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24 Cards in this Set
- Front
- Back
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What does the term "Negative" mean?
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Imprint of teeth
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Why are impressions taken?
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As a record of the teeth (imprint)
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What is impressions poured in? (material wise)
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Stone or plaster
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Why are casts used?
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For diagnostic, survey or construction purposes
Also for record keeping- to have before and after models to show differences i.e. Ortho |
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What material is used to do impressions?
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Alginate
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Define "mixing" alginate
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Alginate powder mixed with water
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Define "setting" alginate and how many stages?
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2 stages: 1st stage SOL
2nd stage GEL |
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What is in the alginate powder?
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Sodium alginate and calcium phosphate
Also filler particles (silica) increase the stiffness and smoothness Potassium sulphate and Fluoride) to allow a hard dense surface on the stone cast |
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Describe what happens in SOL and GEL
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Sodium alginate dissolves in water forming a 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 place in tray
-tray to be positioned in the mouth -the material to flow around the tissues in the arch and then to GEL |
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What class of alginate impression materials belong too?
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Hydrocolloids
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What does Hydrocolloids mean?
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A particle distribution in water
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Alginates are irreversible hydrocolloids, what does this mean?
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The GEL is formed by a chemical change and cannot be returned to the SOL form
All alginates at OHCWA are irreversible |
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Does the pH of alginate change during setting? And how?
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Yes, from quite alkaline to approx neutral
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Alginates are elastic, define this.
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They deform elastically when removed over teeth and other undercut areas, and the rapidly return to their original shape
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What influences strength of alginate material?
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Water/ powder ratio
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What is used to disinfect impressions? And for how long?
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Chlorine solution, 10mins
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What can distort impressions?
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Left to long in disinfectant, time can also distort
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What does Syneresis mean?
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Impression can lose water
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What does Imbibition mean?
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Impression can absorb water
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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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What factors affect Plaster/stone setting time?
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-Temperature
-Rate of mixing -Chemical additives |
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As the Plaster/stone sets its gets warm/hot, Why?
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Plaster/stone+Water= Gypsum + Heat
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