Glass ionomer cements Flashcards
What is a glass ionomer cement (GIC)?
A cement material made out of glass particles embedded in a polyacrylic matrix. (starts and ends up as a polymer)
How does the cement set?
Acid-base reaction - cement sets in a neutralisation reaction
What are the uses of GIC?
• Filling abrasion and erosion lesions – close to gingival margin where moisture control is hard, as GIC is more forgiving to moisture
• Some restorations involving exposed root dentine (moisture control also hard here)
• Temporary fillings
• Various cementation and lining purposes under indirect and direct restorations -> v biocompatible e.g. won’t irritate pulp
• Atraumatic Restorative Therapy (ART):
o Hand instruments only – no electrical tools
o GIC to restore
o No drilling! Anxious patients, environments with no/limited electricity
What is GIC made of?
Filler particles - silica, aluminum, calcium fluoride glasses w additions
Polyacid - polyacrylic acid (polymer w carboxy acid groups attached)
Tartartic acid - helps with the setting process
Water - because the setting reaction requires water to happen
Minor components - sodium, aluminium fluordies, calcium and aluminium phosphates (act as additional fluoride sources and fluxes)
How is GIC made?
Filler particles (Si, Al, CaF) glasses ground, heated and those with lowest melting temp melt = fluxes
CaF is material with lowest MP so melts and flows to embed all of the other materials
Shock cooled and ground again to form a fine powder.
10-50um particle size typical
Smaller for cements, larger for fillings
Heavy metals added for radiopacity
How does GIC work?
- Components mixed by hand or in an amalgamator
- Starts to set rapidly, not command cure
- Dissolution - activated by water
H+ ions dissociate, attack glass particles and dissolve them (COOH groups -> COO- groups) - Gelation - makes gel consistency
Ca2+ forms bridges between 2 polyacid ions - Hardening - Al3+ can cross link 3 ions
More setting than with Ca2+ (Al displaces Ca bridges)
Why do we want to include tartaric acid?
- want to pass the gelation phase as this does not allow good setting
- Tartaric acid forms bonds with the Ca2+ and removes it from the reaction
-Therefore gelation phase is passed and go straight to hardening phase.
What do you end up with at the end of the setting process?
• you finish up with a tooth-coloured, quite opaque, moderately strong cement
• material is vulnerable for first few days, can become porous due to too little water. Therefore, finish w a varnish to lock in right amount of water
• The filler particles are an ACTIVE part of the setting process – not like a composite!
• It contains 8-10% water even after setting – always hydrated solid
• For a time after setting it is prone to desiccation or dissolution
• Inside GICs, the rxn betw fillers and aqueous environment inside material, and the aq environment outside the material continues during its lifetime
What is the purpose of fluoride in this material?
- Fluoride can come out of the material and act on the surrounding tissues - can help to reduce caries
- Can also enter the material by fluoride from the saliva
- Fluoride released throughout life
- The release of F is initially large and then reaches a steady level. Not enough evidence to say that it reduces decay
How does GIC bond to the tooth?
- GIC has an inherent affinity for enamel and dentine
- H bonding to collagen
- Ca bridging to hydroxyapatite
- Immediate layer of polyacrylate w some calcium and phosphate ions from the tooth
- Bond strength lower than for composite but GIC do no fail in adhesion.
What are the advantages of GIC?
- Ok aesthetics
- Adhesion to tooth is more forgiving in application regarding moisture
- Thermally insulating - thermal properties comparable to tooth tissue
- No setting shrinkage, no depth of cure issue
What are the disadvantages of GIC?
- Inferior aesthetics to composite
- Inferior mechanical properties
- More limited range of applications than composite
What are the variants of GIC?
- Chemfil rock - uses zinc based class and claims faster setting time of better mechanical properties
- Resin modified GICs and compomers
- RMGICs more similar to GIC and compomers more similar to composites.
- Resins and photo initiators as in composites. Silica filler as in GICs plus components added to make these components compatible.
What is the main resin in a compomer?
UDMA
What is the main resin in RMGIC?
HEMA as it is usually a water soluble resin (compatible with water as hydrophilic)
