Simulation of cumulative damage associated with long term cyclic loading using a multi-level strain accommodating loading protocol

Objective. To assess step by step the associated cumulative damage introduced in zirconia veneered restorations after long term cyclic loading using a new multi-level strain accommodating loading protocol. Methods. 40 zirconia veneered crowns received thermal and cyclic loading (3.5 million cycles at maximum load of 25 kg representing 70% of the critical load of the veneer ceramic). The used loading protocol allowed for reproduction of the combined damping action of the periodontal ligament, food substance, jaw deformation, and free movement of the mandibular joint. Speed of load application and release was obtained from the chewing cycle of adult patients. Principles of fractographic analysis were used to study the behavior and origin of critical crack and associated structural damage. Results. The multi-level strain damping effect prevented generation of cone cracks and contact damage under the loading indenter commonly associated with fracture strength tests. 29 specimens (73%) survived 3.5 million cycles without fracture, 9 specimens (22%) demonstrated cohesive fracture of the veneer ceramic and limited axial fracture of the framework was observed in two specimens (5%). Of all fractured specimens, 2 restorations (5%) failed after 500,000 cycles while the rest survived at least 3 million cycles before fracture was observed. Fractographic analysis revealed initial wear and abrasion below the loading area, subsurface micro-cracking of the glass matrix followed by slow crack growth that traveled in a stepping pattern till deflection at zirconia veneer interface. Significance. Cyclic loading using multi-level strain accommodating model can reproduce clinical failure. With exception to manufacturing errors, zirconia veneered restoration survived a simulated 7-year service time without fracture. (c) 2012 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.