Edge chipping and flexural resistance of monolithic ceramics☆
Introduction
More than a decade of clinical trials recording survival rates for all-ceramic posterior crowns and fixed dental prostheses (FDPs) indicate vulnerabilities to various failure modes [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. The trend in circumventing this problem has been toward a strong and tough yttria tetragonal zirconia polycrystal (Y-TZP) core veneered with an esthetic porcelain [25], [26], [27]. Such bilayer systems have several major drawbacks: (i) their fabrication is a multistep process; (ii) the veneer has a low toughness and is consequently susceptible to chipping [28]; (iii) the bonding between veneer and core can be weak relative to the toughness of the constituent material layers, with ensuing potential for delamination [29], [30] and (iv) residual tensile stresses can develop during the veneering process, further degrading the porcelain and its bond to the zirconia core [31], [32], [33], [34]. Veneered zirconia prostheses do not appear to perform as well as their veneered metal counterparts [9], [27], [28].
The obvious way to circumvent all these drawbacks is to replace the veneer/core bilayer with a monolithic ceramic. This has not been straightforward, because the microstructural qualities that confer good mechanical properties do not lend themselves to good esthetics, and vice versa. However, some approaches are emerging with the promise of improved clinical performance. One such is to begin with translucent but inherently weak glass–ceramics, such as Dicor [35], [36], and to refine the compositions and microstructures to produce a tougher ceramic without compromising the esthetics. The lithium disilicate IPS e.max glass–ceramics manufactured by Ivoclar-Vivadent fall into this category [37]. Those materials have strong needle-like crystals embedded within a glass matrix, mimicking natural enamel and thereby inhibiting crack propagation. They have performed well in crown applications [13], [23]. However, they are not as strong as the zirconias, and are less suitable for applications where stress concentrations can be high, e.g. FDP connectors [38].
An alternative approach has been to begin with a strong and dense zirconia, and to manipulate additive components and heat treatments to produce an acceptable translucency. The monolithic zirconias BruxZir by Glidewell and LAVA Plus by 3 M ESPE are examples. The translucency of BruxZir is achieved via the elimination of light-scattering alumina sintering aids and porosities, along with the utilization of a higher sintering temperature (1530 °C) and longer dwell time (6 h). The translucency of LAVA Plus is attained by reducing alumina sintering aids, but also by increasing the density of the green compact in order to reduce the sintering temperature (1450 °C) and dwell time (2 h), with resultant finer grain size. Glass-infiltrated zirconia (GZG) is another route to improved translucency [39]. While remaining highly fracture resistant, zirconia-based ceramics do not yet match the esthetics of the glass–ceramics.
In assessing potential lifetimes of any material type it is important to consider the different modes in which fracture may occur. Foremost among these modes is top-surface chipping [40], [41], [42] and subsurface flexural fracture [43], [44], [45]. Resistance to one mode does not necessarily imply resistance to the other. The hypothesis in this work was that monolithic crowns can sustain uncommonly high bite forces, provided certain dimensional requirements are met. To test that hypothesis, the chipping resistance and flexural strength were evaluated for candidate lithium disilicate glass–ceramics and modified zirconia materials. To place the results into perspective, comparative data for a veneering porcelain and unmodified Y-TZP were also evaluated.
Section snippets
Material selection and specimen preparation
The restorative materials selected for this study are listed in Table 1, along with values of Young's modulus E, toughness T and strength S. The properties data for commercial veneering porcelain, as well as for Y-TZP and glass-infiltrated zirconia (GZG), have been documented in earlier reports [42], [46], [47].
Zirconia plates were fabricated in-house from 3 mol% Y-TZP powder (TZ-3Y-E grade, Tosoh, Tokyo, Japan) and sintered at 1450 °C for 2 h in air at a heating/cooling rate of 10 °C/min.
Results
Scanning electron micrographs are shown in Fig. 1 for Y-TZP, GZG, lithium disilicate and porcelain. In contrast to the homogeneous, equiaxed fine-grain structures of the zirconia-based ceramics, lithium disilicate Press and CAD and porcelain have a glass matrix containing ≈70% needle-like elongate and ≈30% equiaxed coarse crystallites, respectively. In the Press grade the crystallites are ≈4 μm long and ≈0.6 μm wide and somewhat aligned parallel to the polished surface, whereas in the CAD grade
Discussion
This study has evaluated the mechanical properties of the latest-generation monolithic ceramics for crowns and bridges, with specific attention to zirconia-based ceramics and lithium disilicate glass–ceramics. The zirconia-based materials have markedly superior resistance to chipping and flexural fracture. However, even the most translucent zirconias produced to date cannot approach the esthetics of glass–ceramics, especially in terms of shade matching. The traditional procedure of adding a
Conclusions
Monolithic ceramic restorations exhibit markedly superior fracture resistance relative to their porcelain-veneered counterparts, from both chipping and radial cracking modes. Zirconia-based ceramic monoliths have higher resistance to failure than lithium disilicate glass–ceramics, but are less esthetic. However, recent developments in sintering treatments and infiltration procedures are producing more translucent zirconia systems, signaling a bright future for monolithic zirconia-based
Conflict of interest
All authors declare no conflict of interest.
Acknowledgements
The authors would like to thank Dr. F.K. Fatehi for performing statistical analysis. Funding was provided by the United States National Institute of Dental and Craniofacial Research (Grant 2R01 DE017925) and the National Science Foundation (Grant CMMI-0758530) and by NIST.
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Certain commercial materials are identified in this paper to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
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Srikanth has contributed considerably to the work upon revision. He conducted experimental work on flexural fracture of ceramic layers cemented to composites.