Effect of layering gingiva-shade composite resin on the strength of denture base polymers.

STATEMENT OF PROBLEM: Acrylic resin denture base materials, fabricated with either a traditional technique or computer-aided design and computer-aided manufacturing (CAD-CAM) technique, layered with different thicknesses of gingiva-shade composite resin may affect the strength of the definitive prostheses and have not been investigated. PURPOSE: The purpose of this in vitro study was to assess the flexural strength of acrylic resin denture base materials modified by layering different thicknesses of gingiva-shade composite resin. MATERIAL AND METHODS: Two denture acrylic resins, heat-polymerized type (Lucitone 199) and CAD-CAM prepolymerized type (AvaDent) polymethyl methacrylate (PMMA) resin, were used as the base materials. Three-millimeter-thick specimens were fabricated and prepared according to the ISO 1567 and ISO 20795-1:2013 and were used as the controls. A proprietary gingiva-shade composite resin (GRADIA gum shades) was used to replace different thicknesses (0.5 mm, 1.0 mm, and 1.5 mm) of the denture base materials, giving 4 groups for each tested material (n=16). A 3-point loading test was conducted by using a universal testing machine and a custom fixture with a crosshead speed of 5 mm/min. The maximum fracture loads were recorded, and ultimate flexural strength values were calculated. The collected data were statistically analyzed with ANOVA and the Tukey honestly significant difference (HSD) tests (alpha=.05). Representative fractured specimens were examined under a stereomicroscope at x20 magnification and a scanning electron microscope to determine the interface and fracture patterns. RESULTS: The mean +/-standard deviation of ultimate flexural strengths for the heat-polymerized acrylic resin specimens ranged from 94.79 +/-9.89 MPa to 40.34 +/-12.79 MPa, and that of the CAD-CAM prepolymerized acrylic resin ranged from 125.98 +/-7.96 MPa to 64.16 +/-20.77 MPa. Acrylic resin denture base materials after layering with gingiva-shade composite resin had a significantly lower mean fracture load than the controls (P<.05). The Tukey HSD test revealed that the control groups had significantly higher flexural strength values compared with the other tested specimens within each denture acrylic resin group layered with gingiva-shade composite resin (P<.05). The SEM images displayed brittle fracture exhibiting well-defined, flat, compact, and organized surface fractures. CONCLUSIONS: The flexural strengths of CAD-CAM prepolymerized acrylic resins were higher than those of the heat-polymerized denture acrylic resins. The flexural strengths of all 3 thicknesses of the CAD-CAM prepolymerized acrylic resins were greater than 65 MPa.