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FP > Final V (Def. casts) > Flashcards

Final V (Def. casts) Flashcards

1
Q

Master Cast

A

Definitive Cast = Working Cast: Replica of the prepared tooth or teeth, ridge areas, and other parts of the dental arch (Sometimes also referred to as “Models.”)

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2
Q

Die

A

Positive reproduction of the prepared tooth.

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3
Q

Master Cast Requirements:

A

Must accurately represent both prepared and unprepared tooth surfaces
Unprepared teeth immediately adjacent to the prepared teeth must be free of voids, defects or other irregularities
All contacting surfaces of opposing teeth in MI and any tooth surfaces involved in anterior guidance must be free of defects and allow for precise articulation
All relevant soft tissues must be accurately reproduced, especially edentulous ridges that will be involved in a fixed prosthesis
Overall cast shape and form must be an accurate representation of the relevant anatomical structures
Must be neatly trimmed and de-bubbleized

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4
Q

minimize bubble at critical areas by

A

wiping with finger

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5
Q

Die Requirements:

A

Must accurately reproduce the prepared tooth or teeth

All marginal areas must be accessible

The unprepared tooth structure immediately cervical to the margins should be easily discernible (0.5 to 1.0 mm beyond prepared margin if possible). This enables the correct cervical contours to be determined.

Must be of a durable material that is not easily abraded

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6
Q

Die materials

A
  • Epoxy Resin
  • Polyurethane
  • Electroplated Silver
  • Copper Over Gypsum
  • Flexible Silicone or
  • Polyether
  • Gypsum
  • Cyanoacrylate Reinforced *most common die
  • Gypsum
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7
Q

Type 4 and 5 gypsum can be

A

abraded

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8
Q

Epoxy Resin

A

More abrasion resistant than gypsum

More expensive than gypsum

Some formulations undergo slight shrinkage upon polymerization

Must use PVS or Polyether impression material (Incompatible with hydrocolloid and polysulfide rubber base).

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9
Q

Polyurethane

A

More abrasion resistant than gypsum

More expensive than gypsum

Must use PVS or Polyether impression material (Incompatible with hydrocolloid and polysulfide rubber base).

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10
Q

Epoxy Resin

A

More abrasion resistant than gypsum

More expensive than gypsum

Some formulations undergo slight shrinkage upon polymerization

Must use PVS or Polyether impression material (Incompatible with hydrocolloid and polysulfide rubber base).

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11
Q

Polyurethane

A

More abrasion resistant than gypsum

More expensive than gypsum

Must use PVS or Polyether impression material (Incompatible with hydrocolloid and polysulfide rubber base).

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12
Q

Electroplated

A

Difficult process

Easily distorted

When done properly, can be very accurate and durable

Incompatible with polyether and Polysulfide rubber base

Silver plating requires a cyanide solution which introduces toxicity and disposal challenges

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13
Q

Flexible Silicone or Polyether

A

Die can be fabricated in a matter of minutes

Used primarily for chairside indirect
composite resin inlays or onlays

Can also be used for interim restorations

Restoration is easily separated from die

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14
Q

Gypsum

A

Inexpensive
Easy to use
Accurate

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15
Q

Gypsum’s downfall.

A

easily

ABRAIDED

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16
Q

Cyanoacrylate Reinforced Gypsum

A
  • Inexpensive
  • Easy to use
  • Accurate
  • Small amount of **low-viscosity cyanoacrylate is applied to die and further thinned with compressed air, making die more resistant to abrasion
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17
Q

Gypsum Products

A

Gypsum (CaSO₄ ∙ 2 H₂O; calcium sulfate dihydrate) is a mineral mined in many parts of the world

Gypsum products used in dentistry are supplied in the form of the hemihydrate (CaSO4 ∙ ½ H₂O; calcium sulfate hemihydrate), which is produced by heating ground gypsum

After mixing a dental gypsum product with water, it reverts back to gypsum

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18
Q

calcium sulfate dihydrate

A

changed to hemihydarte or anhydrite when we use it. Water is taken off.

