Glass ionomer cements Flashcards
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
What are the advantageous mechanical properties of GIC
- 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
Describe GIC flouride release capability
Releases loads initially but after about a week much less.
But, 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
What are the uses of GIC
- dressing
- fissure sealant
- endodontic access cavity temp filling
- luting
- orthodontic cement
- restoration of deciduous teeth
- restoration of permanent teeth
- base or lining
Advantages of GIC
- Stable chemical bond to enamel and dentine
- Low microleakage
- Fluoride release
- Good thermal properties
- No contraction on setting
What are the disadvantages of GIC
- Brittle
- Poor wear resistance
- Moisture susceptible when first placed
- Poor aesthetics
- Poor handling characteristics
- Susceptible to acid attack and drying out over time
- Possible problems bonding to composite
- Etching damages surface
What are cermets
silver coloured GICs
no advantages and worse aesthetics (aim was for silver to overcome brittleness)
Why were RMGICs developed
take advantages of GIC - bonding to tooth, fluoride release
and add advantages of composite:
- light curing (command set)
- improved physical properties
- better aesthetics
What is in the powder of a RMGIC
- fluro-alumino-silicate glass
- barium glass (radiopacity)
- vacuum dried polyacrylic acid
- potassium persulphate (allows resin cure in the dark)
- ascorbic acid
- pigments (shades vary)
What is in the liquid of a RMGIC
- HEMA (water miscible resin)
- polyacrylic acid with pendant methacrylate groups
- tartaric acid
- water
- photo-initiators
Describe the dual curing setting reaction that RMGICs undergo
- acid-base reaction
- light activation a free radical methacrylate reaction occurs resulting in a resin matrix being formed (20s)
- acid base reaction continues
- redox reaction also occurs
- material will set even if insufficient light although will have reduced physical properties
Describe the tri curing setting reaction that RMGICs undergo
- acid-base reaction
- redox reaction begins
- light activation = free radical reaction –> resin matrix formed (20s)
- redox (5mins) and acid-base (hours) reactions continue
- final hardening of the acid/base phase with aluminium polyacrylate formation can take days
In what ways are RMGICs better than conventional GICs
Better physical properties Lower solubility Fluoride release Better translucency and aesthetics Better handling
In what ways are RMGICs worse than conventional GICs
- 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
- Light curing slows down the acid base setting reaction
- Benzoyl iodides and bromides can be released which are cytotoxic
- Fluoride release no better than conventional GIC
What are the main benefits of RMGIC compared to conventional GIC
- better aesthetics
- easier to use
- stronger
What are the main benefits fo RMGIC compared with composite resin
- easier to use
- fluoride release
Uses of RMGIC?
(same as conventional GIC)
- Dressing
- Fissure sealant
- Endodontic access cavity temporary filling
- Luting
- Orthodontic cement
- Restoration of deciduous teeth
- Restoration of permanent teeth
- Base or Lining
Why might you pick conventional GIC or RMGIC
to do with light curing
Which of these ions is not released in the glass ionomer setting reaction: A. Sodium B. Fluoride C. Calcium D. Magnesium E. Aluminium
Magnesium
Which of these is not a constituent of conventional glass ionomers: A. Polyacrylic acids B. Camphorquinone C. Silica D. Alumina E. Calcium Fluoride
camphorquinone
Identify the incorrect statement:
A. The glass ionomer setting reaction involves 3 phases
B. Glass ionomers are brittle with poor tensile strength and wear resistance
C. RMGI’s have poorer mechanical properties than conventional GI’s
D. Glass ionomers are susceptible to acid attack
E. Glass ionomers bond to enamel and dentine, reducing susceptibility to microleakage
C. RMGI’s have poorer mechanical properties than conventional GI’s
What speeds up the setting of GIC
tartaric acid
Which direct restorative material relies on chelation between carboxyl groups and the calcium in the tooth as a major component of the bonding mechanism? Select one: A. Amalgam B. Glass ionomer cements C. Composite resin D. Zinc phosphate E. Zinc oxide eugenol
Glass ionomer cements
