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Questions and Answers

What is the primary purpose of preparing a working cast?

To reproduce both prepared and unprepared tooth surfaces (correct)

To adjust the contour of the tooth and allow for margin adaptation

To create a mold of the entire dental arch

To test the dimensional stability of the impression material

What is the ideal time for pouring an agar agar impression?

As soon as possible (correct)

Up to 7 days

After 24 hours

Within one hour

What is a characteristic of polysulfide and condensation silicone impression materials?

Poor dimensional stability due to loss of water or alcohol (correct)

High strength and high expansion

Can be poured up to 7 days

Excellent dimensional stability

What type of stone is recommended for pouring impressions for the die and cast?

Type IV: High-strength Dental Stone or Type V: Improved Stone Signup and view all the answers

What is the purpose of using a high-strength stone for the die and cast?

To prevent surface abrasion when fabricating the wax pattern Signup and view all the answers

What is the ideal finish line characteristic?

Complete and smooth Signup and view all the answers

What should be easily detected on the die?

The remaining unprepared tooth structure immediately cervical to the finish line Signup and view all the answers

What is a characteristic of addition silicone and polyether impression materials?

Excellent dimensional stability Signup and view all the answers

What is the purpose of following the manufacturer's instructions for the water/powder ratio?

To ensure the correct consistency of the stone mixture Signup and view all the answers

What is the purpose of using vacuum when mixing water and stone?

To remove any air bubbles from the mixture Signup and view all the answers

What is the primary purpose of applying surfactant to the impression?

To reduce the number of voids trapped in the cast Signup and view all the answers

What is the consequence of adding a large amount of stone to the impression at once?

Air will be trapped and voids will result in the cast Signup and view all the answers

Why is it necessary to vibrate the impression until the stone reaches the bottom of the preparation?

To ensure the stone is in contact with the entire preparation Signup and view all the answers

What is the purpose of building the stone up to a height of approximately one inch over the preparation?

To provide a reservoir for excess stone Signup and view all the answers

What is the advantage of using a double pour technique?

It allows for the creation of separate dies for each tooth Signup and view all the answers

Why is it necessary to relieve the stone slightly to seat the pattern?

To compensate for the slight difference in size between the first and second pours Signup and view all the answers

What is the primary advantage of using a sectional cast (separate die)?

It allows for the creation of separate dies for each tooth, keeping the relationship between abutments fixed and immovable Signup and view all the answers

What is the primary disadvantage of using a sectional cast (separate die)?

Fragile wax patterns are difficult to transfer between the two parts, resulting in distortion of some of internal adaptation Signup and view all the answers

What is the purpose of using a full arch cast?

For adjusting proximal contact and occlusion Signup and view all the answers

What is the critical requirement for dies used in a sectional cast?

They must return to their exact positions on the cast Signup and view all the answers

What is the primary purpose of I-Ditching in dental cast construction?

To accentuate the finish line for better wax pattern construction Signup and view all the answers

What is the recommended thickness of the die spacer used in painting the die?

20-40µm Signup and view all the answers

What is the purpose of leaving a 1mm unpainted band at the die margin?

To increase margin adaptation Signup and view all the answers

What is the advantage of using special plates in the construction of removable dies?

They provide a simple and easy method for producing dies and models Signup and view all the answers

What is the purpose of trimming the impression before pouring the stone?

To remove excess impression material Signup and view all the answers

What is the recommended location for the dowel pin holes in the clear plate?

As marked on the clear plate Signup and view all the answers

What is the purpose of using a saw blade in the construction of removable dies?

To section the dies from the cast Signup and view all the answers

What is the advantage of using removable dies?

They provide a fast and simple method for producing dies and models Signup and view all the answers

What is the purpose of painting the die with a die spacer?

To provide space for cement Signup and view all the answers

What is the recommended depth of the circumferential groove created by I-Ditching?

0.3mm Signup and view all the answers

What is the primary advantage of using a single die and working cast system?

Elimination of errors due to impression distortion Signup and view all the answers

What is the purpose of coating the bottom of the cast with separating medium?

To form the base of the working cast Signup and view all the answers

What is the role of the plastic sleeves in the Pindex system?

