Effect of different marginal finish lines and marginal thickness on the fracture resistance of Zirconium reinforced lithium silicate glass ceramic crowns

Aim: The aim of this study is to investigate the fracture resistance of zirconia reinforced lithium silicate glass ceramic (ZLS) crowns with different marginal finish lines and marginal thickness. Material and Method: The dies representing the prepared mandibular first molar tooth with a flat occlusal (non-anatomical) surface at 6 mm crown length and 6°taper angle, from which the specimens will be obtained, were prepared in the CAD/CAM milling unit to provide standardization. Specimens were separated into 8 groups as different marginal finish lines and marginal thickness. In the first group; 1 mm 135 ° shoulder finish line, in the second group; 1 mm 90 ° shoulder finish line, in the third group; 1 mm chamfer finish line, in the fourth group; 1 mm deep chamfer finish line, in the fifth group; 0,5 mm 135 ° shoulder finish line, sixth in the group; 0,5 mm 90 ° shoulder finish line, in the seventh group; 0,5 mm chamfer finish line, in the eighth group; 0,5 mm deep chamfer finish line. A total of 88 epoxy resin die models were obtained, 11 for each group, using silicone molds from these prepared dies. ZLS crowns were cemented with resin cement onto an epoxy resin die. The specimens thermocycled at 5-55 °C for 5000 cycles and then the fracture resistance test was performed with 1 mm/sec load speed by universal testing machine. Fractured specimens are grouped according to Burke classification fracture types. The fractured surfaces were examined in SEM (Scanning Electron Microscopy). The data were statistically evaluated using the Shapiro-Wilk, Oneway Anova tests. Results: Statistically significant difference was found between the groups (p=0,002; p<0.05). According to the data obtained in our study, ZLS crowns prepared as 1 mm chamfer finish line (1749.88±327.69 N) had the highest fracture strength values and ZLS crowns prepared as 0.5 mm 90° shoulder finish line (1030.91±432.84 N) had the lowest fracture strength values. Conclusion: It was concluded that different marginal finish lines and marginal thickness affect the fracture strength of ZLS crowns, 1 mm chamfer finish line design increases the fracture resistance of ZLS crowns. Key words: Chamfer finish line, Shoulder finish line, Fracture resistance, Zirconia-reinforced lithium silicate (ZLS)