Final V (Def. casts) Flashcards

Question

why not use type 4 and 5 for dentures?

Answer 1

too hard, lab will hate it.

Answer 2

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

Answer 3

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

Answer 4

more than 18.6ml

Answer 5

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.

Answer 6

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.

Answer 7

water and crysatlline structre

Answer 8

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

Answer 9

Dentstrone (full, partial denture, cast partial, counter models, type 3) Lab Stone (articulating Mounting stone (low expansion, always use mounting stone to mount

Answer 10

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

Answer 11

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

Answer 12

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

Answer 13

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

Answer 14

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)

Answer 15

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

Answer 16

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)

Answer 17

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

Answer 18

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

Answer 19

The compressive strength of dental stone is also slightly improved with vacuum mixing *because less voids*

Answer 20

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

Answer 21

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

Answer 22

*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

Answer 23

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

Answer 24

gypsum is still proous

Answer 25

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

Answer 26

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

Answer 27

are stones, rest are plasters

Answer 28

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)

Answer 29

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)

Answer 30

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…

Answer 31

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

Answer 32

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

Answer 33

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

Answer 34

No contralateral guidance in excursions | No condylar reference

Answer 35

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.

Answer 36

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,

Answer 37

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

Answer 38

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

Answer 39

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.

Answer 40

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.

Answer 41

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

Answer 42

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.