Acrylic resin denture teeth MAJOR DENT Major Prodotti Dentari

Acrylic resin denture teeth (MAJOR-DENT; Major Prodotti Dentari S.P.A., Moncalieri, Italy) were arranged on the cast. The arrangement of the left anterior teeth began with the carved wax rim serving as Sulfo-NHS-LC-Biotin guide for the positioning of the central and lateral incisors and canine. The same procedure was applied on the right side. The posterior teeth were arranged starting with the first premolar and continuing through the second molar. The same procedure was applied in the right arch. A wax-up was used to form the polished surfaces of the upper dentures. Replicate dentures were made using a silicone matrix (Fig. 1). Two sprue holes were prepared in the resulting mold.
After the placement of artificial teeth and prepared stone casts in the matrix, molten pink base-plate wax (modeling wax; Vertex Dental) was poured into the matrix and allowed to cool before removal. All sets of teeth were from the same mold. Simulated metallic reference pins were placed in the artificial teeth at the mesial aspects of the central grooves of the first molars, and upright on cingula of both central incisors (Molligoda Abuzar et al., 1995) (Fig. 2). Intermolar (M–M) and interincisor (I–I) transverse distances, and anteroposterior distances from the incisors to the molars (LI–LM and RI–RM), were measured with digital calipers (Mitutoyo Corporation, Kanagawa, Japan), which can record changes as small as 0.01mm (Barbosa et al., 2002) (Fig. 3). The measurements were made at the wax denture stage (pre-polymerization) and after denture decasting (post-polymerization). Differences between the final and initial measurements indicated tooth movement.
The sample was divided randomly into three groups (=15 each) according to investing method. In the plaster–plaster–plaster (P–P–P) group, the lower part of flask (the drag) was filled with dental plaster (Pars Dandan, Tehran, Iran). After separating medium was applied to the exposed surface, plaster was poured up to the level of the incisal edges of the anterior teeth and the cusp tips of the posterior teeth (the middle portion of the mold, or the cope), and the remaining portion of the flask (the cap) was also filled with plaster (Fig. 4). The same procedure was applied to the plaster–stone–stone (P–S–S) group, except the middle and upper portions of the mold were filled with artificial stone (Fig. 5). In the plaster–mix (P–M) group, the lower part of the flask was filled with plaster and the upper portion was filled with a single layer of a 50:50 mixture of plaster and artificial stone (Fig. 6).
The compression molding technique was used to preparing all dentures. The heat-polymerized acrylic resin (Vertex Regular; Vertex Dental) was mixed with polymer and monomer at a ratio of 3:1 by volume. When the mixture reached a doughy consistency, it was placed in the mold over the teeth. A sheet of separating plastic was applied between the gypsum and the acrylic resin. The flask was closed and submitted to a trial packing procedure. Final closure was performed at 3500psi and maintained for 30min. The acrylic resin was polymerized in water with a long polymerization cycle. The polymerizing unit (Hanau Engineering Company, Buffalo, NY, USA) was programmed to increase the temperature to 74°C at 1h, which was maintained for 8h. After polymerizing and cooling to room temperature Sulfo-NHS-LC-Biotin in a water bath, deflasking and decasting were carefully completed. The dentures were stored in water at 37°C for 24h. Then, the transverse and anteroposterior distances were measured again.

Results
Tooth movement occurred in two directions in all three groups (Tables 1 and 2). M–M, LI–LM, and RI–RM movement was significantly greater in specimens prepared with dental plaster alone (0.1573mm, 0.1193mm, and 0.1133mm, respectively) than in the other groups; no significant difference in I–I movement was observed among groups (Table 1). Transverse movement along M–M and I–I was significantly greater than anteroposterior movement in specimens prepared with dental plaster alone; no significant difference among measurements was observed in the other two groups (Table 2).