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Questions and Answers
What is imbibition primarily associated with in the context of hydrocolloids?
What is imbibition primarily associated with in the context of hydrocolloids?
Which hydrocolloid is known for poor accuracy and limited shelf life?
Which hydrocolloid is known for poor accuracy and limited shelf life?
What disadvantage is associated with polysulfide elastomers?
What disadvantage is associated with polysulfide elastomers?
What is a significant characteristic of polyether materials in dentistry?
What is a significant characteristic of polyether materials in dentistry?
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Polyvinyl siloxane (PVS) is known for which of the following advantages?
Polyvinyl siloxane (PVS) is known for which of the following advantages?
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What affects the setting time of alginate materials?
What affects the setting time of alginate materials?
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Which material requires the teeth and gingival sulci to be dried before use due to its hydrophobic nature?
Which material requires the teeth and gingival sulci to be dried before use due to its hydrophobic nature?
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What is a unique advantage of modified alginate materials?
What is a unique advantage of modified alginate materials?
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What is a requirement for an ideal impression material?
What is a requirement for an ideal impression material?
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Which property allows impression materials to replicate fine intraoral surface details?
Which property allows impression materials to replicate fine intraoral surface details?
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What type of impression material is Alginate classified as?
What type of impression material is Alginate classified as?
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In terms of surface wetting, hydrophilic materials are characterized by which of the following?
In terms of surface wetting, hydrophilic materials are characterized by which of the following?
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What is the ideal characteristic of tear resistance in impression materials?
What is the ideal characteristic of tear resistance in impression materials?
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Which of the following is true regarding elastomeric impression materials?
Which of the following is true regarding elastomeric impression materials?
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Which impression material is traditionally best suited for edentulous cases?
Which impression material is traditionally best suited for edentulous cases?
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What characteristic defines hydrophobic materials in relation to impression making?
What characteristic defines hydrophobic materials in relation to impression making?
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What is the primary purpose of a primary (diagnostic) impression in fixed prosthodontics?
What is the primary purpose of a primary (diagnostic) impression in fixed prosthodontics?
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Which type of impression tray is designed for capturing both arches simultaneously?
Which type of impression tray is designed for capturing both arches simultaneously?
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What material is typically used to create a positive replica from a dental impression?
What material is typically used to create a positive replica from a dental impression?
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Which tray type is more likely to avoid distortion during the impression process?
Which tray type is more likely to avoid distortion during the impression process?
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What is a significant characteristic of custom trays compared to stock trays?
What is a significant characteristic of custom trays compared to stock trays?
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What is the role of perforated trays in impression making?
What is the role of perforated trays in impression making?
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Which of the following is NOT typically a purpose of dental impressions in fixed prosthodontics?
Which of the following is NOT typically a purpose of dental impressions in fixed prosthodontics?
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What is used to mark the end of the spacer when fabricating a custom tray?
What is used to mark the end of the spacer when fabricating a custom tray?
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Study Notes
Impressions in Fixed Prosthodontics
- Accurate impressions are essential for creating dental restorations like crowns, bridges, and implants.
- Impressions are negative replicas of teeth and surrounding structures used to create positive replicas (casts).
- The cast serves as the base for fabricating the restoration.
Types of Impressions
- Primary (Diagnostic) Impression: Used for initial diagnosis and construction of custom trays.
- Secondary Impression: Used to obtain a master (definitive) cast for the final restoration.
- Tray: A device holding the impression material.
Types of Impression Trays
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Stock Trays (Prefabricated): Pre-made trays available in various sizes, made of plastic, aluminum, or stainless steel.
- Plastic/Aluminum Trays (non-rigid): Can cause distortion
- Stainless Steel Trays (rigid): Help avoid distortion
- Custom/Special Trays: Individualized trays made on a diagnostic cast, offering more accurate adaptation.
Custom Tray Fabrication
- Marking: A line is drawn on the diagnostic cast to mark the desired tray extension, typically 3-5 mm from the gingival margins.
- Baseplate Wax: Two layers of baseplate wax are applied to the cast and trimmed along the marked line.
- Tray Stops: Wax is removed to create tray stops, ideally on non-functional cusps to prevent the tray from seating too deeply.
- Tin Foil: Thin tin foil is adapted to the wax to create a smooth surface for the resin.
- Resin Adaptation: Resin material is applied to the cast, excess material is trimmed, and a handle is added.
Tray Selection
- Coverage: The tray should cover all surfaces of the teeth to be included in the impression.
- Wall Distance: Sufficient distance should be maintained between the tray wall and the teeth on both sides.
- Narrow trays: Can hinder adequate seating of the tray.
Impression Materials
Requirements for an Ideal Impression Material
- Biocompatibility: Safe for the patient.
- Patient comfort: Acceptable taste, odor, and appearance.
- Ease of use: Easy to handle and manipulate.
- Working and Setting Time: Adequate time for manipulation and setting.
- Disinfection: Can be disinfected without altering its properties.
