The Post-Fatigue Load-Bearing Capacity Of A Novel Multilayer Zirconia Restoration For Implant And Its Color Permeability

Objective:More recently,a novel monolithic zirconia named“multilayer zirconia”have been introduced into the dental market.They have polychromatic layers,with color-,transparency-and flexural strength-gradient.The literature on its fatigue performance is scarce;thus,further evaluation is required.Accordingly,in the first section of this dissertation,the effect of fatigue loading on the load-bearing capacity of implant restorations made from a novel multilayer zirconia was evaluated.In addition,when this novel high-transparent zirconia is used as an implant prosthesis,its background color will affect the final color of the restorations.Accordingly,in the second section of this dissertation,the effect of background color of implant abutment and color of cements on the color of high-transparent zirconia restorations with three thicknesses was evaluated,providing a guidance for the application of appropriate cements.Methods:In the first section of this dissertation,identical computer-aided design/computer-aided manufacturing(CAD/CAM)implant-supported posterior single-crowns were divided into 4 groups:crowns milled at 3 axial cutting depths in multilayer zirconia disks(Upcera Meiying)along their thickness direction,i.e.,the top,middle and bottom groups,and crowns milled from conventional zirconia disks(Vita YZ HT)with a homogeneous structure as a control group(the YZ group).After being cemented to each analog,the specimens were then mounted in an electro-magnetic force fatigue testing machine.All crowns were tilted at 15°and subjected to sinusoidal compressive-compressive fatigue loading for various numbers of cycles(0,10~6,2×10~6,3×10~6 or 4×10~6,corresponded to an in vivo chewing stroke of approximately 0,4,8,12 and 16 years respectively)between 50 and 500 N at the center of incline of the cusp.All surviving crowns were subsequently submitted to a static fracture test until failure to determine the load-bearing capacity(N).The data were analyzed:(1)the post-fatigue load-bearing capacity of specimens in each group;(2)the post-fatigue load-bearing capacity of specimens in each subgroup.The failure modes were identified and observed with scanning electron microscopy.In the second section of this dissertation,zirconia restorations milled from high-transparent zirconia disks were made and divided into 3 groups according to thickness(1.5 mm,2.0 mm,2.5 mm).Restorations of each group were bonded in turn with transparent cement,white cement and chromatic cement to the sandblasted abutment analogs,i.e.,the experimental groups.The unbonded restorations with 3 thicknesses were considered as the control groups.Color parameters were measured with VITA Easy Shade to get the color coordinates(CIEL*a*b*)of all specimens.ΔE between the experimental groups and control groups were calculated.The effects of background color of implant abutment and color of cements on the color of high-transparent zirconia restorations were analyzed by comparing perceptibility threshold(PT)and acceptability threshold(AT).Results:In the first section of this dissertation,after 10~6cycles of fatigue loading(corresponded to an in vivo chewing stroke of approximately 4 years),there was not a significant reduction in the fracture load compared with unfatigued specimens(approximately10%in the multilayer zirconia and 5%in the YZ zirconia at P>0.05).However,increasing the number of fatigue loading cycles had a negative effect on the load-bearing capacity.After long-term service of 4×10~6cycles,the load-bearing capacity of all crowns decreased significantly by approximately 50%.Within each subgroup,the YZ group presented the highest load-bearing capacity followed by the bottom,middle,and top groups of the multilayer zirconia disk.For both materials,a catastrophic failure mode produced by cone cracks with radial propagation was identified.In the second section of this dissertation,(1)when the transparent cementation was used,for the restorations with 1.5 mm thickness,ΔE(8.51)were higher than AT;for the restorations with 2.0 mm thickness,ΔE(2.03)were lower than AT but higher than PT;for the restorations with 2.5 mm thickness,ΔE(1.26)were lower than PT.(2)For the restorations with1.5 mm thickness,only when the chromatic cement was used wereΔE lower than AT,but they were still higher than PT.For the restorations with 2.0 mm thickness,ΔE were all lower than AT.Only when the chromatic cement was used wereΔE lower than PT.For the restorations with 2.5 mm thickness,ΔE were lower than PT.Conclusion:1.Attenuation,due to fatigue,in the load-bearing capacity of zirconia restorations subjected to chewing for 4 years may be not detected.However,a long-term period of chewing for 8,12 and 16 years,fatigue degraded the load-bearing capacity of restorations considerably;2.Restorations can be milled at various CAD/CAM axis cutting depths in a novel multilayer zirconia disk depending on the durability and aesthetic requirements;3.The background color of the implant abutment and different colors of cements can affect the color of the high-transparent zirconia restorations with low thickness.When the designed thickness of the restorations is less than 2.5 mm,it is recommended to use chromatic cement with the same color as the restoration;When the thickness is 2.5mm or above,the background color of cement or abutment has little influence on the color of the high-transparent zirconia restoration and all three colors of cements can be applied.