Test 1 Review Flashcards
which two impression materials can be poured multiple times and maintain reasonable accuracy?
PVS and polyether
what 3 things can create distortion in impression material?
time, compression, and temperature extremes
___ is the ability of an impression material to rebound back to its original accuracy
elastic recovery
how long does alginate take for full elastic recovery?
about 10 minutes
between polymethylmethacrilate and polyethylmethacrilate, which has a higher exotherm upon polymerization?
polymethylmethacrilate
between polymethylmethacrilate and polyethylmethacrilate, which one shrinks more upon polymerization?
polymethylmethacrilate
between polymethylmethacrilate and polyethylmethacrilate, which has a higher fracture toughness?
polymethylmethacrilate
between polymethylmethacrilate and polyethylmethacrilate, which one is easier to add to?
polymethylmethacrilate
between polymethylmethacrilate and polyethylmethacrilate, which would you only want to use for a single unit and why?
polyethylmethacrilate because it is quite soft, even in its set form
when taking an impression, what can happen if you allow the light body to start setting before you add the heavy body?
the bite record will be open
when taking an impression, what can happen if the heavy body material starts to polymerize before it is inserted?
the die will be smaller
what are the two biggest drawbacks to hydrocolloids and why?
syneresis and imbibition, because they make the hydrocolloids unstable
what is syneresis?
the expression of fluid from hydrocolloid impression material over time, resulting in dimensional change and distortion
what is imbibition?
the uptake of water by hydrocolloid when immersed in water
what is the modulus of elasticity, and which impression materials has the highest modulus of elasticity?
stiffness of a material
polyether
why shouldn’t you use plastic posterior impression trays in the clinic?
because they can bend - this is why you should use metal
___ can interfere with the setting of PVS
latex gloves
why is it important to capture the area apical to the prepared margin when taking an impression of crown prep?
it is easier to trim and easier to help with contours
when taking indirect crown prep impressions, what 3 things are absolutely necessary for you to capture?
you want to capture the die, the opposing anatomy, and the adjacent teeth
what are some ways that impression material can be kept adherent to impression trays?
adhesive spray, rim lock, and perforated trays
how can gypsum be reinforced, and what does the reinforcement do?
it can be reinforced by adding low viscosity cyanoacrylate, which makes the gypsum stronger and more abrasian-resistant
how many ADA recognized types of gypsum materials are there?
5 types (I-V)
___ is the term used to describe taking water away from gypsum
calcination
dental stone casts reach maximum strength how long after being poured? about what percent of the setting expansion occurs within the first hour of setting?
24 hours (typically, over 75% of the setting expansion occurs within the first hour of setting)
which stone type has the least amount of expansion?
type I mounting stone
what is a master cast, and what are two other names for it?
replica or copy of the prepared tooth or teeth, ridge areas, and other parts of the dental arch. other names include definitive cast and working cast. also sometimes referred to as “models”.
what is a die?
positive reproduction of the prepared tooth
what is the basic gypsum reaction, and what is the process called?
- gypsum (dihydrate) is ground and then heated to drive off water
- the resulting hemihydrate is a fibrous, porous, crystalline aggregate known as plaster of paris or dental plaster
- as the product is heated further, it becomes an anhydrite, known as dental stone
- this process is known as calcination
gypsum (calcium sulfate dihydrate) is heated and converted to ___ (aka plaster or paris or dental plaster) or ___ (no water). when mixed with ___, it reverts back to calcium sulfate dihydrate form as hardened or set gypsum.
- calcium sulfate hemihydrate or calcium sulfate anhydrite
- water
___ is produced when gypsum is heated in an open kettle type environment at temps between 110-130*C. which dental stone types is this used to produce?
