Quiz 2 (Chapters 17 & 18) Flashcards
The dry strength of gypsum is approximately twice that of the wet strength. True or False?
True
At what percentage of weight loss (excess water) does compressive strength of gypsum increase sharply?
7.5%
Increasng the speed of spatulation or length of time spent mixing _____ the setting time:
Shortens
What are the four characteristics of Cyanoacrylate reinforced gypsum?
- 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
Depending upon the method of calcination, how many different forms of the hemihydrate can be obtained? And what are they?
3
- Dental plasters (produced when gypsum is heated in an open kettle at a temperature of 110-130 degrees celsius. This is known as the β-hemihydrate, and is used to produce type II Dental Stone.
- Hydrocal, and is used to make type III dental stone. This second form of product results when gypsum is dehydrated under pressure, and in the presence of water vapor at about 125°C. Is an α-hemihydrate for of calcium sulfate. Type III stone is used in many laboratory procedures, as well as to fabricate diagnostic casts, and partial and complete dentures.
- Densite, used to make type IV and V die stones.
This 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. Type IV and V die stones are both α-hemihydrate forms of calcium sulfate as well.
The set time for dental plaster and stone is largely under the control of the ________:
Manufacturer
What are the two principal objectives of reflowing and refinishing the margins immediately before the investing of the wax pattern?
- Minimizing dissolution of the luting agent
- Facilitating plaque control
What are the main six die materials?
- Epoxy Resin
- Polyurethane
- Electroplated Silver or Copper Over Gypsum
- Flexible Silicone or Polyether
- Gypsum
- Cyanoacrylate Reinforced Gypsum
How can one overcome the problem of distortion or increased elasticity with their wax pattern?
By applying the initial layer of wax in melted increments or drops.
As the solidfying metal (casting, crown) cools to room temperature, it shrinks, so how is the accuracy maintained so it fits on the tooth?
Dimensional accuracy of the casting is achieved by balancing the shrinkage against precisly controlled expansion of the mold.
With gypsum, what is the definition of working time?
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.
With gypsum, increasing temperature _____ solubility of the hemihydrate relative to that of the dihydrate:
Lowers. At 100°C the solubility’s are equal, at which point no reaction occurs and plaster does not set.
With gypsum, a moderate increase in temperature will ______ set time:
Decrease
What is wet strength?
Wet strength is the strength measured when excess water remains in the hardened mass of stone
What is the name of the powder form of calcium sulfate hemihydrate?
Calcium sulfate anhydrite, which is the starting form of calcium sulfate used in dentistry for the production of casts and casting investments.
What are the six factors that increase the luting agent space?
- Increased thermal and polymerization shrinkage
of the impression material (see Chapter 14).
- Use of a solid cast with individual stone dies (see
Chapter 17).
- Use of an internal (initial) layer of soft wax in the
wax pattern.
- Use of die spacers.
- Increased expansion of the investment mold
(see Chapter 22).
- Removal of metal from the fitting surface by
grinding, airborne-particle abrasion, etching with aqua regia, or electrochemical milling.
What is the lost wax technique?
- This technique consists of obtaining an accurate impression of the prepared tooth and making a cast from the impression on which a wax pattern that resembles the shape of the final restoration is shaped. A mold is then made around the wax pattern with a refractory investment material. When the investment 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. The metal casting is retrieved, excess metal is removed, and after polishing, the cast restoration is ready for clinical evaluation.
- As the solidifying metal (casting) cools to room temperature, it shrinks. Dimensional accuracy of the casting is achieved by balancing this shrinkage against precisely controlled expansion of the mold. Wax is used to make the patterns because it can be conveniently manipulated and precisely shaped. By heating, it can be completely elimminated from the mold after investing.
- The lost-wax technique is widely used in industrial and jewelry manufacturing. The first bronze castings reportedly were made in the third millennium B.C.E. with beeswax and clay refractory materials. Ancient lost-wax castings such as Chinese bronzes, Egyptian deities, and Greek statues have withstood the centuries, yielding information about ancient societies and cultures. The lost-wax method may have been used in Sumer as early as the Second Early Dynastic Period for figurines and even larger body parts.
