Dental Impression Materials

Primary Impression and Uses

• Uses of primary impression:
  • Primary cast (diagnostic, study)
  • Special tray
• Types of trays:
  • Stock tray
  • Special tray

Secondary Impression and Uses

• Uses of secondary impression:
  • Secondary cast (working, master)
  • Final applications

Hydrocolloids

• Forms of hydrocolloids:
  • Reversible (Agar-Agar)
  • Irreversible (Alginate)
• Synersis and imbibition lead to dimensionally unstable in storage
  • Synersis: shrinkage due to evaporation of water
  • Imbibition: swelling of the gel due to absorption of water

Casting and Impressions

• Immediate casting within 10 minutes
• To solve tear strength problem in hydrocolloids:
  • Thick sections (4-6 mm)
  • Sharp snap removal
• Disinfection of hydrocolloids:
  • Sprays (Iodophor or glutaraldehyde)
  • Sealed plastic bag with paper towels

Gypsum Products

• Calcination reactions for type 1, 2, 3, 4, and 5 gypsum products
• Particles size and shape:
  • Type 1 and 2: irregular and porous
  • Type 3: regular and less porous
  • Type 4 and 5: most regular and least porous
• Chemical W/P ratio: 18.61 g for 100 g of caso4.½H20
• W/P ratio for different types of gypsum products

Manipulation of Gypsum

• Armentarium:
  • Rubber bowl
  • Stiff plastic spatula
• Manipulation steps:
  1. Proportioning
  2. Mixing
  3. Pouring
• Vigorously advantages:
  • Increase powder wetting
  • Press air bubbles out
• Visual inspection of final mix: glossy surface, smooth and creamy consistency

Investment Materials

• Strength of investment materials:
  • Silicate > Phosphate > Gypsum
• The type of ring with:
  • Gypsum bonded investment: metallic ring with wet paper
  • Phosphate and silicate bonded investments: rubber ring (removed after setting)

Casting Technique

• Steps of casting technique:
  1. Formation of wax pattern
  2. Spruing the pattern
  3. Investing
  4. Wax elimination
  5. Casting
  6. Finishing and polishing
• Enumerate wax pattern methods:
  1. Direct on a patient mouse (accurate)
  2. Indirect on a die (easy)
  3. Indirect-direct (fabrication on a die and adjust in the mouth)

Sprueing

• Enumerate the rules of sprue:
  1. Allow safe handling of wax
  2. Create a channel
  3. Act as a reservoir
• Enumerate sprue materials:
  1. Wax (the best)
  2. Plastic
  3. Metal filled with sticky wax
• Sprue attachment:
  • To the thickest portion
  • 45⁰ to the proximal surface

Elastomers

• Enumerate the tear strength influences:
  1. Strain rate
  2. Rapid rate of removal
• Tear strength of elastomers:
  • Poly sulfide > Poly ether > Silicones

Dental Casting Alloys

• Classification based on:
  • Joining, working requirements, and sag resistance
  • Mechanical properties
  • Specific use
• General classifications:
  • ADA specification No. 1 (High type)
  • ADA specification No. 2 (Noble type)
  • ADA specification No. 3 (Predominantly base metal type)

Gold Alloys

• General composition:
  • Noble metals and their effect on properties
  • Effect of Gold (Au) on properties:
    • High noble metal resists tarnish and corrosion
    • Low strength, hardness, and high ductility
  • Effect of platinum:
    • Increases tarnish and corrosion resistance, strength, and hardness
  • Effect of palladium:
    • Whitens the color of gold alloy
    • Decreases the greening effect of silver and redness effect of copper
  • Effect of copper:
    • Increases strength and hardness
    • Reduces porosity and rough surface casting development

Classification of Gold Alloys

• Classification based on gold content:
  • Carat and Fineness
  • Carat refers to the parts of pure gold in 24 parts of alloy
  • Fineness refers to the parts of pure gold per thousand parts of alloy

Note: These study notes cover the key points and concepts mentioned in the provided text.### Critical Ratios in Dental Gold Alloys

• 1% palladium for every 3% silver to balance the tarnish tendency of silver
• Importance of careful balance: to prevent tarnish of the alloy
• Consequences of imbalance: tarnish of the alloy

