How do all-ceramic crowns fracture?

This summary is based on the research done by the Nordic Institute of Dental Materials (NIOM) and published in the European Journal of Oral Sciences: Simulation of clinical fractures for three different all-ceramic crowns (April 2014)

Marit Øilo, Ketil Kvam, Nils R. Gjerdet

Context

• Comparison of fracture strength and fracture modes of different all-ceramic crown systems is not straightforward.
• Established methods for reliable testing of all-ceramic crowns are not currently available.
• Published in-vitro tests rarely simulate clinical failure modes and are therefore unsuited to distinguish between the materials.
• The in-vivo trials usually lack assessment of failure modes.
• Fractographic analyses show that clinical crowns usually fail from cracks initiating in the cervical margins, whereas in-vitro specimens fail from contact damage at the occlusal loading point.

Purpose of the Study

To compare three all-ceramic systems using a clinically relevant test method that is able to simulate clinical failure modes.

Key Findings

• Ten incisor crowns of three types of all-ceramic systems were exposed to soft loading until fracture. The initiation and propagation of cracks in these crowns were compared with those of a reference group of crowns that failed during clinical use.
• All crowns fractured in a manner similar to fracture of the clinical reference crowns. 
• The zirconia crowns fractured at statistically significantly higher loads than alumina and glass-ceramic crowns. Fracture initiation was in the core material, cervically in the approximal areas.

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