Type 4 and 5 dental stones is anhydrite.

other are hemi (2/3)

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19
Q

Gypsum Products

A

Calcium sulfate hemihydrate (CaSO4 ∙ ½ H₂O) and calcium sulfate anhydrite, the powder, is the starting form of calcium sulfate used in dentistry for the production of casts and casting investments

Calcium sulfate dihydrate (CaSO₄ ∙ 2 H₂O) is the set form of dental stone

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20
Q

Production of Gypsum Products

A

Production of Gypsum Products

Commercially, gypsum (dihydrate) is ground and subject to temperatures of 110°C-130°C in open containers to drive off water

The resulting hemihydrate is a fibrous, porous crystalline aggregate known as plaster of Paris, or dental plaster

As the product is heated further, it becomes an anhydrite known as dental stone

This process is called calcination,

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21
Q

Calcination

A

CaSO4 picture from slide 21

*increase calcium concentration by reducing water to make into gypsum.

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22
Q

first form of calcination

A

Depending upon the method of calcination, three different forms of the hemihydrate can be obtained

The first form, dental plasters, are produced when gypsum is heated in an open kettle at a temperature of 110°C-130°C

This is known as the β-hemihydrate, and is used to produce type II Dental Stone
(more irregular and crystalline in structure)

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23
Q

second form of calcination gypsum product

A

The second form of product results when gypsum is dehydrated under pressure, and in the presence of water vapor at about 125°C

This gypsum product is called hydrocal, and is used to make type III dental stone

Type III dental stone is an α-hemihydrate form of calcium sulfate

Type III stone is used in many laboratory
procedures, as well as to fabricate diagnostic casts, and partial and complete dentures

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24
Q

Third dental product from calcination

A

The third form of dental product results when gypsum mineral is boiled in a 30% calcium chloride solution

-> The chloride is then washed from the rock, and the remaining mineral is ground to the desired level of fineness

This gypsum product is called [densite], and is used to make type IV and V die stones

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25
Q

why not use type 4 and 5 for dentures?

A

too hard, lab will hate it.

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26
Q

4th dental product from calcination

A

Type IV and V die stones are both α-hemihydrate forms of calcium sulfate as well

Type IV and V die stones are used to make many types of fixed indirect restorations

**Type IV die stone is high strength/low expansion stone

**Type V die stone is high strength/high expansion stone

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27
Q

Setting of Gypsum Products

A

When either α or β-hemihydrate is mixed with water, the calcination reaction is reversed
The following reaction is the setting reaction for all dental gypsum products including type I-V stone, and gypsum-bonded investments

28
Q

minimum amount of water to rehydrate stone

A

more than 18.6ml

29
Q

Gypsum product theory to rehydrate stone.

A

The theoretical lowest required volume of water needed to hydrate 100g of hemihydrate in either the α or β form is 18.6 ml

However none of the dental products currently in use can be mixed with this volume of water and still develop a mass that can be manipulated

The reason for differences among the recommended volumes of mixing water for plaster, dental stone and die stone is due to shape and size of the different forms of calcium sulfate hemihydrate particles

*die stones dont need as much water to put into dihydrate form.

30
Q

How to alpha and beta hemihydrate differ

A

The α and β hemihydrates differ based on crystal size, surface area, and lattice perfection.

β-hemihydrate crystals require the most water because they are MORE IRREGULAR in shape and porous. (More water is needed to wet the particles when mixing.)

The powder particles of α-Hemihydrate used for dental stone are SMALLER, more prismatic, and more REGULAR in shape then those for plaster.

The α-hemihydrate produces a much stronger, harder and denser structure than the β-hemihydrate because it is mixed with less water.

31
Q

what 2 things helps stone with density and strength

A

water and crysatlline structre

32
Q

Die stones

A

Die Keen (for optoim expansion, crown and bridge, porcelain, high scarpe resistance, type V)

Die stone (for crown and brige, porcelain, cast partial, high scrape resistance) type 4

33
Q

Model stones

A

Dentstrone (full, partial denture, cast partial, counter models, type 3)

Lab Stone (articulating

Mounting stone (low expansion, always use mounting stone to mount

34
Q

Mixing and Set Time:

A

Increasing the speed of spatulation or length of time spent mixing shortens the setting time

35
Q

increase spatulation causes waht

A

When gypsum product powder is placed in water, the reaction starts and calcium sulfate dihydrate is formed

During spatulation, newly formed dihydrate crystals are broken into smaller crystals, which serve as nucleation sites for further growth

Because increased spatulation causes more nuclei centers to be formed, conversion from hemihydrate to dihydrate is accelerated

36
Q

Effect of Temperature:

A

Moderate increase in temperature will decrease set time (makes it set faster)

However increasing temperature also lowers solubility of the hemihydrate relative to that of the dihydrate

At 100°C the solubility’s are equal, at which point no reaction occurs and plaster does not set

37
Q

Set Time and working time:

A

The reaction begins the moment powder is mixed with water
Working time is the time that elapses from the start of the mix until the mix loses it’s gloss, and the viscosity reaches a point where it will no longer flow into the impression
The time required for the reaction to go to completion is called the final set time

38
Q

set time for dental plaster and stone is largely under control of the what?

