Abstract: Objective：To evaluate the wear intensity and surface roughness of occlusal veneers on premolars made of microhybrid composite resin or two kinds of ceramics in vitro after the thermocycling and cyclic mechanical loading tests. Methods: In the study,24 fresh extracted human premolars without root canal treatment were prepared （cusps reduction of 1.5 mm in thickness to simulate middle to severe tooth wear, the inclinations of cusps were 20°）. The prepared teeth were restored with occlusal veneers made of three different materials: microhybrid composite, heatpressed lithium disilicate ceramic and computeraided design/computer-aided manufacturing (CAD/CAM) lithium disilicate ceramic in the thickness of 1.5 mm. The occlusal veneers were cemented with resin cement. The specimens were fatigued using the thermocycling and cyclic mechanical loading tests after being stored in water for 72 h. The wear of specimens was measured using gypsum replicas and 3D laser scanner before and after the thermocycling and cyclic mechanical loading tests and the mean lost distance (mm) was used to indicate the level of wear. The surfaces of occlusal contact area were observed and the surface roughness was recorded using 3D laser scanning confocal microscope before and after the fatigue test. Differences between the groups were compared using ONE-way ANOVA（P<0.05）. Results: All the specimens successfully survived after the thermocycling and cyclic mechanical loading tests. The mean wear of microhybrid composite group, heatpressed lithium disilicate ceramic group, and CAD/CAM lithium disilicate ceramic group was （-0.13±0.03） mm, （-0.05±0.01） mm and （-0.05±0.01） mm, the wear of microhybrid composite was significantly higher than the two ceramic groups（P<0.001）.The mean surface roughness（Ra）before the fatigue test was(1.24±0.20) μm, （0.75±0.09） μm, （0.73±0.14） μm and it became （1.81±0.24） μm, （1.53±0.26） μm and （1.77±0.23） μm after the test . Before the fatigue test, the surface roughness of microhybrid composite was significantly higher than the two ceramic groups（P<0.001） and after the test, the surface roughness of heat-pressed lithium disilicate ceramic was significantly lower than microhybrid composite（P=0.005） and CAD/CAM lithium disilicate ceramic(P=0.010). Conclusion: From the view of wear speed, microhybrid composite was significantly higher than the two kinds of ceramics, but it was similar to enamel when the opposing tooth was natural. The surface roughness before the themocycling and cyclic mechanical loading test of microhybrid composite was significantly higher than that of the two ceramic groups. After the test, the surface roughness of heat-pressed ceramic was significantly lower than that of the other two groups. From the view of surface roughness, heat-pressed ceramic has more advantage.

Key words: Occlusal veneer, Microhybrid composite, Lithium disilicate ceramic, Wear, Surface roughness, Thermocycling and cyclic mechanical loading test