Wear behavior of dental Y-TZP ceramic against natural enamel after different finishing procedures

Abstract

Objective

The aim of this in vitro study was to evaluate the influence of different finishing procedures on the wear behavior of zirconia against natural enamel.

Methods

64 quadratic specimens (10 mm × 10 mm × 2 mm) were cut from a commercial hipped dental Y-TZP ceramic. Four different groups with 16 specimens each were formed according to the following finishing procedures: PZ (polished), RR (fine-grit diamond), GR (coarse-grit diamond), GZ (glazed). Polished specimens of a leucite-reinforced glass ceramic (Empress CAD) were used as a control (GC). The materials were subjected to the Ivoclar wear method (Willytec chewing simulator, 120,000 cycles, 5 kg weight) with 80 natural caries-free cusps of first upper molars as antagonists. Wear was analyzed for both the enamel cusps and test specimens by measurement of the vertical substance loss with a laser scanner. Surface roughness was measured by means of a white-light interferometer.

Results

The surface roughness was significantly different among the polished, diamond-finished, and glazed ceramic specimens (ANOVA, post hoc Bonferroni p < 0.05). The results of the one-way ANOVA indicated that the finishing technique significantly affected enamel wear ( p < 0.05). The post hoc test indicated that the specimens finished with the coarse diamond caused significantly higher antagonist wear than the polished ones. Polished zirconia showed the lowest wear of the antagonist enamel, with a mean value of 171.74 (SD = 121.68), and resulted in enamel wear that was not significantly different from that of the glass ceramic control group. No significant linear correlation could be found between pre-testing surface roughness and abrasive wear.

Significance

If zirconia is used without veneering material for crowns and fixed dental prostheses (FDPs), the surface must be well-polished if occlusal adjustments with coarse diamonds are performed. The polishing step reduces the wear of the opposing enamel.

Introduction

Yttrium-stabilized tetragonal zirconia (Y-TZP) ceramics are considered highly stable materials and have been established as core materials for all-ceramic crowns and fixed dental prostheses (FDPs), due to their excellent strength and superior fracture resistance as the result of an inherent transformation-toughening mechanism. Compared with other metal-free core materials for FDPs, Y-TZP displays the highest initial and most favorable long-term strength . Zirconia frameworks can be used for FDPs in both the anterior and posterior regions and on implants . In addition, zirconia presents good biocompatibility, low radioactivity, and tooth-like optical properties .

For esthetics, zirconia frameworks must be veneered with translucent feldspathic or glass ceramic materials. The bonding mechanism between Y-TZP and the veneering ceramic is not yet well-known, and the core–veneer interface is among the weakest aspects of these restorations, so ceramic chipping is possible. In in vitro studies of layered zirconia single crowns, the fracture strength was limited mainly to the veneer ceramic . Although special layering ceramics have been developed for Y-TZP to minimize this unfavorable aspect, in clinical applications, the veneering ceramic has also been revealed to be the weakest link in zirconia-supported reconstructions. A systematic review revealed that the frequency of chipping was statistically significantly higher for zirconia-supported FDPs compared with metal-supported FDPs ; however, there was a strong study effect. Different modifications have been sought to address the problem, both in the firing process and in the technical fabrication of the FDPs. Since the thermoconductivity of zirconia is much lower compared with that of metal, considerable internal stress is created . Prolonged cooling phases during firing resulted in less internal stress and less chipping in the laboratory . Furthermore, it has been suggested that: (1) the veneer thickness should be less than two-fold of the core thickness and (2) the veneers’ cusps should also be anatomically supported by the zirconia in the pontic area.

A simple method to avoid chipping is to use non-veneered, full-contour, zirconia materials. Several manufacturers (e.g., BruxZir, Glidewell Laboratories, Newport Beach, CA, USA) offer this possibility to their clients; they have improved the esthetics of zirconia materials, mainly by reducing the opacity of the material and adding coloring pigments. In addition to chipping resistance compared with conventional zirconia-based restorations, there are other possible advantages. For example, it can be assumed that, by omission of the ceramic veneering, a more solid framework could be constructed, and a conservative preparation similar to full-cast gold could be performed, since there would be no need to maintain space for the veneer porcelain. The manufacturing costs could be reduced by automatic designing and milling to a full-anatomical contour by CAD/CAM technology. The technique-sensitive veneering process would no longer be necessary, thus guaranteeing a more consistent quality of the restorations.

