Dental Impression Materials Quiz

Impression Materials

• Set impressions are negative reproductions of oral tissues. A positive reproduction is achieved by pouring a suitable model or die material into the set impression.

Desirable Properties for Impression Materials

• Adequate shelf life: Impression materials should last a sufficient time before use.

• Acceptable odour and taste: The material should not be overly pungent or unpleasant in taste.

• Biologically acceptable: The material shouldn't contain toxic or irritating constituents.

• Easily manipulated: The material should be easy to work with minimal equipment.

• Setting characteristics:

  • Suitable working time: The time the material is workable.
  • Suitable setting time: The time it takes for the material to set.
  • Readily wets oral tissues: The material should have good contact with the tissues
  • Not affected by oral fluids: The material should not react with saliva or other oral fluids.
  • Does not release gas during setting: The material should not generate gases during setting.

• Properties of the set material:

  • Sufficient mechanical strength: Should not break or tear on removal from the mouth.
  • Elastic properties: Should not deform permanently after a strain.
  • Good dimensional accuracy: Should create an accurate representation of the oral tissues.
  • Dimensional stability: Stable over temperature and humidity ranges encountered in clinical and lab settings.

• No ideal impression material satisfies all requirements. Clinicians must select the best material for the clinical situation and techniques.

Classifications of Impression Materials

• Method of setting:

  • Hardening by chemical reaction (thermoset): Plaster of Paris, alginates, and rubber impression materials (irreversible).
  • Softening and hardening by heat (thermoplastic): These materials are reversible.

• Behavior after setting:

  • Non-elastic: Used when there are no undercuts (e.g., Plaster of Paris, Zinc oxide eugenol).
  • Elastic: Used when undercuts exist (e.g., hydrocolloids, rubber impression materials).

• Classification tree displays various impression materials, and their characteristics:

  • Non-Elastic: Waxes, Plaster, Zinc-Eugenol compound
  • Elastic: Alginate, Agar, Hydrocolloid, Rubber base (Elastomers): Poly-sulfides, Silicone (Addition, Condensation), Polyether

Non-Elastic Impression Materials

• Wax: A pliable material used in various dental procedures.
• Impression plaster (Plaster of Paris): A common material for creating models and casts.
• Impression compound: A material used for taking impressions.
• Zinc oxide-eugenol impression material: A compound used in dental practices.

Hydrocolloids

• Definitions: Solutions, suspensions, colloids

• Sol to Gel Transformation:

  • Reduction in temperature
  • Chemical reaction: Agar impression material, Alginate impression material

• Dimensional Effects by Syneresis and Imbibition:

  • Syneresis: Loss of water, leading to shrinkage.
  • Imbibition: Absorption of water, leading to expansion

Alginate Impression Material

• Advantages: Easy to mix and manipulate, minimal equipment, flexible, accurate, inexpensive
• Disadvantages: Limited material options for the models and die material, not accurate in fine details replication
• Uses: Preparing study casts, preparing gypsum models, removable partial denture, athletic mouth protectors
• Storage: Stored in 100% relative humidity to prevent syneresis/imbibition and shrinkage/expansion
• Manipulation Equipment: Powder-dispensing cup, water-dispensing cup, rubber mixing bowel, spatula

Agar Impression Materials

• Advantages: Excellent accuracy, highly accurate, elastic
• Disadvantages: Difficult manipulation, dimensional instability, patient discomfort from thermal shock
• 3-chamber conditioning unit: Boiling, storage, tempering

Rubber Base Impression Materials (Elastomers)

• Definition: Non-aqueous, elastomeric materials that are rubber-like, formed by long chain coiled molecules

• Advantages: High accuracy, more dimensional stability than Agar and alginate, easy manipulation, can be used with undercuts, can be electroplated

• Consistencies: Low (Syringe/wash), Medium (Regular), High (Tray), Putty (Extra high)

• Mixing systems: Hand mixing, Auto-mixing, and Dynamic mechanical mixing

• Types of Elastomeric Impression Materials: Polysulfides, Silicones (Addition, Condensation), and Polyether

Polysulfides

• Advantages: High dimensional stability, high strength and tear resistance, can be electroplated
• Disadvantages: Dimensional instability (water by-product), hydrophobic, bad odor (sulfide odor), staining of clothes. Relatively long setting time

Silicones

• Condensation silicones: Solve many problems of polysulfides. Advantages: moderate working time, better odor and taste, less permanent deformation. Disadvantages: hydrophobic, dimensional instability (alcohol by-product)
• Addition silicones: Advantages: accuracy, Dimension stability. Disadvantages: less flexible, lower flow

Polyethers

• Advantages: Better mechanical properties than polysulfides, less dimensional change than condensation silicone.
• Disadvantages: High working time, high stiffness, high cost
• Properties: High accuracy, dimensional stability with 0.25% shrinkage within 24 hours, least flexibility of all rubbers, viscoelastic with 98.9% elastic recovery, excellent compatibility, better tear strength than silicone, but less than polysulfides, Disinfectants compatible

Recent Advances in Impression Materials

• Optical Impression:
  • Advantages: No need for materials or trays, impressions are transmitted and stored electronically, no mess, 3D/360° view of preparation is easily maneuvered, adjustments are made to virtual impressions (no need for second impression), no disinfection required.
  • Disadvantages: High start-up cost, technique sensitivity, and requires more equipment.

Test your knowledge on various types of dental impression materials and their characteristics. This quiz covers essential aspects such as elasticity, setting methods, and advantages and disadvantages of each type. Perfect for dental students and professionals looking to refresh their understanding.