Abstract
Fit accuracy and fracture strength of milled monolithic zirconia (Zi) and zirconia-reinforced lithium silicate (ZLS) crowns are important parameters determining the success of these restorations. This study aimed to evaluate and compare the marginal and internal fit of monolithic Zi and ZLS crowns, along with the fracture load, with and without mechanical aging. Thirty-two stone dies acquired from a customized master metal molar die were scanned, and ceramic crowns (16 Zi Ceramill Zolid HT+ and 16 ZLS Vita Suprinity) were designed and milled. Absolute marginal discrepancies (AMD), marginal gaps (MG), and internal gaps (IG) of the crowns, in relation to the master metal die, were evaluated using x-ray nanotomography (n = 16). Next, thirty-two metal dies were fabricated based on the master metal die, and crowns (16 Zi; 16 ZLS) cemented and divided into four groups of eight each; eight Zi with mechanical aging (MA), eight Zi without mechanical aging (WMA), eight ZLS (MA), and eight ZLS (WMA). Two groups of crowns (Zi-MA; ZLS-MA) were subjected to 500,000 mechanical cycles (200 ± 50 N, 10 Hz) followed by axial compressive strength testing of all crowns, until failure, and the values were recorded. Independent sample t tests (α = 0.05) revealed no significant differences between Zi and ZLS crowns (p > 0.05); for both internal and marginal gaps, however, there were significant differences in AMD (p < 0.005). Independent samples Mann–Whitney U and Kruskal–Wallis tests revealed significant differences between the two materials, Zi and ZLS, regardless of fatigue loading, and for the individual material groups based on aging (α = 0.05). Multiple comparisons using Bonferroni post-hoc analysis showed significant differences between Zi and ZLS material groups, with or without aging. Within the limitations of this study, the ZLS crown fit was found to be on par with Zi, except for the AMD parameter. As regards fracture resistance, both materials survived the normal range of masticatory forces, but the Zi crowns demonstrated greater resistance to fracture. The monolithic Zi and ZLS crowns seem suitable for clinical application, based on the fit and fracture strength values obtained.
Highlights
Significant differences were found between the mean Absolute marginal discrepancies (AMD) values of
With regard to the internal gap widths, the mean axial gap (AG) values were markedly lower than the mean occlusal gap (OG) values for both Zi and zirconia-reinforced lithium silicate (ZLS) crowns
Scanning electron microscopic examination was not performed on the samples in this study for detailed fractographic analysis and is a minor limitation of the paper. Both Zi and ZLS crowns showed no significant differences between the mechanically aged and non-aged crown specimens, in terms of load-to-failure values, to the results reported in other recent investigations [37,56]
Summary
Complete coverage ceramic crowns are being routinely used in dentistry as a substitute for the traditional ceramo-metal options for multiple reasons, including fine aesthetics [1,2]. Different types of zirconia-based ceramics have been made available in recent years for the restoration and replacement of both anterior and posterior teeth through a variety of fabrication techniques, including computer-aided design and computer-aided manufacture (CAD/CAM) [3,4]. Zirconia (Zi) has favorable mechanical properties for a variety of applications in dentistry, the white color and poor translucency of the material precluded the use for full contoured restorations in the past [5,6,7,8,9].
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