Luting Agents Flashcards
What are properties of a luting agent?
Viscosity and film thickness
Ease of Use
Radiopaque
Marginal seal (impenetrable bond)
Good Aesthetics
Low Solubility
Cariostatic (fluoride releasing, antibacterial)
Biocompatible (non toxic, low thermal conductivity)
Mechanical properties
How viscous should luting agents be and what is this dependant on?
Dependant on the size of powder or filler particles in the material.
Must be low to allow seating of the restoration without interference.
Viscosity increases as material sets → must seat restoration quickly and maintain pressure.
What should film thickness of the luting agent be?
Film thickness should be as thin as possible ideally 25um or less.
Why should it be radiopaque?
easier to see marginal breakdown
What mechanical properties should the luting agent have?
High compressive strength
Dentine around 275 MPa
High tensile strength
Dentine around 50MPa
High Hardness Value
Dentine around 70K
Enamel around 400K
Young’s Modulus similar to tooth Dentine around 15 GPa
What are the types?
3 main with their subsections
- Dental Cement
Zinc Phosphate
Zinc polycarboxylate - Glass ionomer Cement
Conventional
Resin modified - Composite resin luting agents
Total etch for use with DBA
Self etch
Requires etch but has own bonding agent incorporated.
What is zinc phosphate made from and why is it good?
Acid base reaction (powder and liquid)
Excellent clinical service
Easy to use
Cheap
What are the power components of zinc phosphate?
what is their role
Zinc Oxide >90%
- Main reactive ingredient
Magnesium Dioxide <10%
- Gives white colour
- Increases compressive strength
Other Oxides (Alumina and Silica)
- Improve physical properties
- Alter shade of set material
What are the liquid components of zinc phosphate?
Aqueous solution of phosphoric acid (approx. 50%)
- Oxides which buffer the solution
Aluminium oxide
- Ensures even consistency of set material
Zinc Oxide
- Slows the reaction giving better working time
What are the disadvantages of zinc phosphate?
Low initial pH approx. 2
Can cause pulpal irritation as pH can take 24hrs to return to neutral
Exothermic setting reaction
Not adhesive to tooth or restoration
It works like grout on tiles just filling in any spaces.
Retention may be slightly micromechanical due to surface irregularities on prep and restoration
Not cariostatic
Final set takes 24hrs
Brittle
Opaque
What are the advantages of zinc polycarboylate?
Similar material but phosphoric acid replaced by polyacrylic acid.
This material had the advantage of bonding to tooth surfaces in a similar way to glass ionomer cements.
There is less heat of reaction.
The pH is low to begin with but returns to neutral more quickly and longer chain acids do not penetrate dentine as easily.
Cheap.
What are the disadvantages of zinc polycarboxylate?
Difficult to mix
Difficult to manipulate
Soluable in oral environment at lower pH
Opaque
Lower modulus and compressive strength than Zinc Phosphate.
What is the main difference between GI used as cement or as a filling material?
The main difference is particle size of the glass which is less than 20um to allow for suitable film thickness
How does the GI cement bond to the tooth?
Cement bonds to tooth surface through
Ion exchange with calcium in enamel and dentine
Hydrogen bonding with the collagen in the dentine
What are the advantages of GI cement?
Low shrinkage
Long term stability.
Relatively insoluble once fully set.
Aesthetically better than ZnPhos.
Self adhesive to tooth substance.
Fluoride release.
Cheap.
What is added to conventional GIC to make it resin modified?
In addition to conventional GIC powder and liquid the liquid contains a hydrophilic monomer
The monomer must be hydrophilic as GIC is a water based material.
HEMA (Hydroxyethyl methacrylate)
particle size is smaller to allow acceptable film thickness
What is the reaction of RMGI?
The same acid base reaction occurs.
Light activation causes polymerisation of the HEMA and any copolymers in the material leading to a rapid initial set.
The acid base reaction then continues for some time.
Some materials have a secondary cure via a REDOX reaction
This allows ‘Dark Curing’ where material not exposed to light will cure.
What are the advantages of RMGI?
Shorter setting time
Longer working time
Higher compressive and tensile strengths
Higher bond strength to tooth
Decreased solubility
What are disadvantages of RMGI?
HEMA is cytotoxic - Very important that no monomer remains as it can damage the pulp.