However, one concern associated with the use of monolithic zirconia FDPs is the possible abrasiveness of the material toward enamel . A restorative material used for crowns and FDPs should possess microstructural and bio-tribological properties similar to those of natural teeth; otherwise, excessive enamel wear of the antagonist can occur. Enamel wear caused by opposing ceramic restorations is a progressive phenomenon associated with physical, microstructural, chemical, and surface characteristics of dental ceramics . Despite manufacturers’ claims that veneering ceramics possess enamel-like tribological qualities, heterogenic results have been reported. Several investigators have demonstrated that, in general, dental ceramic substrates cause greater abrasive wear of human enamel compared with dental alloys, even though the wear mechanism is unclear. To date, no clinical data about the wear behavior of dental Y-TZP against natural enamel are available.

During manufacturing and clinical use, the surface of a full-contour zirconia restoration could be subjected to numerous surface treatments, such as grinding, polishing, glazing, and heat. The dental literature has focused on the effect of surface treatment on the strength of zirconia-based materials, and the influence of strength has been related to the percentage of the transformation of zirconia into the more unstable monoclinic phase , the severity of surface damage , and local temperatures . Less is known about the wear behavior of zirconia for dental applications. Knowledge gathered from zirconia hip joint tests can hardly be transferred into dental clinical practice for different reasons. While some basic studies of zirconia–zirconia combinations have shown catastrophic wear , others have demonstrated excellent wear resistance , mainly due to different test conditions . In clinical investigations zirconia femoral heads are generally articulated with ultra-high molecular polyethylene cups. Moreover, the different loading conditions in hip joint prostheses and dental restorations result in different wear mechanisms.

As wear measurements for dental materials in vivo are very time-consuming and complicated, wear is generally assessed in wear simulators. Different devices that use different force actuator and wear mechanism principles are available . It has been shown, that the wear results from different wear simulators are not comparable, as all methods follow different wear testing concepts . The IVOCLAR wear method, run on the Willytec chewing simulator proved a good compromise for measuring two-body wear with regard to costs, practicability and robustness.

The aim of this in vitro study was to investigate the abrasive behavior of dental Y-TZP against natural enamel in relation to the influence of different finishing procedures. As control, the leucite-reinforced glass ceramic material Empress CAD was used. The null hypotheses were as follows: (1) there is no difference in enamel wear between zirconia crowns with different surface preparations; and (2) there is no difference between both materials with regard to enamel wear.

Materials and methods

Specimen preparation

A commercial hipped Y-TZP ceramic (Everest ZH, KaVo, Biberach, Germany, Batch No. 6421) and a leucite-reinforced glass ceramic (Empress CAD, Ivoclar Vivadent, Schaan, Liechtenstei, Batch No. K11423) were tested. The test specimens ( n = 80) were prepared by cutting the industrial sintered blanks using a 0.8 mm wide diamond cut-off-wheel saw (Seccotom 10 433CA, STRUERS GmbH, Willich, Germany) with coolant at 1700 rpm and a crosshead speed of 0.080 mm/s. The surfaces of the ceramic specimens were ground and polished with a commercial metallographic preparation system (Tegra Force 5, Struers, Willich, Germany). The sequence of the grinding/polishing procedures is summarized in Table 1 . After the specimen surfaces were ground and polished to a high gloss, the dimensions of the quadratic specimens were 10 ± 0.5 mm × 10 ± 0.5 mm × 2 ± 0.5 mm. Subsequently, specimens were embedded in epoxy resin with the finished surface facing up.

