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 (α = 0.05).

Results: Group O showed significantly higher fracture resistance than groups OG and OSG (p = 0.002 and p = 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.