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Ibbetson, Richard J.
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article
The effect of coping/veneer thickness on the fracture toughness and residual stress of implant supported, cement retained zirconia and metal-ceramic crowns
Abstract
<p>Objectives. The aim of the work was to assess the influence of coping and veneer thickness on the fracture resistance of porcelain-metal and porcelain-zirconia crowns in a clinically representative model.</p><p>Methods. A total of 30 zirconia and 30 precious metal copings were fabricated. There were 10 copings in each group of 0.5 mm, 1 mm and 1.5 mm thickness. Each group of 10 was further divided into two groups with a total thickness of three and four millimeters inclusive of veneering ceramic. The specimens were cemented to a titanium abutment with zinc oxide cement and tested using a microindenter. Crack length, hardness and spalling (chipping) were recorded using an optical microscope from which fracture toughness was calculated.</p><p>Results. Kruskal-Wallis tests and simple linear regression analysis were used to analyze the results, revealing a significant difference between zirconia (ZrCC) and metal (MCC) specimens for crack length. 0.5 mm coping thicknesses and MCC specimens showed the highest fracture toughness values. Simple linear regression analysis showed a limited effect of the overall thickness or veneer thickness on crack length and hardness; however coping thickness showed a positive correlation with both. Spalling was higher in zirconia specimens.</p><p>Residual stresses were higher for ZrCC specimens and showed a positive correlation with crack length. The ratio of veneer to coping thickness was negatively correlated with residual stress.</p><p>Significance. Residual stresses due to thermal mismatch between the coping and the veneering ceramic should be kept to a minimum. The ZrCC specimens were found to have lower apparent fracture toughness than MCC specimens. Thermal mismatch caused a larger drop in apparent fracture toughness than the inherent differences in the materials used. (C) 2012 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.</p>