Introduction
Making a complete denture starts with an exact record of the edentulous jaws. This takes two impressions: a preliminary (anatomical) impression in a stock tray to make a study cast, and a final (functional) impression in a custom tray made on that cast, from which the master cast is poured.
Preliminary impression
- Stock edentulous trays (metal or plastic) in a range of sizes, adapted with compound or wax where needed.
- Material: alginate or impression compound.
- Purpose: a study cast on which the custom tray is made.
Custom (individual) tray
Outlining
On the study cast, the depth of the sulcus is outlined. A second line is drawn about 2 mm short of the sulcus depth (1.0–1.5 mm in some techniques) and around the frena: this is the tray border. On the upper cast the border runs behind the tuberosities and across 1–2 mm behind the vibrating line ("A" line), where both lines coincide.
Materials and making
- Shellac base plate, the most common in practice; also cold-cure acrylic or vacuum-formed sheets.
- Undercuts are blocked out; a wax spacer may be added where room for impression material is needed. No spacer over the posterior palatal seal area.
- The softened plate is pressed through a cloth onto the cast and trimmed to the line; edges smoothed in the flame.
- Upper handle: burnt wire 1.5–2 mm thick, about 10 cm long, bent into an omega shape 10–15 mm long and angled about 30° to the frontal plane, so it doesn't push the lip.
- Lower tray: reinforced with wire following the ridge crest; two cylinders (10–15 mm high, 10 mm across) in the premolar areas serve as finger rests.
- Trays are usually not perforated, because holes weaken the peripheral seal.