Table 1
Preparation procedures for the ceramic specimens.
Step Preparation tool Polishing suspension
Plae grinding MD-Piano: resin bonded diamond disc (grain size 220 μm)
Fine grinding MD-Largo: composite disc (grain size 3–9 μm) Dia Pro Alegro: diamond suspension
Diamond polishing MD-Dac: satin woven acetate OP-S: colloidal silica (0.04 μm)
Oxide polishing MD-Chem: porous neoprene OP-S: colloidal silica (0.04 μm)

Five different groups with 16 samples each were formed according to the finishing procedures and material used ( Table 2 ).

Table 2
Description of the test groups.
Test group Group code
P olished Z irconia PZ
Zircona prepared with R ed R ing 30-μm diamond bur (8879.314.016) a RR
Zircona prepared with Gre en R ing 100-μm diamond bur (S6879.314.016) a GR
G lazed Z irconia GZ
Polished G lass C eramic GC

a Brasseler, Savannah, GA, USA.

The finishing procedures for the groups RR and GR were accomplished with a specially developed device that held both the high-speed handpiece and the embedded ceramic specimen. The device was designed to guarantee a constant pressure of 50 N of the rotating instrument on the test specimen, so that the surface would remain flat after material removal ( Fig. 1 ). The handpiece (KAVO GENTLEpower LUX 25 LP, KaVo, Biberach, Germany) was moved by hand on a sliding platform. Five back-and-forth strokes were performed at the maximum operating speed for the handpiece, 200,000 min −1 . After being processed and before being tested, the flat specimens were kept dry at a temperature of 37 °C for 24 h.

Fig. 1
Schematic presentation of the device used for the finishing procedures.

For Group GZ, the specimens were sandblasted for 30 s with 100-μm Al 2 O 3 particles at a pressure of 2 bar and glazed with a commercial glazing agent (Vita Akzent VM9, Vita Zahnfabrik, Bad Säckingen, Germany, Batch No. 7485) according to the manufacturer’s instructions.

Enamel antagonists

80 mesio-palatal cusps of similar shape were cut from intact human first or second upper molars with completed root development . Teeth with pointed surface configuration, enamel defects, or hypomineralization were not used. The teeth were mechanically cleaned, disinfected in 0.2% sodium azide solution for 1 wk, and stored in tap water. The shape and the surface configuration of the antagonist cusps were evaluated and photographed under a light stereomicroscope (Leica S6D, Leica Microsystems AG, Heerbrugg, Switzerland) at 10× magnification. The antagonists were luted to aluminum SEM holders by means of a chemically curing composite (Rebilda SC, VOCO, Cuxhaven, Germany).

Wear simulation

The ceramic specimens and the enamel antagonists were mounted in a commercially available dual-axis chewing simulator (Willytec, SD Mechatronik, Feldkirchen-Westerham, Germany) and were subjected to 120,000 masticatory cycles of unidirectional antagonist movements with a frequency of 1.6 Hz and a force of 50 N. The samples performed a lateral movement of 0.7 mm upon contact with the antagonist to simulate a wear path. In the test chambers, demineralized water was used to (1) keep the enamel antagonists wet, (2) remove worn particles, and (3) simulate the wet environment of the oral cavity.

Wear quantification

Before and after the wear test procedures, polysiloxane (Identium Medium Soft, Kettenbach, Eschenburg, Germany) impressions of the test specimens and the antagonists were taken, and replicas of the impressions were fabricated with super hard plaster (Type IV, Fuji Superhard Rock, GC Corporation, Tokyo, Japan).

The surfaces of these replicas were analyzed by means of an optical 3D surface scanner, Laserscan 3D (Willytec/etkon, Gräfelfing, Germany) . The maximal vertical loss and the mean vertical loss were then determined by use of the appropriate software, Match 3D, Version 2.3 (Willytec/etkon). Use of the 1% eliminated extreme values produced by fine dust particles as well as other discrepancies.

Surface roughness measurements

The surface of each specimen was analyzed by white-light interferometry (Zygo New View 200 3D Imaging Surface Structure Analyzer, Zygo Corporation, Middlefield, CT, USA). A quantitative 3D image was then calculated by frequency domain analysis. The specimens were measured with a 10× magnifying lens, delivering a vertical resolution of 0.1 nm and a lateral resolution between 0.64 and 11.8 μm. The arithmetic mean deviation of the assessed profile ( R a ) was measured under the following conditions: measurement length of 5.0 mm, and measurement speed of 2 μm/s.

