An impression refers to the negative likeness of a structure (oral tissues). The materials that are used to duplicate the anatomic form of the oral tissue are known as impression materials.
Impression materials are considered as auxiliary dental materials as they are required for the fabrication of prostheses but do not become part of a final prosthesis.
Requirements
To produce an accurate replica of the intra-oral and extra-oral tissues, an impression material should be initially moldable, so that it can be forced against the tissues and then, after removal, it should remain stable while models or dies are being poured.
The impression material should be:
- Sufficiently fluid to adapt to the oral tissues.
- Viscous enough to be contained in a tray.
- Able to transform (set) in a rubbery or rigid solid in the mouth in a reasonable time (less than 7 minutes).
- Resistant to distortion or tearing when removed from the mouth.
- Dimensionally stable long enough to allow one or more cast pouring.
- Biocompatible.
- Cost-effective in terms of the required processing time and the expense of the materials and associated processing equipment.
Classification
Impression materials can be classified in several ways such as based on composition, mechanism of setting, and mechanical properties of the set material.
Based on Composition
1. Non-aqueous elastomers: Cross-linked polymers that can be stretched when set and rapidly recover to their original dimensions when stress is released.
- Polysulfide.
- Condensation silicone.
- Addition silicon.
- Polyether.
2. Hydrocolloids: Consists of a colloidal system where in a substance (dispersed phase) is microscopically dispersed in water (dispersion phase).
- Alginate.
- Agar.
3. Inelastic materials: Brittle materials that exhibit an insignificant amount of elastic deformation before fracture.
- Impression compound.
- Plaster of Paris.
- Zinc oxide-eugenol (ZOE).
Based on Setting Mechanism
1. Irreversible materials: Involves a chemical reaction during impression making, that is irreversible and the set material cannot be reused.
- Plaster or Paris.
- ZOE.
- Alginate.
- Elastomers.
2. Reversible materials: Sets by thermally induced physical reaction. There is no chemical reaction involved and the material soften upon heating and retain the shape when cooled in the oral cavity.
- Agar.
- Impression compound.
Based on Mechanical Properties
1. Rigid (Inelastic) materials: The set material is rigid and highly resistant to flexure, but fracture when deformed.
- ZOE.
- Impression plaster.
- Impression compound.
2. Elastic materials: The material can be flexed easily without fracture and returns to its original form when unstressed.
- Hydrocolloids.
- Elastomers.
Usage
- Elastic materials are suitable for impressions of both the hard and soft structures of the mouth, including the undercuts and interproximal spaces.
- Inelastic materials are ideal for impressions of edentulous jaw structures or soft tissues as they do not compress the tissue during seating of the impression tray.
References
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[](https://amzn.to/3USt5AE) Phillips' Science of Dental Materials, 13th Edition, Shen, Rawls, Esquivel-Upshaw, Elsevier
*This article is an excerpt from the above mentioned book and Medical Sutras does not make any ownership or affiliation claims.