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Introduction

Feldspathic dental porcelains on zirconia are increasingly being used in dental restorations due to their esthetic appearance similar to natural teeth as well as biocompatibility, corrosion resistance and mechanical properties . The fracture of dental porcelains and ceramics is a problem affecting the integrity of the dental components . High masticatory forces may induce fracture or deformation in the dental restoration that can lead to premature failure , but the majority of failures occur after some time. Studies of zirconia-based restorations reported fracture, chipping, and/or delamination as the predominant failure modes primarily in single crowns and bridges, without observing framework failures .

These failures are attributed to different reasons, such as low fracture toughness, inappropriate framework support, low cohesiveness, shear forces between the zirconia framework and veneering porcelain as well as thickness reduction or cracking after occlusal contact in the oral environment .

Different papers in the literature described veneering porcelain failures after 2–5 years of functioning with rates ranging from 0 to 30% depending on the material and service time . One example is the study of twenty-seven patients with 33 zirconia fixed partial dentures (FPDs) examined during five-years, reported by Sailer et al. , where 15.2% of the FPDs presented chipping of the veneering porcelain. Likewise, Vult von Steyern et al. , reported a study of 23 five-unit FPDs that were fabricated for 18 patients on a total of 56 abutments. They were clinically followed for 24 months, where 15% of the failures were observed in the porcelain. Tinschert et al. , reported the study of 46 patients with FPDs monitored for 3 years. In four cases chipping in the veneering porcelain occurred.

Kelly calculated the mean failure load versus days of function, assuming 1 million cycles per year, from data collected of analogs of single-unit prostheses fabricated according to common dental laboratory and clinical practices. Under wet cyclic loading a prediction from 1 to 7 years to failure was determined.

All the previous studies report failure after a time under service, but do not discuss in detail the failure from a material viewpoint, where contact fatigue and stress corrosion cracking (SCC) of the porcelain may be a relevant cause of failure. Stress corrosion cracking refers to crack propagation of small pre-existing flaws under sustained loads usually in presence of water and contact fatigue refers to the crack extension due to the degradation of the microstructure by cyclic contact loading , and it is usual that both mechanisms act simultaneously.

During the mastication process, the magnitude of the forces range between 3 and 364 N over cuspal radii of 2–4 mm with chewing cycles spanning from 0.25 to 0.70 s. It is estimated that the contact loading period per each day is of 15–30 min . Therefore, cyclic contact loads can be a relevant factor for damaging the dental prosthesis.

Masticatory loads and curvature of the teeth can be closely simulated by Hertzian contact loading, where the concentrated stress field result in progressive local damage accumulation . This technique permits to follow the full evolution of contact damage, from initial cracking to final failure .

Recently, we have presented the study of monotonic contact of the feldspathic porcelain on dental zirconia, where the first ring crack appeared in a load range between 40 and 200 N depending on the sphere radii and additional concentric ring cracks and small radial crack that caused removal of material between cracks was observed with higher loads between 80 and 750 N. The aim of the present work is to investigate the contact fatigue response of feldspathic porcelain on dental zirconia, by employing static and cyclic Hertzian indentation tests to assess the stress corrosion cracking and the fatigue of the material, respectively.