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33 Cards in this Set
- Front
- Back
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Is an impression a negative or a positive?
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negative
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Why do we use impressions?
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for ortho diagnostics, survey, retainers
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What is impression poured in to create a cast representative of the teeth?
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plaster or stone
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What is the mixing stage of alginate impression material?
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alginate powder mixed with water
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What is the setting stage of alginate impression material?
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2 stage
1st stage- sol 2nd stage- gel |
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What is sol?
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the alginate to water
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What is in alginate powder?
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sodium alginate and calcium sulphate
filler particles (silica) increase the stiffness and smoothness potassium sulphate (and fluoride) to allow a hard dose surface on the stone cast |
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explain the 2 stage process of setting
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sodium alginate dissolves- runny mixture--> sol
sodium alginate reacts with calcium sulphate to form insoluble calcium alginate--> gel |
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What does the 2 stage set allow for?
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material to be placed in tray
tray positioned in the mouth material to flow around the tissues in the arch and then to gel |
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Alginate impression materials belong to a class of materials called....
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hydrocolloids
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What does the term hydrocolloid describe?
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a particle distribution in water
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Alginates are irreversible hydrocolloids because...
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the gel is formed by a chemical change and cannot be returned to the sol form
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Why should you avoid breathing the dust of alginate powder?
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because of the silica particles could be a health hazard long term.
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What is required for mixing alginate?
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rubber/plastic bowl
metal or plastic spatula alginate powder and measuring scoop water and measuring vial |
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How long should you mix for and why?
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about 1 min to ensure all the powder is dissolved into a smooth creamy mix
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What should you avoid whipping into the mix?
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air
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What changes during the setting?
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pH, some products have additives that change colour with changes in pH
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Why do alginates deform elastically when removed over teeth and other undercut areas, and then rapidly return to their original shape?
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they are elastic impression material
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How long should you wait to pour up if doing a diagnostic cast?
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no longer than 1 hour
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What influences the strength of alginate?
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water/powder ratio- to vary the setting time vary the temp of water
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Explain the 2 types of alginates
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Type 1- gel in about 1-2 min at 20*c
Type 2- gel in about 3-4 min at 20*c |
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How do you disinfect alginates?
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rinse impression in water
disinfect by immersion in chlorine solution for 10 minutes rinse impression in water before pouring |
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What is it called if impression loses water?
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syneresis
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What is it called if impression absorbs water?
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imbibition
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How is plaster/stone produced?
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gypsum + heat--> plaster/stone + water
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What happens when plaster/stone is mixed with water?
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plaster/stone + water--> gypsum + heat
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Explain the mixing/setting of dental plaster/stone.
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when mixed with water it:
forms a runny mixture, that can be poured into the impression before it sets hard as gypsum is reformed |
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Why is water needed?
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to wet the particles to allow to mix and for the setting reaction
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What happens if there is too little water?
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can't form a working mix
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What happens if too much water?
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strength is decreased
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What factors affect setting time of plaster/stone?
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temperature
rate of mixing chemical additives |
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When is the cast strong enough to separate from the impression?
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when it is cool
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explain the strength of the dental cast
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strength will increase as it dries
compressive strength of set gypsum is related to the W/P ratio i.e. less water= more strength dental stone needs less water for mixing compared to plaster and hence stone casts are stronger |
