Stress distribution and failure mode of dental ceramic structures under Hertzian indentation

Objectives. To understand better the clinically-relevant failure of the ceramic in ceramic–cement–substrate structures under Hertzian indentation, including the effects of supporting substrate modulus and ceramic thickness on the stress distribution in the ceramic. Methods. Discs (thickness, Tc=0.2, 0.6, 1.2, 1.6, 2.0, 2.4 mm) of a glass–ceramic material (IPS Empress 2, Ivoclar) were cemented (Variolink II, Vivadent) to flat polymer substrates with modulus of elasticity Es of 2, 6 and 10 GPa. The top surface of the ceramic–cement–substrate structure was loaded by a 20 mm radius spherical indenter until the initial failure of the ceramic occurred. The finite element method was used to analyse the stress distribution under such Hertzian indentation, varying Es and Tc, as well as calculating the maximum tensile stress based on the experimentally observed failure load and contact radius. The failure initiation site of the ceramic was identified by fractography using scanning electron microscopy. Results. The tensile stress concentration at the cementation surface of the ceramic was the predominant factor controlling the ceramic failure. Failure load increased with increase of Es, while the maximum tensile stress at the cementation surface of the ceramic clearly decreased. Failure load increased logarithmically with ceramic thickness, but the critical tensile stress increased linearly. Significance. The failure mode observed clinically for ceramic restorations was reproduced in laboratory tests.