Twenty identical maxillary stone casts were produced from a standard metallic mold. The maxillary residual ridge was free of any obvious ridge undercuts or surface irregularities and had a smooth U-shaped, well-formed arch. The master casts were divided into two groups, 10 casts each. The first group was assigned for construction of 10 denture bases from the conventional heat-cured acrylic resin as a control group (compression molding technique). The second group was assigned for construction of 10 denture bases from the newly introduced Vertex ThermoSense denture base material (injection molding technique).
Baseplate wax was constructed on one master cast using one sheet of wax 2 mm thick (Tru Wax, Dentsply International Inc., York, PA, USA). Anatomic acrylic maxillary second molars were arranged in their respective positions on the ridge (Dentsply International Inc., York, Pa.). The wax base thickness was preserved as 1.25 mm. Small amounts of wax were added to fix the teeth in their respective positions. Two wax sprues with 10 mm diameter were attached on the back of tuberosities. The system (model, base plate wax and teeth) was duplicated using polyvinyl siloxane (Silastic E; Dow Corning, Midland, Mich, USA) to prepare 20 identical denture base wax. After silicon curing, waxed denture base and model has been removed. This silicon was used as a standard template for next waxed denture base specimens. The teeth of the same sizes were arranged in their respective positions in the silicone mold. This silicone mold was placed on 20 stone casts where the teeth and model were fitted in location. Molten base plate wax was poured through the sprues and after cooling the wax replicas of denture were obtained.
The waxed denture bases were invested with dental plaster and dental flasks using compression molding technique. Stone cap were prepared over the cusps of the maxillary teeth. Waxed denture bases were washed up in boiling water for 10 minutes. The flasks were opened and wax was eliminated. After removing the wax with boiling water and separating medium application, the first group was constructed from heatpolymerized resin, and Major.base 20 (Major Prodotti Dentari S.p.A. Italy) were packed according to the manufacturer, s instructions. The powder/liquid was mixed at a ratio (3:1) andpacked, and the flasks were dipped in boiling water at 100 ℃ for 30 minutes. The flasks were cooled slowly and the denture bases were deflasked with their respective casts in position. While the denture bases remained in their position, the casts were trimmed from the cast-base to expose the border of the denture flanges.
The second group was constructed from Vertex ThermoSens (Vertex™ ThermoSens, Vertex-Dental, Netherlands) according to the manufacturer, s instructions. This system used special metallic flasks with posterior wax sprue for injection of the material inside the plaster molds. VertexTM ThermoSens is based on injection technique, with an automatic or manual injection machine. The model and flasks were prepared according to the standard procedures of the dental technique. Since there is no chemical bonding between synthetic acrylic teeth and Vertex™ ThermoSens, a mechanical bonding must be obtained. For injection of the Vertex™ ThermoSens into the flask, wax sprue should be used. The main sprue was about 9.5 mm and side sprues was 4.5 mm. The material was heated at 270 ℃–280 ℃ within 18 minutes and injected automatically at a pressure of 8.5 bar. The flasks were cooled slowly and the dentures were deflasked with their respective casts.
The models were trimmed to expose the denture borders at the regions of the tuberosities, labial frenum notch and the postdam. To assess the anterior-posterior dimensional changes, two reference points were prepared in the midline at the fitting surface of the bases. The first point located at the labial frenum notch (point A) and the second point in midline of the postdam (point B). Also, two reference points (C and D) were prepared in the internal surface of the buccal pouch part of the border of the denture flange (Fig. 1).
The antero-posterior reference points (A-B) and the right to left (C-D, flange-flange) reference points
The lengths between A, B, C and D points were standardized in all dentures by using metallic bar. The distances between these points in the fitting surface were measured by dial caliper (Mitutoy, Us Ms00 13, 500 series, Japan). The caliper has two jaws where one is fixed and the other is movable. The sliding jaw was moved by pressing the thumb on the bump on the bottom. The caliper was used for reading of the distance between centers of these points in antro-posterior direction (A and B) and right-left direction (C and D) and the results were recorded. The unit of measurement was the millimeter with precision of 0.01. The distances were measured before deflasking, while the dentures were still on their models, after removal of the denture from their casts, and 1 week and 2 weeks of water storage after decasting.
Immediately after the stone casts were deflasked with the dentures still in place, reference notches were prepared in the lingual aspect of the most distal molar tooth on either side of the arch (Fig. 2). With a caliper device, the molar-molar (M-M) cross arch linear distance was measured. The dentures were removed from their casts, the M-M cross arch linear distance was again measured. Measurements were done after storage in water for 1 and 2 weeks.
Reference points for molar-molar measurements