The growing demand for minimally invasive dentistry allows the use of vertical preparation as a more conservative approach toward tooth structure. Vertical preparation with a reverse shoulder is a recent approach that is supposed to enhance the biomechanics of zirconia crowns. The aim of this study was to evaluate the influence of different marginal designs (deep chamfer, vertical, vertical with reverse shoulder) on the fracture strength of monolithic zirconia crowns.
Materials and methods: Thirty human maxillary first premolar teeth were used in this study. The teeth were divided into three groups according to the margin design (n=10), group A : deep chamfer; group B: vertical, and group C: modified vertical with a reverse shoulder of 1 mm on the buccal surface of the tooth. All samples were scanned using an intra-oral scanner and milled with a 5-axis machine. The crowns were cemented with resin cement. A single load to failure test was used to assess the fracture load using a computerized universal testing machine that automatically recorded the fracture load of each sample in Newton (N).The data were analyzed using One-way analysis of variance (ANOVA) test and Bonferroni test.
Results: The highest mean of fracture load was recorded by chamfer (2969.8 N), followed by modified vertical (2899.3 N), and the lowest mean of fracture load was recorded by vertical (2717.9 N). One-way ANOVA test revealed a significant difference among the three groups. Bonferroni test showed a significant difference between group A and B, while a non-significant difference was revealed between group C with group A and group B.
Conclusion: Within the limitations of this in vitro study, the modification of the vertical preparation with a reverse shoulder placed at the buccal surface improved the fracture strength up to the point that it becomes statistically non-significant with the chamfer group.
Keywords: Zirconia, Vertical, Preparation, Reverse shoulder