Fixed Prosthodontics Concepts

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Questions and Answers

What is the primary requirement for a dental cast in fixed prosthodontics?

To allow for slight distortions for easier fabrication.

To only reproduce prepared tooth surfaces accurately.

To reproduce only soft tissues, excluding tooth structures.

To reproduce both prepared and unprepared tooth surfaces accurately. (correct)

Why is it essential for the unprepared teeth adjacent to the preparation to be free of voids in a dental cast?

To ensure proper articulation with opposing casts. (correct)

To reduce the need for additional adjustments.

To minimize the amount of material required for the model.

To simplify the trimming process for the technician.

What is the role of accurately reproducing soft tissues in the definitive cast?

To allow for easier removal of the cast.

To provide a detailed reference for the technician. (correct)

To allow adjustments in the prosthesis.

To improve the aesthetic appearance of the cast.

Why is it beneficial to have a solid model in addition to a working model when creating dental restorations?

To provide a reference for testing the fit of the final restoration before patient try-in and avoid patient appointments. (B) Signup and view all the answers

What is the main purpose of a die in fixed prosthodontics?

To represent the prepared tooth accurately. (A) Signup and view all the answers

Which gypsum type is primarily used for fabricating diagnostic models and taking impressions of edentulous arches?

Type 1 (D) Signup and view all the answers

Why is it important that any unprepared tooth structure immediately cervical to the finish line be visible on the die?

To ensure the technician can establish the correct cervical contour of the restoration. (B) Signup and view all the answers

What is a disadvantage of using epoxy resin for die creation compared to gypsum?

It can not be used with polysulfide impression materials. (A) Signup and view all the answers

What is the 'cuff' in the context of dental impressions and dies?

The area of the impression that goes beyond the finish line. (D) Signup and view all the answers

Why is it important for a die to return to its exact original position after removal?

To ensure precise work and accurate replication of the margins. (D) Signup and view all the answers

What impact does the lack of a cuff on the impression have on the resulting die?

It causes the area beyond the finish line to be poorly represented. (D) Signup and view all the answers

What material is most commonly used for the fabrication of dental casts and dies?

Gypsum (D) Signup and view all the answers

Why is adequate access to the margin imperative on the die?

To easily identify the finish line. (B) Signup and view all the answers

Which of the following is a characteristic of higher gypsum expansion when fabricating a dental restoration?

A looser fit of the restoration. (B) Signup and view all the answers

What is the primary purpose of sectioning a die from the working model when creating a crown or bridge?

To separate the prepared tooth from the entire arch for more precise work. (B) Signup and view all the answers

Which of the following die materials is known for producing a tighter fit of the restoration compared to gypsum dies?

Epoxy resin (A) Signup and view all the answers

What is the primary function of the flat side on a dowel pin?

To provide an anti-rotational feature (B) Signup and view all the answers

Which system uses a laser pointer to indicate where to drill holes for the pins?

Pindex dowel system (C) Signup and view all the answers

What feature distinguishes the Pindex dowel system pins from standard dowel pins?

They consist of two joined pins, providing an anti-rotational function (B) Signup and view all the answers

What is the primary advantage of using pre-made plastic bases in the Di-Lok and DVA model systems?

They facilitate accurate repositioning of dies after sectioning (D) Signup and view all the answers

What does the term 'ditching' refer to in the context of die preparation?

Trimming the area below the finish line (C) Signup and view all the answers

Which of the following is a purpose of marking the finish line with a colored marker?

To enhance visibility for accurate margin creation (C) Signup and view all the answers

What is the cuff in the impression?

The area beneath the finish line (B) Signup and view all the answers

Which of the following systems does NOT use a plastic base for die repositioning?

Dowel Pin system (A) Signup and view all the answers

What is the primary purpose of marking the finish line with a red or blue marker?

To enhance clarity and ensure precise work (D) Signup and view all the answers

Why is an accurate bite registration crucial when mounting casts on an articulator?

To minimize the required chairside adjustments (D) Signup and view all the answers

How should undercuts on the die be corrected?

By blocking them out with wax or another material (A) Signup and view all the answers

What happens if there is inadequate space for luting agent when seating a restoration?

The restoation will not seat properly (D) Signup and view all the answers

What is the correct range of space that should be provided for the luting agent?

20-40 micrometers (D) Signup and view all the answers

What is the consequence of creating a luting agent space that is too wide?

