Effect of Tooth Preparation Design on Fracture Resistance of Zirconia-Reinforced Lithium Silicate Overlays

Purpose To assess the effect of tooth preparation design on fracture resistance of zirconia-reinforced lithium silicate overlays. Materials and Methods This study evaluated 50 human maxillary first molars with no caries, restorations or anatomical defects. The teeth were randomly divided into five groups (n = 10/group) based on preparation design for the fabrication of overlay restorations: (O) anatomical occlusal reduction, (OS) anatomical occlusal reduction with round shoulder preparation, (OG) anatomical occlusal reduction with a central groove, (OSG) anatomical occlusal reduction with round shoulder preparation and central groove, and (C) no preparation of tooth (control group). All restorations were fabricated using zirconia-reinforced lithium silicate (Vita Suprinity). The specimens underwent thermomechanical fatigue loading in a masticatory simulator (1.2 million cycles at 98 N). Fracture resistance was measured using a universal testing machine. The mode of failure was determined as well. Data were analyzed using one-way ANOVA followed by Tukey's post-hoc test, paired t-test and Fisher's exact test (alpha = 0.05). Results Group O showed significantly higher fracture resistance than groups OG and OSG (p= 0.002 andp= 0.001, respectively). The fracture resistance of group OS was significantly higher than that of group OSG (p= 0.008). The fracture resistance of the control group was significantly higher than that of OG and OSG (p= 0.001) and had no significant difference with other groups. Conclusions Group O (anatomical occlusal reduction alone), which had the most conservative preparation design, yielded the highest fracture resistance.