8. Glass Ionomer Cements Flashcards
Types of GICs (2)
Conventional GI
RMGI
Uses for GI (4)
Restorations
Core build-up
Lining
Luting
Chemical components of GIs (2)
Acid (liquid)
Base (glass powder)
Components of acid (2)
Polyacrylic acid (ionic monomers) Tartaric acid
Function of polyacrylic acid
Usually copolymers of acrylic and itaconic acid or acrylic and maleic acid
Function of tartaric acid
Added to control the setting characteristics of the material
Components of base/powder (6)
Silica (silicon dioxide) – 30-40% Alumina (aluminium dioxide) – 15-30% Calcium fluoride – 15-35% Aluminium fluoride – 2-10% Aluminium phosphate – 4-20% Sodium fluoride – 4-10%
Effect of adding strontium and lithium salts to the base (2)
Can increase the radiopacity
Play no part in the reaction chemistry
Effect of alumina/silica ratio (2)
Ratio of alumina/silica alters the translucency
More silica, more translucent
GI setting reaction (2)
MO.SiO2 + H2A –> MA + SiO2 + H2O
Glass + acid –> salt + silica gel
M – metal; A – polyacid
GI setting reaction phases (3)
Dissolution
Gelation
Maturation/hardening
Process of dissolution (4)
The acid is added into the solution The H+ ions interact and attack the glass surface Glass ions (Ca, Al, Na, F) are released/leach out This leaves silica gel around unreacted glass
Process of gelation (4)
The initial part of the setting reaction is due to Ca2+ ions crosslinking with the polyacid by chelation with the carboxyl groups (quite quick), forming calcium polyacrylate (causing the material to appear quite hard in the mouth)
Ca ions are bivalent, so can react with two molecules and join them
Issue with crosslinking
Crosslinking is not ideal as calcium can chelate with two carboxyl groups on the same molecule
Process of maturation/hardening (2)
Trivalent Al3+ ions ensure good crosslinking with an increase in strength
Aluminium polyacrylate formation takes a while – it does not start for at least 30 mins and can take around a week to complete
Effect of aluminium reaction
The aluminium reaction ensures a much higher degree of crosslinking, greatly improving the mechanical, physical and aesthetic properties of the material
Effects of contamination, by moisture and desiccation (4)
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
Materials used to protect GIC following placement (3)
Varnishes (copper ether, acetate)
Resins (DBAs, unfilled Bis-GMA)
Greases/gels (vaseline)
Varnishes and resins provide better protection; petroleum gel is quickly removed by the action of the lips and tongue and offers little protection
Protection is also required at a later date if desiccation of a GIC restoration is possible during work on other areas in the mouth. A thin layer of varnish or resin should be applied at this time to prevent surface damage due to excessive drying
Bond strength of conventional GIC to enamel/dentine
5MPa
Features of GIC
Good sealing ability with little leakage around margins
Conventional GIC bonding mechanism (3)
Chelation between carboxyl groups in the cement and calcium on the tooth surface
Re-precipitation of complex mixture of calcium phosphate (from apatite) and calcium salts from the polyacid onto and into the tooth surface
Hydrogen bonding or metallic ion bridging to collagen
Requirements of a good bond (2)
Clean surface
Conditioned (not etched) surface (to produce a clean, smooth surface, removal of little/no tissue, best conditioner appears to be polyacrylic acid)
GIC aesthetics (4)
Colour is acceptable
Lack translucency
Material with a higher silica content are better
Translucency improves over 24+ hours when extra crosslinking occurs. GICs are not suitable when aesthetics is of prime importance
Mechanical properties of GIC (10)
Poor tensile strength Lower compressive strength than composite Poorer wear resistance than composite Lower hardness than composite Higher solubility than composite Good thermal properties No contraction on setting Once set, less susceptible to staining and colour changes than composite Fluoride release Lower modulus can be a good thing
Compressive strength of GIC
80-110MPa
Features of high GIC solubility (2)
Dissolution of unprotected material during gelation phase
Long-term erosion by acids
Features of fluoride release (4)
Can release fluoride without damage to their structure
Beneficial against secondary caries (inhibits bacteria growth)
However, fluoride is released in negligible amounts
Initial fluoride release is high, but this diminishes very quickly, over the first week
Features of GIC fluoride reservoir (4)
Can take up fluoride from the environment
Can recharge their fluoride when the F- concentration around them is higher than that in the cement.
