31. Technology of dental prostheses. Types of impressions. Impression trays. Choice of impression material and impression technique.
Explains why the impression is the critical first step in making a dental prosthesis, then sets out the three types of impression (preliminary, final and occlusal registration) and the stock, sectional, triple and custom trays, including how tray perforations or adhesive prevent distortion. It compares impression materials in rigid and elastic families through a table of strengths and weaknesses for alginate, polyether and silicones, and closes with one-step and two-step silicone techniques for crowns and bridges, plus a checklist for judging an impression before disinfection.
Topic 31 of 75 in the MU Sofia syllabus for Preclinics of Dental Prosthetics (Dental Medicine, Year 2). The free AceMed note covers it in 6 sections, about 205 words (3 min of reading), with 14 flashcards.
In this note
Introduction
What a good prosthesis must offer
Types of impression
Impression trays
Choosing the material
Impression techniques for crowns and bridges
Introduction
Almost every prosthesis is made outside the mouth, so the laboratory needs an exact copy of the patient's teeth and tissues. That copy begins with an impression: a negative imprint of the hard and soft tissues, from which a positive cast is poured.
What a good prosthesis must offer
Restore function, above all chewing, with correct occlusal contacts built to the patient's individual geometry.
Strength, resistance to fatigue and to ageing in the mouth.
Biocompatibility, comfort, and protection of the remaining teeth and tissues.
All of this depends on the accuracy of the first step: the impression.
Impression trays
Perforations or adhesive keep the material from pulling away from the tray, which would distort the impression.
Impression techniques for crowns and bridges
Elastic materials, usually silicones, are used in two consistencies: a stiff putty (heavy body) that carries a fluid light body into the fine detail of the preparation.
One-step, two-layer: both mixed at the same time, light body syringed around the preparations, tray seated once. The best choice for inlays, onlays and partial crowns.
Two-step (putty-wash): putty impression first, then light body added inside and the tray reseated.
Check every impression before it leaves the surgery: the whole finish line visible, no bubbles or tears on the preparations, the material firmly attached to the tray. Then disinfect it.
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