Outcome 3 - Fabricate various casts including study models from alginate impressions

Question 1

What are study models?

Answer 1

Study models are vital diagnostic records and may be used in the construction of intra oral appliances and restorations

Question 2

5 Types of Gypsum Products used in dentistry:

Answer 2

1. Impression Plaster
2. Model Plaster
3. Dental Stone
4. Die Stone
   5 High-Strength, High-Expansion Die Stone

Question 3

What is Impression Plaster?

Answer 3

Used to take impressions of dentulous patients

Question 4

What is Model Plaster?

Answer 4

Essentially the same as impression plaster but is stronger and has a longer working time. This plaster is used to make study models, or in the use of orthodontics or case presentation.

Question 5

What is Dental Stone?

Answer 5

-Used to make study casts; it is harder and stronger than plaster and the resulting casts are more durable
-it used for study models that require more strength, working casts and models for partial and full dentures

Question 6

What is Die Stone?

Answer 6

A very high-strength gypsum, it has a great hardness and low expansion
-Stone is used to make dies upon which wax patterns are made for crowns, bridges, and inlays.

Question 7

What is High-Strength, High-Expansion Die Stone?

Answer 7

Also known as Densite or improved dental stone. Ideal for use by the dental technicians in creating a die for use in fabrication of wax pattern for crowns, bridges

Question 8

What must gypsum be mixed with?

Answer 8

Each type of gypsum must be mixed with water to initiate a chemical reaction

Question 9

What chemical reaction happens when gypsum is mixed with water?

Answer 9

The hemihydrate crystals within the gypsum dissolve in the water and form clusters known as nuclei of cyrstallization. As these clusters expand during reaction, they become entangled and form a solid meshwork.

Question 10

What happens to the excess water in Gypsum?

Answer 10

Gypsum products, when mixed with water, set to a hard mass. The amount of water required to mix the material is greater than the amount required for the chemical reaction to occur.

The required water, in addition to that necessary for the chemical reaction, is called excess water. This excess water evaporates during the setting reaction, leaving voids (air bubbles) in the cast

Question 11

How is gypsum stored?

Answer 11

Store gypsum in airtight containers, only dispensing the amount required. Contamination by humid air will allow the powder to absorb moisture from the atmosphere. This will affect the setting time and strength of the cast.

Question 12

What are the 3 methods of pouring study models?

Answer 12

1. Double Pour Method
2. Boxing Method
3. Inverted-Pour Method

Question 13

What is the Double Pour Method?

Answer 13

The anatomic portion of the cast is poured first, and then a second mix is used to prepare the art portion. A commercial rubber mold can be used as a base. The double-pour method is not difficult to use; two mixes are used.

Question 14

What is the Boxing Method?

Answer 14

The impression is surrounded with a “box” made from boxing wax. The box should extend above the palatal area of the maxillary impression and above the tongue area of the mandibular impression. One large mix of gypsum material is used to pour both portions of the model.

Question 15

What is the Inverted-Pour Method?

Answer 15

One large mix of gypsum material is used to pour both portions of the model. Once the anatomic portion has been poured, the mix is slightly thickened to create the base or art portion of the model. The anatomic portion is inverted into the base.

Question 16

Separating Gypsum Models

Answer 16

Before evaluating gypsum models, separating of the models must be done first. When separating dental models/casts from the impression tray, extra caution must be mantained. Laboratory knife is used to remove the cast from the impression.

Question 17

Evaluating pouring gypsum models

Answer 17

If critical areas are not present (tooth anatomy broke during separation or large distortion is present etc.), patient will be asked to visit dental office to retake dental impressions. Dental assistant will repeat the process of pouring.

Question 18

Evaluating Anatomic Portion of Gypsum Model

Answer 18

• All anatomical features are present. Hold the model in your hand. Look at each tooth and all the gingiva.
• Is any of the tooth anatomy missing? How much?
  -Air bubbles have been eliminated from the anatomical portion. Look closely at each tooth and at the gingiva. Are there any holes? The holes may vary considerably in size. Tiny, frothy bubbles are over-vibration, not air voids.
• The occlusal plane is parallel to the base.

Question 19

Evaluating Art Portion of Gypsum Model

Answer 19

• The sides and hells of the art portion extend sufficiently to allow for removal of material during trimming.
  -Look at your model/cast from all directions. Is there enough material beyond all anatomy?
  -Sufficient material for the next step (trimming) is required.

Question 20

Evaluating Consistency of Gypsum Model

Answer 20

The appearance o the set material is smooth and even in both texture and colour. Look at and feel the clay model. Is it chalky or streaked? Texture? Does it rub off or crumble easily? Is it mottled or streaked in colour?

Question 21

What happened if impression material has separated from the impression tray and appears dry?

Answer 21

Too much time has elapsed between the taking of the impression and the pouring of it. The alginate has undergone dehydration, or syneresis. Pour alginate impressions as soon as possible.

Question 22

What happened if the gypsum model surface appears chalky and is soft or crumbly when touched?

Answer 22

• the impression was not separated from the poured model within one hour. After this time, the alginate begins to absorb moisture from the gypsum.
• the incorrect powder-to-water ratio was used, making the mix too thin. Make sure you weigh the stone and mix with the recommended amount of water.
• the impression was not cleaned and dried before pour-up, leaving saliva, blood, food debris, water or disinfectant next to the gypsum.

Question 23

What happened if there is voids within the anatomy of gypsum model?

Answer 23

-place material at a controlled rate in small increments. Control the rate of flow into eacch tooth so that air can escape the depression. (Check vibrator speed, quantitiy of material being added, and angle of the impresion on the vibrator.)
- review techniques for reduction of surface tension

Question 24

What happened if there are frothy bubbles within the anatomy of gypsum model?

Answer 24

Could be caused by excessive vibrator speed or prolonged contact between tray & vibrator.

Question 25

What happened if the gypsum model base is too thin?

Answer 25

The base material was dispensed too soon. The material was manipulated too vigorously, causing flow. The impression was pushed too far into the base material. The occlusal plane is not parallel to the counter

Question 26

What to do if the gypsum model anatomy has been removed during the rough trip?

Answer 26

Carve a little at a time, occlude and check. Re-carve as necessary

Question 27

What happend if gypsum model is easily broken?

Answer 27

• the impression was removed before the final set. The impression was pivoted during removal. An improper powder-water ratio was used.
• Air has been incorporated into the mix, producing a weakend final product. This can happen due to a poor spatulation technique or due to over vibration. Spatulate the material against the side of the rotating rubber bowl. Always lift the impression of the vibrator as you reach for an additional increment of plaster or stone to add to it.

Question 28

Armamentarium for Pouring Study Models?

Answer 28

-PPE
-Plaster Spatula
-Lab Knife
-Dental Stone
-Water (room temp)
-Plaster vibrator with plastic protective cover
-Weigh Scale
-Graduated cylinder (water measure)
-Stone mixing bowl
-Alginate impressions (Mand and/or Mand)
-Moist paper towels
-Plastic Tile(s)