HEMA swells, it expands in a wet environment - It cannot be used to cement conventional porcelain crowns as they may crack, It shouldn’t be used to cement posts as it may split the root
No bond to indirect restoration
What are composite luting agents and what must they be used with?
The simplest of these materials are variants on composite filling materials with suitable viscosity and filler particle size.
They must be used in conjunction with a suitable DBA.
They can be light cured or dual cured
How are composite luting agents technique sensitive?
Although dual cured the physical properties are reduced by 25% if they are not light cured.
How does bonding of CLA occur with indirect composite?
Composite bonds to composite
Bond strength is lower to inlay fitting surface than to
new composite
Bond is micromechanical to rough internal surface of inlay
Bond is also chemical to remaining C=C bonds on the fitting surface of the inlay
Use a dual curing cement as light penetration through the inlay will be poor
How does bonding of CLA occur with porcelain?
what is required?
Porcelain is brittle and requires to be bonded to tooth to prevent fracture
Untreated porcelain is smooth and non retentive.
It can be treated with HF to etch the surface (v toxic).
This produces a rough retentive surface but it is still not hydrophobic and compatible with composite resin luting agents.
A surface wetting agent is required.
What is applied to the etched porcelain surface?
how does this help in bonding?
silane coupling agent
Applied to the etched porcelain surface, ideally as a
monolayer
Very strong bond between oxide groups on the porcelain surface and the silane.
The other end of the silane molecule has C=C bond which reacts with the composite resin luting agent.
This works in the same manner as a dentine bonding agent does with tooth.
Is the bond to porcelain with CLA good?
yes dual cured
requires surface wetting agent
What does bonding to metal occur with CLA require?
Like porcelain composite materials do not bond directly to metal
Metal surface needs to be roughened.
This can be done by etching
More usually done by sandblasting
What does etching metal do and why is it sometimes inconvient?
Electrolytic etching removes the different phases of the
alloy at different rates
This gives a very retentive surface BUT
Technique sensitive
Beryllium containing alloys work best Cannot etch precious metals at all
After sandblasting, what is required to strengthen the bond?
Sandblasting roughens the surface but does not give the undercut surface of etching.
Chemical bonding is required to strengthen the bond
How does bonding to non precious metals occur?
Materials with carboxylic and phosphoric acid derived resin monomers.
MDP and 4-META
These molecules have an acidic end and a C=C end
The acidic end of the molecule reacts with the metal oxide and renders the surface hydrophobic.
This is the same as DBA and Silane
How is bonding to metal cured?
Must use a dual curing material as light will not penetrate metal.
How is bonding to precious metals carried outt?
Change precious alloy composition to allow oxide formation
Increase copper content and heat 400oC in air
Tin Plate
Sulphur based chemistry of bonding agent
All complicated and technique sensitive.
What is self adhesive composite resin?
The metal coupling agent is incorporated into the composite resin.
This simplifies the bonding process
What is an example of a self adhesive composite resin?
MDP is used in Panavia
What are the advantages of self adhesive composite resins?
This is an anaerobic self cured material
Consistent results over many years
Good film thickness
Opaque
Moisture sensitive
How do self etching composite resin luting agents work?
Attaches like a self etching primer (sort of )
Acidic groups bind with calcium in hydroxyapatite forming a stabilising attachment between the tooth and the resin
Ions from dissolution of filler neutralise the remaining acidic groups forming a chelate reinforced methacrylate network
Limited removal of smear layer or significant infiltration into the tooth surface. (only a couple of microns)
Good bond strength to dentine
What are temporary cements?
Made to cement temporary restorations in place while permanent restoration is fabricated.
Soft for easy removal some do not set at all.
Prep must be physically retentive or they will not work.
Can be used for trial lute of permanent restoration to allow assessment by patient or clinician.
What is the form of temporary cements?
two paste system base and catalyst/accelerator
What are the components of temporary cements?
Base contains ZnO, Starch and mineral oil
Accelerator contains resins, eugenol or ortho-EBA and
carnauba wax
The wax weakens the structure of the set cement and makes it easier to remove
Material can be modified to make it weaker still by incorporating petroleum jelly into the mixture
What are the two main types of temporary cements?
those with and those without eugenol
When should eugenol temorary cements not be used and why?
Eugenol containing materials should not be used to cement the provisional restoration where the permanent restoration will be cemented with a resin cement.
Any residual eugenol may interfere with the setting of this type of luting agent.
Irrespective of the type used complete removal of the temporary cement is essential.
