Effect of a continuous mechanical polishing protocol on the color stainability, microhardness, mass, and surface roughness of denture base acrylic resin

Abstract

Statement of problemThe acrylic resin used in dental prostheses, which is subject to changes in its properties caused by hygiene chemicals, brushing, and colored beverages, can benefit from mechanical polishing methods. However, studies evaluating whether such procedures can be performed continuously without damaging the prosthetic materials are lacking. PurposeThe purpose of this in vitro study was to evaluate the effect of a continuous mechanical polishing protocol on the color stainability, surface roughness, microhardness, and mass of a denture base acrylic resin. Material and methodsThe polishing protocol was tested in specimens submitted to one of the following conditions: immersion in coffee (CF) (n=20); toothbrushing (Br, 35 600 cycles in a brushing simulation machine) (n=20); and immersion in 1% sodium hypochlorite (HYP) (n=20). For each condition, half of the specimens were polished (aluminum oxide paste and felt wheel attached to an electric motor, 3000 rpm) at simulated biweekly intervals. Two groups (n=10) of specimens were immersed in distilled water and used as the control for the immersion groups. Thus, the present study consisted of a total of 8 groups (n=80). Before and after simulated periods of 12 and 24 months, surface roughness was evaluated by using a contact profilometer; a spectrophotometer was used to calculate color changes (ΔE00); and a Knoop microdurometer and a precision balance were used to assess microhardness and mass changes. Data were compared by using 2-way mixed ANOVA. The Tukey HSD and Student t tests were used for post hoc analysis (α=.05). ResultsThe polishing influenced surface roughness (Br, CF, HYP: P<.001) and color stainability (Br: P=.008; CF: P<.001). Significant increase in roughness was observed for the Br and HYP groups, and the polishing significantly reduced roughness under all experimental conditions. In general, the polishing did not affect the microhardness and caused minimum wear of the acrylic resin. ΔE00 values were above the clinical perceptibility threshold (ΔE00>1.30) for the Br group, exceeding the clinical acceptability threshold for the CF group (ΔE00= 2.51). Polishing reduced ΔE00 in these groups, making values similar to those found in the groups immersed in distilled water. The color in the HYP group was not influenced by polishing. ConclusionsIn general, the tested polishing protocol reduced the deleterious effects of brushing and contact with 1% sodium hypochlorite and coffee, reducing surface roughness and color change without affecting microhardness and mass in any clinically relevant way.