Stress distribution and failure mode of dental ceramic structures under Hertzian indentation.
OBJECTIVE
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, T(c)=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 E(s) 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 E(s) and T(c), 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 E(s), 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.


CONCLUSIONS
The failure mode observed clinically for ceramic restorations was reproduced in laboratory tests.
Dong
X D
XD
Dental Materials Science, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong, People's Republic of China.
Darvell
B W
BW
D000538:Aluminum Silicates; D001840:Dental Bonding; D003776:Dental Porcelain; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D004548:Elasticity; D020342:Finite Element Analysis; D006244:Hardness; D006245:Hardness Tests; D008422:Materials Testing; D019279:Resin Cements; D013314:Stress, Mechanical; D013718:Tensile Strength
D000538:Aluminum Silicates; C075815:IPS-Empress ceramic; D019279:Resin Cements; D003776:Dental Porcelain