Glass ionomer cements
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What are the two types of Glass ionomer cements
• Conventional GI - Resin modified GI (self and light cure)
What are the uses of Glass ionomer cement
• restorative filling material • core build up prior to restoration with crown • lining underneath permanent fillings • luting (cementing indirect restorations)
How did GIC originate
The original cement came from two different materials: • zinc polycarboxylate cement • silicate cement
What are the components of GIC
• acid (liquid polyacrylic acid and tartaric acid) - base (powder alumina/silica base)
What is the function of tartaric acid
added to control the setting characteristics of the material
How does the ratio of alumina/silica affect the translucency?
more silica more translucent
What is the function of strontium and lithium salts in the powder base of GIC
increase the radiopacity
What are the different variations in composition of GIC
anhydrous materials encapsulated materials
What is the benefit and disadvantage of encapsulated GICs
you get the same product every time (good and a bad thing)
What is the name of the machiene that mixes encapsulated GIC
amalgamator
What impact does the size of the particles have on GIC
Bigger particles = better aesthetic smaller particles = good for cementing (quicker setting but more opaque)
What impact does the length/molecular weight of the chain have on GIC
Longer chain = better mechanical properties but harder to mix so need a compromise
Describe the acid base reaction which occurs
MO.SiO2 + H2A –> MA + SiO2 + H2O Glass + acid –> salt + silica gel
What are the 3 phases involved in the setting reaction
• dissolution • gelation • hardening
Describe the dissolution phase
acid dissolves the glass particles, ions go into solution, end up with silica gel outside of unreacted glass core
Describe the gelation phase
ca ions cross link with polyacrylic acid molecules (bivalent)
Describe the hardening phase
Trivalent aluminium ions ensure good crosslinking with an increase in strength. Aluminium Polyacrylate formation takes a long time but greatly improves mechanical properties While it’s setting its moisture sensitive, relatively soluble even though it feels hard (for about a week). If it’s too wet to begin with it will be weaker so you need to protect it
What can be used to protect GIC from contamination while setting is completed?
Varnishes Resins - e.g. bonding agent Greases or gels - vaseline will not last long (varnishes and resins are best)
How has tartaric acid improved handling properties
working time largely unchanged but setting time is shortened
Describe the adhesion properties of GIC
• can bond to enamel and dentine • not as strong as composite • good sealing ability with little leakage around margins
What is the bonding mechanism of GIC
• chelation between carboxyl groups in the cement and Ca on the tooth surface • Re- precipitation of complex mixture of calcium phosphate and calcium salts from the poly acid onto and into the tooth surface • hydrogen bonding or metallic ion bridging to collagen
What does good bonding require
• clean surface - conditioned surface (not etched, polyacrylic is best conditioner)
What are the aesthetics of GIC like
Colour okay but lacks translucency suitable for a posterior tooth but not where aesthetics are of prime importance
What are the disadvantageous mechanical properties of GIC
• poor tensile strength • lower compressive strength than composite • poorer wear resistance than composite • lower hardness than composite • higher solubility than composite