Reduced retention and resistance of the restoration (B) Signup and view all the answers

What is the main factor that determines the precision of luting agent space?

Materials and techniques used in the indirect process (C) Signup and view all the answers

According to the context, what is the consequence of a defect within 1 mm of the cavity margin?

It may be ignored (C) Signup and view all the answers

What is the primary purpose of using the solid model during the restoration evaluation process?

To check if the restoration fits the patient's mouth accurately. (C) Signup and view all the answers

After firing a porcelain restoration, which step is crucial before stain and glaze applications?

Allowing the restoration to cool down completely. (B) Signup and view all the answers

What is the main reason for applying stains to a restoration, especially for anterior teeth?

To create incisal translucency, mimicking natural teeth. (D) Signup and view all the answers

What is the next step after polishing a porcelain restoration?

Applying the glaze and then the stain. (A) Signup and view all the answers

What is the purpose of the porcelain firing (or crystallization) process?

To solidify the layered porcelain and give it strength. (D) Signup and view all the answers

Which of the following contributes to an increased space for the luting agent between a restoration and the die?

Application of die spacer on the die. (C) Signup and view all the answers

What is the purpose of applying die spacer in the fabrication of a dental restoration?

To increase the space for the luting agent, facilitating cementation. (A) Signup and view all the answers

In the context of wax patterns, what does 'cut back' refer to?

Removing wax to create space for porcelain layering. (A) Signup and view all the answers

Why is it essential to ensure correct occlusal contact on the wax pattern?

To ensure the final restoration will have the correct occlusal contact. (A) Signup and view all the answers

What role does the casting ring liner play in the investment process?

It allows for more expansion of the castings to obtain a larger restoration. (B) Signup and view all the answers

What is the function of the casting ring when using the lost wax technique for dental restorations?

To serve as a container for the investment while it sets. (A) Signup and view all the answers

If a full-contour wax-up is done for a PFM restoration, what is the next step to make space for porcelain?

Cutting back some of the wax from the full-contour. (D) Signup and view all the answers

What is the purpose of airborne particle abrasion on the fitting surface of the restoration?

To remove metal, creating more space for the luting agent. (C) Signup and view all the answers

Study Notes

Laboratory Procedures

The slides are in black ink, doctors' notes are in red.
Mohammad Abushehab wrote the notes, Yazeed Al-Jammal corrected them.
Dr. Islam Abd Alraheam is the presenter, and he has qualifications from the American Board of Operative Dentistry and the Jordanian Board of Restorative Dentistry.

Communicating with the Dental Laboratory

Effective communication between dentist and lab technician is essential for fixed prosthodontics.
The dentist's prescription must include all necessary details, and sometimes direct communication with the lab is needed.
Lab technicians have limitations, so understanding these limitations is crucial for the dentist to make clear requests.
Everyone on the dental team needs to understand what each member can reasonably expect.
Clinicians need in-depth understanding of lab work to make appropriate clinical decisions.
Key elements to include in the prescription are: the material, occlusal/proximal contact type (e.g., heavy, medium, light, or out of occlusion), shade, design specifications (e.g., ridge, modified ridge), and delivery date.

Definitive Casts and Dies

After taking an impression, the lab first makes the definitive cast.
The cast needs precise reproduction of both prepared and unprepared teeth (including adjacent teeth).
Adjacent unprepared teeth should not have voids.
Surfaces related to anterior guidance and occlusion should allow for precise articulation.
Soft tissues relevant to the work should be recorded well in the impression.
The die specifically needs to reproduce the prepared tooth precisely, with no bubbles or voids.
Easily visible structure (0.5-1mm) immediately around the finish line should be apparent on the die.
The die should have a clear finish line that allows good access to the margins.
The lab should create a working model and a solid model.

Materials Science

The ideal cast and die materials display dimensional accuracy and resistance to wear.
Gypsum is the most common material for cast and die fabrication (types 3, 4, 5).
Gypsum expansion properties vary; technicians must be aware of these properties for accurate restorations.
Resin (epoxy resin) is another material, which is less compatible with some impressions and slightly tighter fitting than those made with gypsum.
Electroplated dies are used less frequently.

Fabricating a Crown or Bridge

Separating the prepared tooth from the rest of the arch is essential for precise work and margin replication.
A die (positive replica of the prepared tooth) enables repeated removal and repositioning without any rotation or displacement.

