key points dental ceramics Flashcards
what is a ceramic?
crystalline minerals and a glass matrix
what do decorative ceramics contain?
kaolin - opaque
dental ceramic constituents
kaolin <5% silica 12-25% feldspar 70-80% metal oxides 1% glass up to 15%
feldspar
flux - melts and embeds other particles
borax
lowers fusing temp
metallic oxides
strengtheners
making powder
heat
cool rapidly (fritting)
mill frit
add binder
powder and distilled water built up into restoration
sintering
heat
glass phase softens - coalesce
form solid ceramic mass
20% contraction
aesthetics
best
colour stable
v smooth surface
reflectance
% of light striking a surface which is reflected off it
translucency
allows light to pass through, objects behind can’t be seen
opacity
lacking transparency/translucence
transparency
allows light to pass through, objects behind can be seen
opalescence
scattering of visible light wavelengths, bluish appearance in reflected colour and orange/brown in transmitted colour
chemical stability
v stable
thermal properties
similar to tooth
TEC similar to D
thermal diffusivity low
dimensional stability
once fired fully v stable
mechanical properties
high compressive strength high hardness - too high tensile strength v low flexural strength v low fracture toughness v low static fatigue surface microcracks slow crack growth - cyclic fatigue
high hardness
abrasion of opposing teeth esp if not glazed
static fatigue
time dependent decrease in strength even in the absence of any applied load
layers
opaque porcelain - provides porcelain metal bond and masks dark oxide colour
body/dentine porcelain
incisal/enamel porcelain
alumina core - why can it only be used as a core?
opaque
alumina core - mechanism
alumina particles act as crack stoppers - prevent propagation
- necessitates more energy to propagate a crack so more resilient
alumina core - strength
flex strength double porcelain >120 MPa
not strong enough for posteriors
INCERAM
Spinel and Zi
increased alumina content
single posterior crowns
PROCERA
increased alumina content
single posterior crowns
overcoming problems with conventional ceramics
strong coping resistant to fracture, cover in conventional porcelain - metal coping - alumina core - zirconia core cast/press a block of harder ceramic mill a lab prepared block of ceramic
Zi core and bonding
difficult to bond to
Zi core
probably most popular ceramic core material
v hard
use Yttria-stabilised zirconia
- <1% Yttria
tetragonal crystal to monoclinic structure
- slight expansion of the material and closes crack tip - like a self-healing crack
strong enough to use as bridge framework
problems with zirconia cored crowns
£££ equipment
potential for veneering porcelain to debond from core
- can inbuild stresses between the 2 materials on
manufacture
opaque core
- are aesthetics much better than metal ceramic?
inert fitting surface, can’t etch or bond
- use conventional cement
milled core crowns and bridges
zirconia
lithium disilicate
precious metal
non-precious metal
Ti
= all have scintered surface layer for best aesthetics
- but ones milled from a single block are stronger than if you put the layer of sinter on
milled Rx design feature
need to round internal angles in prep as sharp shoulder can’t be replicated in a milled Rx
all ceramic crowns
1 - glass infiltrated ceramics - etched with HF acid and treated with silane coupling agent, bonds to tooth 2 - no glass content (Zi and alumina) - luted with cement - stronger but less aesthetic
sintered vs milled
for the same material milled stronger than built up/pressed
block has had ideal heat txs to maximise its properties and all blocks will be consistent
lithium disilicate glasses e.g. Emax
have unique needle-like crystals - crack propagation v difficult - good flexural strength better aesthetics than Zi - translucency stronger - crystalline structure 2nd gen need 1.5mm occ reduction contains silica heat pressed
what structure do strong materials have?
small crystal size and high vol fraction of crystals
cast and pressed
most often cut back labially and veneered with feldspathic porcelains
ceramic used - glass ceramics
- lithium disilicate glass
- leucite reinforced glass
= becoming a bit redundant due to milled ceramics
reinforcing ceramics
dispersion strengthening: alumina often used
monolithic e.g. milled from a ceramic block
core (coping)
leucite
1st gen crystalline reinforcing phase v good aesthetics raises TEC - good for MCCs heat pressed weaker than lithium disilicate
design features
rounding internal line angles
occlusal contacts at least 1.5mm away from porcelain metal jct
bonding to metal
needs to be a mononuclear oxide layer
adhesive failures
between different materials
cohesive failures
between same material
luting any silica-containing ceramic
can etch with HF acid - retentive surface
- can bond to it using a silane coupling agent
- bond to tooth with bonding agent
luting Zi-cored crowns
don’t contain silica - not affected by acid
- strong enough to be self-supporting, can be luted with conventional cement
difficult to bond to
