Fracture resistance of different overlay designs with novel lithium disilicate materials on 3D printed dies

Moderate coronal destruction cases have recently been repaired using minimally invasive adhesive overlays with varying cavity depths and occlusal preparations. The optimal cavity preparation (criteria and dimensions) varies according to clinical crown length, patient age and inter occlusal space. Consequently, design and thickness of restoration will affect esthetics, function and fracture resistance necessitates the employment of new materials capable of meeting their full potential. However, there are numerous available materials and different preparation designs. However, there is insufficient data on the selection of appropriate material, preparation thickness and their effect on fracture resistance. To evaluate the fracture resistance of chairside (CAD/CAM) lithium disilicate using two diverse overlay restoration designs for premolars. Specimens were prepared using a CNC milling machine, according to two different overlay designs: (A) overlay with 2 mm cavity depth & 1.5 mm occlusal thickness, (B) overlay with 1.5 mm cavity depth & 2 mm occlusal thickness. 42 restorations were designed and fabricated with a chairside CAD/CAM system (Mc XL, Dentsply Sirona) using 3 CAD/CAM lithium disilicate blocks (IPS e.max CAD, Amber Mill, CEREC Tessers) (14 specimens/group). Restorations were luted to 3D printed resin dies using standard resin luting cement (Breeze, Pentron) and then loaded with a steel indenter until fracture. The group findings were analyzed using one-way analysis of variance, and the medians were assessed independently using Kruskal-Wallis. The null hypothesis states that there will be no significant difference in the fracture resistance between the three CAD/CAM lithium disilicate materials and the two preparation designs. The fracture force of CAD/CAM lithium disilicate restorations varied significantly based on the restoration design. Among the two overlays, the restorations with design A showed significantly higher fracture force than the restorations with design B (p < 0.001). Within design A: there was no significant difference between the materials. Within design B: there was a significant difference in fracture resistance between materials. Design A, including more cavity depth (2 mm) related to less occlusal reduction(1.5 mm) provides more fracture resistance values than design B using lithium disilicate overlay preparation design. Preparation design has a great effect on fracture resistance values of overlay restorations, while material type has negligible effect. Despite the presence of additional virgilite crystals in CT, it did not produce any increase in fracture resistance.