Glass Ionomers Flashcards
What are the types of GI
Conventional
Resin modified
What are the uses of GI
Restorative
Core build up
Lining
Luting (cementing indirect restorations)
What are the two components of GI
Acid (liquid)
Base (powder)
What acids are used in GI
Polyacrylic acid
Tartaric acid
What is the composition of the powder base
Silica, SiO2 (Silicone dioxide) 30% - 40%
Alumina, Al2O3 (Aluminium dioxide) 15% - 30%
Calcium Fluoride, CaF2 15% - 35%
Aluminium Fluoride 2% - 10%
Aluminium phosphate 4% - 20%
Sodium fluoride 4% - 10%
What ingredients alter the translucency of GI
The ratio of alumina/silica alters the translucency.
More silica more translucent.
How do anhydrous and encapsulated materials vary in composition
Anhydrous materials
-The acid is freeze dried and added to the powder
-The liquid is distilled water
-This makes for easier handling of the material, particularly mixing
Encapsulated materials
-Consistent powder/liquid ratio
-Easier to use
-Should be more consistent properties of the mixed material
When is a smaller particle size needed
For luting cements for a low film thickness and quicker setting reaction
What does a higher molecular weight mean
Better mechanical properties however too high can make the material difficult to mix
What is the GI setting reaction
MO.SiO2 + H2A —> MA + SiO2 + H2O
glass + acid —> salt + silica gel
What are the phases to setting GI
Dissolution
Gelation
Hardening
What occurs during dissolution
Acid into solution
H+ ions attack the glass surface
Ca, Al, Na & F ions are released
Leaves silica gel around unreacted glass
What occurs during gelation of GI setting
Initial set is due to calcium ion crosslinking with the polyacid by chelation with the carboxyl groups.
Calcium ions are bivalent so they can react with two molecules joining them
Crosslinking is not ideal as the Ca can chelate with two carboxyl groups on the same molecule
This GELATION equates to the initial set of the material and takes several minutes depending on the particular material.
This initial set is caused by formation of Calcium polyacrylate
Following this reaction the material will appear hard in the mouth.
What causes GI hardening
Trivalent Aluminium ions ensure good crosslinking with an increase in strength.
Aluminium Polyacrylate formation takes a long time
This process does not start for at least 30 minutes and can take a week or longer to be complete.
The Aluminium reaction ensures a much higher degree of crosslinking
This process greatly improves the mechanical properties of the material.
How long does hardening take in GI
Up to a week or longer
When does hardening start
Not for atleast 30 mins
What happens when GI is contaminated with water
Aluminium ions diffuse out of the material
Excessive drying means water will be lost
Saliva contamination causes absorption of water
All lead to a weak material which will be rough, break up and have poorer aesthetics
How is GI protected
Varnishes
-Copal ether
-Acetate
Resins
-Dentine/Enamel bonding agents
-Unfilled Bis-GMA resins
Greases or gels
-Vaseline
How does tartaraic acid affect GI property
Increases viscosity
What are the adhesion properties of GI
Can bond to enamel and dentine without the need for use of an intermediate material
Bond strength not high compared with composite to acid etched enamel
-About 5MPa vs 20MPa
-Probably not measuring the bond itself as GIC tends to fail cohesively
Good sealing ability with little leakage around margins
What groups within GI does the Ca in enamel bond to
COO-
Carboxyl groups
How is the best bond acheived with GI
Clean surface
Conditioned surface
-Conditioned, not etched.
-Little or no tissue is removed.
-Best conditioner appears to be polyacrylic acid
-Purpose is to produce clean smooth surface
What are the issues with aesthetics in GI
Lack translucency
Not suitable where aesthetics are of prime improtance (anterior teeth)
What are the sub-optimal mechanical properties of GI
Poor tensile strength
Lower compressive strength than composite
-Less than half. 80 – 110MPA vs 300+MPa
Poorer wear resistance than composite
-Subject to abrasion
Lower hardness than composite
Higher solubility than composite
-Dissolution of unprotected material during gelation phase
-Long term erosion by acids
What are the mechanical advantages of GI
Good thermal properties
-Expansion similar to dentine
No contraction on setting
Once set less susceptible to staining and colour change than composites
Fluoride release
Lower modulus can be a good thing
What are the benefits of fluoride release
They can recharge their fluoride when the Fl concentration around them is higher than that in the cement
They then release Fl again when the ambient concentration falls
They can act as a fluoride reservoir or fluoride sink
May have anti-caries effect. Not certain
How is GI used
Dressing
Fissure sealants
Temp filling
Luting
Orthodontic cement
Base/lining
What are the general advantages of GI
Stable chemical bond to enamel and dentine
Low microleakage
Fluoride release
Good thermal properties
No contraction on setting
What are the general disadvantages of GI
Brittle
Poor wear resistance
Moisture susceptible initially
Poor aesthetics
Susceptible to acid attack/drying out
What are the ingredients in RMGIC powder
Fluoro-alumino-silicate Glass
Barium Glass
-Provides Radiopacity
Vacuum dried Polyacrylic acid
Potassium persulphate
-Redox catalyst to provide resin cure in the dark
Ascorbic acid
Pigments
-Varies shade
Howis the liquid component modified in RMGIC
Contains HEMA, water, photo-initiators (light curing)
What are the dual cure setting reactions og RMGIC
Acid base reaction initially
Light - free radical methacrylate reaction forming a resin matrix
What are the tri-curing steps
Initial acid base reaction
Redox reaction
Light activates free radical methacrylate reaction
What are the properties of RMGIC
Good bond to enamel and dentine
Superior to conventional GIC ??
-Difficult to know what is being measured
-Definitely better initially
Better physical properties
Lower solubility
Fluoride release
Better translucency and aesthetics
Better handling
What are the poor properties of RMGIC
Polymerisation Contraction
Exothermic setting reaction
-both polymerisation and dark cure
Swelling due to uptake of water
-HEMA is extremely hydrophilic
Monomer leaching
-HEMA is toxic to the pulp it must be polymerised completely
Reduced strength if not light cured
What are the benefits of RMGIC compared to GI and composite
Compared with conventional GIC
-Better aesthetics
-Easier to use
-Stronger
Compared with Composite Resin
-Easier to use
-Fluoride release
