Materials for crown and bridgework Flashcards
what are the 4 components needed for the choice of materials for indirect restorations
- Allow the realisation of patients’ cosmetic expectations, but not necessitate preparations involving excessive removal of sound tooth tissue.
- Facilitate optimum tissue response.
- Take account of:
- The materials and tissues forming opposing and adjacent contacts.
- Technical considerations. - Be limited to those which satisfy the relevant standards
what are the 6 material requirements for indirect restorations
- Accuracy of fit.
- Strength to resist occlusal forces.
- Rigidity to avoid flexure and hence cement failure.
- Thermal expansion comparable to tooth.
- Should not attract plaque.
- Biocompatibility
what are the 4 options for crown and bridge materials
- metal
- ceramics
- metal-ceramics (porcelain fused to metal)
- indirect dental composite
what is a good metal material option for crowns and bridges
gold
what are the 2 types of metals used for crown and bridges
- gold and palladium alloys (precious alloys)
- base metal alloys (non-precious alloys)
name examples of gold and palladium alloys (precious alloys)
Type III gold alloys*
Silver-palladium
Palladium-silver-gold
Gold-silver-copper-palladium (Au>40%)
Palladium-copper
Palladium-tin
name examples of base metal alloys (non-precious alloys)
Nickel-chromium
Nickel-chromium-beryllium
Titanium
what 3 factors contribute in choosing a material for crowns and bridges
- cost
- corrosion resistance
- strength, stiffness, hardness, ductility
- low-stress bearing inlay
- posterior bridge
what is 1 star
- medium-gold casting alloy
- extra hard
- yellow-gold
- high proportions of gold with elements of silver and copper
what are the 4 roles of gold as a material in 1 star
- colour
- tarnish and corrosion resistance
- malleability of an alloy
- increases density of an alloy
what are the 2 roles of copper as a material in 1 star
- strengthener
- colour enhancer in Au-Ag- Cu crown and bridge alloys
copper rich Au-Ag-Cu alloys tend to have what colour
reddish colour
what are the 2 roles of silver as a material in 1 star
- balances the reddening effect of copper
- together with copper, used to control strength and hardness of crown and bridge alloys
silver rich Au-Ag-Cu alloys tend to have what colour
greenish
why may someone choose precious alloys over Au-Ag-Cu alloys
cheaper (cost of gold is steadily increasing)
what is an issue with reducing the gold content in alloys
- reduces density which may make casting more unreliable
- palladium and silver absorb oxygen when molten which can result in porous castings (esp. if buttons are re-used)
2 characteristics of base metal alloys
- Have a higher modulus of elasticity than noble alloys i.e. they are more rigid
- Implies less flexing of , e.g., a long span bridge
what are 2 disadvantages of base metal alloys
- the need for very carefully controlled casting conditions
- increasing concern BIOCOMPATIBILITY of dental alloys
what is the definition of biocompatibility
the ability of a material to perform with an appropriate host response in a specific application
3 biocompatibility issues
- Grinding and casting fumes can result in conjunctivitis, dermatitis and bronchitis (which may not express itself for several years after exposure)
- High levels of nickel are carcinogenic
- More commonly, nickel is well known to cause contact dermatitis (a host response).
why may a patient opt for a porcelain fused to metal instead of metal
aesthetics and functional longevity
who introduced porcelain fused to metal
Weinstein, Katz and Weinstein (1962).
what % of porcelain fused to metal lasts over 7 years
95-97%
5 examples of choices for metal-ceramic alloys
High gold (88%Au, 1%Ag, 6%Pd, 4%Pt)
Gold-palladium (50%Au, 38%Pd, 10%Ag)
High palladium (80%Pd)
Silver-palladium (60%Ag, 30%Pd)
Nickel-chromium (70%Ni, 20%Cr, 10%Mo)
why is the porcelain in the alloys in PFM restorations modified
to more closely match thermal expansions
difference between alloys for PFM restorations and alloys for precious metals
the bond to base metal alloys is not as good as that of the precious metals.
what is V-Delta SF
- metal ceramic material
- high proportion of gold and palladium with some indium and gallium
what is the role of indium in PFM alloys
- strengthens and hardens both gold and palladium and raises the thermal expansion of both
- indium lowers the melting range of both gold and palladium, and contributes to the formation of the bonding oxide
why does metal-ceramic materials (such as V-Delta SF alloy) have a higher melting range than 1 star
stable metal framework on which a porcelain veneer will be baked, therefore alloy needs a higher melting temperature so that it can retain its structure
what is it that makes metal ceramic work
the coping distributes stresses and provides rigid support
- Inhibits propagation of cracks from small faults at the ceramic/metal interface.
what 3 things allow a good bond to be achieved in a metal ceramic crown/bridge
- Mechanical retention (roughness).
