Apparent fracture toughness of metal ceramic restorations with different manipulative variables

Apparent fracture toughness (Kc) is a measure of the strain absorption capacity of brittle materials. It relates to the level of tensile stress that must be exceeded at the tip of a crack before failure occurs. Using an indentation technique, this study measured Kc and elastic modulus-to-hardness ratio (E/H) of 65 metal ceramic crowns made with 13 different variables. The variables studied were (1) brand of porcelain, (2) firing temperature, (3) condensation, and (4) modeling liquid. Statistically significant differences in Kc and E/H were found in respect to porcelain brand and between two of the modeling liquids. No significant differences were found in respect to altering firing temperature or condensation.