glass ionomer cements Flashcards
what are the 2 types of glass ionomer cements
> conventional GI
> resin modified GI
what are the uses of GIC
> restorative
- filling material
eg riva, vitremer
> core build up
- prior to restoration with crown
eg vitremer, crown core
> lining
- underneath permanent fillings
eg vitrebond, ionoseal
> luting
- cementing indirect restorations
eg fuji luting, vitremer luting cement, aquachem
originally what materials did conventional GIC come from
- zinc polycarboxylate cement
- silicate cement
what property did zinc polycarboxylate cement bring to conventional GIC
ability to stick to teeth
what property did silicate cement bring to conventional GIC
fluoride release
it is an anterior filling material based on fluoro-alumino-silicate glass combined with phosphoric acid
what is the chemistry behind the conventional GIC
2 components
> acid = liquid
> base = glass powder
what are the acids included in the chemistry of conventional GIC
> polyacrylic acid (ionic monomers)
- usually copolymers of acrylic and itaconic acid or acrylic and maleic acid
> tartaric acid
- added to control the setting characteristics of the material (shorter setting time, same working time)
what is included in the powder of the base in the chemistry of conventional GIC
○ Silica, SiO2 (silicone dioxide) 30-40% ○ Alumina, Al2O3 (aluminium dioxide) 15-30% ○ Calcium fluoride, CaF2 15-35% ○ Aluminium fluoride 2-10% ○ Aluminium phosphate 4-20% ○ Sodium fluoride 4-10%
what materials are added to the glass powder base of conventional GIC to increase the radiopacity
strontium and lithium salts (heavy metals)
these play no part in the reaction chemistry (setting reaction)
what alters the translucency of conventional GIC
the ratio of alumina / silica
more silica = more translucent
explain conventional GIC as an anhydrous material
the acid is freeze dried and added to the powder
the liquid is distilled water
this makes for easier handling of the material, particularly mixing
why is encapsulated materials of conventional GIC used in op tech and clinics
- consistent powder / liquid ratio
- easier to use
- should be consistent properties for the mixed material
what are the advantages and disadvantages of encapsulated conventional GIC
advantage
> no variation
> consistent result
disadvantage
> cannot change consistency of material - sometimes want it to be thicker the make the procedure easier
what can the powder particle size in the glass powder cause to vary
- need <20 micrometres for luting cement to give a low film thickness (needs small size so it wont interfere)
- the smaller the particle size the quicker the setting reaction and the more opaque the set cement
- bigger particles = better aesthetics
how can the molecular weight of the acid cause changes to the conventional GIC
- generally the higher the weight the better the mechanical properties of the set material
- longer chain acid = better properties but the liquid is less liquid so it is more difficult to mix
- however the higher molecular weight acids are viscous and difficult to mix
- need a happy medium between the mechanical properties and the mixing of the material
what is the setting reaction / acid base reaction of the conventional GIC
MO.SoO2 + H2A -> MA + SiO2 + H2O
Glass + acid -> salt + silica gel
M = metal A = polyacid
water absorbed into surface of silicone
what are the 3 phases of the setting reaction of conventional GIC
- dissolution
- gelation
- hardening
what happens in the dissolution stage of the setting reaction of conventional GIC
acid into solution
H+ ions attack the glass surface (acid dissolves the surface of the glass particles)
Ca, Al, Na and F ions are released (go into solution)
leaves silica gel around unreacted glass
what happens in the gelation stage of the setting reaction of conventional GIC
- initial set is due to calcium ion crosslinking with the polyacid by chelation with carboxyl groups
- calcium ions are bivalent so they can react with 2 molecules joining them
- crosslinking is not ideal as the calcium can chelate with 2 carboxyl groups on the same molecule
- this gelation equate to the initial set of the material and takes several minutes depending on the particular mateial
- this initial set is caused by formation of calcium polyacrylate (calcium bonding happens quickly)
- following this reaction the material will appear hard in the mouth (although it is not properly hard at this point)
what happens in the hardening stage of the setting reaction of conventional GIC
- trivalent aluminium ions ensure good crosslinking with an increase in strength
(sticks to 3 molecules at the same time, greater degree of crosslinking, makes for a stronger material) - aluminium polyacrylate formulation takes a long time
