Objectives: To study influence of different designs of zirconia substructures on color and fracture resistance in bilayered zirconia CAD CAM restorations.
Materials and Methods: Fifty identical PMMA dies were 3D printed using stereolithography, to mimic a prepared maxillary first molar with chamfer finish line of 1 mm, axial reduction of 1.5 mm and occlusal preparation of 2 mm. CAD CAM (Wieland dental) was used to digitally scan the fifty dies in order to fabricate 50 zirconia copings. They were assigned to 5 groups according to coping design (n=10): Gp FV (control): full-veneer coping covering to finish line, Gp CS: 1mm cervical-shoulder, GP BW: monolithic zirconia with window cut-back on the buccal surface, GP 3W: monolithic zirconia with window cut-back on buccal, lingual and mesial surfaces and Gp MM: circular projections of 1 mm on palatal cusps and mid-palatal surface. All copings were then veneered, air-abraded using 50μm- Al2O3, primed and cemented to their matching dies using resin cement (Rely X U200, 3M-ESPE). Specimens were tagged and thermo-cycled for 10000 cycles. Spectrophotometer (Easy Shade V, VITA Zahnfabrik) was used to digitally analyze the color for all groups. Delta E was calculated in-comparison to the control group. Compressive load was applied to all specimens in the central fossa, parallel to long axis of each die at a crosshead speed of 0.5 mm/min till failure. Level of significance between groups was calculated using One Way ANOVA (P<0.05).
Results: Regarding the fracture resistance, a significant difference among groups was revealed by ANOVA(P=0.000). Gp MM recorded the highest mean fracture resistance (2102.3) whereas Gp 3W recorded the least (1363.2). Delta E showed no significant difference in color among all tested groups (P=.219).
Conclusions: Fracture resistance is significantly affected by the core design. MM design increased the mean fracture of Bilayered zirconia without affecting negatively its color, therefore it can be considered as a replacement of the traditional coping design.