Lists the properties an impression material must have, from patient safety and colour-coded mixing to working time, tray adhesion, disinfection, strength and fine detail reproduction down to 20 µm. Gives a table of the factors that decide accuracy, including viscosity, temperature, thixotropy, wetting, particle size and pressure. Closes with residual deformation for elastic materials, showing a diagram of the three stages of removal, the 3% permanent deformation limit and tips for removing the impression.
Topic 28 of 52 in the MU Sofia syllabus for Dental Materials (Dental Medicine, Year 1). The free AceMed note covers it in 4 sections, about 227 words (2 min of reading), with 14 flashcards.
In this note
Introduction
General requirements
What decides accuracy
Residual deformation
Introduction
An impression carries clinical information to the laboratory: the shape, volume and relationships of the tissues. A model poured from it must be an exact copy of the prosthetic field. Impression materials therefore have to meet a long list of requirements.
General requirements
Harmless to patient and dental team, with a pleasant taste, smell and color.
Different colors in two-part materials, so that mixing can be checked by eye.
Long enough working time to form the impression while the material is plastic.
Adhesion to the tray, but not to the teeth or mouth.
Compatible with model materials, not disturbing their setting and separating easily.
Able to be disinfected: in 1% sodium hypochlorite for 10 minutes, or 2% glutaraldehyde for 30 minutes.
Strong enough not to tear on removal, in transport or during pouring.
Accurate, reproducing details down to 20 µm.
Volume changes that can be compensated: gypsum expands by 0.1 to 0.3%, so shrinkage within this range is compensable.
Minimal residual deformation in elastic materials.
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