Focused Quiz: 9/29 Flashcards
Gypsum is made of:
calcium sulfate dihydrate (CSD - think cast)
We use gypsum for:
casts and investment material
how is a dihydrate made to hemi?
reduction/ oxidation
Both alpha and beta hemihydrate have this driven off:
water
Which is the natural form, di or hemi hydrate?
di
How is a-hemihydrate produced?
steam and pressure
Shape of crystalline HH:
rods, prisms (a-HH can be cuboidal)
How is b-HH produced?
heating in air (dessication)
Shape of b-hh:
irregular
What is b-hh?
fibrous aggregate of fine crystals
How many times more soluble is hh than dh?
4 times
What precipitates when water is added to HH?
DH
As more DH precipitates:
more HH dissolves
What initiates setting expansion?
DH formation, molecules bumping into one and other
How to increase nucleation:
over spatulation, less water, dirt, slurry water, finer particle size of HH
How to inc reaction rate:
inc nucleation, additives (inorganic salts, K2SO4)
How to decrease the rate of reaction:
more water, shorter, slower mixing, blood, borax, coarser particle size of HH
How to enhance setting expansion:
by nuclei forming close together, less water, over spatulation
surface tension is aka:
restriction
How to remove surface tension on gyspum:
put setting gypsum under water
Does surface tension restrict or aide movement?
restricts
hygroscopic:
tending to absorb moisture from the air
Setting expansion of gypsum:
0-0.3%, very small
How will increased porosit affect strength?
decrease
What affects amt of porosity?
w/p ratio, vaccum mixing
How do accelerators and retarders affect compressive stregnht?
decrease
Surface area of particles is proportional to:
amt of water needed to “float and coat”
“Float and coat”
getting particles into suspension, not dissolving, supersaturated solution that can precipitate, denser = stronger
w/p ratio affects
set strength, ratio of hemihydrate or dihydrate in the mix
How much dimensional change does gypsum undergo if under running water for 20m?
0.1%
What should you soak a gypsum cast in?
saturated calcium sulfate, best: a container with gypsum pieces on bottom
Why does snap stone set faster?
additives added
Dental stone, high strength:
Type 4
Dental plaster:
Type 2
Impression plaster:
Type 1
Dental stone, high strength, high expansion:
Type 5
Dental stone:
Type 3
Type I plaster are made of:
polysulfides and polyethers
Type I dental stone:
impression plaster, rarely used, lowest strenth, porous b-HH, 0.5W/ 0.75P, setting expansion: 0-0.15%
Type II:
Dental plaster, usually white, mounting plaster, denture flasking, weak, 0.45W/ 0.5P, setting expansion: 0-0.3%, accuracy not required
Type II, a-hh or b-hh?
b-hh
Type III, a-h or b-hh:
a-hh
Type III:
dental stone, calcination (reduction/ oxidiation) under steam, a-hh, 0.28W/ 0.3P, settting expansion: 0-0.2%
Type III stone has half the w/p ratio as these
Type I and almost Type II (0.5 vs. 0.45)
What accounts for the drop in the w/p ratio for Type III stone?
a-hh, reduced sa
Type IV dental stone:
densite a-hh, cuboidal-shaped particles (reduced sa), 0.22W/ 0.24P, setting expansion: 0.0.1%
Type V dental stone:
stronger that IV, 0.18W/ 0.22P, setting expansion: 0.1-0.3%, expansion compensates for base metal shrinkage
What does Type V stone expansion compensate for?
base metal shrinkage in casting
Type IV, a-hh or b-hh?
a-hh
Which is more accurate, weight or volumetric measurements?
weight
Next ppt:
investment materials and metal alloys
the sprue is cast onto this (not the tooth):
crucible former
What happens after the sprue is attached to the tooth?
burnout and sectioned
Steps to lost wax technique:
impression, cast, restoration in wax, invest wax pattern, burnout, cast
How should investment be mixed?
under vacuum
Big contact angle:
Big glob of water (check, this was backwards in my notes)
Function of investment:
preservation of the wax pattern after melting
Material that can be used for lost wax tech:
alloy or ceramic
Wax, high or low surface energy?
low, needs wetting agent
Why do we need wetting agent?
lots of water in investment material, doesn’t spread well over wax (lipid), this helps preserve the pattern
Ideal properties of investment:
heat resistabt, expands, accurate reproduction, strength, porosity for gas escape, ability to recover casting
What stone to use to compensate for alloy shrinkage:
Type 4
What would make recovering the cast difficult?
material that is too strong
2 main components of investment:
refractory material, binder
refractory material:
resistant to high temps, not easily melted or worked
Is investment heat resistant?
yes
Refractory used in investment:
silica, quartz or cristobalite
How is thermal expansion controlled in investment?
