrate
over-etching - too deep an etch and the primer cannot penetrate the full depth of the etch
moisture dependent - too dry and the surface of dentine collapses, too wet and the primer is diluted
how does separate etch with primer and sealer work?
2
still a total etch techinque with same chemicals used
primer and sealant are combined and applied to the etched, washed dentine and light cured
how do self-etching primers work?
they do not remove the smear layer, it infiltrates it and incorporates into it
it is not washed off and creates a hybrid layer
what is shown in this picture?
hybrid layer and resin that has entered tubules
what are the advantages to self-etching bonding agents?
2(3,1)
less sensitive technique
* no rinsing
* no excess drying
* no dentine collapse leading to low bond strength
simultaneous demineralisation and resin infiltration
* less chance of post-op sensitivity
what is the disadvantage of self-etching bonding agents?
1(2)
variable initial pH between products
* different pH results in different etch and penetration of resin
* materials with lower pH may not etch enamel efficiently
how do self-etching bond agents work?
3
- contain bi-functional monomers that etch and infiltrate the dentine surface with the hydrophilic end
- acidic groups react with Ca2+ ions in the tooth and create an amorphous calcium chelate
- smear layer is dissolved and incorporated into the hybrid layer
what is the adhesion-decalcification concept?
3
- interaction between dentine bonding agent molecules and HA
- all acid monomers bond to the calcium in HA ionically initially
- monomers with a lower pK (acidic) do not form a stable bond as they continue to dissolve HA which leads to a hybrid layer with unstable calcium phosphates incorporated
strong self-etch disadvantages
3
- etching by-products are not washed away so tooth is continually etched
- by-products are soluble and weaken the integrity of the bond
- if too much HA is dissolved, the collagen is vulnerable to breakdown and the bond will fail
what are problems with the smear layer?
- if smear layer is thick then mild self etch adhesives may not penetrate
- need to penetrate smear layer without decalcifying the tooth surface too much and removing HA
