How to explain fracture toughness

“Fracture toughness is a measure of how difficult it is topropagate a crack through a material; it describes the ability of a material containing a crack to resist fracture,” writes Dr. J. Robert Kelly, DDS, MS, DMedSc, in his book, Ceramics in Dentistry: Principles and Practice (from Quintessence Publishing Co. Inc.).

While measured strengths—such as flexural strength—are very sensitive to flaw size, fracture toughness is not. Strength is measured using simplified bars or discs and doesn’t take into account influential variables in crown failure, such as water, sandblasting, etching, silanization and the elastic modulus of the cement and buildup material.

The bottom line: when it comes to ceramic restorations, fracture toughness is a much better indicator of the long-term clinical success than flexural strength, says Kelly. 

Based on numerous clinical...