Denture Bases in Prosthodontics

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Questions and Answers

What is one of the main functions of a denture base?

Provide nutrition to the wearer

Enhance phonetics

Create a suction effect

Support and retain denture teeth (correct)

Which of the following is NOT a requirement for denture bases?

Absence of taste and odor

High thermal conductivity (correct)

Biocompatible

Strength and durability

What is an advantage of non-metallic denture bases?

Increased risk of allergies

Excellent heat transmission

High mechanical strength

Light in weight (correct)

Which type of acrylic resin is activated by heat?

Heat cured acrylic resin Signup and view all the answers

What is one of the disadvantages of non-metallic denture bases?

Moderate mechanical properties Signup and view all the answers

Which initiator is used in heat cured acrylic resin?

Benzoyl peroxide Signup and view all the answers

What can be a consequence of the residual monomer in chemical-cured acrylic plastics?

Irritation of soft tissues Signup and view all the answers

Which of the following steps is NOT part of the polymerization reaction?

Bending Signup and view all the answers

What are the main components of light-activated acrylic resin?

UDMA and PMMA Signup and view all the answers

What is one of the advantages of using light-activated acrylic resin?

No MMA monomer found Signup and view all the answers

What is the primary disadvantage of light-activated acrylic resin?

Lower mechanical and physical properties Signup and view all the answers

What is the first step in the compression molding denture base construction technique?

Flasking procedure Signup and view all the answers

What is the purpose of the separating medium in the flasking process?

To prevent the attachment of plaster to the cast Signup and view all the answers

How is the wax eliminated during the denture processing?

By immersing the flask in boiling water Signup and view all the answers

What process follows the dewaxing step in denture base construction?

Curing Signup and view all the answers

Which denture processing technique is the most common?

Compression molding Signup and view all the answers

What is the proportion of polymer to monomer during mixing?

3:1 Signup and view all the answers

What characterizes the sticky stage of the mixing process?

The mixture is stringy and adheres to surfaces. Signup and view all the answers

What is the main goal of packing the denture base resin?

To completely fill the mold cavity without voids. Signup and view all the answers

What is the result of under-packing during the process?

Internal voids in the denture base. Signup and view all the answers

What is the main purpose of the curing cycle in the denture-making process?

To control the polymerization reaction of the dough. Signup and view all the answers

What is an important step to avoid excessive heat production during finishing and polishing?

Water cooling the polishing wheel. Signup and view all the answers

What can cause porosity in the denture base?

Under-packing the mold cavity. Signup and view all the answers

Which stage does the mass become more cohesive and rubber-like with evaporating monomer?

Rubbery stage Signup and view all the answers

What is one of the causes of crazing in denture bases?

Alternative drying and wetting Signup and view all the answers

Which property of acrylic denture bases primarily affects their strength?

Brittleness on impact Signup and view all the answers

What can happen if the flask is cooled too rapidly in water?

Warpage of the denture Signup and view all the answers

What occurs due to the differences in thermal expansion coefficients between acrylic and porcelain?

Crazing of denture bases Signup and view all the answers

How can the mechanical properties of acrylic denture bases be enhanced?

Adding carbon and glass fibers Signup and view all the answers

What is the recommended condition for storing dentures when not in use?

Maintain a wet environment Signup and view all the answers

Which property makes acrylic denture bases good insulators?

Low thermal conductivity Signup and view all the answers

What type of reaction can arise from methyl methacrylate monomer in dentures?

Immediate allergic reaction Signup and view all the answers

Study Notes

Denture Base Definition

The denture base is the part of the denture that rests on the tissues and supports the artificial teeth.

Denture Base Functions

Supports and retains denture teeth.
Distributes stress.
Improves aesthetics.

Denture Base Requirements

Strength and durability.
Satisfactory thermal properties.
Processing accuracy & dimensional stability.
Chemical stability.
Insolubility in oral fluids.
Absence of taste and odor.
Biocompatible.

Advantages of Non-Metallic Denture Bases

Color matches gingival tissues.
Easy to construct and repair.
Easy to finish and polish.
Easy to reline.
Lightweight.
Not expensive.

Disadvantages of Non-Metallic Denture Bases

Moderate mechanical properties.
Can be scratched.
Do not transmit heat.
May cause allergies in patients.
Crazing and bleaching may occur.
Bacterial and fungal colonization may occur.

Types of Acrylic Resin

Heat-Cured Acrylic Resin
- Polymerized with heat.
Chemical-Cured Acrylic Resin
- Polymerized with chemical activation at room temperature.
- Also known as self-curing, cold-curing, or autopolymerizing resins.
Light-Cured Acrylic Resin
- Polymerized with light activation using a blue light source.

Polymerization Reaction Steps

Initiation: The initiator is activated by heat, light, or chemicals, producing free radicals.
Propagation: Each free radical reacts with a monomer, creating a new free radical, repeating the process.
Termination: Two growing chains react to form the polymer.

Heat-Cured Acrylic Denture Base Ingredients

Powder

Pre-polymerized poly-methyl methacrylate (PMMA).
Initiator: Benzoyl peroxide.
Plasticizers: Make the material resilient.
Pigments: Provide various tissue-like shades.
Acrylic fibers: Simulate minute blood vessels.
Glass fibers: Increase stiffness.

Liquid

Monomer: Methyl methacrylate.
Inhibitor: Prevents premature polymerization.

Chemical-Cured Acrylic Denture Base

The liquid contains a chemical activator, tertiary amine, that reacts with the peroxide initiator, producing free radicals to initiate polymerization at room temperature.

Chemical-Cured Acrylic Denture Base Uses

Denture bases.
Repair of broken dentures.
Special trays.