inversion: low temp (alpha) and high-temp (beta) phases
Function of refractory:
strenght, resistant to heat, and thermal expansion
Greater expansion: cristobalite or quartz?
cristobalite
Undergoes thermal expansion at a lower temp, cristobalite or quartz?
cristobalite (250C vs 573C)
Cristobalite can get up to __% expansion:
2%
Binder:
added to form dry ingredients to maintain even consistency
Function of binder:
strength for investment
2 types of binder used in investments:
gypsum, phosphate
How are investment materials classified?
binder type
What compensates for alloy shrinkage during cooling from liquid ot solid phase?
expansion of investment
3 types of expansion:
setting, hygroscopic, thermal
Molecular level change occuring when gypsum udergoes setting expansion:
HH to DH
What allows for equal expansion in the casting ring?
liner
When does setting expansion occur?
as binder sets
How to maximize hygoscopic expanion:
get into water bath sooner
When will hygroscopic expasion occur in gypsum?
if it sets under water
When does most hygroscopic setting expansion occur?
during initial set
How to increase expansion of gypsum setting under water?
heat the water
Theory of hygroscopic expansion:
reduced surface tension means less confinement of growing crystals
How many times more expansion can you get, normal vs. hygroscopic?
4 times more
Normal expansion:
none at beginning, 1st 10 minutes very little, most occurs bw 10 and 20 min
Hygroscopic expansion:
majority of expansion bw 5-10min, up to 1.3% at 10min,
2% max
What is thermal expansion dependent upon?
the refractory material, which type of silica, quartz or cristobalite?
Quartz transition temp:
1067F, almost 3 times higher than cristobalite
Cristobalite transition temp:
About 400F
1st investment used in dentistry:
gypsum bonded
When does the refractory material lose stability?
1200F
What do we add to refractory material to increase the temperature it which it can remain stable?
gold alloys
heating gypsum converts:
DH to HH
What type of refractory does beauty cast have?
quartz refractory
Is gypsum bonded with cristobalite hygroscopic expansion used?
no, the addition of cristobalite would lead to too much expansion
Can all alloys be cast in gypsum?
no
Binder for phosphate bonded:
magnesium oxide and phosphate
When is phosphate bonded investment necessary?
materials that melt over 1200C/ 2200F, most metal ceramic alloys and cast ceramics
What refractory is used in phosphate bonded investment?
silica, as always
What types of expansion occur with phosphate bonded investment?
setting and thermal
What is thermal expansion controlled by in phosphate bonded investment?
amount of silica, amount of special liquid that comes with phoshate bonded investment, mix before use to equally suspend silica
What is special liquid:
colloidal silica
Does setting expansion increase or decrease with the addition of the special liquid?
increase
Does thermal expansion increase or decrease with the addition of the special liquid?
increase
Phosphate investment:
Stable at higher temps, allows greater thermal expansion, more prone to surface nodules (harder to adapt wax), difficult to breakout casting, stronger, harder
Commercial labs typically use this investment:
phosphate
Recommended max investment thickness:
6mm. to allow gas release scratch off top of investment, decrease chance of air bubbles
Liners are made of:
aluminum silicate ceramic or paper
Functions of liner:
allow expansion, recovery of casting is easier, can push cast out of ring and break up investment
What can lead to rough casting:
improper finish of wax pattern, excess surfactant, wrong w/p ratio, excess temperature during burnout
Most common casting failure:
rough casting/ not good finish
Casting failures:
rough casting, fins, nodules, incomplete casting, porosity
Causes of fins (cracks that get filled w material):
increased w/p ratio, bad pattern position, premature heating, rapid heating, dropping ring, investment not fully set, too much moisture,
The (investment?) is more prone to cracking if the wax pattern/ outline is less than __ mm from the top:
6
All nodules are due to:
air
What better o have on the casting, neg or pos?
neg
What to do when removing a positive nodule:
remove a little extra, neg better than pos
Cause of nodules:
inadequate vacuum, no surfactant
These can lead to incomplete casting:
wax pattern too thin, incomplete wax elimination/ burnout, cool mold or melt, inadequate metal
Area of crown this is most often too thin:
occlusal surface
How does the density of the alloy affect the amt of the alloy required for a crown?
more dense, less alloy needed
These can lead to porosities:
improper melting temp, pattern position or sprue location or thickness
How is metal bonded to ceramic in PFM?
intermediate oxide layer
We could start to make PFM restos when:
decreasing the coefficient of thermal expansion of metal
What would happen if metal and procelain didn’t have similar coefficients of thermal expansion?
