Strength influencing variables on CAD/CAM zirconia frameworks
Introduction
The introduction of zirconia to the dental field opened up the design and application limits of all-ceramic restorations. The superior mechanical properties of zirconia combined with the state-of-the-art CAD/CAM fabrication procedure allowed for the production of large and complex restorations with high accuracy and success rate [1].
The strength of zirconia can be directly influenced by different surface treatment methods which exert different degrees and types of surface damage. These areas of surface flaws act as stress concentration sites and even though they are microscopic in nature, they act as potential sites for crack initiation and propagation [2]. Dental literature has focused on studying the effect of different surface treatments on the strength of zirconia-based materials and reduction in strength was generally associated with the degree of surface damage [3], [4], [5]. On the other hand, some studies reported an increase in strength after air-borne particle abrasion and related such finding to the creation of compressive fields as a result of the induced tetragonal–monoclinic transformation of the surface crystals [6], [7], [8], [9].
A point worth noting is that in some of the previous studies polished zirconia was used as a reference point, while grinding with different grits of silicon carbide paper and air-borne particle abrasion at a high pressure were commonly used as examples of different levels of surface damage. On the other hand, under daily circumstances, zirconia is subjected to a different type of surface damage as a result of the milling procedure, which leaves behind characteristic trace lines and different patterns of surface damage and flaws [5]. Additionally, the combined effect of the CAM milling procedure and common laboratory procedures such as air-borne particle abrasion and multiple firing cycles used in baking the ceramic veneer, could be different from that expected for polished or disc-ground zirconia [10]. The mechanical properties and the long-term stability of CAD/CAM zirconia will be a function of the exerted surface damage, the degree of transformation, and the loading environment in terms of peak stresses and number of cycles [11].
The aim of this study was to evaluate the damage induced by the CAD/CAM milling procedure, combined with different surface finishing procedures, on the mechanical properties of zirconia and to analyze the interaction between these variables.
Section snippets
Materials and methods
At least 20 zirconia bars (17 mm × 2 mm × 1 mm) for each group where prepared by either of the following methods: cutting zirconia milling blocks in a sawing machine (ISOMET Q2 1000, Buehler Ltd., Lake Bluff, IL) using a diamond coated disc saw (ground bars) or by using CAD/CAM technology (Cercon, Degudent GmbH, Hanau-Wolfgang, Germany) where individual bars were milled by the machine using wax replicas (CAM bars). The bars were sintered in the relevant manufacturer equipment (Cercon Heat uses a 6.5 h
Results
Statistical analysis revealed significant differences in the three-point flexure strength values between the groups tested as a result of the different surface treatment methods applied (F = 35.5, p < 0.000). According to strength values, the groups tested could be divided into the following categories: (1074–1166 MPa) for polished zirconia and CAM and polished bars which were particle abraded (50 μm Al2O3), (936 MPa) for ground bars that were particle abraded (50 μm Al2O3), (708–794 MPa) for CAM bars,
Discussion
As the main function of the underlying framework is to support the ceramic veneer and to carry the loading forces, different laboratory tests were used to evaluate the internal strength of zirconia frameworks. The design of fixed partial dentures can be considered as a simple beam and different flexure strength tests are frequently used for strength evaluation [4]. On the other hand, standard flexure strength tests do not take into account important factors such as the effect of design,
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