A

The set time for dental plaster and stone is largely under the control of the manufacturer

Factors under the control of the operator include

Changing the temperature of the water to accelerate or retard the set

Increasing or decreasing spatulation (mix) time can accelerate or retard the set
Substituting 2% K2SO4 for water will accelerate the set time
If you grind a small amount of set dental stone and create a slurry in the water used to mix with the gypsum product, it will accelerate the set
Adding more water to the mix slows the set time (Table 12-2)

39
Q

Compressive Strength

A

Gypsum products are popular in dentistry because, when set, they show relatively high values of compressive strength

As mentioned earlier, water in excess of 18.6 ml/100 grams of product is excess water

When set stone dries, excess water evaporates and voids are left in the remaining stone

Dense stone has the fewest voids and the highest compressive strength

Plaster has the greatest quantity of excess water, whereas high strength dental stone contains the least excess water

40
Q

Set Time:
changing the temp will do what?
what can you substititute to accel the set?
what does extra water do?

A

The set time for dental plaster and stone is largely under the control of the manufacturer
Factors under the control of the operator include
Changing the temperature of the water to accelerate or retard the set
Increasing or decreasing spatulation (mix) time can accelerate or retard the set
Substituting 2% K2SO4 for water will accelerate the set time
If you grind a small amount of set dental stone and create a slurry in the water used to mix with the gypsum product, it will accelerate the set
Adding more water to the mix slows the set time (Table 12-2)

41
Q

Compressive Strength

A

Gypsum products are popular in dentistry because, when set, they show relatively high values of compressive strength

As mentioned earlier, water in excess of 18.6 ml/100 grams of product is excess water

When set stone dries, excess water evaporates and voids are left in the remaining stone

Dense stone has the fewest voids and the highest compressive strength (type 4/5 alpha especialy)

Plaster has the greatest quantity of excess water, whereas high strength dental stone contains the least excess water

42
Q

effect of water powder ratio

A

The effect of water/powder ratio on compressive strength is given in table 12-16

Note that when dental stone and high strength dental stone is mixed with the same water/powder ratio as model plaster, the compressive strength is almost the same

43
Q

benefit of vacuum mixing

A

The compressive strength of dental stone is also slightly improved with vacuum mixing

because less voids

44
Q

effects of water content

A

Two strength properties of gypsum product are reported: wet strength and dry strength

Wet strength is the strength measured when excess water remains in the hardened mass of stone

The dry strength of gypsum products is the strength with all of the excess water driven out
The dry strength is approximately twice that of the wet strength

At 1-2 hours after mixing and pouring, hardened gypsum product appears to have reached maximum strength

However at room temperature, about seven days are needed for an average denture flask filled with gypsum product to lose excess water

45
Q

effct of drying on compressive strength

A

The effect of drying on compressive strength is shown in figure 12-22
Theoretically, about 8.8% of excess water is found in the mass of stone
As the mass loses up to 7% of excess water, no appreciable change in strength is observed
When 7.5% of excess water is lost, strength increases sharply

46
Q

Surface Hardness and Abrasion Resistance

A

*arent necessarily the same, but correlated

Hardness is a surface measurement of the resistance of one material to be deformed by indenting or scratching another material
Scratch test: First hardness scale proposed by Fredrich Mohs
Indentation test: Knoop and Vickers
Depth of surface penetration is reported in units of force/area
Abrasion, or wear, is the removal of material that occurs when surfaces slide across or against each other
Two body abrasion
Three body abrasion
N/A

47
Q

Hardness has been correlated with what?

Surface hardness?

A

Hardness has been correlated with wear resistance
Surface hardness has been shown to be a poor predictor of material wear (Peyton et al, JPD, 1952)
Abrasion resistance or wear resistance is a complex process that is influenced by material
Strength
Surface hardness and roughness
Third-body wear particle size
Mode of fracture (ductile vs. brittle material)
Applied force
Poor abrasion resistance is a disadvantage of gypsum materials
N/A

48
Q

on microscopic level

A

gypsum is still proous

49
Q

When set what does gypsum show?

