Comparison of four monolithic zirconia materials with conventional ones: Contrast ratio, grain size, four-point flexural strength and two-body wear

Abstract

Objectives

To test the mechanical and optical properties of monolithic zirconia in comparison to conventional zirconia.

Materials and methods

Specimens were prepared from: monolithic zirconia: Zenostar (ZS), DD Bio ZX² hochtransluzent (DD), Ceramill Zolid (CZ), InCoris TZI (IC) and a conventional zirconia Ceramill ZI (CZI). Contrast ratio (N=75/n=15) was measured according to ISO 2471:2008. Grain sizes (N=75/n=15) were investigated with scanning electron microscope. Four-point flexural strength (N=225/n=15/zirconia and aging regime) was measured initially, after aging in autoclave or chewing simulator (ISO 13356:2008). Two-body wear of polished and glazed/veneered specimens (N=108/n=12) was analyzed in a chewing simulator using human teeth as antagonists. Data were analyzed using 2-/1-way ANOVA with post-hoc Scheffé, Kruskal–Wallis-H, Mann–Whitney-U, Spearman–Rho, Weibull statistics and linear mixed models (p<0.05).

Results

The lowest contrast ratio values were found for ZS and IC and CZ. IC showed the largest grain size followed by DD and CZI. The smallest grain size was observed for ZS followed by CZ. There was no correlation between grain size and contrast ratio. The aging regime showed no impact on flexural strength. All non-aged and autoclave-aged specimens showed lower flexural strengths than the control group CZI. Within groups aged in chewing simulator, ZS showed significantly lower flexural strength than CZI. CZI showed higher material and antagonist wear than monolithic polished and glazed groups. Glazed specimens showed higher material and antagonist loss compared to polished ones. There was no correlation between roughness and wear.

Conclusions

Monolithic zirconia showed higher optical, but lower mechanical properties than conventional zirconia.

Introduction

Patients appreciate dental prostheses which are durable and at the same time aesthetically pleasing. For this, dental materials with tooth-like optical properties such as ceramics are preferred. Oxide ceramics, particularly zirconia, are gaining attention because of their good biocompatibility, high strength and excellent load-bearing capacity (Piconi and Maccauro, 1999; Sailer et al., 2006; Vult von Steyern et al., 2005). Prospective studies reported about reliable clinical results of 3- and 4-unit partial fixed dental prostheses (FDPs) with frameworks made of zirconia (Heintze and Rousson, 2010; Rinke et al., 2013). The only problems encountered were fractures in the veneering ceramic, so-called chipping (Heintze and Rousson, 2010; Rinke et al., 2013). To avoid such complications, the use of anatomic contour zirconia FDPs (monolithic restoration) without additional veneering was proposed. High-translucency zirconia is a newly introduced material which enables esthetic improvement for the fabrication of posterior monolithic zirconia FDPs. The first steps to get translucent zirconia went through the optimization of sintering parameters (first generation). By increasing the sintering temperature and the sintering time, the zirconia became translucent and showed good esthetic results (Janney et al., 1992, Stawarczyk et al., 2013a, Stawarczyk et al., 2014). The translucency of zirconia can be improved by reducing residual porosity and through the generation of a nanometric microstructure because the in-line transmittance of 50% at the visible wavelength range is expected for grain sizes <40 nm (Klimke et al., 2011, Zhang et al., 2011). The characteristics of porosity and nanometric microstructure can be manipulated as mentioned by sintering parameters and through the resulting enlargement of the zirconia grain size (Jiang et al., 2011, Stawarczyk et al., 2013a, Stawarczyk et al., 2014, Zhang et al., 2012). However, recent studies showed that the increase of grain size decreases the flexural strength and the reliability of the conventional zirconia materials (Stawarczyk et al., 2013a). Even worse, the higher sintering temperature affected negatively the low-temperature degradation of zirconia materials (Hallmann et al., 2012).

In addition, the translucency of zirconia materials can be influenced by the type and the amount of additives (Hallmann et al., 2012). Frequently used additives such as alumina dopant, although efficient in enhancing the densification rate of zirconia owing to an enhanced grain boundary, also reduce material translucency (Matsui et al., 2008, Zhang et al., 2012). With these findings, a second generation of zirconia has been developed. In this generation, the proportions of alumina were lowered and its particle size reduced. In addition, according to the manufacturer (TOSOH, Japan), the alumina oxide particles were placed to the grain boundaries of the zirconia. Because of this optimization the zirconia grain size is relatively small, hence it can be expected a good translucency with good mechanical properties.

Particularly mechanical properties are supposed to strongly affect the wear resistance of such monolithic zirconia materials (Albashaireh et al., 2010, Heintze et al., 2008, Preis et al., 2011). Before insertion, monolithic restorations have to be polished or glazed using a layering technique or a glaze spray (Stawarczyk et al., 2013b). Restorative materials should possess wear properties similar to those of natural teeth. This is essential for the reduction of induced pathological consequences (Heintze et al., 2006). The wear behavior of polished zirconia of the first generation showed only marginal wear rates on enamel antagonists and almost no wear in the zirconia itself (Stawarczyk et al., 2013b). In contrast, glazed and veneered zirconia led to higher wear rates on the enamel antagonist and the ceramic (Stawarczyk et al., 2013b). However, currently, insufficient information is available about the mechanical and optical properties of monolithic zirconia materials of the second generation. Therefore, the null hypothesis stated that contrast ratio, grain size, flexural strength and two-body wear rate of monolithic zirconia materials of the second generation are comparable with conventional zirconia of the first generation.