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IPS e.max CAD and IPS e.max Press (Ivoclar Vivadent AG) are lithium disilicate glass ceramics marketed as interchangeable materials indicated for the same clinical uses. However, different crystal sizes of lithium disilicate are formed during the processing of each of these materials, a factor that could lead to significantly different mechanical properties. As mechanical failure is always associated with a crack-initiation/crack-propagation process, fracture toughness (KIC) values could be useful in comparing different ceramics and possibly predicting clinical performance.The purpose of this in vitro study was to determine and compare the KIC of IPS e.max CAD and IPS e.max Press.The notchless triangular prism (NTP) specimen KIC test was used to determine and compare the KIC of IPS e.max Press and IPS e.max CAD. Twenty 6×6×6×12-mm NTP specimens of each material were prepared. IPS e.max CAD blocks were cut, ground, and then crystallized, while IPS e.max Press specimens were prepared by pressing IPS e.max Press ingots into molds obtained from 6×6×6×12-mm wax prisms, using the lost wax technique. Each specimen was mounted into a specimen holder, and custom grips were used to attach the specimen holder assembly to a computerized universal testing machine (model 4301; Instron Canada, Inc). The assembly was loaded in tension at a crosshead speed of 0.1 mm/min, and the KIC value was calculated based on the recorded maximum load at fracture. Fractured surfaces were characterized using scanning electron microscopy (SEM). Results were statistically analyzed using Weibull statistics and the Student t test (α=.05).Significantly (P<.05) higher KIC value was determined for IPS e.max Press than for IPS e.max CAD and, based on the Weibull modulus (m), IPS e.max Press was also more reliable. Fractured surfaces, characterized by SEM, showed a marked difference between the 2 materials, suggesting a more complete crystallization in IPS e.max Press, which was most likely responsible for the higher KIC determined.Within the limitations of this in vitro study, the results suggest that IPS e.max Press is superior to IPS e.max CAD with regard to the KIC and characteristic Weibull parameters.