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