Hygiene Atlas

Atlas · Plate XVI · Supportive Treatment Services · 02

Impressions and study models

Alginate is mixed for about 30-60 seconds and sets faster in warm water. Because it loses and gains water, rinse, disinfect and pour it within about 15 minutes. Take the mandibular impression first. Study models use Type II model plaster, and extra water makes a weaker cast. Polyvinyl siloxane is the most dimensionally stable elastomer.

High-yield facts · 10

  1. Alginate (irreversible hydrocolloid) mix time about 30-60 s; working time about 1-2 min; regular set 2-4.5 min, fast set 1-2 min.T2
  2. Cold water slows alginate setting; warm water speeds it.T2
  3. Alginate loses water (syneresis) and gains water (imbibition), so pour it as soon as possible (within about 15 minutes).T2
  4. Take the mandibular impression first (less gag); seat the maxillary tray from posterior to anterior with the patient leaning forward.T2
  5. Rinse and disinfect impressions with an EPA-registered intermediate-level disinfectant (spray or short immersion per the manufacturer) before pouring.S7
  6. Polyvinyl siloxane is the most dimensionally stable elastomer and can be poured days later; polyether is stiff and absorbs water.T2
  7. Gypsum types: I impression plaster, II model plaster (study models), III dental stone (working casts), IV high-strength die stone, V high-expansion die stone.T2
  8. Water-to-powder ratios per 100 g: plaster about 45-50 mL; stone about 28-32 mL; die stone about 19-24 mL. More water = weaker cast.T2
  9. Study models: the art (base) portion is about one-third and the anatomic portion about two-thirds of the height; bases are trimmed parallel to the occlusal plane.T2
  10. A wax bite registration records centric occlusion for articulating casts.T2

Facts are written in our own words; each tag links to its source. Values marked "sources vary" differ between references, so check the one your program uses.