- A direct chemical bond (ion diffusion).
- Mismatch of the coefficient of thermal expansion (CTE) (metal higher than ceramic).
- The mismatch in CTE creates tangential compressive stresses in the ceramic.
what are the 4 disadvantages of metal ceramic as a material
- Absence of light transmission (light doesn’t pass through metal), especially in cervical and proximal areas.
- Reduced depth of translucency.
- Presence of grey line at gingival margin.
- Sensitivity or allergy to alloys.
what is an issue with nickel in terms of biocompatibility
nickel sensitivity causing acute inflammation
what is an issue with non-precious metal bonded restorations in terms of biocompatibility
chronic discolouration
from metal ceramic and ceramic, which has better optical properties
ceramic (light can pass through)
what is the gold standard material in terms of aesthetics
all ceramic
who created all ceramic
C H Land, 1901
list ceramics with increasing toughness and bend strength
conventional porcelain
glass-ceramic
glass-infiltrated alumina
high-tech ceramics (e.g. zirconia)
what is a fundamental issue with ceramics
brittleness
what is the Griffith equation in material science
σf = K1c / Y √c
Where:
K1c = fracture toughness
σf = failure stress
c = critical flaw size
Y = constant
how can you overcome issues such as when a material has a fixed strain
increasing the stress to failure
how can you use the Griffith equation to increase the stress failure
increase fracture toughness
increase impact of critical flaw
What reason are ceramics better than glass as a material
Ceramic consists of a crystalline material in a glassy matrix. If a crack starts to propagate in the glass, the crystalline domains deviate propagation, so progression of crack is hindered. => crack will stop before complete failure.
what is aluminous porcelain
high strength ‘core’ porcelain containing up to 50% fused alumina crystals onto which a matched expansion veneer is baked
what is the benefit of adding alumina to porcelain
acts as a crack stopper, preventing cracks propagating through the material, thus increasing strength
what are glass ceramics
Fine grained polycrystalline materials which are obtained from an initially glassy phase following a heat treatment cycle
why re large numbers of fine crystals useful in glass ceramics
crystals limit the propogation of flaws through the glass-ceramic which has far superior mechanical properties compared to the base glass
what is IPS E.max
what does it consist of
what is the layering material
glass ceramics designed for crown and bridgework
- 60% Lithium disilicate (main phase)
- Lithium orthophosphate (secondary)
Core material strength ~400 MPa
fluoroapatite
how are lithium silicate glass ceramics produced
- glass powder melted and cooled in mould
- block produced used in CAD/CAM screening
- crystals are nucleated and then crystallised
- lithium silicate glass ceramic produced
list 5 indications for lithium silicate glass ceramics
- (thin) veneers
- inlays/onlays
- posterior crown
- anterior crown
- anterior bridge
superior aesthetics
med strength
2 characteristics of lithium silicate glass ceramics
v good aesthetics
med strength
describe the CAD/CAM process
- CCD camera (e.g. Cerec 3)
- video processing
- computer design
- computer miller
name a alumina core ceramic material
techceram
name 3 zirconia core ceramic materials
Lava
Cercon
IPS e.max ZIRCAD
What phenomenon gives zirconia its strength
transformation toughening
zirconia can exist in different crystal phases depending on what 2 things
time and pressure
what happens to zirconia when you cool it below 1000C
goes for tetragonal to monoclinic (opposite above 1000C)
what is the benefit of adding itria or magnesia to the composition of zirconia
tetragonal phase can be maintained at lower temperatures
So, if a crack encounters the tetragonal crystal, the constraints on the zirconia are released:
tetragonal → monoclinic
→ monoclinic occupies a greater volume in the bulk material than tetragonal
→ compressive forces
→ these counteract the tensile forces causing the cracks
→↓ suppressing crack propagation.
which material has the best aesthetics
glass ceramics (lithium disilicate)
which material has the best toughness
zirconia (zirconium oxide)
what is an issue with veneered zirconia
delamination and bonding issues between zirconia core and porcelain
what is the strength of lithium disilicate
360-400MPa
what is the strength of veneered zirconium oxide
zirconia core= >900MPa
porcelain veneer= 80-120MPa
