Effect of the shades of background substructures on the overall color of zirconia-based all-ceramic crowns

PURPOSE: The objective of this study was to determine the effect of the color of a background substructure on the overall color of a zirconia-based all-ceramic crown. MATERIALS AND METHODS: Twenty one posterior zirconia crowns were made for twenty subjects. Seven premolar crowns and six molar crowns were cemented onto abutments with metal post and core in the first and second group. In the third group, eight molar crowns were cemented onto abutments with a prefabricated post and composite core build-up. The color measurements of all-ceramic crowns were made before try-in, before and after cementation. A repeated measure ANOVA was used for a statistical analysis of a color change of all-ceramic crowns at alpha=.05. Twenty four zirconia specimens, with different core thicknesses (0.4-1 mm) were also prepared to obtain the contrast ratio of zirconia materials after veneering. RESULTS: L*, a*, and b* values of all-ceramic crowns cemented either on a metal cast post and core or on a prefabricated post did not show significant changes (P>.05). However, the slight color changes of zirconia crowns were detected and represented by DeltaE*ab values, ranging from 1.2 to 3.1. The contrast ratios of zirconia specimens were 0.92-0.95 after veneering. CONCLUSION: No significant differences were observed between the L*, a*, and b* values of zirconia crowns cemented either on a metal cast post and core or a prefabricated post and composite core. However, the color of a background substructure could affect the overall color of posterior zirconia restorations with clinically recommended core thickness according to DeltaE*ab values.

Clinical performance and failures of zirconia-based fixed partial dentures: a review literature

PURPOSE: Zirconia has been used in clinical dentistry for approximately a decade, and there have been several reports regarding the clinical performance and survival rates of zirconia-based restorations. The aim of this article was to review the literatures published from 2000 to 2010 regarding the clinical performance and the causes of failure of zirconia fixed partial dentures (FPDs). MATERIALS AND METHODS: An electronic search of English peer-reviewed dental literatures was performed through PubMed to obtain all the clinical studies focused on the performance of the zirconia FPDs. The electronic search was supplemented by manual searching through the references of the selected articles for possible inclusion of some articles. Randomized controlled clinical trials, longitudinal prospective and retrospective cohort studies were the focuses of this review. Articles that did not focus on the restoration of teeth using zirconia-based restorations were excluded from this review. RESULTS: There have been three studies for the study of zirconia single crowns. The clinical outcome was satisfactory (acceptable) according to the CDA evaluation. There have been 14 studies for the study of zirconia FPDs. The survival rates of zirconia anterior and posterior FPDs ranged between 73.9% - 100% after 2 - 5 years. The causes of failure were veneer fracture, ceramic core fracture, abutment tooth fracture, secondary caries, and restoration dislodgment. CONCLUSION: The overall performance of zirconia FPDs was satisfactory according to either USPHS criteria or CDA evaluations. Fracture resistance of core and veneering ceramics, bonding between core and veneering materials, and marginal discrepancy of zirconia-based restorations were discussed as the causes of failure. Because of its repeated occurrence in many studies, future researches are essentially required to clarify this problem and to reduce the fracture incident.

The Effect of Closed Fitting Mouth Guard with Fluoride Gel on Surface Hardness of Enamel after Soaking in Chlorinated Water

The aim of this study was to assess the effect of closed fitting mouth guard with fluoridegel (1.1% NaF gel) or fluoride toothpaste (0.11% NaF) on surface hardness of enamel aftersoaking in chlorinated water at pH 3.4 for 2 hours. Fifty two human premolars extracted fororthodontic reasons were divided into 5 groups (13 specimens in each group): enamel slabswithout mouth guard (control group; C), enamel slabs with mouth guard only (experimentalgroup 1; E1), enamel slabs with mouth guard and fluoride gel (experimental group 2; E2),enamel slabs with mouth guard and fluoride toothpaste (experimental group 3; E3) andenamel slabs with mouth guard and Vaseline (experimental group 4; E4). Before and aftersoaking in chlorinated water, ten specimens of each group were measured for enamelmicrohardness and 3 specimens of each group were prepared to examine the enamelappearance using scanning electron microscope (SEM). The results showed that enamelmicrohardness decreased in all specimens significantly (p \< .05) except E2. Group C had thehighest means of difference Vickers microhardness number (VHN) before and after soaking inchlorinated water. (147.75ฑ11.25) E1, E4, E3 and E2 had lower means of difference in VHN.The lowest one was E2 (10.36ฑ6.74). There were significant differences between groups(p \< .05) except between E3 and E4. After soaking in Chlorinated water, SEM revealedhoneycomb-liked etched patterns on enamel surface only in group C. This study indicated thatenamel which exposed to strong acidic water had decreased surface hardness and hadchanges in enamel surface significantly. Using mouth guard either with or without other agentscould maintain enamel surface hardness and enamel appearance better than not using mouthguard. Using mouth guard with fluoride gel provides the most effective result.Key words: dental erosion; enamel microhardness; erosion; fluoride gel; mouth guard