Inﬂuence of Core-Veneer Thickness Ratio on the Fracture Load and Failure Mode of Zirconia Crowns Luciana Mara Soares, DDS, MS, 1 Cristiano Soares, 2 Milton Edson Miranda, DDS, MS, PhD, 3 & Roberta Tarkany Basting, DDS, MS, PhD 4 1 Department of Restorative Dentistry, Herman Ostrow School of Dentistry of USC, Los Angeles, CA 2 Dental Technician, Campinas, S ˜ ao Paulo, Brazil 3 Department of Prosthodontics, S ˜ ao Leopoldo Mandic Institute and Dental Research Center, Campinas, S ˜ ao Paulo, Brazil 4 Department of Restorative Dentistry, S ˜ ao Leopoldo Mandic Institute and Dental Research Center, Campinas, S ˜ ao Paulo, Brazil Keywords Ceramic crowns; chipping; thermomechanical cycling. Correspondence Profa. Dra. Roberta Tarkany Basting, Faculdade de Odontologia e Instituto de Pesquisas S ˜ ao Leopoldo Mandic, Departamento de Odontologia Restauradora– Dent´ ıstica, Rua Jos ´ e Rocha Junqueira, 13. Bairro Swift, Campinas – SP, CEP: 13045–755, Brazil. E-mail: rbasting@yahoo.com. The authors deny any conﬂicts of interest. Accepted January 7, 2017 doi: 10.1111/jopr.12601 Abstract Purpose: To evaluate the inﬂuence of different core-veneer thickness (CVT) ratios on fracture loads and failure modes of zirconia crowns with or without thermomechanical cycling (TC). Materials and Methods: Forty yttrium-stabilized tetragonal zirconia polycrystalline (Y-TZP) cores were veneered with a pressable ceramic and divided into four groups (n = 10): group 1 - 1:3 CVT ratio (0.5-mm core, 1.5-mm veneer); group 2 - 1:1 CVT ratio (1.0-mm core, 1.0-mm veneer); groups 3 and 4 - identical to groups 1 and 2, respectively, except that they underwent TC (1,000,000 cycles with an 84-N load applied at a 4-Hz frequency, temperature cycling at 4°C, 37°C, and 55°C – 30 seconds each). Loads were applied axially to the central fossa of the crowns with a 1 mm/min crosshead speed at 200 N until failure. Results: Two-way ANOVA showed that the crowns with a 1:3 CVT ratio had a lower mean fracture load than crowns with a 1:1 CVT ratio (group 1 = 1666.4 ± 466.4 N; group 2 = 2156.6 ± 472.3 N). TC had no effect on the fracture load for crowns with the same CVT ratio. The failures observed for the crowns in groups 1, 3, and 4 were fractures within the veneering ceramic, which exposed the core. The failure severity of fractures for group 2 was less extensive, characterized by chipping within the ceramic veneer. Conclusion: Crowns with a 1:1 CVT ratio had higher fracture load values and more conservative failures. TC had no effect on the fracture load of the ceramic crowns. Although all-ceramic restorations have long been used in den- tistry for their mechanical properties and biocompatibility, 1,2 clinical studies have endeavored to document and understand the nature of ceramic veneer fractures, especially yttrium- stabilized tetragonal zirconia polycrystalline (Y-TZP) bilay- ered systems. Chipping and delamination of the veneer have been reported as the major clinical reasons for technical com- plications in performing restorations in the posterior region and could affect the prognosis of the restoration to a varying degree, depending on its extension. 3-7 For this reason, great efforts have been expended to improve the performance of Y-TZP systems and to minimize failures. Several possible explanations for this fracture behavior include: the quality and homogeneity of the coefﬁcient of thermal expansion (CTE), and the elastic modu- lus of veneering ceramic and zirconia to increase the durable bond strength between the two materials; suitable cooling rate to avoid residual stresses at the interface; different cementation techniques; effects of preparation and ﬁnish lines; differences in veneering methods and materials; and issues related to the manufacturing process, for instance, porosities and microstruc- tural defects. 8-18 Studies have emphasized the importance of design and thick- ness of the cores and of the veneering ceramics 12,14,19-24 on the mechanical strength of restorations and stress distribution. The core-veneer thickness ratio and the core design are reported as inﬂuencing the reliability of all-ceramic restorations, particu- larly Y-TZP restorations. 25 The recommended approach to pre- vent chipping and residual thermal stresses between core and veneer is to ensure optimal support to veneering ceramic by providing a thicker core, yet avoiding excessive thickness. 15,20 Aside from material thickness, cooling rate is another factor that could strongly affect the clinical longevity of restorations, and that plays an important role in stress accumulation in the structure. 13,14,25-28 Alternatives have been studied for reduc- ing residual thermal stresses in Y-TZP restorations by manag- ing the cooling rates. It has been reported that overall thermal 1 Journal of Prosthodontics 00 (2017) 1–8 C  2017 by the American College of Prosthodontists