The Preliminary Impression

The preliminary impression, which records the conformation of that portion of the residual ridge supporting the denture, is typically made with alginate impression material and is used in the fabrication of a diagnostic cast from which a custom tray is made.

The alginate mix is placed into stock impression trays that have been designed for the edentulous ridge. Thermoplastic impression compound can also be used in stock metal trays. While this method yields poor tissue detail, it involves less clinical time and may be indicated for the patient who gags or is uncooperative. Additionally, a thermoplastic impression does not have to be poured immediately, which lends to its usefulness in dental care delivered outside the dental office (e.g., in a home, hospital, or convalescent facility).

Impression Armamentarium

The preliminary alginate impression requires:

• Alginate powder and measuring scoop
  
• Graduated water measuring cylinder
  
• Mixing bowl and spatula
  
• Impression tray

Evaluating the Mandibular Impression Tray

The impression tray should match the shape and size of the denture-bearing area of the mandible as closely as possible. The length of the tray can be increased with wax or compound to extend as necessary into the retromolar or sublingual (retromylohyoid) areas.