one would crack
Disadvantages to PFM compared to CCC:
remove more tooth structure, risk of porcelain fracture
These help determine the risk of porcelain fracture:
design of framework, how the porcelain was applied
Advantages and disadvantages of PFM vs. al ceramic:
adv: more conservative prep, stronger, disadv: poor esthetics (usually)
Dentistry is going in the direction of using this crown type:
all ceramic (check, why? less conservative than PFM)
Alloy:
mutually soluble in each other in molten state
Function of gold in alloy:
soft, inc resistance to tarnish and corrosion, increase ductility and malleability
Function of copper in alloy:
hardener, neccesary for heat tx for conc greater than 12 wt%
Function of silver in alloy:
dec melting temp, inc ductility and maleability
problem with adding silver to alloy:
any alloy that has silver must use a compatible ceramic, comes form the oxidation of the silver
Platinum:
increases melt temp, increases tensile strength, decreases CTH, reduces tarnish and corrosion
Palladium:
increases melt temp, increases hardness, absorbs H2 gas which can cause porosity, prevents tarnsih and corrosion, strong whitening effect on gold alloys
Zinc:
scavenger, prevents oxidation, increases fluidity , decreases st, which improves castabilitiy
Iridium:
grain refiner, increases tensile strength and % elongation (ductility)
Tin and indium:
allow oxidation, added to high noble alloys to get oxidation, the fusing of porce to metal requires an oxide layer
High noble:
60% min, at least 40% of that is gold
2 metals in all high noble alloys:
gold and palladium, just those 2, add silver, or add platinum
Metal type most often used:
noble
Why do you need to choose porcelain based on the oble metal cois>
greening problem
All noble metals have this metal:
pladium, that and silver, or that and galllium, or that and copper and gallium
Noble metal requireent for base metals:
none
Metal found in base metals:
chromium, that and cobalt or that and nickel
Issue with using some base metals:
some have nickel in them, allergies
Most difficult base metal to use:
Co-Cr
What metal causes greening in some porcelain systems?
silver
How to melt traditional high gold alloys:
gas-air torch
How to melt metal ceramic alots:
gas-oxygen torch, gas-air won’t get hot enough to melt
Color of burning gas:
blue
what type of atmosphere does burning create
redeced
Hottest, best zone or melting:
transition bw blue are and the area just supperior to it (burnt gases, oxidizing atmosphere
This forms reducing atmosphere and this is the oxidizing atmosphere:
burning gases, burnt gases
To minimize oxidation oxidation of the alloy:
use the hottest zone, best for melting
Relationship bw strength and esthetics:
inverse relationship, strength increases, esthrtics decreases
% of high noble in base metal:
less than 25%
Colors of high gold metals:
yellow or white (gold or silveer)
captek:
we will not use, very esthetic metal ceramic crown, copper/gold material, chipping is common
Titanium alloy:
creates thick oxide layer, not good bonding between ceramic and titanium
The working cast is made from:
definitive impression (heavy and light body)
Indications for complete cast crown:
extensive coronal destruction (caries, trauma, resto), max retention and resistnace form is needed, short crown, correction of axial (wall height, too tilted) is needed, to restor teeth w endo
Crown type for a bruxer:
full coverage crown
Crown type for short crownx
Zirconia, requires low reduction for posterior teeth, similar to CCC reduction
Can angulation of a tilted tooth be corrected with direct restorations?
no
Greater occlusal reduction leads to decreased:
retentive and resistance forms
Conctraindications for CCC:
active caries, untreated pd, more conservative resto will suffice (intact B or L walls, less need for retention or resistance form), anterior teeth (esthetics)
Advantages of CCC:
longevity, greater resistnace and retention forms than more conservative restos, stronger, adaptable to proper contours, better control of the occlusion - modifications compared to more conservative restorations, more conservative prep than other crown preps
Finish line design or CCC:
feather edge
Crown type to use when you need good adaptation to a furcation area:
CCC
Crown type for pts with larger pulp:
CCC, less preparation
Disadvantages to CCC:
more tooth removeal than more conservative preps, inflammation of gingiva, can’t do pulp testing bc of the conductivity of the alloy, allergy to base metals, esthetics
How is pulp testing altered with a CCC?
amplified, unreliable response
Only do CCC’s on these teeth:
Max 2nd molar, man molars and premolars
More esthetic, all ceramic or metal ceramic?
all ceramic
Better marginal fit, all ceramic or metal ceramic:
metal ceramic
more durable, metal ceramic or all ceramic:
metal ceramic
Indications for metal ceramic crown:
same as CCC
Contraindication for metal ceramic restoration:
active caries, untreated pd, young pts with large pulp chamber, when you can be more conservative
more durable, all ceramic FPD or metal ceramic FPD?
metal ceramic, connectors can also be soldered or cast
Crown type to use for survey crowns:
metal ceramic, not all ceramic
