glass ionomer cements Flashcards
what are the uses of GIC?
restorative core build up -prior to rest. w/ crown lining luting -cementing indirect rest.
what are the 2 materials in GIC?
zinc polycarboxylate cement
silicate cement
what happened when silicate & zinc polycarboxylate combined?
glass from silicate was dissolved in polyacrylic acid from zinc cement
why was this viewed as a good material?
resulted in a fluoride releasing filing material which had the ability to bond to tooth structure
what are the 2 components?
acid
-liquid
base
-glass powder
what acids are used?
polyacrylic acid
-usually copolymers of acrylic and itaconic acid or acrylic and maleic acid
tartaric acid
-added to control the setting characteristics of the material
what are the powder components?
silica (SiO2) 30-40% alumina (Al2O3) 15-30% CaF2 15-35% aluminium F 2-10% AlPO 4-20% NaF 4-10%
what ratio alters the translucency and how?
alumina/silica
silica more translucent
describe anhydrous material composition
acid is freeze dried and added to powder
liquid is distilled water
easier handling of material, particularly mixing (& storage)
describe encapsulated materials?
consistent powder/liquid ratio
easier to use
should be more consistent properties of mixed materials
-this can be a disadvantage as clinically may need more wet/dry composition
what are some variations in composition?
due to composition of powders
powder particle size
-smaller = quicker setting + more opaque
molecular weight of acid, higher = better mechanical properties of set material, but more viscous
what are the 3 phases of the setting reaction?
dissolution
gelation
hardening
what is the basic acid base reaction?
glass + acid -> salt + silica gel
describe dissolution
acid into solution
H+ ions attack the glass surface
Ca, Al, NA + F ions are released
leaves silica gel around unreacted glass
describe gelation
initial set due to Ca ion crosslinking with polyacid by chelation with carboxyl groups
Ca ions bivalent so they can react with 2 molecules joining them
crosslinking not ideal as the Ca can chelate w/ two carboxyl groups on the same molecule
several minutes
forms calcium polyacrylate
material -> hard
describe hardening
trivalent Al ions ensure good crosslinking with an increase in strength
Al polyacrylate formation takes a long time from 30 mins- a week
aluminium reaction causes much higher degree of crosslinking
improves mechanical properties of material
what happens if moisture contamination occurs during the setting reaction?
Al ions diffuse out of the material
excessive drying means water will be lost
saliva contaminations causes absorption of water
all lead to a weak material which will be rough, break up and have poor aesthetics
what can be used for protection following placement?
varnishes -copal ether -acetate resins -dentine/enamel bonding agents -unfilled Bis-GMS resins greases or gels -vaseline
why is Vaseline not ideal protection?
quickly removed by the action of lips and tongue
what other situation is protection required?
is desiccation of a GIC restoration is possible during work on other areas in the mouth
-thin layer applies to prevent surface damage due to excessive drying
describe the handling properties
tartaric acid gives ease of use
setting time good
working time too long
describe the adhesion properties
can bond to enamel and dentine w/o need for use of an intermediate material
low bond strength
good sealing ability w/ little leakage around margins
what is the bonding mechanism?
chelation between carboxyl groups in the cement and Ca on the tooth surface
re-precipitation of complex mixture of Ca phosphate and Ca salts from polyacid onto and into the tooth surface
hydrogen/metallic ion bridging to collagen
what is required for a good bond?
a clean surface conditioned surface -polyacrylic acid no etch little/no tissue removed clean smooth surface
what are the mechanical properties?
poor tensile strength low compressive strength poor wear resistance low hardness high solubility good thermal expansion no setting contraction less susceptible to staining and colour change fluoride release low rigidity modulus
how does it act as a fluoride reservoir?
initial release
recharge Fl when conc. around them is higher than in cement
release again when conc. gradient changes
what are the uses?
dressing fissure sealant endodontic access cavity temporary filling luting orthodontic cement rest. deciduous rest. base/lining
what are the advantages of GIC?
stable chemical bond to enamel and dentine low microleakage Fl release good thermal properties no setting contraction
what are the disadvantages of GIC?
brittle poor wear resistance moisture susceptible when first placed no command set poor aesthetics poor handling characteristics susceptible to acid attack and drying out over time possible problems bonding to composite -etching damages surface
