Bonding systems
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What are the properties of a dental adhesive
Provide a high bond strength to tooth tissues Immediate high strength bond Durable bond Impermeable bond Easy to use Safe
Why is bonding to enamel easy
Heterogeneous structure: densely packed prismatic Highly mineralised: 95% inorganic ‘Dry’
What percentage aqueous solution of phosphoric acid is normally used in practice for enamel etching
30-50%
What does the acid etch technique do
Long enamel prisms filled with imperfectly packed hydroxyapatite crystals This surface can be modified by applying acid This roughens the surface producing an etched pattern This allows micromechanical interlocking ofresin filling materials The etching also allows for greater wettability of the enamel by removing contaminants This allows the resin to adapt better to roughened enamel surface
What is the composition of dentine
20% organic (mostly collagen) 70% inorganic (mostly hydroxyapatite) 10% water
What makes dentine an inconsistent material
Aged dentine is more mineralised. Dentine near the pulp has more tubules and increased moisture content
What are the requirements of a dental bonding agent
Ability to flow Potential for intimate contact with dentine surface Low viscosity Adhesion to substrate -Mechanical -Chemical -Van der Waals -Combination of the above
What bonds are formed within organic dentine
Covalent
What bonds are formed within mineralised dentine
Ionic
When is the best adhesion/bonding within dentine acheived
When Van der Waals forces are optimised (strength depends on contact angle <90% indicates solid surface is hydrophilic)
What is critical surface energy
The surface tension of a liquid that will just spread on the surface of a solid
When will a liquid flow over a surface and stick to it
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.
Why are dental bonding agents required in restoring dentine
Dentine bonding agents increase the surface energy of the dentine surface and allow composite to flow and stick to the surface. They are SURFACE WETTING AGENTS
What is molecular entanglement
Adhesive is absorbed and can penetrate into surface of dentine where it can form a long chain polymer which meshes with the substrate
Why is molecular entanglement desired
Leads to a high bond strength
What is the smear layer
The smear layer is an adherent layer of organic debris that remains on the dentine surface after the preparation of the dentine during the restoration of a tooth
What is the thickness of the smear layer
0.5-5 microns
What do we 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 it to the dentine below
Why did earlier fillings using (Phosphorylated Bis-GMA : bisphenol-A glycidyl methacrylate (Scotchbond), NPG-GMA: N-phenylglycidyl methacrylate (Crevident)) tend to fail more fresuently
The bond was only to the top of the smear layer 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
What do materials today rely on for bond strength
Total etch Self etch To remove and/or modify the smear layer
What generation of DBA are we on currently
8th
What are examples of total etch bonding agents
Scotchbond multipurpose Clearfil photo bond Optibond FL
What are the 3 components of a total etch DBA
Dentine conditioner Primer Adhesive
What does a dentine conditioner do
Removes smear layer Opens dentinal tubules by removing smear plugs Decalcifies the uppermost layer of the dentine The etchant is washed off with water. The collagen network in this top 10um of the dentine is exposed and subsequently penetrated by the next two components.