Alginate and Stone

Question 1

What are the charateristics of the ideal impression material?

Answer 1

• 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

Question 2

What are the key properties of impression materials?

Answer 2

• accuracy
• dimensional stability
• tear resistance

Question 3

What are the nonelastic options for impression materials?

Answer 3

gypsum (plaster)
impression compound
zinc oxide eugenol
impression wax

Question 4

What are the elastic options for impression materials?

Answer 4

• hydrocolloids (agar/alginate)
• non-aqueous elastomers (polysulfides, polyethers, condensation silicone, addition silicone)

Question 5

What weird ingredient is alginate made of?

Answer 5

based on a natural substance extracted from brown seaweed (aqnhydro-B-D-mannuronic acid; alginic acid)

Question 6

What was alginate developed as a substitute for?

Answer 6

agar impression material (it became scarce during WW2)

Question 7

Why is alginate successful?

Answer 7

easy to manipulate, comfortable for patient, relatively inexpensive

Question 8

What are the chief active ingredients of irreversible hydrocolloid material (alginate)?

Answer 8

sodium phosphate
diatomaceous earth
potassium titanium fluoride
zinc oxide
potassium alginate
calcium sulfate
triethanolamine alginate

Question 9

What is the safest way to alter the setting time of alginate?

Answer 9

changing the temperature of water (hot = faster; cold = slower)

Question 10

Is alginate impression material hydrophobic or hydrophilic?

Answer 10

hydrophilic

Question 11

What type of impressions are alginate mostly used for?

Answer 11

preliminate impressions (used to construct a custom tray)

Question 12

What are the steps to making an alginate impression?

Answer 12

1. measure powder in a clean rubber bowl
2. powder is incorporated into the water by carefully mixing with a spatula until all of the powder is dissolved
3. mixing time of 45 seconds-60 seconds
4. result is smooth, creamy mixture
5. should have sufficient body so it does not flow out of tray and gag patient
6. place in a tray and then into mouth
7. at least 3 mm of thickness between tray and tissues

Question 13

What are the different types of stock trays?

Answer 13

rim lock tray
perforated tray

Question 14

How should you disinfect an impression?

Answer 14

disinfectant-soaked paper towel/plastic bag for 10 minutes

Question 15

What is the future of digital impressions?

Answer 15

digital impressions (scan and then send electronic file)

Question 16

What are some current digital impression systems?

Answer 16

Sirona (CEREC, Omnicam, bluecam)
Align Tech (Itero)
Midmark (LAVA True Definition Scanner)

Question 17

What is gypsum almost entirely made of?

Answer 17

calcium sulfate dihyrate

Question 18

What is a gypsum dental investment?

Answer 18

plaster is mixed with fillers (different forms of silica; high heat resistant)

Question 19

What is the principle constituent of gypsum-based products?

Answer 19

calcium sulfate hemihydrate

Question 20

What is the colloidal therory of gypsum?

Answer 20

when mixed with water, plaster enters into the colloidal state through a sol-gel mechanism

Question 21

What is the hydration theory of gypsum?

Answer 21

rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material

Question 22

What is the dissolution-precipitation theory of gypsum?

Answer 22

dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystal to fm the set solid

Question 23

What is the setting reaction of gypsum?

Answer 23

1. dissolution of calcium sulfate hemihydrate
2. formation of saturated solution of calcium sulfate
3. aggregation of less soluble calcium sulfate dihydrate
4. precipitation of the dihydrate crystals

Question 24

What is the mixing time (MT)?

Answer 24

the time from the addition of the powder to the water until the mixing is completed

Question 25

What is the average mechanical mixing time of stones and plasters?

Answer 25

20-30 seconds

Question 26

What is the average hand-spatulation mixing time of stones and plasters?

Answer 26

1 minute

Question 27

What is the working time (WT)?

Answer 27

time available to use a workable mix, noe that maintains a uniform consistency to perform one or more tasks

Question 28

What is the setting time (ST)

Answer 28

time that elapses form the beginning of mixing until the material hardens

Question 29

What is the best way to control the setting time of stone/plaster?

Answer 29

the addition of certain chemical modifiers such as accelerators or retarders

Question 30

What is the crystallization?

Answer 30

outgrowth of crystals from nuclei of cyrstallization

Question 31

What is the most effective method for controlling the setting expansion?

Answer 31

addition of chemicals from the manufactor (potassium sulfate, sodium chloride, borax)

Question 32

What are the types of gypsum?

Answer 32

• 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)

Question 33

What is centric relation?

Answer 33

• condyle is in the most anterior-superior position
• first contact when leaf gauge is in place on the anterior teeth

Question 34

What are the characteristics of impression plaster (type 1)?

Answer 34

• made of plaster of paris
• rarely used anymore

Question 35

What are the characteristics of model plaster (type 2)?

Answer 35

• white color
• relatively weak

Question 36

What are the characteristics of dental stone (type 3)?

Answer 36

• used to process dentures

Question 37

What are the characteristics of dental stone type 4?

Answer 37

• hard surface
• ncessary for a die stone

Question 38

What are the characteristics of dental stone type 5?

Answer 38

• high strength and expansion