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Purpose: This in-vitro study evaluated the effect of different thicknesses on load to failure of monolithic zirconia crowns (MZC), produced by computer-aided design/computer-aided manufacturing (CAD/CAM) after the combined effects of thermocycling and compressive load-cycling.
Materials and Methods: An Ivorine molar (#19) was prepared to receive an all- ceramic crown and replicated 60 times using highly filled epoxy resin (Viade Products Inc., Camarillo, California, USA). Sixty MZC crowns- of two different companies BruxZir and Diazir with 3 different thicknesses of 0.5 mm, 1 mm, 1.5 mm, were tested. All crowns were cemented to their dies using resin cement (Rely-X Unicem Clicker) and subjected to the following accelerated artificial aging program: water storage at 37ºC for 5 weeks, thermocycling (5-55ºC) for 8,000 cycles and load-cycling for 250,000 cycles (20 to 270 N). All samples were subsequently loaded in the central fossa area to fracture at a crosshead speed of 1 mm/min with a tungsten carbide ball of 3 mm in an MTS universal testing machine. Data was analysis by the Kruskal-Wallis test, followed by a Mann–Whitney test with Bonferroni correction, p < 0.05.
Results: Crown survival rates after artificial aging was 100%. Regarding the fracture loads there was a statistically significant difference among the three thicknesses of MZC, with a mean fracture loads in Newton (N) of: 0.5 mm- 1119.3 N (373.4 N), 1 mm- 1820.3 N (266.1 N), 1.5 mm- 2071.2 N (136.8 N). However, there were no significant statistical difference between the brands.
Conclusions: The thickness of MZC is a significant factor for the fracture strength of the restoration. Long term clinical studies are necessary before recommending all ceramic MZC for daily practice, especially the 0.5 mm thickness.
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