Comparison of Dimensional Accuracy of Two Acrylic Resins Polymerized by Injection Molding

Abstract

Statement of Problem: Despite the fact that polymethyl methacrylate is the most popular material in denture base construction, one of its main defects is the polymerization shrinkage that frequently leads in time consuming clinical problems. Several ways have been suggested to solve this problem; among them is "injection molding method".Purpose: The purpose of this study was the assessment of dimensional accuracy of the two acrylic resins cured by injection molding method.Materials and Method: In this experimental study, the dimensional accuracy of two acrylic resins after curing by injection molding method (Unipress system) was assessed by profile projector measurement. Twenty master casts were made of a master dye simulating both maxillary and mandibular edentulous arch. They were divided in two groups, each group containing 10 casts used for making denture bases. Three reference points were determined, one in the central incisor region and the other two symmetrical points in the posterior region in molar area. Three measurements were made: 1. on the master cast, 2. on the denture base just after removing the denture base from the master cast, and 3. after immersion in water for one week. The measurements were analyzed using Friedman and Mann-Whitney tests. Results: All the denture bases in both groups, revealed significant shrinkage after removing from casts (p<0.05). After immersion in water, there was no significant dimensional change (p>0.05) in any of the groups. Statistical analysis revealed no significant difference between the two acrylic resins too.Conclusion: Since there was no significant difference between dimensional accuracy of the two acrylic resins, it could be conclude that using Bayer acrylic resin with Unipress injection molding system would lead in acceptable results concerning dimensional accuracy. Key words: Dimensional accuracy, Injection molding, Acrylic resin