ABSTRACT
AIM: To evaluate the effect of water temperature on the marginal fit of bis-acrylic composite provisional crown during resin polymerization. MATERIALS AND METHODS: Precisely machined 10 brass master dies were designed to simulate molar teeth. Five brass dies were selected and precisely machined to simulate all ceramic crown preparation. An acrylic jaw replica was made in which brass dies were arranged equidistant from each other. A custom-made metallic tray was fabricated on the acrylic jaw replica to make polyvinyl siloxane impression matrix. Bis-acrylic composite resin provisional crowns were made using polyvinyl siloxane impression matrix. Provisional crowns were polymerized at room temperature (Group I direct technique, on dental stone cast; Group I indirect technique crowns) and at different water temperatures (Group II direct technique crowns). The vertical marginal gap between all the provisional crown margins and the finish line of brass dies was measured using a Research Stereomicroscope System. RESULTS: The results were statistically analyzed using one-way analysis of variance (ANOVA) test and Newman-Keul's test. The results showed that crowns polymerized in 20°C and 30°C water had marginal gap approximately three times smaller than those polymerized in 30°C air, due to the reduced polymerization shrinkage. CONCLUSION: This study shows that crowns polymerized in 20°C and 30°C water had mean vertical marginal gap approximately three times smaller than those polymerized in 30°C air. It was approximately closer to that of crowns fabricated by indirect technique. Warmer water also supposedly hastens polymerization.
ABSTRACT
CONTEXT: To understand the effect of masticatory and parafunctional forces on the integrity of the prosthesis and the underlying cement layer. AIMS: The purpose of this study was to evaluate the stress pattern in the cement layer and the fixed prosthesis, on subjecting a three-dimensional finite element model to simulated occlusal loading. MATERIALS AND METHODS: Three-dimensional finite element model was simulated to replace missing mandibular first molar with second premolar and second molar as abutments. The model was subjected to a range of occlusal loads (20, 30, 40 MPa) in two different directions - vertical and 30° to the vertical. The cements (zinc phosphate, polycarboxylate, glass ionomer, and composite) were modeled with two cement thicknesses - 25 and 100 µm. Stresses were determined in certain reference points in fixed prosthesis and the cement layer. STATISTICAL ANALYSIS USED: The stress values are mathematic calculations without variance; hence, statistical analysis is not routinely required. RESULTS: Stress levels were calculated according to Von Mises criteria for each node. Maximum stresses were recorded at the occlusal surface, axio-gingival corners, followed by axial wall. The stresses were greater with lateral load and with 100-µm cement thickness. Results revealed higher stresses for zinc phosphate cement, followed by composites. CONCLUSIONS: The thinner cement interfaces favor the success of the prosthesis. The stresses in the prosthesis suggest rounding of axio-gingival corners and a well-established finish line as important factors in maintaining the integrity of the prosthesis.
