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Occlusion and Fixed > Lecture 4 Alginate and Stone > Flashcards

Lecture 4 Alginate and Stone Flashcards

1
Q

what is meant by accuracy in impression materials

A

ability to replicate the intraoral surface detaisl

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2
Q

what is meant by dimensional stability in impression materials

A

ability to retain its absolute dimensional size over time

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3
Q

what is meant by tear resistance in impression materials

A

ability to resist tearing in thin sections

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4
Q

what are the 2 classes of impression material

A

elastic and non elastic

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5
Q

what are the classes of elastic material

A

hydrocolloids and non aqueous elastomers

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6
Q

what are the classes of hydrocolloids

A

agar reversible and alginate irreversible

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7
Q

what are the non aqueous elastomers

A

-polysulfides
-polyethers
-condensetation silicone
-addition silicone

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8
Q

what are the non elastic classes

A

-plaster (gysum)
-impression compound
-zinc oxide eugenol
-impression waxes

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9
Q

when was alginate made

A

as a substitute for the agar impression material when its supply became scarce during world war 2

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10
Q

what is alginate based on

A

a natural substance extracted from brown seaweed called anhydro-B-D-mannuronic acid or alginic acid

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11
Q

what are the principle factors responsible for success of alginate

A

easy to manipulate
-comfortable for the patient
-relatively inexpensive

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12
Q

what are the chief active ingredients of the irreversible hydrocolloid impression materials

A

sodium, potassium, or triethanolamine alginate

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13
Q

what is the composition of alginate with respective function

A

-potassium alginate- 15% - soluble alginate
-calcium sulfate - 16% - reactor
-zinc oxide - 4% - filler particles
- potassium titanium fluoride - 3% - accelerator
- diatomaceous earth - 60% - filler particles
- sodium phosphate - 2% - retarder

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14
Q

describe the typical sol-gel reaction

A

a reaction of soluble alginate with calcium sulfate and the formation of an insoluble calcium alginate gel

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15
Q

what can modifying the W/P ratio or mixing time have on alginate

A

effects properties of the gel, the tear strength, and the elasticity

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16
Q

how can you safely influence setting time

A

altering the temperature of the water

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17
Q

what is the polarity of alginate impression material

A

hydrophilic

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18
Q

what are the steps in making an alginate impression

A
  • measured powder is sifted into a clean rubber bowl
  • powder incorporated into water by mixing with metal or plastic spatula- mixing time about 45 seconds to 1 minute
    -result from mixing should be a smooth creamy mixture that does not readily drip off the spatula when it is raised from the bowl
    -before seating the material should have enough body so it will not flow out of the tray and gag the patient
  • mixture is placed in a suitable tray which is then placed in the mouth, ensuring the material adheres to the tray
  • tray must fit patients arch so there is enough thickness of material - thickness between tray and the tissue should be at least 3 mm
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19
Q

when is the tear strength increased in alginate hydrocolloid

A

when the impression is removed with a snap

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20
Q

how do you disinfect impressions

A

soaked paper towel/plastic bag for 10 minutes

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21
Q

is VPS affected by disinfection

A

no

22
Q

when do hydrocolloids expand

A

after 10 minutes

23
Q

when do polysulfides and polyethers expand

A

with increased disinfectant exposure time

24
Q

what are some current digital impression systems

A

-CEREC, omnicam (sirona)
- itero element (align technology)
- LAVA true definition scanner (midmark

25
Q

what is gypum made of chemically

A

pure calcium sulfate dihydrate

26
Q

what is gypsum dental investment

A

when plaster is mixed with fillers such as different forms of silica

27
Q

what is the principle constituent of gypsum based products such as plasters and stones

A

calcium sulfate hemihydrate

28
Q

what is the colloidal theory

A

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

29
Q

what is the hydration theory

A

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

30
Q

what is the most widely accepted mechanism of setting gysum and what is it

A

dissolution precipitation theory- based on dissolution of plaster and instant recrystallization of gypsum followed by interlocking of the crystals to form the set solid

31
Q

how does the setting reaction of gypsum occur

A

-dissolution of calcium sulfate hemihydrate
- formation of a saturated solution of calcium sulfate
- aggregation of less soluble calcium sulfate dihydrate
-precipitation of the dihydrate crystals

32
Q

what is mixing time defined by

A

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

33
Q

how long does it take to mix stones and plasters

A

20-30 seconds

34
Q

what is working time defined as

A

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

35
Q

what is setting time defined as

A

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

36
Q

what is the most effective and practical method for controlling the setting time

A

the addition of certain chemical modifiers to the mixture of plaster or dental stone

37
Q

what is the chemical called if it decreases the setting time

A

an accelerator

38
Q

what is the chemical called if it increases the setting time

A

a retarder

39
Q

what is the crystallization process

A

an outgrowth of crystals from nuclei of crystallization. on the basis of entanglement of the dihydrate crystals, crystals growing from the nuclei can intermesh with and obstruct the growth of adjacent crystals

40
Q

when sufficient crystals form to produce the outward thrust by impingement ____

A

setting expansion follows

41
Q

what is the most effective method for controlling the setting expansion

A

through the addition of chemicals by the manufacturer

42
Q

how can the setting expansion be reduced

A

by adding either potassium sulfate, sodium chloride or borax

43
Q

what are the types of gypsum

A

-impression plaster (type 1)
- model plaster (type 2)
- dental stone (type 3)
- dental stone, high strength (type 4)
-dental stone, high strength, high expansion (type 5)

44
Q

what is impression plaster type 1 made of

A

composed of plaster of paris, to which modifiers have been added to regulate the setting time and the setting expansion

45
Q

describe model plaster type 2

A

-usually marketed in the natural white color
- relatively weak with compressive strength as low as 9 MPa and a tensile strength of 0.6 MPa

46
Q

when are type 3 stones preferred and why

A

for casts used to process dentures because the stone has adequate strength for that purpose and the denture is easier to remove after processing

47
Q

when do you use type 4 gypsum

A

a hard surface is necessary for a die stone because the cavity preparation is filled with wax that is carved flush with the margins of the die

48
Q

describe the compressive strength of type 5

A

higher than type 4

49
Q

describe the setting expansion of type 5

A

higher than type 4

50
Q

what is maximum intercuspation

A

all the teeth contact in a stable way

51
Q

what is centric occlusion contact

A

first contact with leaf gauge is in place on the anterior teeth is in posterior teeth

52
Q

what is centric relation

A

condyle is in the most anterior superior position

Occlusion and Fixed (14 decks)

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