Impact of different layers within a blank on mechanical properties of multi-layered zirconia ceramics before and after thermal aging.

OBJECTIVES: To compare the mechanical properties of different layers of multi-layered zirconia materials. METHODS: 720 cylindric test plates were fabricated from four defined layers of three multi-layered zirconia ceramics (IPS e.max ZirCAD Prime, Optimill Multilayer 3D; Ceramill zolid fx multilayer) and divided into two equal groups. One group underwent thermal cycling (5-55 °C, 10 000 cycles; "TC") and one did not ("no TC"), before density, flexural strength, Weibull modulus, and Vickers hardness were evaluated. EDX analysis was conducted using an additional cylinder of each material. Data were analyzed using Kruskal-Wallis and Mann-Whitney U tests. Statistical analysis was performed with Bonferroni correction (α < 0.001). RESULTS: After aging, ZirCAD layer 4 showed the overall highest density (6.04 ± 0.02 g/cm3), which was significantly higher than density of layer 4 of Optimill (6.02 ± 0.06 g/cm3) and Ceramill (5.80 ± 1.08 g/cm3) (both p < 0.001). Flexural strength of ZirCAD and Optimill increased consecutively after thermal aging. ZirCAD layer 4 had the overall highest flexural strength before and after artificial aging. After thermal cycling, the Weibull modulus ranged between 4.32 (ZirCAD layer 1) and 13.58 (Ceramill layer 4). ZirCAD had the overall highest Vickers hardness: in layer 1 (1579.18 ± 47.14 HV) before aging, and in layer 2 (1607.1 ± 149.71 HV) after aging. Flexural strength and Vickers hardness differed significantly between the four ZirCAD layers (p < 0.001). Thermal ageing had no significant impact on mechanical properties (p > 0.001). SIGNIFICANCE: Mechanical properties were affected by plate position within the blank. When nesting a restoration within a multi-layered zirconia blank, the mechanical properties required should be considered.

Influence of dental tissue substructure and dimension on the fracture strength of lithium disilicate and zirconia ceramics.

PURPOSE: To assess the influence of substructure and dimension on the fracture strength of ceramic discs made from both lithium disilicate ceramic and zirconia. MATERIALS AND METHODS: A total of 128 intact maxillary third molars were collected, and standardized enamel and dentin discs were fabricated. Lithium disilicate ceramic (IPS e.max CAD, Ivoclar Vivadent; n = 64) and zirconia (Katana, Kuraray; n = 64) discs with 0.5-mm (n = 32 IPS and n = 32 Katana) and 1-mm (n = 32 IPS and n = 32 Katana) thickness were produced, and each group was divided into two subgroups (n = 16 each) that were luted to the enamel or dentin discs using Panavia V5 (Kuraray). Half of the specimens in each subgroup were aged (chewing simulation and thermocycling), and all specimens were loaded until fracture in a universal testing machine (Z010, Zwick/Roell). Differences between Katana and IPS with respect to enamel and dentin as substructure and the thickness of the ceramic were analyzed by use of a nonparametric test (Mann-Whitney U test). RESULTS: In un-aged specimens, fracture loads were not significantly (P > .05) different between zirconia and IPS specimens for 1- or 0.5-mm thickness. However, fracture loads were significantly higher (P < .001) in specimens supported by enamel, independent of the ceramic material. In aged specimens, the fracture loads of all specimens were significantly (P < .01) higher when supported by enamel; however, in the 0.5-mm groups, zirconia achieved significantly higher breaking loads than IPS when luted to dentin. CONCLUSION: When 0.5-mm ceramic discs were luted to dentin, zirconia outperformed IPS with respect to breaking loads.

In vitro investigations on retention force behavior of conventional and modern double crown systems.

OBJECTIVE: This study aimed to investigate the effects of long-term use on the retention force and wear behavior of double crown systems. METHODS: Based on a common double crown design sixty pairs of telescopic crowns were fabricated and divided into six groups, each consisting ten samples: "Gold standard" cast gold alloy primary and secondary crown (GG) and cast non-precious alloy (NN), computer-aided design (CAD)/computer-aided manufacturing (CAM)-milled zirconia primary crown and galvanoformed secondary crown (ZG), CAD/CAM-milled non-precious alloy primary and secondary crown (CC NN), CAD/CAM-milled zirconia primary crown and non-precious alloy secondary crown (CC ZN) and CAD/CAM-milled zirconia primary crown and polyetheretherketone (PEEK) secondary crown (CC ZP). In the constant presence of artificial saliva, all samples were subjected to 10,000 joining-separation cycles at a velocity of 120 mm/min. Wear was analyzed by reflected light microscopy and confocal microscopy before and after artificial aging. RESULTS: Retention force losses were observed in each group after long-term use, with significant losses in the groups ZG and CC ZP (pZG = 0.01, pCC ZP = 0.049). During artificial aging, no significant differences in pull-off force were recorded for groups GG, NN and CC ZN. Regarding wear, merely the Y-TZP primary crowns of the CC ZP group displayed no surface changes. SIGNIFICANCE: All tested production methods and material combinations seem to be suitable for clinical practice. CAD/CAM technology allows similarly predictable results to be achieved as the gold standard. Confocal microscopy is recommended for surface examinations of double crowns.

The flexural strength of CAD/CAM polymer crowns and the effect of artificial ageing on the fracture resistance of CAD/CAM polymer and ceramic single crowns.

OBJECTIVES: The purpose of this study was to investigate the fracture resistance, flexural strength and Weibull modulus of an innovative CAD/CAM polymer and to compare its fracture resistance with that of glass ceramics. MATERIALS AND METHODS: A total of 32 (n = 16 IPS e.max CAD (LIDI); n = 16 LuxaCam Composite (LUXA)) first mandibular molar crowns were fabricated and cemented onto metal dies by use of luting composite. Half of the specimens were loaded until fracture without prior artificial ageing. The other half were subjected to thermal (5°/55 °C) and mechanical (1,200,000 cycles, 80 N) cycling before fracture loading. Scanning electron microscopy was used to analyse fracture behaviour. A three-point bending test of the flexural strength of LUXA was performed according to ISO 6872:2008. Data were analysed by means of the Kolmogorov-Smirnov test, Mann-Whitney U-test (p < 0.05) and Weibull statistical analysis. RESULTS: Initial fracture resistance of LIDI was significantly higher than that of LUXA. However, the initial fracture resistance of LIDI decreased significantly after artificial ageing. After ageing, fracture resistance was 1050.29 ± 325.08 N for LUXA and 1250.09 ± 32.53 N for LIDI. Three-point bending test yielded a mean flexural strength value for LUXA of 145.28 ± 18.21 MPa and a Weibull modulus of m = 9.51. CONCLUSIONS: Polymer-based material tested in this study had a lower fracture resistance than that of the glass-ceramic material. Fracture resistance and flexural strength of LuxaCam Composite are sufficient for use in the first molar region. CLINICAL RELEVANCE: The mechanical properties of this innovative polymer-based material indicate it can be used in the first molar region as a suitable alternative to glass ceramics. Further clinical studies are required to confirm this. The study presents an innovative material as an alternative to glassceramic for the clinical use in dentistry. The materials investigated were differently affected by artificial aging. Clinical use for patients with bruxism may be considered.