Alginate and Stone Flashcards
What are the charateristics of the ideal impression material?
- easy to mix/handle
- suitable working time
- suitable setting time
- compatible with stone
- not toxic or allergenic
- dimensionally stable
- accurate to record fine details
- has acceptable odor/taste
- adequate strength
- adequate shelf life
- economical
- can be disinfected
- exact record of all aspects
- free from air bubbles
What are the key properties of impression materials?
- accuracy
- dimensional stability
- tear resistance
What are the nonelastic options for impression materials?
gypsum (plaster)
impression compound
zinc oxide eugenol
impression wax
What are the elastic options for impression materials?
- hydrocolloids (agar/alginate)
- non-aqueous elastomers (polysulfides, polyethers, condensation silicone, addition silicone)
What weird ingredient is alginate made of?
based on a natural substance extracted from brown seaweed (aqnhydro-B-D-mannuronic acid; alginic acid)
What was alginate developed as a substitute for?
agar impression material (it became scarce during WW2)
Why is alginate successful?
easy to manipulate, comfortable for patient, relatively inexpensive
What are the chief active ingredients of irreversible hydrocolloid material (alginate)?
sodium phosphate
diatomaceous earth
potassium titanium fluoride
zinc oxide
potassium alginate
calcium sulfate
triethanolamine alginate
What is the safest way to alter the setting time of alginate?
changing the temperature of water (hot = faster; cold = slower)
Is alginate impression material hydrophobic or hydrophilic?
hydrophilic
What type of impressions are alginate mostly used for?
preliminate impressions (used to construct a custom tray)
What are the steps to making an alginate impression?
- measure powder in a clean rubber bowl
- powder is incorporated into the water by carefully mixing with a spatula until all of the powder is dissolved
- mixing time of 45 seconds-60 seconds
- result is smooth, creamy mixture
- should have sufficient body so it does not flow out of tray and gag patient
- place in a tray and then into mouth
- at least 3 mm of thickness between tray and tissues
What are the different types of stock trays?
rim lock tray
perforated tray
How should you disinfect an impression?
disinfectant-soaked paper towel/plastic bag for 10 minutes
What is the future of digital impressions?
digital impressions (scan and then send electronic file)
What are some current digital impression systems?
Sirona (CEREC, Omnicam, bluecam)
Align Tech (Itero)
Midmark (LAVA True Definition Scanner)
What is gypsum almost entirely made of?
calcium sulfate dihyrate
What is a gypsum dental investment?
plaster is mixed with fillers (different forms of silica; high heat resistant)
What is the principle constituent of gypsum-based products?
calcium sulfate hemihydrate
What is the colloidal therory of gypsum?
when mixed with water, plaster enters into the colloidal state through a sol-gel mechanism
What is the hydration theory of gypsum?
rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material
What is the dissolution-precipitation theory of gypsum?
dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystal to fm the set solid
What is the setting reaction of gypsum?
- dissolution of calcium sulfate hemihydrate
- formation of saturated solution of calcium sulfate
- aggregation of less soluble calcium sulfate dihydrate
- precipitation of the dihydrate crystals
What is the mixing time (MT)?
the time from the addition of the powder to the water until the mixing is completed
What is the average mechanical mixing time of stones and plasters?
20-30 seconds
What is the average hand-spatulation mixing time of stones and plasters?
1 minute
What is the working time (WT)?
time available to use a workable mix, noe that maintains a uniform consistency to perform one or more tasks
What is the setting time (ST)
time that elapses form the beginning of mixing until the material hardens
What is the best way to control the setting time of stone/plaster?
the addition of certain chemical modifiers such as accelerators or retarders
What is the crystallization?
outgrowth of crystals from nuclei of cyrstallization
What is the most effective method for controlling the setting expansion?
addition of chemicals from the manufactor (potassium sulfate, sodium chloride, borax)
What are the types of gypsum?
- Impression plaster (type 1)
- Model plaster (type 2; white)
- Dental stone (type 3; yellow stone)
- Dental stone, high strength (type 4)
- Dental stone, high strength and expansion (type 5)
What is centric relation?
- condyle is in the most anterior-superior position
- first contact when leaf gauge is in place on the anterior teeth
What are the characteristics of impression plaster (type 1)?
- made of plaster of paris
- rarely used anymore
What are the characteristics of model plaster (type 2)?
- white color
- relatively weak
What are the characteristics of dental stone (type 3)?
- used to process dentures
What are the characteristics of dental stone type 4?
- hard surface
- ncessary for a die stone
What are the characteristics of dental stone type 5?
- high strength and expansion
