| AIM:By simulating the chewing wear process of lithium disilicate glass ceramic and zirconia ceramic crowns of CAD/CAM in oral environment,and studying the wear loss,micro-morphology and the fracture resistance,to reveal the relation between wear characteristics and fracture resistance,in order to provide reference data for clinical application and improvement of all-ceramic materials.Methods:1.Study on the time-dependent wear performance of lithium disilicate glass ceramic crowns:12 pieces of lithium disilicate glass ceramic crowns were fabricated by CAD/CAM,12 pieces of PMMA abutments were fabricated by 3D printing,the crowns and abutments were bonded by Multilink N bonding system.The crowns were loaded 1million cycles by stainless balls with a diameter of 10mm,with the loading force of 350N and frequency of 1.7Hz,in artificial saliva environment.The three-dimensional topography scanner was used to scan and calculate the wear loss of the species at the selected cycle nodes?the interval of the measuring nodes was 50,000 cycles at the initial wear stage,and 100,000 cycles after the wear rate increased slowly?,then the wear curves were ploted.2.Study on the time-dependent wear performance of zirconia ceramic crowns:12zirconia ceramic crowns were fabricated by CAD/CAM,12 PMMA abutments were fabricated by 3D printing,the crowns and abutments were bonded with Multilink N bonding system.According to the method of experiment one,3.6 million cycle were performed?the interval of the measuring nodes was 0.3million cycles?then the wear curves were ploted.3.Sixteen lithium disilicate and zirconia all-ceramic crowns were fabricated and divided into four groups,with 4 specimens in each group.In the group of lithium disilicate glass ceramics crowns,the machine was suspended at the cycle nodes of0.2million,0.8million and 1 million,while in the group of zirconia ceramic crowns,the machine was suspended at the cycle nodes of 0,0.9million,2.4 million and 3.6 million,then the specimens were disassembled.Acetone was used to dissolve the resin embedded in the metal base,and then SEM was used to observe the microstructure of specimens of each group.4.Sixteen lithium disilicate and zirconia all-ceramic crowns were fabricated and divided into 4 groups according to experiment three.After loaded to the corresponding nodes the specimens were disassembled and then cleaned by an ultrasonic cleaner.The specimens were placed on the universal test airborne platform to test the fracture load with 6 mm diameter stainless-steel ball load on the occlusal surface along the long axis of abutment at a crosshead speed of 1 mm min-1.The fracture load curves of lithium disilicate and zirconia crowns were plotted.Results:1.During the 1 million cycles,the wear rate between the three stages divided by the nodes of 0.2 million cycles and 0.8 million cycles was statistically different?P<0.05?,and there was no statistical difference between nodes in each stage?P>0.05?.It is suggested that the wear of lithium disilicate glass-ceramic crowns in this experiment can be divided into three stages:0-0.2 million?running-in period?,0.2-0.8 million?stable wear period?and 0.8-1 million?severe wear period?,which conforms to the classical three-stage theory of tribology.2.During the 3.6 million cycles,the wear rate between the two stages divided by the node of 0.9 million cycles was statistically different?P<0.05?,and there was no statistical difference in each stage?P>0.05?.It is suggested that the wear of zirconia ceramic crowns in this experiment can be divided into two stages of 0-0.9 million?running-in period?and 0.9-3.6 million?stable wear period?,and the severe wear period had yet occurred.3.The microstructure of the lithium disilicate crowns during the running-in period,stable wear period and severe wear period:flat surface without obvious wear marks?closely adjacent strip-like wear scars?sheet exfoliation and extensively extended fissures.The microstructure of zirconia all-ceramic crowns in the running-in period,stable wear period:scattered uplift with dense parallel wear marks?a small amount of scratch-like wear marks?smooth wear surface with scattered cracks.4.The fracture load of lithium disilicate crowns showed no statistical difference between the running-in period?1424±43?N and the pre-wear period?1454±35?N?P>0.05?.The fracture load was statistically different between the running-in period?1424±43?N,the stable wear period?1308±65?N,and the severe wear period?1061±74?N?P<0.05?.The fracture load of zirconia crowns showed no statistical difference between the running-in period?3955±150?N and the pre-wear?4082±177?N?P>0.05?.The fracture load was statistically different between the running-in period?3955±150?N,the stable wear period?3377±216?N,and the stable wear period?2202±191?N?P<0.05?.Conclusion:1.The dynamic wear performance of lithium disilicate crowns exhibited:running-in period,stable wear period and severe wear period.The dynamic wear performance of zirconia crowns exhibited:“running-in period”,and“stable wear period”.The wear loss and rate of lithium disilicate crowns were higher than that of zirconia in the same wear period.2.The microstructures of the lithium disilicate crown in different wear stages showed:sparse wear scars?strip-like wear scars?cracks and stripping,while that of zirconia ceramic crowns showed:intensive wear scar?sparse shallow wear scar.3.There was no significant difference between the fracture load of the running-in period and the pre-wearing period for both ceramic crowns.The fracture load of lithium disilicate crowns has decreased in the running-in period and has decreased significantly in the severe wear period.The fracture load of zirconia crowns has a significant decline in stable wear period.The reduction of the strength of all-ceramic restorations was related to the wear loss,the cycles and the period of wear... |
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