Electron backscatter diffraction (EBSD)

For quantification of the microstructure with respect to grain size, grain morphology, and texture, the EBSD technique was used. A high spatial resolution of up to 50 nm could be achieved. EBSD was applied based on an automated recording of individual Kikuchi patterns on a user-specified grid, with subsequent automatic indexing. Since Kikuchi patterns are generated within a 50 nm volume under the surface of the sample, the metallographic preparation of the sample is crucial when the EBSD technique is used. In this study, samples were ground to 0.25 μm and mechanically polished by standard metallographic techniques. Afterward, a final preparation step involving ion-beam polishing of the region of interest was performed.

The EBSD system in this study consisted of a FEI Dual Beam Workstation (Strata DB 235, FEI, Hillsboro, Oregon, USA) equipped with a TSL OIM analysis unit. The acceleration voltage to generate Kikuchi patterns was 20 kV. All patterns were recorded with a Hikari camera at a scanning speed of 450 patterns/s. Orientation maps were generated by scanning of the electron beam over a selected area and the simultaneous analysis of the Kikuchi patterns. EBSD results are typically presented as so-called ‘inverse pole figure’ (IPF) maps. They chiefly provide information concerning crystallographic orientation; however, by means of software analysis, the distribution of grain sizes in the selected area can also be determined.

Microstructural analysis

The microstructure of finished ceramic specimen surfaces as well as their topography was analyzed by standard SEM techniques (Strata DB 235, FEI Company, Hillsboro, OR, USA). As for the zirconia test specimens from the groups PZ, RR and GR no wear could be detected by means of 3D scanning, the wear areas were marked and for selected specimens the surfaces within the markings were inspected for wear traces using SEM.

Statistical analysis

Statistical evaluation was performed with SPSS for Windows, Release 17.1 (SPSS Inc., Chicago, IL, USA). Differences in means of each group were compared by analysis of variance (ANOVA). Post hoc comparisons were performed with the Bonferroni correction. Level of significance was set at 0.05. Correlation between the outcome variables for vertical loss and the surface roughness was evaluated by Spearman’s correlation test. The relationship between pre-testing roughness and wear was assessed by Spearman’s correlation test.

Materials and methods

Specimen preparation

A commercial hipped Y-TZP ceramic (Everest ZH, KaVo, Biberach, Germany, Batch No. 6421) and a leucite-reinforced glass ceramic (Empress CAD, Ivoclar Vivadent, Schaan, Liechtenstei, Batch No. K11423) were tested. The test specimens ( n = 80) were prepared by cutting the industrial sintered blanks using a 0.8 mm wide diamond cut-off-wheel saw (Seccotom 10 433CA, STRUERS GmbH, Willich, Germany) with coolant at 1700 rpm and a crosshead speed of 0.080 mm/s. The surfaces of the ceramic specimens were ground and polished with a commercial metallographic preparation system (Tegra Force 5, Struers, Willich, Germany). The sequence of the grinding/polishing procedures is summarized in Table 1 . After the specimen surfaces were ground and polished to a high gloss, the dimensions of the quadratic specimens were 10 ± 0.5 mm × 10 ± 0.5 mm × 2 ± 0.5 mm. Subsequently, specimens were embedded in epoxy resin with the finished surface facing up.

Table 1
Preparation procedures for the ceramic specimens.
Step Preparation tool Polishing suspension
Plae grinding MD-Piano: resin bonded diamond disc (grain size 220 μm)
Fine grinding MD-Largo: composite disc (grain size 3–9 μm) Dia Pro Alegro: diamond suspension
Diamond polishing MD-Dac: satin woven acetate OP-S: colloidal silica (0.04 μm)
Oxide polishing MD-Chem: porous neoprene OP-S: colloidal silica (0.04 μm)
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Wear behavior of dental Y-TZP ceramic against natural enamel after different finishing procedures
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