Luting Agents
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What is luting agent another word for?
• Dental cement
What is the ideal film thickness of luting agent?
• 25um or less
What is the ideal viscosity for luting agent?
• Must be low to allow seating of restoration without interference- Viscosity increases as material sets therefore must seat restoration quickly and maintain pressure
What ease of use properties should luting agents have?
• Easy to mix i.e via clicker system in encapsulated ones- Working time long to allow for seating of restoration- Setting time short
Why must luting agents be radiopaque?
• Some ceramic crowns are radiolucent- Makes it easier to see marginal breakdown
Ideally how should a luting agent bond to tooth and indirect restoration?
• Bond chemically to tooth and indirect rest with a permanent and impenetrable bond
What is the ideal aesthetics of luting agents?
• Tooth coloured with variation in shades and translucency- Non staining
What is the ideal solubility of luting agent?
• Low
What is the ideal cariostatic properties of luting agents?
• Fluoride releasing- Antibacterial - Important in preventing secondary caries around crown margins
What is the ideal biocompatibility of luting agents?
• Non toxic- Not damaging to the pulp (inappropriate pH and heat on setting)- Low thermal conductivity
What are the ideal mechanical properties of luting agents?
• High compressive strength (dentine 275MPa)- High tensile strength (dentine 50MPa)- High hardness value (dentine 70K and enamel 400k)- Young modulus similar to tooth (dentine 15GPa)* No luting agent gets close to tooth values for more than one or two physical properties*
What are the types of dental cement?
• Zinc phosphate- Zinc polycarboxylate
What are the types of glass ionomer cement?
• Conventional- Resin modified
What are the types of composite resin luting agents?
• Total etch for use with DBA- Self etch- Requires etch but has own bonding agent incorporated
What is zinc phosphate?
• In use for 100+years- Acid base reaction- Powder and liquid- Excellent clinical service- Easy to use- Cheap
What is the constituents of zinc phosphate powder?
• Zinc oxide >90% (main reactive ingredient)- Magnesium dioxide <10% - Other oxides (alumina and silica)
What is magnesium dioxide used for in zinc phosphate powder?
• Gives white colour- Increases compressive strength
What are alumina and silica used for in the powder of zinc phosphate cement?
• Improves physical properties- Alter shade of set material
What are the constituents of zinc phosphate liquid?
• Aqueous sol of phosphoric acid (approx 50%)- Aluminium oxide that ensures even consistency of set material- Zinc oxide slows the reaction giving better working time
What are the reactions of the zinc phosphate cement?
• Initial acid base reaction- Followed by hydration reaction - Results in formation of crystallised phosphate matrix
What does the aluminium oxide prevent in Zinc phosphate?
• Prevent crystallisation leading to amorphous glassy matrix of acid salt surrounding unreacted ZnO powder
What happens when zinc phosphate cement matures?
• Matrix originally insoluble but it is porous and contains free water from setting reaction- When it matures it binds the water- Leads to stronger, less porous material
What are the disadvantages of zinc phosphate cement?
• Low initial pH approx 2- Exothermic setting reaction (release heat)- Not adhesive to tooth or restoration- Not cariostatic- Final set takes 24hrs- Brittle- Opaque
What can a low pH lead to?
• Pulpal irritation