To facilitate die removal and reassembly Signup and view all the answers

What type of arch is restricted to using die stone in the Pindex system?

U-shaped arch Signup and view all the answers

What is the purpose of applying wax to the sleeve end?

To facilitate die removal Signup and view all the answers

What is the primary disadvantage of using a single die and working cast system?

Increased risk of introducing errors during die reseating Signup and view all the answers

What is the function of the light in the Pindex machine?

To indicate the location of the drill Signup and view all the answers

What type of cement is used to attach the pins to the cast?

Cyano-acrylate Signup and view all the answers

What is the purpose of using cutting saws in the Pindex system?

To section the dies Signup and view all the answers

What is the function of the di-lok tray in the Pindex system?

To hold the cast in place during pouring Signup and view all the answers

Study Notes

Working Cast and Die Preparation

A working cast is a positive reproduction of the prepared teeth, ridge areas, and other parts of the dental arch, obtained from pouring the final impression.
The cast should:
- Reproduce both prepared and unprepared tooth surfaces
- Be free of voids or defects
- Include all soft tissues, including edentulous spaces and residual ridge contours

Ideal Time for Impression Pouring

Agar agar: pour as soon as possible (dimensionally unstable, undergoes synersis and imbibition)
Polysulfide and condensation silicone: pour within one hour (poor dimensional stability, due to loss of water or alcohol as a byproduct)
Addition silicone and polyether: pouring can be delayed up to 7 days and multiple pours can be done (excellent dimensional stability, no byproduct from polymerization reaction)

Die Stone Characteristics

High-strength stone (type IV) or high-strength, high-expansion stone (type V) should be used for pouring the impression
Gypsum products:
- Type I: Impression Plaster
- Type II: Model plaster
- Type III: Dental stone
- Type IV: High-strength dental Stone
- Type V: Improved Stone, Die Stone

Pouring the Impression

Follow the manufacturer's instructions for the correct water/powder ratio
Mix water and stone by hand with a spatula until the powder is completely wet, then use vacuum
Impressions should be completely dry and the surface should be free of visible water
Apply surfactant on the impression by spraying to reduce the number of voids trapped in the cast

Working Cast and Sectional Cast

The working cast and the sectional cast (separate die) can be obtained from separate impressions or by pouring the impression twice
If a double pour is used, the first pour is used for fabrication of the die
The double pour can be used only with elastomeric impressions because hydrocolloid is torn and distorted too much to be used for an accurate second pour

Advantages and Disadvantages of Sectional Cast

Advantages:
- Ease of fabrication
- Keep the relationship between abutments fixed and immovable
Disadvantages:
- Fragile wax patterns are difficult to transfer between the two parts (from cast to die), so distortion of some of internal adaptation will occur
- The second pour of the impression may be different (slightly larger) than the first, therefore, it may be necessary to relieve the stone slightly to seat the pattern

Single Die

One die is used for proximal contacts, occlusion, and margins
Dies must return to their exact positions on the cast
Dies must remain stable even when inverted
Dies should be accurately related to adjacent and opposing teeth

Advantages and Disadvantages of Single Die

Advantages:
- No need to transfer wax pattern from dies to working cast
- Eliminate errors between separate die and working cast caused by impression distortion between pours
Disadvantages:
- The risk of introducing an error if the die doesn’t reseat accurately in the working cast
- Debris may prevent complete reseating of the dies

Pindex System

A Pindex machine drill parallel holes from the underside of the trimmed cast by using a reverse drill press, creating a master cast with dies that can be removed and replaced repeatedly with great precision
Advantages:
- No need to transfer wax pattern from dies to working cast
- Eliminate errors between separate die and working cast caused by impression distortion between pours

Di-Lok System

Uses special plates which are aligned and drilled to provide removable dies
Advantages:
- Fast
- Simple and easy method for producing dies and model
- Only one pour is required

Ditching and Die Spacer

Ditching: a circumferential groove 0.3mm in depth below the finish line
Function: accentuate finish line, therefore, allow better wax pattern construction
Die spacer: a 20-40µm thick layer to provide space for cement and allow complete seating of restoration during cementation

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