Flashcards in Alginate Deck (54):
what is an impression
the negative duplication of the oral tissue -->filled with model material (dental stone) for the production of a positive cast
what is a cast?
the positive duplication of the oral tissues
what happens when an impression shrinks?
the cast will be oversized because when the impression material shrinks, it shrinks towards the tray and the space in the impression will be bigger therefore wheen gypsum is poured in you will have an oversized cast
why do casts need to be created in dentistry?
to perform indirect procedures
what do casts do?
the casts duplicate the geometry and dimensions of the oral tissue so laboratory work can be performed.
-needed for diagnosis, treatment planning, occlussal evaluation and laboratory fabrication of restoration
what does an impression do?
captures the intraoral details -teeth, gingiva, alveolar bone or residual ridge, hard/soft palate tissue
impression materials vary in composition and properties and are selected on the basis of relative need for?
-is the perfect accuracy, why?
no impression material produce 100 percent accuracy because of the operator and the materials
affected by temperature change
-ie) in mouth its 37 degree when impression is removed from mouth its coler therefore shrinkage occurs
impression materials have two sub categories what are they?
1. elastic ("rubber like" when set)
2. non elastic (hard and rigid)
impression materials: non elastic
plaster, impression compound, zinc-oxide eugenol
impression material: "elastic" --break down into two further sub-sub categories. what are they ?
2. non-aqueous elastomers
elastic impression material: hydrocolloids consist of what to materials?
elastic impression material: non equeous elastomers consist of what 3 material?
for occlusal relations of the maxilla and mandible the requirement for accuracy are relatively LOW...what type of impression would be acceptable?
elastic vs. inelastic impression
1.elastic materials can make impressions of "undercuts"
2.inelastic materials can make impressions of surfaces but no "undercuts"-->alginate
who would we use non elastic material on?
endontuleous clients- no teeth
who would we use elastic impression material on? (hydrocolloids and the non aqueous elastic polymeres)
-people with teeth
what properties are desirable for a impression material?-13
-odor and taste
-adequate shelf life- economic
-easy to use-satisfactory consistency
-readily wets oral tissue (water friendly)
-setting charatceristics that meet clinical requirement
-elastic properties, no permanenet deformation or strain
-adeqquate strength-so it doesnt break upon removal from mouth
-dimensional stability over temperature and humidity ranges
-compatbility with cast and die materials
-accuracy in clincal use
-readily disinfected without loss of accuracy
what is the degree of elastic reocvery?
a materials ability to return to original dimensions followingc compression
how is elastic recovery represented?
percent recovery or percent deformation
factors that influence permanent deformation?
1. percent compression
2. time under compression
3. time after removal of load
what do impressions trays do?
carry impression material
-stock trays: plastic or metal
alginate impression materials ---are they commonly used in dentistry?
what are hydrocolloids
-hydro = water
-colloids=intermediate estate of matter- you can have a mix of solids or liquids--ie)toothpaste and aliginate
alginate --what is it made from?
what is alginates pros?
2.easy to use
when is alginate used?
casts made from alginate impressions can be used for?
2. bleaching and flouride trays
how is alginate supplied?
powder that is mixed with water
ingredients of alginate
1.sodium or potassium alginate: seaweed
2. calcium sulphate dihydrate: reactor: gypsum
3.filler (silica powder) -viscosity
5.moistening:makes powder dustless
6. flavoring agent
alginate: what are the two types of ISO/ADA alginate
1. Type I: fast setting time, mixing time is 45 seconds, setting time 30-75 seconds
2. Type II: regular set, mixing time 1 minute, setting time 2-3.5 mins
-when would we use type I alginate? for kids and gaggers
-compatible with model materials?
-no permananent deformation after strain
-adequate elastic strength-will not tear upon removal
-does not adversely affect model materials
-accuracy: dimensionally stable, adequate flow, readily wets oral tissues
alginate and accuracy?
because no matter what the impression is made of the alginate will always have a 5 % deformation
alginate and dimensional stability?
-why is alginate unstable?
the percentage of change in water content of an impression material
-because water is 85 percent of its composition
alginate and dimensional stability:
what is syneresis?
syneresis: tendency of hydrocolliod materials to contract after setting
alginate and dimensional stability:
imbibition: tendency of the hydrocolloid to absorb water, swell and distort after setting
how do we try to control imbibition and syneresis?
by wrapping alginate in a wet paper towel--however alginate is still not accurate and must be used on low resolution applications
manipulation of alginate:effects of w/p ratio
-water and powder must be measured because this ratio affects the consistency, setting time, strength, quality-->deviation of +- 15 % effects flow,tears,strength, setting rate, flexability
-must avoid contiminated equipement this can accel or retard the reaction
manipulation: effects of water temperature on setting time of alginate
the best temp is between 15-25 degrees
1. below 15-25 it will be uncomfortable for patient
2. above 15-25 it will be fast setting
what is compression set?
measure of the ability of a material that was compressed to return to its original shape
alginate and compression set
-when removing alginate impressions it deforms when trying to pass the crest of contour and will not return
manipulation: compression set
-what does compression set depend on?
-strain and time
for compression set what does strain depend on?
-the shape and tilt of the tooth
-thickness of material between the tooth and the tray
how to reduce compression set? 5 points
1. use recommended mixing time
2. maximize tray- tooth distance
3.material must be completely set before removal
5.provide recovery time : 8-30 min before pouring
t/f: alginate is a weak material that tears easily if force is applied for a long time?
-prevention of cross contamination
-rinse to remove body fluids
-soak up to 30 mins in 1% sodium hypochlorite,iodophors or 2% glutaraldehyde
storage of alginate
cool dry place for max 1 year
common causes of alginate impression failure?
4.irregular shape voids
6.rough chalky cast
factors to maximize accuracy
2.use reasonable bulk of alginate
3.rapid snap removal
4.time follwing impression
what are the most common impressions?
what are the two main components
1. potassium (K) alginate (carbohydrate polymere)
2.CaSO4 (reactor) --react and form a gel-->irreverible reaction
how can water affect your alginate?
it affects the accuracy greatly by either loosing or gaining water