Influence Of Polishing And Glazing On The Wear Of Zirconia And Enamel

Objectives Nowadays zirconia is proverbially used in dental restorations. As far as we concerned, zirconia restorative materials are satisfied in the esthetic zone and have superior biocompatibility and mechanical properties. However, high strength and hardness lead to the poor polishing performance. In clinical, try-in crowns usually need to adjust the surface. The occlusal adjustment of ceramic crowns may roughen the occluding surface. Because of its wear potential zirconia may be harmful to the opposing enamel. According to investigate its properties of abrasion and attrition, find a way of zirconia surface treatment which is more compatible with dental enamel and reduce wear of enamel to protect the antagonists.Methods Zirconia specimen preparation:Twenty-five plates(5×10×15mm) made of zirconia were divided into five equal groups by a random number table method. Group a was only ground, group b was ground and polished, group c was ground and polished by prophylaxis paste, group d was ground and glazed, group e was ground, polishsed then reglazed. Five central incisors were embedded in autopolymerizing acrylic resin with a size of 5×10×15mm as the control group f. For each group five upper premolars buccal cusps were prepared as the antagonist. Control enamel specimen preparation: Five central incisors were freshly extracted from human.They were embedded in autopolymerizing acrylic resin and revealed labial surface with a size of 5×10×15mm as the control group f. Antagonist specimen preparation: Thirty premolar teeth were prepared from youngsters about thirteen to fifteen years old who pulled out teeth for orthodontics. Then the non functional cusps of each tooth was divided with complete form. Cusps were embedded in autopolymerizing acrylic resin with a shape of circular column whose diameter were 3.1mm. For each group five cusps were prepared as the antagonist. The wear test was performed in a Universal Micro-Tribotester by pin-on-disk, contact with a vertical load of 15 N, horizontal sliding of 1 mm. The test was performed at a frequency of 2hz and continued for 6000 cycles. After sliding 1 mm, the load was removed, the liquid bath returned to its original position and the cycle repeated. Performed the test in artificial saliva, use enamel as antagonists, investigate its properties of abrasion and attrition of zirconia and enamel. The relation of horizontal sliding, friction force and coefficient of friction were recorded. The coefficient of friction changed every cycle index, drew the curve by computer. Roughness test: The pretest surface roughness(Ra) of all the zirconia and control enamel were determined with a roughmeter. Five spots of an area in the middle of each specimen was selected for testing. The mean value were computed as the roughness result.Volume measurement: Use a 3D scanner to scan the contour profile of the antagonist before and after the wear test. Measured the specimens in the same horizontal plane. Use a software of 3Dmax to calculate the volume of the antagonist. Scanning electron microscope observation: The wear surface of zirconia and enamel were observed by scanning electron microscopy to determine the wear characteristics, after the wear test, SPSS 17.0 statistical software was used for statistical analysis.Results The result of roughness test: ground group had the highest surface roughness. The polished then reglazed zirconia and ground then glazed zirconia had a similar roughness. Polished by prophylaxis paste group had the lowest Ra value than the other groups. The control group and polished by prophylaxis paste group had a similar roughness. The 1-way ANOVA demonstrated no differences between the Ra values of the two glazed groups and no differences between polished by prophylaxis paste group and control group(P?0.5). Ra values of the other groups showed significant differences between(P?0.5). Volume loss of the antagonists:group a 0.736±0.031 mm~3, group b 0.595±0.023 mm~3, group c 0.413±0.017 mm~3, group d 0.905±0.018 mm~3,group e 0.818±0.015 mm~3, group f 0.235±0.017 mm~3. The date arranged from large to small in was group d, group e, group a, group b, group c, group f. The 1-way ANOVA showed significant differences among groups(P<0.5). The result of scanning electron microscopy observations: Many cracks and loss of small fragments were observed on the wear area of a,b,c groups. The degree of wear of group a,b,c was gradually reduce. The wear area of glazed groups d and e showed the glaze layer destructed, small fragments of the glaze layer were observed on the wear area. The underlying surface of the ceramic becomes exposed, the furrows of ground then glazed group were more deep than polished then reglazed group. The wear surface of control group presented flat. It can not see furrows but can find frgments and cracks of enamel. The surface presented more furrows, small fragments and heteroplasy when compared to the areas of zirconia. Antagonist of group a showed more serious than antagonist of group b. The surface of antagonist of group c can not see furrows but can find fragments and cracks of enamel. The surface of antagonists of group d and e presented more furrows, small fragments and heteroplasy. Antagonist of group d showed more serious furrows, heteroplasy and fragments than antagonist of group e. The furrows antagonists of group f showed the minimum depth and least quantity than other antagonists.Conclusions To the surface treatment of zirconia, polishing is better than glazing. The way of polishing by prophylaxis paste can decrease wear of the antagonist.