Stress Distribution and Reliability of Bioinspired Zirconia Crowns

Objectives: Biological materials are naturally graded to prevent failures between heterogeneous structures. This study evaluated the failure probability of crowns with conventional and graded bioinspired layers, after fatigue, using Weibull analysis. The stress distribution in the crown (Maximum Principal Stress criteria) was assessed by finite element modeling.
Methods: Fifty anatomical preparations were made and divided into two groups according to the type of crown: Traditional- infrastructure in zirconia and superstructure in veneering porcelain; Bioinspired- infrastructure in porcelain and superstructure in zirconia. The fatigue load was 100N. Specimens were mechanically cycled in a sliding fatigue machine (2.10⁶ cycles, 4 Hz). Every 5.10⁵ cycles, the cycling was interrupted and the crowns were assessed for the presence of failures in a stereomicroscope. For the samples that did not show any failure (cracks, chipping and debonding between porcelain and zirconia or catastrophic failures), the fatigue test continued until they failed or until the test was finished. Specimens that did not fail were considered "suspended" in the analysis.
Results: The types and number of failures according to the groups were: cracking (Conventional- 3) and feldspathic porcelain chipping (Conventional- 3), followed by delamination at the veneer/core interface (Conventional- 8). Weibull parameters (beta-Weibull modulus and eta-characteristic number of cycles), with a two-sided confidence interval of 95%, were: Conventional – 1,29 and 0,23 x10⁷ and Bioinspired- 1,00 and 1,67 x 10⁷. Bioinspired crowns showed higher reliability under fatigue than conventional ones. The bioinspired design less stress concentration at the interfacial zones.
Conclusions: The two crown designs presented significantly different behavior under fatigue. The Bioinspired crows presented more favorable stress state and higher reliability after 2x10⁶ cycles than veneered zirconia crowns.