- beta-hemihydrate
- types I and II
___ is produced when gypsum is dehydrated under pressure and in the presence of water vapor at about 125*C; it is also referred to as hydrocal and is used to make type ___ dental stone
- alpha-hemihydrate
- type III
what is type III dental stone used for?
study model impressions
it is medium strength and hardness, so is not appropriate for dies
___ is produced when gypsum is boiled in a 30% calcium chloride solution. the chloride is then washed off from the rock and the remaining gypsum is ground to the desired level of fineness. which dental stone types are produced, and which form of calcium sulfate are they?
- densite
- types IV and V
- both are alpha-hemihydrate forms of calcium sulfate
generally speaking, what are each of the dental stone types used for, and what are their corresponding properties?
- type I - impression plasters and mounting stone
- low expansion, soft
- type II - model plasters
- high expansion, soft
- type III - model stones
- intermediate hardness
- type IV - die stones
- high strength, low expansion
- type I - die stones
- high strength, high expansion
what would be the purpose for using type IV dental stone vs type V dental stone for dies?
- type IV - cast inlays and onlays, implant prostheses, RPD frameworks, milled restorations
- type V - full coverage crowns, inlays and onlays, and FPDs with full coverage cast retainers
type IV and V dental stones are alpha or beta hemihydrate?
alpha
when either alpha or beta hemihydrate is mixed with water, the ___ reaction is reversed
calcination
the theoretical lowest required volume of water needed to hydrage 100g of hemihydrate in either alpha or beta form is ___ml
18.6 (this is the REQUIRED amount; anything more than this is considered “excess”)
what is the total volume of mixing water needed for model plaster, dental stone, and high strength dental stone gypsum products?
- model plaster - 37-50ml
- dental stone - 28-32ml
- high strength dental stone - 19-24ml
- *remember than 18.6ml is the required amount, so you can calculate the excess needed for each of these types by subtracting 18.6ml
what are some of the factors that effect setting time of gypsum products?
- increased water temperatures (less than 100*C) will reduce set time (faster)
- increasing total spatulation time or the speed of spatulation reduces set time (faster)
- adding more water will increase set time (slower)
- substitute 2% K2SO4 for water will decrease set time (faster)
- creating a slurry in the water will decrease set time (faster)
increasing water temperature can accelerate setting time for gypsum materials. what is the upper limit of the temperature increase, and why does the limit exist?
at 100*C, the solubility of of the hemihydrate and the dihydrate are equal, so at this temperature, no reaction occurs and the stone/plaster will not set
describe wet strength vs dry strength of stone; how much stronger is the dry strength compared to wet strength?
- wet strength is the strength measured when excess water remains in the hardened mass of stone
- dry strength is the strength measured with all of the excess water removed
- dry strength is twice that of wet strength
the ___ strength of dental stone is improved with vacuum mixing
compressive strength
vacuum mixing reduces the ___ trapped within the mixture
volume of air
if you are preparing a posterior tooth for a zirconia crown, where should the margin be placed?
1mm supragingival because it is in a non-esthetic zone; normally the margin would be subgingival
what are the two most common ways that ceramics are created?
milled 3D printed crown and heat-pressed ceramics (emax)
why are zirconia crowns milled in a larger form?
because zirconia shrinks when it is sintered (fired in the oven)
what is the purpose of retraction cord?
it temporarily retracts soft tissues and also carries the hemostatic agent
retraction cord can be ___ or ___
braided or woven
what are 3 examples of hemostatic chemicals?
epinephrine, aluminum chloride, and ferric sulfate
what are some considerations when using epinephrine-impregnated retraction cord?
- can cause tachycardia in patients with heart conditions
- if you leave the cord in too long, it can cause tissue necrosis
- should not be used in lacerated tissues
what is tripodization?
when in occlusion, the cusp of one tooth engages the valley on the opposing tooth in 3 places around the tip, but not at the tip
what is a centrum?
a flat area at the marginal ridge of a crown preparation, where the cusp tip of the opposing tooth then makes contact with that area during maximum intercuspation
what is the order for scanning a die?
