Fracture Load in Monolithic Multilayer Zirconia Crowns From Three Manufacturers: an in Vitro Study.

Abstract

Introduction: Multilayer zirconia crowns have gained popularity in oral rehabilitation due to their combination of gradient aesthetics and mechanical strength. However, gaps exist in the literature regarding differences in fracture load among commercial brands, a critical variable for their selection in patients with parafunctional habits. Previous studies have evaluated monolithic zirconia, but few have examined multilayer zirconia under standardized conditions. The present in vitro study compared the fracture load of monolithic crowns fabricated from multilayer zirconia discs of three commercial brands: BeautyZir, Upcera, and Yucera.

Methods: A comparative experimental study was conducted using 30 monolithic crowns made from multilayer zirconia discs, distributed as follows: 10 crowns from ST + Multilayer® BeautyZir, 10 from 4D Pro® Yucera, and 10 from Explore® Upcera. Abutments were fabricated with a chamfer finish preparation on an ivorine tooth (#16) and scanned with the TRIOS 3 intraoral system (3Shape®). The resulting STL file was laser-sintered in cobalt‒chromium to obtain the abutment for crown placement. Crown design was performed in Exocad® software with a central fossa depth of 2.1 mm. The crowns were milled according to each brand's specifications and subjected to fracture load testing on a universal Instron® machine, applying axial force using a 12-mm-diameter metallic sphere at a crosshead speed of 3 mm/s. Data were collected using Excel and analyzed using one-way ANOVA and post hoc Tukey tests (α=0.05), following verification of normality with the Kolmogorov-Smirnov and Shapiro-Wilk tests.

Results: Mean fracture loads (±SD) were: ST + Multilayer®: 1807 N (±463), Explore®: 1682 N (±368), Yucera®: 1173 N (±183). ANOVA revealed significant differences between groups (P=0.001). The Tukey test showed significant differences between Yucera and Upcera (P=0.010) and Yucera and BeautyZir (P=0.001), with Upcera and BeautyZir demonstrating significantly higher fracture loads than Yucera; however, no differences were observed between Upcera and BeautyZir (P=0.719). All the tested brands exceeded physiological masticatory forces (133-727 N), but Yucera had the lowest safety margin. The high standard deviation in BeautyZir (±463 N) suggests variability in manufacturing quality.

Conclusion: The three multilayer zirconia brands tested withstood normal occlusal forces, but Upcera and BeautyZir were better suited for patients at risk of higher loads. The lack of difference between Upcera and BeautyZir supports their interchangeable clinical use. The lower resistance of Yucera and high dispersion in BeautyZir emphasize the need for stringent quality control during manufacturing. Future studies should incorporate thermocycling, cementation, and in vivo follow-up to validate these findings under clinical conditions.