Systems for Separating and Repositioning the Die

Dowel pins: placed in the base, extending through the model with one flat side for anti-rotational reference.
Pindex (whaledent) dowel: uses a machine with a laser pointer to drill the holes for the pins, each pin consists of two joined ones.
Di-Lok (DentiFax/Di-Equi): utilizes a pre-made plastic base for the model.
DVA model system: uses multiple plastic parts with the gypsum model on top for die repositioning.

Ditching the Dies

Trimming the area below the finish line with a round bur helps to give emphasis to the finish line.
The finish line is marked (red/blue marker).

Mounting Casts on Articulator

Accurate bite registration is essential for precise mounting on an articulator.
Final outcome heavily depends on the quality of the bite registration.
Significant occlusal adjustments might be required if the bite is inaccurate.

Before Starting the Waxing Procedure

Correct any undercut imperfections with wax, to prevent final restorations from locking in die, so that areas with undercuts are identified and blocked, ideally, there should be no undercut defects.
Adequate luting agent space (20-40 micrometers) is essential to allow proper seating for the restoration.

Luting Agent Space

A space between the casting and the prepared tooth, except immediately adjacent to the margin.
Ideal space is 20-4 micrometers.
Too narrow: improper seating during cementation.
Too wide: reduced restoration form and longevity.
Various factors influence the luting agent space (e.g., impression material, investment/casting alloys thermal and polymerization shrinkage, expansion of the investment mold).

Wax Patterns

Full-contoured crown wax-ups (except for upper framework only).
Accurate occlusal contacts on the wax pattern are crucial for correct reproduction in the final restoration.

Cutting Back

Preparing the veneering area by cutting back the full anatomical wax-up for porcelain layering.
Removing wax to create space for layering porcelain.
The core material might occasionally be zirconia (radiopaque) rather than metal (translucent).

Lost Wax Technique

Surrounding the wax pattern with heat-resistant investment material, heating to melt the wax, and injecting molten metal into the mold through channels called the sprue.
Small variations can affect the quality of the final restoration.

Casting Machine

Features a rotating arm to introduce molten metal into the cavity.
The arm rotates clockwise a set number of times.
Modern machines might eliminate the need for direct flame.

Metal Casting

The wax pattern is attached to the sprue.
The sprue serves as the channel for molten metal.
The casting ring has a base, upon which the casting ring is placed.
The upper part of the image shows the final casting emerge.
The sprue needs to be cut off.
The metal (e.g., gold) can be remelted and reused.
The finished casting should be smooth, without nodules.

Pressed Ceramics

Similar steps to metal casting, but ceramic material is pressed into the investment mold instead of casting metal.
The sprue channel is cut.

Evaluation of the Casting

Possible casting defects include generalized roughness, nodules (gas trapped), fins (cracked/filled investment), and incompleteness.
Excessive burnout temperature, incorrect investment materials, gas formation in investment mix, distortion during removal of wax, are some causes of issues.
Proper investment and casting technique are crucial, as dimensional issues result from factors like shrinkage/expansion of investment and the presence of die spacers.

Finishing and Polishing the Casting

Polishing all casting surfaces (occlusal, buccal, lingual).
applying glaze for shiny finish (after polishing).
stains (optional) to enhance incisal translucency in anterior restorations.

Porcelain Layering over Metal Core

Layering porcelain over the (metal) core using a powder and liquid form.
Opaque layer, body dentine porcelain, and enamel shade are layered.
Proper layering technique is crucial for creating an aesthetically pleasing outcome.

Porcelain Layering over Zirconia Core

Similar to layering over metal cores.
Zirconia core materials are layered with porcelain.
This creates a restoration closely resembling natural teeth.

Porcelain Firing (Crystallization)

Porcelain needs to be solidified or crystallized in an oven with appropriate precise temperature and time settings.
The restoration is allowed to cool down after firing.

References

Contemporary fixed prosthodontics textbook, Chapters 16, 17, 18, 22, and 29.

Restoration Evaluation on the Solid Model

Final evaluation of the restoration involves checking the fit on a solid model.
The solid model replicates the patient's mouth precisely.
Adjustments might be necessary based on the model's evaluation and any needed modifications.

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Description

Test your understanding of essential concepts in fixed prosthodontics, focusing on dental casts, unprepared teeth, and the significance of accurate impressions. This quiz covers the materials and methods critical for creating effective dental restorations. Ideal for dental students and professionals looking to enhance their knowledge.