DM - cement: crowns, posts and bridges Flashcards
When cementing a post and core what surfaces does the luting cement lie on?
all surfaces of post
all contacting surfaces of core
what marker can be placed to accommodate luting cement thickness?
die relief
what is the function of luting cement?
added retention
seals the space between restoration and tooth
what are the 2 types of luting cements?
active
passive
what is an active luting cement?
bonds to tooth and restoration chemically
has a role in retention - mechanical interlocking
creates a marginal seal
what is a passive luting cement?
fills gap between tooth and restoration
no bond between tooth and restoration
retention due to tooth prep
in what ways can tooth prep create renetion?
taper
prep height
surface roughness and mechanical interlocking
list the general ideal properties of a luting cement
biocompatible
retentive
mechanical
marginal seal
low film thickness
ease of use
pseudoplastic
radioopaque
aesthetic
antibacterial
why must a luting cement be biocompatible?
it contacts tooth tissue and periodontal tissues
what mechanical properties should a luting cement have?
high tensile strength
fracture toughness
fatigue strength
wear resistance
what properties must a luting cement have to create a marginal seal?
low solubility
what does low film thickness allow for in a luting cement?
full seating of restoration
good marginal adaptation
why should a luting cement be pseudoplastic?
coats fit surface of restoration without slumping but flows readily on fitting under pressure
list types of passive luting cements?
zinc phosphate
zinc polycarboxylate
GI
RMGI
all are water based as aqueous acid
what luting cement is the oldest and routinely used by 30% UK dentists?
zinc phosphate
what are the components of zinc phosphate luting cement?
powder: zinc oxide, 10% magnesium oxide
liquid: aqueous phosphoric acid
what is the working time of zinc phosphate?
3-6mins
how is zinc phosphate mixed?
gradual incorporation of powder over large area on mixing slab
fluid is slaked - small amount powder added to fluid 1 min before to extend setting time
what can be used when mixing zinc phosphate to increase its working time?
chilled slab
also increases its strength
describe the setting reaction of zinc phosphate luting cement
acid base reaction
exothermic
viscosity increases rapidly
shrinks slightly on setting
is zinc phosphate antibacterial?
no
what does the pH of zinc phosphate depend on?
thickness of mix
describe the mechanical properties of zinc phosphate luting cement
good compressive strength
low tensile strength (brittle)
high solubility
what are the components of zinc polycarboxylate?
powder: zinc oxide, 10% magnesium oxide
liquid: aqueous copolymer of polyacrylic acid (30-40%)
alternatively - acid freeze dries and added to powder, distilled water added
describe the setting reaction of zinc polycarboxylate luting cement
acid dissolves in zinc oxide
what is the working time of zinc polycarboxylate luting cement?
30-40 seconds
can be extended by adding tartaric acid, mixed on a cold glass slab
what pH is zinc polycarboxylate?
3-4
less injurious on pulp
is zinc polycarboxylate antibacterial?
yes
what is zinc polycarboxylate adhesive to?
enamel and dentine
some metals (base metal alloys via oxide layer)
describe the mechanical properties of zinc polycarboxylate luting cement
lower compressive strength
higher tensile strength
reaches 80% full strength in 1 hour
soluble in acid
describe the presentation of GI luting cement
powder: fluoro-alumino-silicate glass
liquid: aqueous poly alkenoic acid
alternatively - acid freeze dried and added to powder, distilled water added
describe the setting reaction of GI luting cement?
chemical set
dissolution of calcium ions, then aluminium
gelation
hardening
setting time of GI luting cement?
initial set 3-6 mins
hardening can take 7 days
why may GI initially cause pulpal inflammation?
acidity
compared to ZnO cements, describe the mechanical properties of GI
better compressive strength
low tensile strength and fracture toughness
less soluble
what are the constituents of RMGI cement?
GI plus monomer e.g., HEMA, Bis-GMA
What are the advantages of RMGI compared to GI?
low solubility
improved biocompatibility
improved fluoride release
improved physical properties (tensile strength)
improved adhesion to tooth tissue
what are the disadvantaged of RMGI?
can undergo hygroscopic expansion - may lead to crack propagation
avoid under conventional all ceramic crowns
what is the name of RMGI luting cement?
FUGI PLUS
what indirect restorations are suitable for the use of RMGI luting cement?
zirconia core/ CAD CAM crowns
what are the constituents of resin based luting cements?
silanated filler
resin - Bis GMA
how are resin-modified luting cements cured?
chemical and light
what indirect restorations are suitable for conventional resin based luting cements?
all ceramic restorations
indirect composite or quartz fibre posts - veneers, dentine bonded crowns
what indirect restorations are suitable for chemically adhesive resin based luting cements?
resin retained bridges
metal veneers
poorly retained indirect restorations
how are resin based luting cements bonded to ceramic?
bond to tooth in conventional way - acid etch, rinse, dry, dentine bonding agent
bond to fit surface of ceramic - etch with hydrofluoric acid, just before cementing silane coupling agent air dry
why can hydrofluoric acid only be used by the lab?
very toxic
must be neutralised
tends to slump
how do you bond to a base metal alloy?
roughen fit surface with 50um aluminia grit
chemically adhesive resin luting cement - modified bis GMA resin
how do you bond to precious metal alloys?
chemically adhesive resin luting cement
modify fit surface
