Effect of Deep Margin Elevation on the Fracture Resistance of Maxillary Molar Zirconia Crowns

Background: The purpose of this in vitro study was to evaluate the fracture resistance of maxillary molar zirconia crowns cemented with deep margins relocated with resin composite. Methods: A maxillary right first-molar typodont tooth was prepared for a complete-coverage zirconia crown and modified to have: (1-DME) a 3 mm deep and 3 mm wide box on the mesial margin; (2-DME) 3 mm deep and 3 mm wide boxes on both the mesial and distal margins; and (N-DME) no box modification. Before the first modification, 45 zirconia (4 mol% yttria) crowns were fabricated. The die was scanned before each modification to fabricate the corresponding 3D-printed dies (n = 15 per group). The DME boxes were restored with universal resin composite, and the crowns were then cemented to their respective dies using a resin luting cement. Then artificial aging was performed by thermocycling for 10,000 cycles between 5 °C and 55 °C with a dwell time of 30 s. The cemented crowns were then loaded to failure in a universal testing machine. Maximum fracture load was analyzed with one-way ANOVA and the post hoc Tukey HSD test (α = 0.05). In addition, fracture patterns were evaluated descriptively with scanning electron microscopy images. Results: The fracture resistance of maxillary molar zirconia crowns differed significantly among the groups. Group 2-DME demonstrated the highest mean fracture resistance (5370 N), followed by Group 1-DME (5083 N). The lowest mean value was recorded for the control group, N-DME (2961 N). Conclusions: Zirconia crowns cemented on teeth with deep margin elevation exhibited higher fracture resistance than crowns cemented without deep margin elevation.