- this process does not start for at least 30 minutes and can take a week or longer to complete
- the aluminium reaction ensures a much higher degree of cross linking
- this process greatly improves the mechanical properties of the material
what does contamination during setting of the conventional GIC cause
> aluminium ions diffuse out of the material
excessive drying means water will be lost
saliva contamination causes absorption of water so extra ions will be flushed out
all of these lead to a weak material which will be rough, break up / be weaker and have poorer aesthetics
name the materials that can be used to protect the conventional GIC following placement
- varnishes
> copal ether
> acetate - resins
> dentine / enamel bonding agents (layer of resin then light cured - will last a little longer than other methods but still not last a week - remember aluminium takes this long)
> unfilled Bis-GMA resins - greases or gels
> vaseline
what materials gives the best protection to the conventional GIC after placement and why
varnishes and resins provide the best
vaseline is quickly removed by the action of the lips and tongue and offers little protection
when should protection be applied to the GIC restoration that is not after placement
if desiccation of the 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 (vaseline does work in this scenario)
what is the handling of conventional GIC like
tartaric acid greatly improves the ease of use
working time is largely unchanged
setting time shortened
what is the adhesion of conventional GIC like
can bond to enamel and dentine without the need for an intermediate material (easily bonds without need for acid etch or a bonding agent)
bond strength is not high compared with composite on acid etched enamel
about 5MPa v 20 MPa
good sealing ability
little leakage around margins
gives a good seal
what is the bonding mechanism of conventional GIC
- 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
what does a good bond need (conventional GIC)
- A clean surface
- a conditioned surface
what is meant by a conditioned surface for conventional GIC
> not etched (dont want to take out the calcium ions)
little or no tissue removed (dont want to lose the calcium ions)
best conditioner = polyacrylic acid
- mild solution
- resin - little bit of primer too
- guarantees a clean surface
many modern materials omit the conditioning stage
what is aesthetics like as a property for conventional GIC
- problem
- colour is okay but lacks translucency
(opaque in colour, looks chalkish when dried out) - new materials have improved
- materials with higher silica content and bigger particle sizes = better
- translucency improves over a 24 hour period when the extra X-linking occurs (improves over aluminium bonding stage so restoration may look rubbish initially but will look better when patient returns)
- not a good material for anterior teeth but good for cervical posterior restorations
what are the negative mechanical properties of conventional GIC
- poor tensile strength
- lower compressive strength than composite (less than half = 80-110MPa v >300MPa)
- poorer wear resistance
- lower hardness
- higher solubility (dissolution on unprotected material during gelation phase, long term erosion by acids)
what are the positive mechanical properties of conventional GIC
- good thermal properties
- expansion and contraction similar to dentine = doesnt stress the bond as much
- no contraction on setting
- one set it is less susceptible to staining and colour change
- fluoride release
- low modulus = allows a little bit of flex eg at the cervical margin when biting the teeth will flex slightly so conventional GIC allows a slight bend (it is not too rigid)
explain fluoride release from conventional GIC
can release fluoride without damaging their structure
has benefits against secondary caries
however lots is released straight away but over a short time it will disappear so there only be a little fluoride release then
it has been established that GIC can take fluoride up from the environment
they recharge their fluoride when the FI concentration around them is higher than that in the cement (takes it in) then they release FI again when the concentration falls
acts as a fluoride reservoir
anti-caries effect
when is conventional GIC and RMGIC used
- dressing
- fissure sealant
- endodontic access cavity temporary filling
- luting
- orthodontic cement (this has gone out of favour, not strong enough to stick a bracket on directly and to be able to transfer forces)
- restoration of deciduous teeth (often used) and permanent teeth (only really for cervical restorations)
- base or lining
what are the advantages of conventional GIC
- stable chemical bond to enamel and dentine
