Impression materials: alginate, silicones and polyether

What it is

An impression records the teeth and surrounding tissues as a negative mould that is poured in gypsum to make a model. Materials are chosen by the accuracy needed. Elastic materials can be pulled past undercuts and spring back; rigid ones cannot. Intraoral scanners now replace many impressions with a digital model.

Composition and properties

• Alginate (irreversible hydrocolloid): cheap and quick, for study models, opposing models and preliminary denture impressions; it shrinks or swells as it loses or takes up water, so it is poured soon.
• Addition silicone (polyvinyl siloxane): the most dimensionally stable; can be poured later and more than once; used for crowns and bridges.
• Its setting can be blocked by sulphur compounds from some latex gloves.
• Polyether: accurate and naturally wettable, but stiff and harder to remove from undercuts.
• Rigid materials (impression compound, zinc oxide–eugenol paste, plaster) are used mainly for edentulous mouths.

Uses, limits and safety

• A distorted impression gives a crown that does not fit, however good the laboratory.
• Tray choice, retraction of the gum around the margin and a dry field matter as much as the material.
• Every impression is contaminated with saliva and blood and must be rinsed and disinfected before it goes to the laboratory.

Next step / Initial management

• Rinse and disinfect each impression with a disinfectant compatible with the material before sending it.
• Pour alginate soon after it is taken; record which material was used for the laboratory.
• Crown or denture that does not fit or rocks: refer back to the dentist; a new impression or scan is often needed.

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Educational content; it does not replace assessment by a dentist or doctor and is not a clinical instruction. For facial or neck swelling with difficulty swallowing or breathing, a knocked-out tooth or bleeding that will not stop, go to an emergency department at once.

Sources: Dental Impression Materials (StatPearls Publishing (NCBI Bookshelf), 2023); Dental Alginate Impressions (StatPearls Publishing (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.