An impression aims to record the oral tissues in such a way as to produce maximum retention and stability in the completed denture. Usually two sets of impressions of the edentulous mouth are recorded.
The primary impression, taken in an edentulous stock tray, is needed to record enough detail for a special tray to be made. The primary impression should reproduce :
1. Sulcus depth
2. Sulcus width along the entire periphery
3. Certain anatomical landmark which indicates the correct extension of the special tray (eg maxillary tuberosity and retromolar pads.
It is usually taken in impression compound or impression plaster (not suitable if undercuts present).
These aim to record the maximum denture bearing area and develop an effective border seal. The special tray should be modified by reducing any over extension and the peripheries adapted by the addition of green stick compound. Impression plaster is usually used for the upper edentulous jaw in a spaced special tray. Zinc oxide eugenol impression paste is used for the lower jaw in a close fitting special tray. There are a number of special impression techniques for complete dentures, each of which have been devised to solve a particular problem or problems. The following techniques are covered in this program.