Impression Materials in Dentistry

Impression Materials Overview

• Impression materials are utilized to create negative reproductions (impressions) and positive reproductions (casts).
• Accurate copying of the oral cavity is essential for effective dental procedures.

Setting Mechanism

• Irreversible setting indicates materials cannot revert to their original state.
• Reversible setting signifies no chemical change occurs during the hardening process.

Classification of Impression Materials

• Study casts are made during the fabrication of individual trays.
• Impression materials categorized by their hardening method: either thermoplastic or thermoset.
• Classification based on physical properties: rigid/inelastic for edentulous patients & elastic for dentate patients.

Types of Impression Materials

• Thermoset (Irreversible):
  • Includes Plaster of Paris, ZOE paste, and rubber impression materials like polysulfide, polyether, condensation silicone, addition silicone.
• Thermoplastic (Reversible):
  • Comprises modeling compounds and agar.

Manner of Withdrawal

• Rigid materials require teasing movement while elastic materials need a sudden pull parallel to the tooth's long axis.

Impression Trays

• Various types of impression trays:
  • Stock trays
  • Rim-locked trays
  • Individual trays
  • Perforated trays
  • Water-cooled trays

Impression Compound

• Has multiple names and classifications based on usage.
• Can be dispensed in various colors.
• Uses: Custom fitting for prosthetics, diagnostics, and treatment planning.

Softening Methods for Impression Compound

• Two primary methods for softening:
  • Dry Heat Method: Utilizes direct flame.
  • Moist Heat Method: Involves hot water.

Disadvantages of Moist Heat Method

• Potential for overheating and alteration of physical properties.
• Can lead to inadequate impressions.

Properties of Impression Compound

• Shrinkage rates range from 0.3% to 0.4%.
• Exhibits good flow properties but possesses disadvantages such as poor compatibility.

Cast Construction and Separation

• Use of plaster of paris is common; it can be applied in thin pastes without conventional washes.
• Two-paste system identified as Type I and Type II.

Components of Impression Compound

• Base materials typically include zinc oxide and inert oils.
• Reactors or catalysts like oil of cloves may be added alongside accelerators for enhanced performance.

Additional Component Functions

• Fillers, coloring materials, and flavoring agents enhance usability and appeal.

Manipulation Techniques

• The proper technique involves a flexible stainless steel spatula with broad strokes, mixing for 3-5 minutes.
• Teasing methods can influence setting time, impacting consistency and material strength positively and negatively.

Temperature Effects on Setting Time

• Hot water application for 5-10 minutes can affect the setting characteristics of impression materials in Types II, III, and IV.