Dental Materials Week 3 Reviewnh Chpt. 9

Front 1

The desirable strength of gypsum materials is ______ related to the amount of water used.

(a) Directly
(b) Indirectly
(c) Not
(d) Partially

Back 1

(a) Directly

The amount of water added to a gypsum product is directly related to its strength. The higher the W/P ratio, as in plaster, the softer and weaker is the resulting gypsum product.

Front 2

For gypsum products, a suitable accelerator and retarder, respectively, would be:

(a) Ethyl alcohol and oleic acid
(b) Oleic acid and glycerin
(c) Borax and potassium sulfate
(d) Potassium sulfate and borax

Back 2

(d) Potassium sulfate and borax

Potassium sulfate is a common accelerator for gypsum products, as is borax, a retarder. Oleic acid, glycerin, and ethyl alcohol are not used to change the setting time of gypsum products.

Front 3

The gypsum material known as “high-strength stone” may also be referred to as:

(a) Plaster
(b) Dental stone
(c) Type III stone
(d) Improved stone

Back 3

(d) Improved stone

High-strength stone may also be called die stone, improved stone, Type IV gypsum, Densite, or modified alpha-hemihydrate. Plaster is referred to as beta-hemihydrate or Type II gypsum. Dental stone may also be called Hydrocal, Type III gypsum, or alpha-hemihydrate.

Front 4

The final setting time for gypsum products typically is:

(a) 15 to 30 minutes
(b) 30 to 45 minutes
(c) 45 to 90 minutes
(d) 90 to 120 minutes

Back 4

(b) 30 to 45 minutes

As a standard setting time for gypsum products, 30 to 45 minutes for a final set is given. The Gillmore needle (or fingernail) should not leave indentations and can be used to determine a final setting time.

Front 5

To make a correct mix for dental stone when using 50 g of powder, the amount of water would be approximately:

(a) 10 to 12 ml
(b) 14 to 15 ml
(c) 28 to 30 ml
(d) 45 to 50 ml

Back 5

(b) 14 to 15 ml

For 50g of stone, 14 to 15 mL of water is required. For 100g of stone, 28 to 30 mL of water is used. For 50g of improved stone, 10 to 12 mL of water is used. For 100g of plaster, 45 to 50 mL of water is needed.

Front 6

6. Cathy and her parents will meet with an orthodontist to discuss an orthodontic treatment plan. At a previous appointment, the orthodontist took impressions of Cathy's maxillary and mandibular arches. The replicas made from each impression to discuss the treatment plan are termed:

(a) Casts
(b) Dies
(c) Study models
(d) Molds

Back 6

(c) Study models

Study models are used for observation, diagnosis, and treatment planning. Restorations or appliances are not made on them, as they are usually constructed from weaker gypsum products. Casts are replicas that are usually constructed with improved stone, and appliances and restorations are fabricated on them. A die is a working replica of a single tooth and is usually part of a cast.

Front 7

Decreasing the setting time of a dental material results in a product that:

(a) Will set faster
(b) Will set slower
(c) Does not affect the setting time
(d) Enhances the properties

Back 7

(a) Will set faster

Increasing the setting time would mean that it takes more time for a product to set. Thus, decreasing the setting time results in a product that sets faster, requiring less time to set. Manipulating the setting time does not enhance the properties of a gypsum product; in fact, it can be deleterious to them.

Front 8

The weakest gypsum product is:

(a) Improved stone
(b) Dental stone
(c) Plaster
(d) Die stone

Back 8

(c) Plaster

Because plaster requires the most water for a correct mix (proper W/P ratio), it would be the weakest, softest, and most porous of the three common gypsum products. Improved stone would be the most dense and the strongest, and dental stone would fall between plaster and improved stone.

Front 9

When excess water is not present in a gypsum product, it is known as the:

(a) Wet strength

(b) Dry strength

(c) Initial setting time

(d) Surface strength

Back 9

(b) Dry strength

Dry strength refers to the strength when excess water is not present. This may be two or three times the wet strength. The wet strength is measured at the final set (30-45 minutes). The initial strength occurs when loss of gloss is present and is an indication of the working time.

Front 10

The best way to mix gypsum products is to:

(a) "Eyeball" the amount of powder added to water
(b) Add water to the powder
(c) Add powder to the water
(d) Add powder and water to bowl simultaneously

Back 10

(c) Add powder to the water

It is recommended to first add the water and then the powder to the mixing bowl. As a result, less air is incorporated
into the mix. "Eyeballing" is not recommended because the W/P ratio will not be correct and inferior properties will result. Adding water to the powder or adding powder and water to the bowl simultaneously tend to increase the porosity of the resulting mix.

Front 11

Initial and final setting times of gypsum can be determined with the use of Gilmore needles. A practical, easy method to determine final setting time in a dental office would be to:

(a) Set a timer for 20 minutes
(b) Observe the change from wet to dry strength
(c) Watch for the loss of gloss
(d) Attempt to penetrate the material with the metal spatula

Back 11

(d) Attempt to penetrate the material with the metal spatula

Final setting time can be determined by attempting to penetrate the material with a dull instrument or a fingernail. Initial setting time is said to occur when the material loses its shine, or "loss of gloss." Final setting time takes approximately 30-45 minutes. The change from wet to dry strength usually occurs after 8 hours.

Front 12

When the setting time of a dental material is increased, which of the following is true?

(a) The material sets slower
(b) The material sets faster
(c) The setting reaction does not change
(d) The setting reaction is increased

Back 12

(a) The material sets slower

When setting time is increased, it takes the material longer to set. A decreased setting time yields a faster setting material, or takes less time to set. If the setting reaction is decreased, the material will take longer to set, and if it .is increased, it will take less time to set.