RESUMO
Objective To investigate the variations of elastic modulus and hardness of human dentin with aging and location.Methods The non-carious third molars were collected and divided into youth group,middle-aged group and elderly group.Nanoidentation test was conducted on the dentin specimens from multiple regions to test their mechanical properties.Results The elastic modulus and hardness in outer and middle dentin were greater than those in inner dentin;the elastic modulus and hardness increased with the aging in each region of the dentin.Conclusions The dentin shows a gradient mechanical property.The stiffness and hardness in middle and outer dentin are greater than those in inner dentin.The deformation resistance in middle and outer dentin is stronger than that in inner dentin.Meanwhile,elastic modulus and hardness of the dentin increase with the aging.
RESUMO
Objective To investigate the variations of elastic modulus and hardness of human dentin with aging and location. Methods The non-carious third molars were collected and divided into youth group, middle-aged group and elderly group. Nanoidentation test was conducted on the dentin specimens from multiple regions to test their mechanical properties. Results The elastic modulus and hardness in outer and middle dentin were greater than those in inner dentin; the elastic modulus and hardness increased with the aging in each region of the dentin. Conclusions The dentin shows a gradient mechanical property. The stiffness and hardness in middle and outer dentin are greater than those in inner dentin. The deformation resistance in middle and outer dentin is stronger than that in inner dentin. Meanwhile, elastic modulus and hardness of the dentin increase with the aging.
RESUMO
Objective To analyze the contact failure of monolithic lithium disilicate CAD/CAM crowns by experiment and numerical simulation, and explore the influences of adhesives aging in water on load-bearing capacity of the crowns. Methods The specimens of sectioned monolithic lithium disilicate crowns were designed and manufactured, and evenly divided into two groups and stored in the air and in the distilled water for 30 days, respectively. The specimens were then subjected to monotonic contact loads to compare and analyze their load-bearing capacity. The fractured surfaces and adhesive interfaces of the specimens were observed by scanning electronic microscope. Meanwhile, the stress distribution on the crowns was calculated by numerical simulation to analyze the adhesives aging influence on load-bearing capacity of the crowns. Results The fracture loads on crowns stored in the air and in the water were (561.51 ± 65.66) N and (398.09 ± 90.20) N, respectively, indicating a significant difference. The tensile stress increased considerably at lower surface of the ceramic crown due to the reduction of adhesive strength at the interface of ceramic crown and substrate, which could increase the propensity of contact failure. Conclusions The adhesives aging in water reduces the bonding strength, and accordingly changes the tensile stress distributions, which can lower the load bearing capacity of the lithium disilicate crowns. The research finding provides references for the design and manufacturing of all-ceramic CAD/CAM restored crowns in clinic.
RESUMO
The purpose of this study was to investigate how cyclic loading influenced the fracture toughness of hot-press lithium disilicate and zirconia core materials and whether there was an increase in the propensity for crown failure. Two types of all-ceramic crowns including the IPS e.max Press system (n=24) and the Lava zirconia system (n=24), were selected. Sectioned specimens were subjected to cyclic loading with the maximum magnitude of 200 N (R=0.1) until two million cycles. The material properties including Young's modulus (E) and hardness (H) and the fracture toughness (KIC) of the core materials were evaluated using indentation methods (n=12 each). The load-bearing capacities of the specimens were examined by means of monotonic load to fracture (n=12 each). It was found that the material properties, including E, H and KIC, of the two types of dental ceramics, were reduced. Statistical analysis indicated that there were no significant influences of fatigue loading on material properties E and H for both types of dental ceramics or KIC for zirconia, while for the IPS e.max Press core, KIC, which was parallel to the direction of the lithium disilicate crystals, was significantly reduced (P=0.001). A conclusion was drawn that zirconia possesses high mechanical reliability and sustainable capacity to resist fatigue loading, while fatigue loading remarkably degraded the anisotropic mechanical behaviour of hot-press lithium disilicate ceramics.
Assuntos
Humanos , Coroas , Mastigação , Teste de MateriaisRESUMO
Objective To analyze the compressive strength and fracture mode between four kinds of dental zirconia CAD/CAM ceramic materials by Hertzian contact test. Methods Four CAD/CAM ceramic materials including Cercon smart, Lava, Porcera, and CEREC 3 were selected. Six zirconia sectioned flat specimens were prepared in each group. The critical load and fracture load as well as the fracture mode for each specimen were analyzed with digital image correlation (DIC) technique. Results It was found that cracks were initiated at the middle level of the veneer below the contact zone and propagating along an arc curve into the core/veneer interface, which eventually caused the debonding of the core/veneer layers. For the four ceramic systems, there were no significant differences in the critical load（P＞0.05）, while there were significant differences in the fracture load（P＜0.05）. Conclusions The compressive strength of the four zirconia CAD/CAM ceramic materials could substantially meet the clinic requirement for oral functions regarding the maximum occlusal load. Crack initiation and complete fracture only occurred in the veneer, which indicated that the strength of the veneer should be further reinforced.