Properties of Economy Gold Alloys

• Comparable to Type III and Type IV gold alloys
• Replacement for reduced gold: palladium, silver, and other metals

Casting Ring Selection

• Factors to consider: investment material strength, expansion, and alloy to be cast (solidification shrinkage)
• Functions of a ring liner: creates space for investment, allows setting of investment under water, and facilitates removal of investment from the casting ring

Wax Pattern Handling

• Importance of immediate investment: to avoid distortion due to release of internal stresses affected by time and temperature

Investment Mixing Techniques

• Techniques: regular mix investment technique and double mix investment technique (improves wettability & avoids distortion)
• Use of wetting agent: to reduce contact angle and provide better wetting of investment

Investment and Casting Procedures

• Steps: casting ring and liner, investing, wax elimination, casting, devesting, finishing, and polishing
• Importance of gradual heating during wax elimination: to vaporize all traces of wax and allow for thermal expansion of the investment
• Consequences of improper selection of investment type: dimensional inaccurate casting
• Purpose of heating/burnout: to vaporize all traces of wax and allow for thermal expansion of the investment

Melting and Casting

• Requirements: heat source to melt the alloy and casting force to drive the alloy into the mold
• Importance of melting machine selection: to provide the necessary heat source to melt the alloy
• Consequences of under-heating: incomplete casting and rounded margin
• Consequences of overheating: possible volatilization of the alloy and decomposition of the investment

Casting Machines

• Types: air pressure casting machines using compressed air, centrifugal casting machine, and a combination of both with an attached vacuum system
• Consequences of failing to select the proper casting machine: incomplete casting with rounded margins

Post-Casting Procedures

• Recommended process: leaving the casting ring and contents standing until they have lost all traces of redness, then quenching in cold water
• Resolution of dark appearance of gold casting due to oxide deposits: immersion in warm HCl solution until lustrous gold color appears

Dental Casting Alloys

• Uses: constructing indirect metallic restorations like inlays, onlays, crowns, bridges, and removable partial denture frameworks
• Functional requirements: stiffness and resilience
• Definition of stiffness: resistance to elastic deformation, allowing equal distribution of stresses
• Importance of resilience: to support the brittle porcelain veneer by absorbing energy without plastic deformation

Properties of Dental Casting Alloys

• Importance of high yield strength: to resist permanent deformation under masticatory stresses in the mouth
• Importance of high fatigue strength and fatigue limit: to resist cyclic loading
• Importance of high tarnish and corrosion resistance: to prevent dissolution in oral fluids and liberation of toxic corrosion products
• Compatibility with porcelain: in terms of coefficient of thermal expansion, elastic modulus, and forming of oxides to help bonding

Fit and Working Requirements

• Definition of fit: the ability of the casting to reproduce accurately the pattern from which it is constructed
• Working requirements: ease of casting, ease of soldering, and ease of burnishing
• Importance of high ductility: for proper marginal adaptation and adjustment of the restoration
• Definition of sag resistance: the ability of the alloy to resist plastic flow under its own weight at high temperatures### Setting Reaction of Gypsum
• The temperature rise during the reaction of hemihydrate and water indicates setting, but it is not an accurate way to measure setting time.

Penetration Test

• The penetration test is used to determine if the gypsum has reached a measurable strength.

Vibrate and Gillmore Tests

• The Vicate test measures the initial setting time of gypsum.
• The Gillmore test measures the initial and final setting times of gypsum.

Controlling Setting Rate of Gypsum

• The setting rate of gypsum can be controlled by altering the solubility of the hemihydrate, changing the number of nuclei of crystallization, and varying the rate of crystal growth.

Effects of Solubility, Nuclei of Crystallization, and Crystal Growth on Setting Time

• Increasing the solubility of the hemihydrate achieves supersaturation of the dihydrate faster, accelerating the rate of dihydrate crystal deposition and decreasing the setting time.
• Increasing the number of nuclei of crystallization leads to faster formation of dihydrate crystals, decreasing the setting time.
• Faster crystal growth rate decreases the setting time.

