5-1. Introduction

a. Complete dentures which replace the lost natural dentition are fabricated by a series of steps or procedures performed by the dental officer and the dental removable prosthetic specialist as a team. Each step from the preliminary impression to the finished prosthesis must be performed accurately and precisely. A slight error during any procedure will result in an unsatisfactory prosthodontic restoration. Therefore, the dental removable prosthetic specialist must have a thorough knowledge of the materials used and of the dental laboratory procedures which are assigned to him. The dental laboratory procedures specifically are stressed in this chapter.

b. The first major step in the fabrication of complete dentures is the making of impressions of the patient's edentulous maxillae and mandible. Impressions are accurate, negative reproductions of these areas and are made by the dental officer. Several materials and combination of materials are used for making impressions. The dental removable prosthetic specialist must be familiar with these materials so that he can handle them properly during the dental laboratory procedures.

5–2. Preliminary Impressions

The final impression of an edentulous arch may be obtained by the use of one of two methods. The first method involves only one step; the final impression being made with the impression material (s) in one sitting. The more commonly used method, involves first making a preliminary impression and later making a final impression. Materials commonly used for preliminary impressions for complete dentures include modeling plastic (modeling compound) and irreversible hydrocolloid (alginate hydrocolloid) (fig. 5-1). A gypsum product, usually artificial stone, is poured into the preliminary impression forming the preliminary cast, which is a positive reproduction of the form of the tissues of the edentulous arch. On this preliminary cast a tray is

fabricated which follows the

outline of the patient's arch. These trays enable the dental officer to use final impression materials that will record minute details of the oral tissue (mucosa). 5-3. Handling Preliminary Impressions

The first dental laboratory procedure performed by the dental removable prosthetic specialist in the fabrication of complete dentures is the preparation of a preliminary impression for the pouring of a cast. Extreme care must be exercised in handling the fragile, easily-distorted impression materials. No part of the inner surface or the border of the impression should ever be touched with the fingers or with instruments. The preliminary impressions are washed with tepid water or a slurry solution to remove mucous and saliva and so preserve all surface detail (A, fig 5–2). A slurry solution is made by grinding discarded gypsum product casts and mixing the material with water. All excess water should be removed from the impressions, but they should never be allowed to become dehydrated. Materials such as modeling plastic impression material, metallic oxide pastes (zinc oxide-eugenol) and polysulfide or silicone base impression material require no special preparation before the cast is poured. Impressions made of gypsum products, however, must be painted with separating material (B, fig 5-2). If this is not done, the gypsum products will bond firmly together and the cast cannot be separated from the impression.

5—4. Pouring the Preliminary Cast

a. Artificial stone (hydrocal) is usually used for the preliminary cast, but other gypsum products may be used. The material is mixed according to the manufacturer's instructions and carefully vibrated into the impression (A, fig 5-3). After the impression is filled, a portion of the mix is placed on a glass slab or parchment paper pad and the impression is inverted onto this (B, fig 5-3). The mix is shaped around the impression