A

When set, all gypsum products show a measurable linear expansion

The percentage of setting expansion varies from one gypsum product to another

Typically, over 75% of the expansion observed at 24 hours occurs in the first hour of setting

50
Q

Manipulation

A

Water should be added to the mixing bowl first, and powder added to the water

Mechanical spatulation requires that the powder particles first be wetted by water through hand mixing
Vacuuming during mixing reduces the volume of air trapped in the mix

51
Q

type 3,4,5

A

are stones, rest are plasters

52
Q

Type I Dental Stone

A

Impression plaster
Composed of plaster of Paris (β-hemihydrate)
Modifiers have been added to regulate the setting time and expansion
Historically, it was used to make impressions of intra-oral structures, but has been replaced by hydrocolloid and elastomeric impression materials

(too inaccurate, too soft)

53
Q

Type II Dental Stone

A

Laboratory plaster
Composed of plaster of Paris (β-hemihydrate)
Used commonly in the laboratory in removable denture construction
Commonly used to articulate casts (but should not be due to significant expansion!)
Usually marketed in a white color so that it can easily be distinguished from other types of stone, which are usually colored

(too inaccurate, too soft)

54
Q

Type III Dental Stone

A

α-hemihydrate
Wide range of uses, where strength in excess of plaster is needed, but not to the level provided by the high-strength die stones
-Diagnostic casts
-Working casts for -removable complete dentures
-Altered cast segments for removable partial dentures
-Casts for general utility purposes including
Bleaching trays
Athletic mouthguards
Nightguards
Duplicate casts
etc…

55
Q

Type IV Dental Stone

A 
High-strength low-expansion
α-hemihydrate
Uses
Cast inlays and onlays
Implant prostheses
Removable partial denture frameworks
Milled restorations
56
Q

Type V Dental Stone

A

High-strength & high-expansion
α-hemihydrate
Used with cast full-coverage restorations to compensate for casting shrinkage
Uses
Full coverage cast crowns, inlays and onlays
Fixed partial dentures with full coverage cast retainers

57
Q

Die system

A

Must accurately maintain relationships of prepared tooth or teeth to surrounding structures!!

Must allow easy retrieval and replacement of die(s)
Numerous systems commercially available

58
Q

What are the limitations of the partial arch articulation system?

A

No contralateral guidance in excursions

No condylar reference

59
Q

triple tray technique

A

Can provide an extremely accurate bite record when done correctly
Error is an inherent aspect of any copy
Fewer interactive surfaces can increase accuracy
Reasonable quality with minimal time investment.

60
Q

What has the lowest and highst contact angle?

A

Of the elastomers, poly- ethers have the lowest contact angle, which means they are the easiest to pour29,30; silicones have the highest contact angle and are the most difficult to pour,

61
Q

lost wax technique

A

This technique consists of obtaining an accurate impression of the prepared tooth (Fig. 18-1A) and making a cast from the impression (Fig. 18-1B) on which a wax pattern that resembles the shape of the final restoration is shaped (Fig. 18-1C). A mold is then made around the wax pattern with a refractory investment material (Fig. 18-1D). When the invest- ment has set, the wax is vaporized in an electric furnace. The hollow mold is then filled with molten casting alloy, reproducing every detail of the wax pattern

62
Q

if luting agent space on crown is too wide

A

Conversely, if the luting agent space is too wide, the casting is loose on the tooth, resistance form (see Chapter 7) is reduced, and the position of the casting is difficult

*we correct this by putting a spacer (green stuff 2 coats) and an initial soft wax dip prior to actual wax

63
Q

Problems when fitting casting are seen when

A

Problems with fitting castings become apparent at two stages of the indirect procedure: when the casting is evaluated on the die and when it is cemented.

64
Q

Sealer should go all the way down to margin T/f?

A

FALSE

However, it should NOT coat the entire preparation. For adequate marginal adaptation, a band of about 1 mm imme- diately adjacent to the preparation margin must be left unpainted.

65
Q

Inlay casting wax

A

Inlay casting wax (the name given all wax used in forming the pattern for cast restorations)

66
Q

Why do we dip the prep in wax first?

A

If a casting is to be accurate, the wax pattern must not become significantly distorted. One reason for distortion is that wax has “memory,” which means that it exhibits some elasticity unless it is thor- oughly liquefied. This problem can be overcome by applying the initial layer of wax in melted incre- ments or drops.

Dipping the lubricated die in a pot of melted wax is an alternative method for making well-adapted internal surfaces (Fig. 18-17). This method is particularly suitable for complete- coverage restorations.

FP (11 decks)

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