What are the ideal properties of a dental luting agent?
- low viscosity and film thickness
- easy to use
- radiopaque
- good marginal seal
- aesthetics
- low solubility
- biocompatible
- cariostatic
what influences the viscosity and film thickness of a luting agent?
the size of powder or filler particles in the material
What are the ideal mechanical properties of a dental luting agent?
- high compressive strength
- high tensile strength
- high hardness value
- Young’s modulus similar to tooth
Types of materials that can be used as dental luting agents
- dental cement
- GIC
- composite resin luting agents
Constituents of zinc phosphate cement
powder
- zinc oxide >90%
- magnesium dioxide <10%
- alumina and silica oxides - improves physical properties
liquid
- aqueous solution of phosphoric acid approx 50%
- aluminium oxide
- zinc oxide
why is magnesium oxide added to zinc phosphate cement?
- gives white colour
- improves compressive strength
What are the downsides of zinc phosphate cement?
- initial pH approx 2 = causes pulpal irritation
- exothermic setting reaction
- not adhesive to tooth or restoration
- not cariostatic
- final set takes 24 hours
- opaque
- brittle
How does zinc polycarboxylate cement differ form zinc phosphate cement?
- phosphoric acid replaced by poly acrylic acid
what are.the advantages of zinc polycarboxylate cement over zinc phosphate?
- bonds to tooth
- less heat of reaction
- pH returns to neutral more quickly
what are the downsides of zinc polycarboxylate cement?
- difficult to mix
- difficult to manipulate
- soluble in oral environment at lower pH
- opaque
- Lower modulus and compressive strength than zinc phosphate
How does GIC bond to the tooth surface?
- ion exchange with calcium in enamel and dentine
- hydrogen bonding with collagen in dentine
Advantages of glass ionomer cements
- low shrinkage
- long term stability
- relatively insoluble once fully set
- beter aesthetics than zinc phosphate
- self adhesive to tooth substance
- fluoride releasing
- cheap
What is meant by the term ‘marginal seal’?
the luting agent should bond chemically to the tooth and the indirect restoration with a permanent and impenetrable bond
How does GIC bond to tooth surfaces?
ion exchange with calcium in enamel and dentine
hydrogen bonding with collagen in the dentine
Glass ionomer cement advantages
clinically easy to use and durable
low shrinkage
long term stability
relatively insoluble when set
aesthetically better than zinc phosphate cements
self adhesive to tooth surface
fluoride releasing
cheap
How does GIC/RMGIC differ from GI/RMGI filling material?
glass particle size is smaller to allow acceptable film thickness
Benefits of RMGIC?
incorporation resin improves some material properties:
- shorter setting time
- longer working time
- higher compressive and tensile strengths
- higher bond strength to tooth
- decreased solubility
RMGIC potential problems
HEMA is cytotoxic
- important no monomer remains as it can damage the pulp
- HEMA swells, expands in a wet environment
- cannot be used to cement convention porcelain crowns as they may crack
- may split the root if used to cement posts
no bond to indirect restoration
Composite luting agents - advantages
better physical properties
aesthetics
lower solubility
downsides of composite luting agents
dual cure composite must be used with DBA
- physical properties are reduced by 25% if they are not light cured
technique sensitive
features of composite luting agents
can be dual cured or light cured
must be used in conjunction with suitable DBA
why is a silane coupling agent used to bond porcelain to composite?
porcelain is smooth and non retentive
must be etched with HF
produces a rough retentive surface but still not hydrophobic and compatible with composite resin luting agents
Function of a silane coupling agent
applied to etched porcelain surface
forms very strong bond between oxide groups on porcelain surface and the silane
other end of silane molecule has c=c bond which reacts with composite resin luting agent
features of self adhesive composite resin
the metal coupling agent is incorporated into the composite resin
- simplifies bonding process
