A recent study revealed that Dentsply Sirona’s Celtra Duo (ZLS) ranked highest in fracture toughness (KIC) when compared with five different chair-side CAD/CAM materials used for crown fabrication. Conducted by the University of Toronto, the complete study report entitled, “Fracture toughness of chair-side CAD/CAM materials – Alternative loading approach for compact tension test,” is published in the July 2016 edition of Dental Materials, the official publication of The Academy of Dental Materials.
The study applied a modified compact tension (CT) test in order to measure fracture toughness of the group of restorative CAD/CAM materials that included both ceramics and nanoceramic resin composite materials. Five different chair-side CAD/CAM blocks including VITA® Mark II, Lava™ Ultimate, VITA Enamic, IPS e.max CAD, and Celtra Duo (ZLS) were tested. Means and standard deviations of fracture toughness measured in MPa m1/2 for the materials are as follows:
- Celtra Duo (ZLS) (fired and unfired) – 2.65 (0.32) and 1.01 (0.15), respectively
- IPS e.max CAD (crystallised and non-crystallised) – 1.88 (0.62) and 0.81 (0.25), respectively
- VITA Enamic – 1.02 (0.19)
- Lava Ultimate – 0.85 (0.21)
- VITA Mark II – 0.73 (0.13)
Celtra Duo (ZLS) and e.max had the notably highest mean toughness values, with Celtra Duo (ZLS) found to be higher than that of e.max. Both leading materials had a substantial increase in fracture toughness after firing. Although the mean KIC values of the tested materials varied considerably, none were able to reach the mean KIC of natural dentin, previously reported at 3.08 MPa m1/2.
According to the report, the difference in IPS e.max CAD and Celtra Duo (ZLS) fracture toughness is most likely due to Celtra Duo’s (ZLS) composition. Celtra Duo (ZLS) is mainly composed of 58 per cent silica, lithium-metasilicate, -disilicate, and -phosphate crystals, and 10 per cent zirconia crystals in addition to other minor ingredients. The highly dispersed zirconia content is completely dissolved in a glassy matrix and acts as a crystal nucleus. Zirconia-based ceramics are known for their enhanced load resistance and fracture toughness compared to alumina- or lithium-disilicate ceramics.
Dental restorations must be extremely durable in order to resist damaging effects of a harsh oral environment. Fracture toughness is an essential property of any material that relates its resistance to crack propagation which imminently causes failure. A restorative material with high fracture toughness shows better fracture resistance and longevity as compared to materials with lower fracture toughness. Moreover, during the CAD/CAM milling process, grinding of the blocks creates micro-cracks and flaws that are routinely smoothed with polishing and glazing. – Krysten McCumber