Loading capacity of zirconia implant supported hybrid ceramic crowns
Introduction
As an alternative to the well-established dental implants made from titanium, zirconium dioxide has been introduced. Zirconia is an inert, non-resorbable and biocompatible metal oxide, which facilitates osseointegration in the form of 3–5 mol% yttria stabilized polycrystalline tetragonal zirconia (Y-TZP) [1], [2]. Esthetic considerations or potential allergies indicate the use of zirconium dioxide instead of titanium, and clinical short-term success rates are promising [3], [4], [5]. However, due to the lack of long-term data the use of zirconia implants in routine clinical practice is not yet recommended [3].
Implant and suprastructure consist of different materials functioning together as a complex system in withstanding strong intraoral bite forces. Maximum biting forces are reported to be in the range of 286–847 N in the anterior and molar region, respectively [6], [7]. Due to its excellent characteristics such as esthetical properties, chemical stability, biocompatibility, and a coefficient of thermal expansion similar to the natural tooth [8] ceramic might be the material of choice for implant restorations. Unfortunately, under tensile stress ceramic is susceptible to fracture as a result of its brittleness, surface and bulk defects and crack propagation under oral function [9].
In order to improve the reliability of ceramics a novel polymer infiltrated ceramic was developed [10], [11], [12]. For this material a fracture toughness of 1.21 MPa m1/2 [12] is reported, which is higher than the one of a typical dental feldspar ceramic (0.92–1.12 MPa m1/2) [13]. In parallel the hybrid ceramic showed a three times lower hardness value (2.92 ± 1.92 GPa) compared to the feldspar ceramic (10.64 ± 0.46 GPa) [14], [15]. In a three point bending test a flexure strength of 144.44 ± 9.61 MPa was measured [12]. Due to its low modulus of elasticity of 31.72 ± 1.43 GPa [12] the hybrid material may work as a buffer area to counterbalance the stiffness of zirconia implants, which is owed to the high elastic modulus of zirconia in the range of 200 GPa [16] and the ankylotic connection to the bone as a result of osseointegration.
When investigating the performance of hybrid ceramic restorations the influence of the cement as an intermediate layer has to be considered. The impact of the cement type on fracture load values of tooth supported restorations has been analyzed in several investigations [17], [18], [19]. The use of a conventional zinc phosphate cement resulted in lower fracture load values for feldspar and resin composite crowns than a cementation with adhesive cement [18]. Glass-infiltrated alumina as well as lithium disilicate and leucite reinforced ceramic crowns luted with a resin composite cement showed higher fracture load values than those luted with a resin modified glass ionomer cement [19]. On a steel analogue of a prepared upper canine, fracture load values of zirconia, lithium disilicate or ceramic fused to metal crowns were not influenced by the type of cement [20]. This was explained by the fact that the intrinsic strength of these materials was so high that cementing with an adhesive cement could not contribute to the fracture strength. In contrast the fracture load of leucite reinforced glass-ceramic crowns significantly increased by the use of adhesive cement when being compared to glass ionomer cement [20].
The test design has a strong impact on fracture load test results. For instance feldspar ceramic as one of the weaker materials among ceramic systems has been tested with fracture load values of 300–1279 N while using different fracture load test designs [18], [21], [22]. Fracture load values of 833.4 ± 147.5 N [22] and 1272 ± 109 N [21], respectively were found in two different studies where machined feldspar ceramic crowns were cemented with an adhesive resin composite cement on human teeth or epoxy duplicates of a prepared tooth. These observations indicate that test results cannot easily be matched and a control group is essential in every investigation.
The objective of this study was to compare the fracture load values of a new hybrid ceramic material with a feldspar ceramic on zirconia implants while using different luting cements and to detect any correlation between the fracture load values of the ceramics and mechanical properties of the cements. The hypotheses are that (1) the fracture load values of hybrid ceramic crowns are higher than those of feldspar ceramic crowns and (2) fracture load values of feldspar and hybrid ceramic crowns are influenced by mechanical properties of the cement.
Section snippets
Implant preparation
Ten one-piece zirconia implants (ceramic implant, VITA Zahnfabrik, Bad Säckingen, Germany) with a diameter of 4.0 mm, a length of 10 mm in the endosseous part and a machined abutment surface (Ra = 0.42 ± 0.06 μm) were used for this study. All implants were embedded according to ISO 14801:2008 in epoxy (RenCast CW 20/Ren HY 49, Huntsman Advanced Materials, Duxford, UK) in order to simulate the elasticity of human bone. The implants were inserted with a 3 mm clearance between implant neck and resin
Fracture load
All crowns fractured during the test under a load ranging from 249 to 1239 N, forming 2–5 fragments. No damage was detected at the implant or the epoxy resin block. The hybrid material (VE) exhibited significantly higher fracture loads than the feldspar ceramic (VM) for the respective cement (p = 0.037). The highest values were achieved with VE machined and MLA (1253 ± 148 N) and the lowest values for VM etched without cement (249 ± 58 N) (Fig. 6 and Table 3). In no group etching improved the fracture
Discussion
The present study was designed to show how the new hybrid ceramic material performs within an implant system and in what way cements influence the fracture load of crowns. The first hypothesis that the fracture load values of hybrid ceramic crowns are higher than those of feldspar ceramic crowns was confirmed. A correlation between mechanical properties of the cements and fracture load of the crowns was only found for the compressive strength. Hence, the compressive strength of the cement has
Conclusions
In the present test design hybrid ceramic provides higher loading capacity than feldspar ceramic. The fracture load of hybrid ceramic and feldspar ceramic crowns is correlated to the compressive strength of the cement. In the test design used, etching of the intaglio surface of hybrid or feldspar ceramic crowns had no effect on the fracture load of the restoration.
Acknowledgements
The authors are grateful to VITA Zahnfabrik for supporting this study with materials and to Fredy Schmidli (University Hospital for Dental Medicine) for the lab support. The statistical analysis was kindly performed by Marek Mrzyk, VITA Zahnfabrik.
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