- preparation arch - buccal placed to back of the scanner
- opposing arch - buccal placed to the back of the scanner
- both arches together - buccal placed at the back of the scanner
- die - doesn’t matter how this is placed in the scanner
what are the 4 types of elastomeric impression materials?
- addition silicone (aka polyvinylsiloxanes aka PVS)
- polyether
- polysulfide rubber base
- condensation silicone
what can you do to addition silicones (PVS) to add up to two minutes of working time?
refrigerate the material
of the elastomeric impression materials, what is the order of most to least stable/accurate?
from most to least:
- addition silicone (PVS)
- polyether
- polysulfide
- condensation silicone
an impression material with a higher elastic limit should exhibit less ___
permanent deformation
a permanent deformation of 1% has an elastic recovery of ___%
99%
of the elastomers, which has the least permanent deformation, and subsequently the most elastic recovery?
addition silicones (PVS)
wettability is assessed by measuring the ___ on the surface of the set impression material
advancing contact angle
describe the wettability of an impression material with a lower advancing water contact angle
lower contact angle = better wettability = better castability
of the elastomers, which ones have the best wettability?
- polyether - lowest contact angle = best wettability
- hydrophilic addition silicones (PVS) = second lowest contact angle
elastomers are supplied in which two components?
catalyst paste and base paste
elastomers are available in a variety of viscosities, including a ___ version for the addition and condensation silicones
putty
elastomers can be mixed using which 3 possible methods?
automixing, mechanical mixing, hand mixing
the base paste of polysulfide rubber base consists of a polysulfide polymer, a filler, and a small amount of sulfur which acts as a ___
accelerator
polysulfure rubber base catalyst paste contains lead dioxide, and either oleic acid or stearic acid which serves to ___
slow the setting of the reaction
water is a byproduct of the polymerization reaction polysulfide rubber bases. why is this a problem?
the material won’t stay dimensionally stable for this reason
what are the advantages of polysulfides?
excellent tear strength, relatively inexpensive, long working time
what are the disadvantages of polysulfides?
stinky, poor dimensional stabiity (water loss), must hand mix, and slow setting time
the setting reaction of condensation silicones produce ___ as a byproduct. why is this a problem?
ehtyl alcohol, which accounts for much of the contraction and distortion of this impression material
what is the best use for condensation silicones?
ONLY for silicone matrices
polyether base paste consiste of ___, and the catalyst contains ___
- a long chain polyether polymer
- an alkyl-aromatic sulfonate
what makes polyether the most hydrophilic of the elastomers?
the ether-dominated polymer backbones
what is the stiffest of all the elastomers?
polyether
this could be problematic - it makes it more likely to break off teeth when you are separating the cast from the impression material, and can also make it difficult to remove from the patients mouth if there are undercuts or large embrasure spaces
what are the advantages of polyether?
most hydrophilic of the elastomers, and intermediate dimensional stability
what are the disadvantages of polyether?
- intermediate tear strength
- stiffest of elastomers
- nasty taste
- can cause delayed soft tissue reaction on some susceptible individuals
the base paste of addition silicones (PVS) consists of ___, and the accelerator paste contains ___
- polymethylhyrosiloxane divinylpolysiloxane and a filler
- divinylpolysiloxane and a platinum salt catalyst
the setting reaction of addition silicones is based on the ___ between divinylpolysiloxane and polymethylhydroxiloxane
addition polymerization
___ is a byproduct of addition silicone (PVS) polymerization. ___ is often included as a scavenger to absorb this byproduct. what is the problem with this byproduct?
- H2 gas
- palladium
- H2 gas can create bubbles in stone if impression is poured too soon
what are the advantages of addition silicone (PVS)?
excellent dimensional stability, high tear strength, excellent elastic recovery, pleasant taste
what are the disadvantages of addition silicones (PVS)?
- hydrophobic (relative to hydrocolloids)
- sulfure contamination from latex gloves can inhibit set