What are the six steps in order for waxing a tooth to anatomic form?
- The die is modified as necessary and lubricated
- An initial coping is waxed, forming the internal surface
- The proximal surfaces are developed, with correctly located contact areas.
- The axial surfaces are waxed. Overcontouring near the gingival margin must be avoided.
- The occlusal surfaces are developed with a wax addition technique, which makes it easier to determine the best location of cusps and occlusal contacts.
- The margins are reflowed, and the wax pattern is finished.
The maxillary cusps become shorter, farther distally. True or False?
True. The mandibular cusps become taller, farther distally.
What is another name for the curve of Spee?
The anteroposterior curve. The wax cones (if used) should line up with the curve of Spee.
In what third of the crown are most posterior contact areas located? Any exceptions?
Occlusal third of the crown. However, contact between the maxillary first and second molar occurs in the middle third.
What are the four main characteristics of regular gypsum?
- Inexpensive
- Easy to use
- Accurate
- Easily abraided
What happens during spatulation with gypsum product powder?
Newly formed dihydrate crystals are broken into smaller crystals, which serve as nucleation sites for further growth. Slurry water sets faster because of this. Because increased spatulation causes more nuclei centers to be formed, conversion from hemihydrate to dihydrate is accelerated.
Type IV die stone is ___ strength and ___ expansion?
High, Low
The surface of set gypsum is porous on a microscopic level. True or False?
True
What is the definition of a die?
Positive reproduction of the prepared tooth.
Do most adults with a class I occlusion and unworn teeth have a cusp-fossa scheme or a cusp-marginal ridge scheme?
Cusp-marginal ridge. Cusp-fossa is only with slight class II. But the cusp-fossa arrangement is recommended for the casting to prevent food impaction, help with centric relation, and improve stability.
What type of hydrate are stone II and III?
Hemihydrate
When set, about 50% of gypsum products show a measurable linear expansion. True or False?
False, they all do.
What happens when the luting agent space is too narrow?
The casting does not seat properly during cementation because of hydraulic pressure that develops when the viscous mass of luting agent cannot escape through the narrow gap between the crown and preparation as the restoration is seated.
What are the three main characteristics of Polyurethane?
- More abrasion resistant than gypsum
- More expensive than gypsum
- Must use PVS or Polyether impression material (Incompatible with hydrocolloid and polysulfide rubber base).
What is centrum?
Occlusal contacting scheme or pattern for posterior teeth. Cusp tip to flat receiving area, no contacts on inclines, 2 to 4 per tooth. Receiving area can be a marginal ridge, or any artificially created area in a fossa (make it flat!)
What happens when gypsum product powder is place in water?
The reaction starts and calcium sulfate dihydrate is formed.
What are the characteristics of type IV dental stone?
- High-strength low-expansion
- α-hemihydrate
- Uses
- Cast inlays and onlays
- Implant prostheses
- Removable partial denture frameworks
- Milled restorations
Overtimming the die directly beneath the margin will result in?
Overcontoured restorations.
What are the five types of gypsum products established by the ADA?
- Type I - Plaster, impression
- Type II - Plaster, model
- Type III - Dental stone
- Type IV - Dental stone, high strength
- Type V - Dental stone, high strength, high expansion
What is the hottest point on the bunsen burner?
The meniscus
What is the definition of abrasion?
- It is the removal of material that occurs when surfaces slide across or against each other. There is two-body abrasion and three-body abrasion.
- Poor abrasion resistance is a disadvantage of gypsum materials
How can one increase the expansion of the investment mold?
Heat the mold to a slightly higher temperature during the wax elimination phase.
Gypsum dies usually provides better surface detail than electroplated or epoxy dies. True or False?
False, it is the opposite.
What are the four characteristics of Flexible Silicone or Polyether?
- 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