- low microleakage
- fluoride release
- good thermal properties
- no contraction on setting
what are the disadvantages of conventional 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 when wanting to use as a lining because you need a retentive surface (etching damages surface)
what is cermet
- developed to overcome glass ionomer brittleness
- silver added to the glass in equal volumes to increase toughness and wear resistance but there was no evidence that this was the case
- ended up as a silver coloured GIC so it looked like amalgam but functioned like GI
- no advantages and worse aestehtics
can still be used but not common
what is resin modified glass ionomer cement
developed to overcome the short comings of conventional GIC (like long setting time, poor physical properties and aesthetics)
wanted to take advantage of the bond to the tooth and fluoride release then add more advantages like light curing (command set), improved physical properties and better aesthetics
what is found in the powder of RMGIC
○ Fluro-alumino-silicate glass ○ Barium glass § Provides radiopacity ○ Vacuum dried polyacrylic acid ○ Potassium persulphate § Redox catalyst to provide resin cure in the dark ○ Ascorbic acid ○ Pigments § Varies shade
what is found in the liquid of RMGIC
○ HEMA
§ Water miscible resin
§ Only resin that works in water
○ Polyacrylic acid with pendant methacrylate groups
§ This can undergo both acid base and polymerisation reactions
○ Tartaric acid
§ Speeds up setting reaction
○ Water
§ Allows the reaction between polyacid and glass
§ Acid base reaction, hydration reaction
○ Photo-initiators
§ Enables light curing
§ Same as the one in composite materials so use the same light to cure it
explain the dual curing of RMGIC
- initially on mixing the acid base reaction begins the same as the conventional GIC
- on light activation a free radical methacrylate reaction resulting in a resin matrix being formed
- quick light activation is complete (20 seconds)
- acid base reaction continues within the resin matrix for several hours
what is the problem with light penetration in RMGIC and how is this over come
these materials are quite opaque so light does not penetrate deeply into the material (should not 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 (only important in the methacrylate polymerisation reaction)
gives confidence that the material will be set even if insufficient light penetration occurs
in the absence of light the physical properties of the set material are reduced by 25%
explain the tri curing of RMGIC
as the initial acid base reaction takes place the redox reaction begins (also takes place straight away)
on light activation a free radical methacrylate reaction occurs resulting in a resin matrix being formed
quickly light activation is complete (20 seconds)
the redox reaction continues for about 5 minutes after initial mixing (forming free radicals where the light does not penetrate)
acid base reaction continues within the resin matrix for several hours (calcium bonding quickly)
final hardening of the acid base phase with aluminium polyacrylate formation can take days
diagram from lecture explains it better maybe im pretty confused w all this lol xoxoxo
what should you do with regards to conditioning of the tooth prior to the cement placement
follow manufacturers advice
Vitrebond = None Fuji cement LC = Optional Fuji II LC = Yes Vitremer = Yes Vitremer luting = No mention
what are the good properties of RMGIC
- good bond with enamel and dentine (bond strength is higher = cohesive strength higher than conventional GIC)
- better than conventional GIC initially
- better physical properties
- lower solubility
- fluoride release
- better translucency and aesthetics
- better handling
what are the bad properties of RMGIC
- polymerisation contraction
- exothermic setting reaction
- swelling due to uptake of water (HEMA is extremely hydrophillic)
- monomer leaching (HEMA is toxic to the pulp so must be polymerised completely, if it is unreacted then it can kill the pulp)
- reduced strength if not light cured
- light curing slows down the acid based setting reaction
- benzoyl iodides and bromides can be released which are cytotoxic
compare RMGIC to conventional GIC
better aesthetics in RMGIC (can be used in anterior teeth) easier to use RMGIC stronger than RMGIC same fluoride release
compare RMGIC to composite resin
easier to use RMGIC
RMGIC bonds itself
fluoride release
which GIC is better
clinically there are times when one material is the correct choice
eg conventional GIC would be best to restore an 8 as it is difficult to access and a moisture problem is less of a problem
often either material would be appropriate and ease of use is often the deciding factor