Chemical-Cured Acrylic Denture Base Advantages

Denture base fabrication in a short time.

Chemical-Cured Acrylic Denture Base Disadvantages

Lower mechanical properties, particularly stiffness.
Higher porosity.
Color instability due to tertiary amine.
Unreacted monomer causes two major difficulties:
- Acts as a plasticizer, decreasing strength.
- Irritates the patient's soft tissues.

Light-Activated Acrylic Resin Composition

UDMA (Urethane dimethacrylate) and PMMA.
A photoinitiator, such as camphorquinone, is added.

Light-Activated Acrylic Resin Properties

When exposed to blue visible light, free radicals are released.
A sheet of the material is adapted to the working cast and cured for 5 minutes in a light-curing unit.
Rope material is adapted as a horseshoe over the cured base, and artificial teeth are arranged over it.
Curing for another 5 minutes occurs.

Light-Activated Acrylic Resin Applications

Denture bases.
Repair of broken dentures.
Special trays.

Light-Activated Acrylic Resin Advantages

No MMA monomer is found, reducing the possibility of allergic reactions.
Lower rate of polymerization reaction, leading to better denture base fit.
Short processing time with no need for flasking.

Light-Activated Acrylic Resin Disadvantages

Lower mechanical and physical properties.

Denture Processing Techniques

Compression molding: Most common technique.
Injection molding.

Denture Base Construction Using Compression Molding

Flasking Procedure: The process of investing the cast with the waxed denture in a flask to create a mold for the acrylic resin denture base.
- The inner surface of the flask is coated with Vaseline.
- The base of the cast is painted with separating medium.
- The first layer of gypsum investment is poured into the lower half of the flask, and the cast is placed on top.
- After the first gypsum investment layer sets, a separating medium is applied to prevent the second layer from sticking.
- The body of the flask is put in place, and the second mix of gypsum investment is placed on top of the first layer, covering the wax, denture base, and teeth.
- The lid is placed onto the body.
Wax Elimination: The flask is immersed in boiling water (100°C) for 5 minutes to remove the wax pattern, creating a mold space for the acrylic to fill.
- A separating medium is applied to the dewaxed mold space prior to packing the acrylic resin.
Proportioning and Mixing: The powder and liquid are mixed in a 3:1 ratio with a stainless steel spatula, kept in a sealed glass jar during initial stages to prevent monomer evaporation.
- The mixture goes through five stages:
  - Sandy Stage: Polymer settles into the monomer, forming a grainy mass.
  - Stringy or Sticky Stage: The mixture becomes stringy and sticky.
  - Dough Stage: The mix becomes smooth and dough-like.
  - Rubbery Stage: The mass becomes more cohesive and rubber-like.
  - Stiff Stage: The monomer evaporates, and the mass becomes stiff.
Packing: The denture base resin is introduced into the mold cavity at the dough stage.
- Over-packing (too much material) results in excessive thickness and misaligned teeth.
- Under-packing (too little material) results in denture base porosity.
- To minimize over-packing and under-packing:
  - Pack resin in the dough-like stage.
  - Shape and adapt the resin over the teeth.
  - Place a polyethylene sheet over the resin.
  - Press the flask closed until metal-metal contact is achieved.
  - Open the flask, remove excess resin, and close the flask again.
  - Repeat until there is no excess resin.
Curing: The flask is heated to control the polymerization reaction of the dough, transforming methyl methacrylate into Poly-methyl methacrylate.
Cooling & Deflasking: The flask is cooled slowly, then submerged under tap water for 15 minutes. The denture is then carefully removed from the flask.
Finishing & Polishing: A polishing wheel and a suspension of fine pumice in water are used. Water cooling is essential to avoid excessive heat production and denture warpage.

Defects During Denture Processing

Porosity: Small voids present internally or externally on the denture surface.
- Causes:
  - Excessive or elevated curing temperature, causing monomer boiling.
  - Decreased powder/monomer ratio.
  - Lack of pressure during packing and curing.
  - Under-packing.
  - Increased powder/monomer ratio.
  - Packing in the sandy stage.
Warpage: Change of the denture's shape due to dimensional changes and internal stress release.
- Causes:
  - Too rapid cooling of the flask in tap water.
  - Excessive heat release during polishing.
Crazing: Small linear cracks that appear on the denture surface.
- Causes:
  - Alternating drying and wetting of the denture.
  - Thermal stresses due to differences in the coefficient of thermal expansion between acrylic denture base and porcelain teeth.

Properties of Acrylic Denture Bases

Mechanical Properties:
- Heat-cured acrylic resins have higher mechanical properties than chemically cured resins.
- Acrylic resins are generally low in strength and brittle.
- They are fairly resistant to fatigue during repeated bending.
- Incorporation of fibers, such as carbon and glass, can strengthen the denture base.
Thermal Properties:
- Poor thermal and electrical conductors.
- Low thermal conductivity acts as an insulator between the oral tissues and hot or cold materials.
- High thermal coefficients of expansion can lead to cracking of the denture base attached to porcelain teeth due to differences in expansion rates.
Water Sorption:
- Water sorption increases the resin's weight, causing expansion.
- This expansion compensates for polymerization shrinkage.
- Drying out the resin causes shrinkage, so dentures should be kept wet when not in use.
Tissue Compatibility:
- Completely polymerized PMMA rarely causes allergic reactions.
- Methyl methacrylate monomer or other trace components might produce an allergic reaction.
- Allergic reactions tend to be immediate and are more likely to occur with chemically cured resin.

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Description

Explore the essential aspects of denture bases, including their definition, functions, requirements, and advantages and disadvantages of non-metallic materials. This quiz will enhance your understanding of acrylic resins used in prosthodontics, highlighting their properties and applications in dental practices.