Properties of Ideal Impression Materials
- Dimensional stability: Maintains its size and shape over time.
- Accuracy: Replicates fine details of the intraoral structures.
- Tear Resistance: Resists tearing, especially in thin sections like those within the gingival sulcus.
- Wettability: Ability of the material to spread evenly and completely cover the surface.
Hydrophilic vs Hydrophobic Materials
- Hydrophilic Materials: Have a high affinity for moisture, allowing for good surface wetting and excellent detail reproduction.
- Hydrophobic Materials: Have a low affinity for moisture, resulting in poor surface wetting and less detailed reproduction.
Classification of Impression Materials
- Non-Elastic (Rigid): Materials that set hard and cannot be removed without breaking (e.g., plaster of Paris, impression compound, zinc oxide eugenol, waxes).
- Elastic: Materials that can be deformed and regain their shape (e.g., hydrocolloids, elastomers).
Non-Elastic Impression Materials
- Plaster of Paris (Type I Gypsum): Used for preliminary impressions, but not widely used in fixed prosthodontics due to its brittleness.
- Impression Compound: Used primarily for edentulous cases due to its brittleness.
- Zinc Oxide Eugenol Impression Material: Used mainly for study models; not commonly used for fixed prosthodontics.
- Waxes: Used for temporary impressions and custom tray fabrication.
Types of Elastomeric Impression Materials
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Aqueous (Hydrocolloids): Water-based materials that set to a gel-like consistency.
- Agar (Reversible): Requires heating and cooling to transition between sol and gel states.
- Alginate (Irreversible): A commonly used material for diagnostic impressions that sets permanently.
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Non-Aqueous Elastomers: Materials based on silicone and polyether, do not use water for setting.
- Polysulfide: Contains lead peroxides as a catalyst; available in different viscosity ranges.
- Condensation Silicone: Sets through a condensation reaction, releasing a byproduct.
- Addition Silicone (PVS): Sets through an addition reaction, no byproduct formation.
- Polyether: Hydrophilic, offers dimensional stability and accuracy.
Hydrocolloid Materials
Alginate (Irreversible Hydrocolloid)
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Advantages:
- Material of choice for diagnostic impressions.
- Easy to use.
- Rapid setting time.
- Hydrophilic.
-
Disadvantages:
- Poor accuracy and detail reproduction.
- Low tear resistance.
- Not suitable for definitive casts.
- Must be poured immediately (within 30 minutes) due to imbibition and syneresis.
- Limited shelf life.
Factors Affecting Alginate Setting Time
- Water Temperature (Hot/Cold): Impacts setting time.
- Water Amount: Too much or too little water can alter setting time.
Modified Alginate
- A type of alginate incorporating silicone polymer, providing improved detail reproduction, tear resistance, and dimensional stability.
Non-Aqueous Elastomers
Polysulfide
- Characteristics: Brown in color, sets with the aid of lead peroxides, available in different viscosities.
- Advantages: High tear resistance.
-
Disadvantages:
- Unpleasant sulfide odor. - Messy. - Relatively long setting time (about 10 minutes). - Hydrophobic, requiring meticulous drying of the prepared surfaces.
Condensation Silicone
- Characteristics: Supplied as putty, heavy body, and light body.
-
Advantages:
- Odorless. - Shorter setting time (6-8 minutes). - Adequate tear resistance. -
Disadvantages:
- Hydrophobic, requiring thorough drying of the prepared surfaces.
- Alcohol byproduct (shrinkage).
- Should be poured immediately.
Addition Silicone (PVS)
- Characteristics: Supplied as putty, heavy body, regular body, and light body. Automix versions are available to reduce waste, improve mixing, and minimize air bubbles.
-
Advantages:
- No byproduct. - Highest accuracy and detail reproduction. - Excellent dimensional stability and tear resistance.- Available in a hydrophilic form.
-
Disadvantages:
- Costlier than other materials.
- Sensitive to sulfur in latex gloves, requiring vinyl or nitrile gloves for mixing.
Polyether
- Characteristics: Hydrophilic, available in automix versions.
-
Advantages:
- Excellent detail reproduction.
- No byproduct.
- Excellent dimensional stability.
- Short setting time (about 5 minutes).
-
Disadvantages:
- Stiffness can lead to broken stone casts during separation from the impression.
- Expensive.
- Susceptible to moisture absorption (imbibition), requiring dry storage.
Polyvinyl Ether Silicone (PVES)
- Characteristics: A combination of vinyl polysiloxane (VPS) and polyether (PE) elastomers.
-
Advantages:
- Hydrophilic.
- Highest accuracy and detail reproduction.
- Excellent flow.
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Description
This quiz explores the various types of impressions used in fixed prosthodontics, including primary and secondary impressions. It covers the importance of accurate impressions in creating dental restorations like crowns and bridges, as well as the differences between custom and stock impression trays. Test your knowledge on the techniques and materials used in the dental field.
