Objective To compare four different types of monolithic zirconia crowns in terms of survival rate and fracture resistance after thermocycling and/or thermo‐mechanical loading in a chewing simulator. Methods Partially stabilized zirconia (PSZ) crowns with fiber‐reinforced resin die assemblies (n = 80) were fabricated using: multi‐yttria‐layered 5Y‐PSZ/3Y‐PSZ, multi‐yttria‐layered 5Y‐PSZ/4Y‐PSZ, monolithic 4Y‐PSZ, and monolithic 3Y‐PSZ as control (n = 20). Half of the samples in each group were subjected to thermo‐mechanical loading under 110 N, 1.4 Hz, 1.2 million cycles with simultaneous thermocycling (10,000 cycles, 5–55°C). The other half were subjected to thermocycling alone. The samples were loaded to failure to measure their fracture resistance. The data were analyzed using by two‐way ANOVA and Tukey's HSD post‐hoc test (α = 0.05). Results All specimens survived the aging protocols. The yttria content significantly affected the fracture resistance of the crowns (p < 0.0001). The mean fracture resistance, from highest to lowest: 3Y‐PSZ, 4Y‐PSZ, followed by the two multi‐yttria‐layered systems. The mean difference between the two multi‐yttria‐layered systems were not statistically significant (p = 0.98). The mechanical loading protocol did not affect the mean fracture resistance within each group (p = 0.18). Conclusions Within each group, there was no difference in fracture resistance after thermocycling alone and thermo‐mechanical loading. However, increasing the yttria concentration at the occlusal third of the crown decreased its fracture resistance. Clinical Significance The term “monolithic zirconia” alone without specifying the actual yttria content is misleading. This term represents different materials with different mechanical properties. The yttria content has an inverse relationship with the fracture resistance of zirconia crowns. The fracture resistance of multi‐layer zirconia crowns is determined by the amount of the weaker zirconia phase at the occlusal part of the restoration rather than enforced by the stronger zirconia at the cervical part of the crown.
Background: Tobacco smoking and diet have been both linked to increased oral Candida carriage. The aim of this study is to investigate the distribution of Candida species in the oral cavity of young individuals smoking cigarettes and waterpipe who have various dietary habits.Methods: Participants were recruited from university students during December 2013, and were divided into three groups; a control nonsmoking, a cigarette smoking, and a waterpipe smoking groups. The study sample completed a pre-structured questionnaire on smoking, and dietary habits. Oral swab samples were collected from all participants to determine oral Candida carriage and the colonizing Candida species. Data were statistically analyzed to determine the significance of association between Candida carriage on one hand and smoking status, and dietary habits on the other hand.Findings: A total of 238 students participated in the study and among those only 30 (12.8%) students had oral Candida carriage. Candida albicans was the most common species to be recovered (56.7%), followed by Candida dubliniensis (23.3%). The association between smoking, and perceived dietary habits on one hand and oral Candida carriage on the other was not statistically significant. Conclusions :Smoking, and perceived dietary habits seem to have no association with oral Candida carriage among a young cohort of university smokers. It is recommended that similar studies are conducted on older age groups to investigate if age has any influence.
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