Operative/Single Crown Nourelhoda

324-350

From page 325-330

Page 329:

6-Which of the following materials are LEAST

suitable for impressions for cast gold

restorations?

A. Polysulfides.

B. Polyvinyl siloxanes.

C. Polyethers.

D. Irreversible hydrocolloids

Non aqueous elastomeric material

aqueous elastomeric material

Answer:

Therefore, alginate is an acceptable

impression material for study models, but it

is an unacceptable impression material for

the fabrication of bridges, crowns, and inlays.

Nearly all impression materials are acceptable

for full-denture impressions.

Clinical aspect of dental material chapter 8 page 117

The disadvantage of the material being predominantly water is that water evaporates

from the surface of the impression if it is left

exposed to air. When the water evaporates,

the impression shrinks and is no longer

accurate

Hydrocolloid materials contract slightly

after setting and exude water. This process

is called syneresis. Syneresis occurs very

slowly, but it is a second reason to pour

hydrocolloid impressions as soon as possible

When disinfecting a hydrocolloid impression,

however, it is important to limit the

time that the impression is exposed to an

aqueous disinfecting solution. The hydrocolloid

will absorb water, swell, and distort.

This is called imbibition.

7-In metal-ceramic crowns, the bond between the

materials is

A. mainly mechanical.

B. mainly chemical.

C. decreased by oxides on the metal surface.

D. decreased when the metal has a high yield point.

Introduction to metal ceramic restoration page 83

8-Nickel-chromium alloys designed for porcelain

bonded to metal crowns should be used with

caution because

A. nickel is an allergen.

B. the modulus of elasticity is low.

C. these alloys cannot be soldered.

D. None of the above.

Introduction to metal ceramic restoration page 35

9-For application of porcelain to a ceramo-metal

alloy, the correct viscosity is achieved by

mixing the porcelain powder with

A. a porcelain modifier.

B. pure methyl alcohol.

C. a mild detergent.

D. distilled water

Introduction to metal ceramic restoration page 17

Page 330

1-A model prepared from a vacuum mixed stone

has higher strength because

A. less water is required for vacuum mixing.

B. there is less porosity.

C. some of the water is removed by the vacuum.

D. the nuclei for crystallization are more numerous.

Stone is made from gypsum by carefully controlled calcination under steam pressure in a closed container. This method of calcination slowly releases the water of crystallization from the crystal so that the resultant powder particle (Fig. 9.4B)is more regular, more uniform in shape, and less porous compared to that of plaster.

Stone is stronger and more expensive than plaster. It is used mainly in making casts for diagnostic purposes and casts for complete and partial denture construction, which require greater strength and surface hardness than that of plaster

The dense particles of stone require less gauging water to float them, and their regular shape allows them to roll over one another more easily.

This difference in the amount of measured water that is required to make a workable mix results in different consistencies for the products when first mixed at the proper water/powder ratio. Plaster is usually thin in consistency, like a ‘smoothie,’ whereas improved stone is like thick cake batter. Dental stone has an intermediate consistency. The water/powder ratio has a direct effect on the properties of each gypsum product and must be controlled for optimum

The strength depends on the porosity of the set material, and the porosity relates to the water/powder ratio necessary to make a workable mix. Plaster,which requires the most gauging water to make a fluid mix, is the weakest in strength, with improved stone being the strongest and stone being intermediate between the two.

Table 9.1 One-Hour Compressive Strengths and Water/Powder Ratios of Gypsum Products

Minimum 1-Hour Compressive

Gypsum Strength (lb/in2 [Mpa])a Water/Powder Ratiob

Plaster (Type II) 1,300 (9) 45–50 ml/100 g (0.45–0.50)

Dental stone (Type III) 3,000 (21) 28–30 ml/100 g (0.28–0.30)

Dental stone, high-strength (Type IV) 5,000 (34) 19–24 ml/100 g (0.19–0.24)

Clinical aspect of dental material chapter 9 page 132

2-Impression trays should be

A. rigid.

B. flexible enough to permit easy insertion.

C. carefully polished on the interior.

D. held in place by the patient.

With any system, tray rigidity is important, because even slight flexing of the tray will lead to a distorted impression. This is particularly frustrating because the errors are usually undetectable until the practitioner attempts to seat the restoration.

For this reason, thin, disposable plastic trays are unacceptable . Resin thicknesses of 2 to 3 mm are needed for adequate rigidity. Clearance between the tray and the teeth should also be 2 to 3 mm; however, greater clearance is necessary for the more rigid polyether Materials.

Contempory fixed prosthesis section II page 365

3-A ceramometal posterior fixed partial denture

pontic should ???

A. A. be constructed to have an occlusal surface one quarter the width of the tooth it replaces.

B. be constructed to have an occlusal surface wider than the width of the tooth it replaces.

C. cover as much mucosa as possible.

D. provide adequate embrasure spaces.