Release F- again when the ambient concentration falls
Can act as a fluoride reservoir or fluoride sink
Uses of GICs (8)
Dressing Fissure sealant Endodontic access cavity temporary filling Luting Orthodontic cement Restorations of deciduous teeth Restorations of permanent teeth Base or lining
Advantages of GICs (5)
Stable chemical bond to enamel and dentine Low microleakage Fluoride release Good thermal properties No contraction on setting
Disadvantages of GICs (7)
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)
Features of cermets (4)
Developed to overcome GI brittleness
Silver was added to glass to increase toughness and wear resistance - no evidence this worked.
No advantages
Worse aesthetics
Development of RMGICs (3)
Developed to overcome the shortcomings of conventional GIC
Advantage of existing GIC technology (bonding to tooth, fluoride release)
Advantages of composite technology (light curing, improved physical properties, better aesthetics)
Composition of RMGICs (2)
Powder (fluoro-alumino-silicate glass, barium glass, vacuum dried polyacrylic acid, potassium persulfate, ascorbic acid, pigments)
Liquid (HEMA, polyacrylic acid with pendant methacrylate groups, tartaric acid, water, photo-initiators)
Function of barium glass in RMGIC powder
Provide radiopacity
Function of potassium persulfate in RMGIC powder
Redox catalyst to provide resin cure in the dark
Function of HEMA in RMGIC liquid
Water miscible resin
Function of polyacrylic acid with pendant methacrylate groups in RMGIC liquid
Can undergo both acid base and polymerisation reactions
Function of tartaric acid in RMGIC liquid
Speeds up setting reaction
Function of water in RMGIC liquid
Allows reaction between polyacid and glass
Function of photo-initiators in RMGIC liquid
Enables light curing
Curing types of RMGICs (2)
Dual-curing
Tri-curing
Curing process of dual-cure RMGIC materials (4)
Initially on mixing, the acid base reaction begins in the same way as conventional GIC
On light activation, a free radical methacrylate reaction occurs resulting in a resin matrix being formed
Quickly, light activation is complete (20s)
Acid base reaction continues within the resin matrix for several hours
Features of dual-curing RMGIC materials (3)
Quite opaque so light does not penetrate deeply into the material
Should consequently be placed in layers or it may not set
To counteract the problem, a redox (reduction oxidation) reaction also occurs in some of these materials
Curing process of tri-cure RMGIC materials (7)
Initially on mixing, the acid base reaction begins in the same way as conventional GIC
The redox reaction begins
On light activation, a free radical methacrylate reaction occurs resulting in a resin matrix being formed
Quickly, light activation is complete (20s)
The redox reaction continues for about 5 minutes after initial mixing
Acid base reaction continues within the resin matrix for several hours
Final hardening of the acid/base phase with aluminium polyacrylate formation can take days
Advantages of RMGICs (7)
Good bond to enamel and dentine Potentially superior to conventional GIC Better physical properties Lower solubility Fluoride release Better translucency and aesthetics Better handling
Disadvantages of RMGICs (5)
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 (redox cure is not as strong as light cure)
Features of RMGICs (3)
Light curing slows down the acid base setting reaction
Benzoyl iodides and bromides can be released, which are cytotoxic
Fluoride release is no better than conventional GIC
RMGIC benefits over conventional GICs (3)
Better aesthetics
Easier to use
Stronger
RMGIC benefits over composite resins (2)
Easier to use
Fluoride release
Uses of RMGICs (8)
Dressing Fissure sealant Endodontic access cavity temporary filling Luting Orthodontic cement Restoration of deciduous teeth Restoration of permanent teeth Base or lining
