Dental Impression Materials: Properties and Types

Essential Requirements for Dental Impression Materials

• Ability to Record Fine Details: Essential for accurate reproduction.
• Patient Acceptability: Material should be comfortable and well-tolerated.
• Suitable Setting Time: Allows for proper manipulation and setting in the mouth.
• High Tear Strength: Prevents tearing during removal from undercuts.
• Long Shelf Life: Ensures material remains viable for extended periods.
• Easy Manipulation & Inexpensive: Practical for clinical use and cost-effective.
• Accuracy: The ability to precisely record fine anatomical details.
• Stability: The ability to maintain recorded fine details over time.

Factors Affecting Accuracy and Stability

• Sufficient Fluidity: Must be fluid enough to capture intricate details.
• Minimal Dimensional Changes: Little to no change during setting and storage.
• Adhesion to Tray: Secure attachment to the impression tray.
• Elasticity on Removal: Must be elastic enough to be removed from undercuts without permanent deformation.
• Compatibility with Model Materials: Should not adversely react with or inhibit the setting of gypsum or other model materials.

Types of Dental Impression Materials

Non-Elastic (Rigid) Impression Materials

Zinc Eugenol Impression Paste

Zinc Eugenol is a non-elastic material primarily used for edentulous impressions.

Components

• Base Paste: Consists of zinc oxide powder and an inert oil.
• Catalyst Paste: Contains eugenol, water, gum (for body, coherence, and adhesion), accelerators, and talc powder.

Manipulation

• Vaseline can be used for easier cleaning of instruments.
• A small drop of water can accelerate the setting time.
• Glycerine can be used to retard the setting time.

Properties

• Fluidity: Sufficiently fluid to record fine details.
• Dimensional Changes During Setting: Exhibits very little shrinkage (approximately 0.1%).
• Adhesion to Tray: Adheres well to the impression tray due to the gum content.
• Elasticity: Non-elastic, making it suitable for edentulous areas without undercuts.
• Dimensional Change During Storage: Considered one of the most dimensionally stable materials.
• Cast Compatibility: Compatible with gypsum model materials and does not require a separate medium. Typically used with non-perforated trays.

Elastic Impression Materials

Alginate Impression Materials

Alginate is a widely used elastic impression material known for its low cost and ease of manipulation.

Ingredients

• Sodium Alginate: Dissolves in water to form a sol.
• Calcium Sulfate: Reacts with sodium alginate to produce insoluble calcium alginate, forming the gel.
• Sodium Phosphate: Acts as a retarder, controlling the working time.
• Potassium Sulfate: Counteracts the inhibiting effect of alginate on gypsum setting.
• Glycol: Added to make the powder dustless.
• Chlorhexidine: Provides self-disinfection properties.
• Flavor & Color: For patient comfort and visual identification.

Manipulation

• The powder ingredients should be thoroughly shaken before mixing.
• Use a perforated impression tray for better retention.
• Mix with water at room temperature.
• The posterior section of the tray should be seated first in the mouth.
• It is preferable to leave the impression in the mouth for at least 2 minutes after initial set, followed by a sharp, snap removal. This technique reduces permanent deformation and increases tear strength.
• Pour the cast immediately after taking the impression (do not store) to prevent syneresis (water loss) and imbibition (water absorption), which lead to dimensional instability.

Properties

• Fluidity: Sufficiently fluid to capture details.
• Shrinkage During Setting: Negligible shrinkage during setting.
• Adhesion: Requires a perforated tray for mechanical retention.
• Elasticity: Exhibits approximately 97.3% elasticity.
• Dimensional Stability: Suffers from syneresis and imbibition, leading to dimensional changes if not poured immediately.
• Separating Medium: No separating medium is needed for gypsum casts.

Disadvantages

• Lower tear strength compared to elastomeric materials.
• Significant dimensional instability if not handled correctly (immediate pouring).

Elastomeric (Rubber Base) Impression Materials

Elastomeric materials are highly elastic and offer excellent detail reproduction.

Advantages

• High tear strength.
• Adequate dimensional stability.

Types of Elastomeric Materials

Polysulphide Rubber Impression Material

• Reaction Type: Condensation reaction, with water as a by-product.
• Base Paste: Composed of polysulphide liquid and silica fillers.
• Catalyst Paste: Contains lead dioxide.

Condensation Silicone Impression Material

• Reaction Type: Condensation reaction, with ethyl alcohol as a by-product.
• Base Paste: Consists of silicone liquid (with hydroxyl groups) and silica fillers.
• Catalyst Paste: Contains tin octoate.

Addition Silicone Impression Material

• Reaction Type: Addition reaction (no by-products).
• Base Paste: Composed of silicone liquid (with vinyl groups) and silica fillers.
• Catalyst Paste: Contains chloroplatinic acid.

Polyether Impression Material

• Reaction Type: Addition reaction.
• Base Paste: Consists of polyether (with imine groups) and silica filler.
• Catalyst Paste: Contains sulfonic acid ester.