Fineness of Hemihydrate Powder and Setting Time

• The finer the hemihydrate particles, the faster the rate of dissolution, increasing the number of nuclei of crystallization formed and decreasing the setting time.

Impurities as Nucleating Agents

• A small amount of CaSO4.2H2O (Terra alba) can be added as a nucleating agent to accelerate the setting time.

Effect of Moisture Contamination on Hemihydrate Particles

• Moisture contamination during storage can change some hemihydrate particles into dihydrate, decreasing the number of nuclei of crystallization and setting time.

Retarders and Their Action

• Retarders form an adsorbed layer on the hemihydrate or on the growing dihydrate crystals, inhibiting their growth and increasing the setting time.

Mechanical Properties of Denture Base Materials

• High modulus of elasticity, adequate resilience, adequate strength, high impact strength, high fatigue strength, high flexure strength, hardness, and good abrasion resistance are required.

Importance of Modulus of Elasticity

• A high modulus of elasticity ensures rigidity and the ability to be made in thin sections while evenly distributing stresses.

Function of Plasticizers and Dyed Organic Fibers

• Plasticizers improve polymer resiliency.
• Dyed organic fibers simulate blood vessels.

Purpose of Pigments and Opacifier

• Pigments give a natural tissue-like appearance.
• The opacifier imparts radioopacity.

Liquid Monomer and Inhibitor in Acrylic Resin Denture Base Materials

• Methyl methacrylate (MMA) is the liquid monomer used.
• The inhibitor prevents the polymerization of the monomer during storage.

Cross-Linking Agent and Initiator in Acrylic Resin Denture Base Materials

• The cross-linking agent improves mechanical properties.
• The initiator is a chemical activator (N,N dihydroxy ethyl para-toluidine).

Polymerization Reaction for Heat-Cured Resin

• Polymer Peroxide Initiator and Monomer Inhibitor.

Components of Self-Cured Resins

• Powder, Liquid, Polymer Peroxide initiator, Monomer Inhibitor, and chemical activator.

Nature of Polymerization Process and Shrinkage

• The polymerization process is exothermic and accompanied by polymerization shrinkage.

Issues with Self-Cured Resins and Residual Monomer

• Self-cured resins are not used for construction of denture bases due to the greater amount of residual monomer and limited to denture repair.
• There is always a residual monomer remaining after polymerization, and its amount depends on the degree of polymerization.

Effects of Excess Free Monomer in Acrylic Resin

• Excess free monomer can release from the denture and irritate tissue.
• It can act as plasticizers and make the resin flexible and weak.
• It may evaporate leaving porosity which decreases mechanical properties.

Thermal Shrinkage and Linear Expansion

• Thermal shrinkage of the resin as the denture cools from the processing temperature to room temperature is 0.44%.
• Linear expansion of the resin for each 1% increase in weight due to water sorption is 0.23%.

Solubility and Mechanical Properties of Acrylic Resin Denture Base Materials

• Acrylic resin denture base material is insoluble in most fluids that may come into contact with it in the oral cavity.
• It has low mechanical properties due to the presence of weak secondary bonds.

Impact Strength and Overcoming Brittleness

• The impact strength of conventional acrylic denture base is low, and the denture will fracture when accidentally dropped.
• Incorporating butadiene styrene rubber inserts can overcome the low impact strength.

Gypsum Products in Dentistry

• Gypsum products are used to create models or casts.
• The chemical composition of gypsum is calcium sulphate dihydrate (CaSO4.2H2O).

Characteristics of Gypsum Products

• High strength to resist breakage and hardness to resist scratching.
• Ability to reproduce fine details and sharp margins.
• Little dimensional change on setting and on storage.
• Compatibility with impression materials (no interaction with the surface of the impression and no need for a separating medium before pouring the impression).
• Good color contrast with other materials (e.g., impression materials).

Classification of Gypsum Products

• Plaster, Impression Plaster (Plaster of Paris), and Model Plaster (Calcined Gypsum) according to ADA specification No. 25.

Dental Stone and Hydrocal Hemihydrate (ISO Type III)

• High strength, low expansion, improved stone (ISO Type IV), high expansion (Type V).
• Characteristics: high strength, low expansion, improved stone (ISO Type IV), high expansion (Type V).