Impact of high-speed sintering, layer thickness and artificial aging on the fracture load and two-body wear of zirconia crowns

Abstract

Objective

To investigate the impact of high-speed sintering, layer thickness and artificial aging in a chewing simulator on the fracture load (FL) and two-body wear (2BW) of 4Y-TZP crowns.

Methods

4Y-TZP crowns (Ceramill Zolid HT+, Amann Girrbach AG) in three different layer thicknesses (0.5, 1.0, 1.5; N = 192, n = 64/group) were manufactured using CAD/CAM technology and sintered at 1580 °C (high-speed sintering) or 1450 °C (control group). Specimens were polished in two-steps and bonded to standardized CoCr abutments with Multilink Automix (Ivoclar Vivadent). 2BW after 6000 thermo- and 1,200,000 chewing-cycles employing enamel antagonists was determined using best fit machining. FL was tested before and after artificial aging. Univariate ANOVAs, post hoc Scheffé, unpaired t -, Kruskal–Wallis- and Mann–Whitney- U -test were computed ( p < 0.05).

Results

High-speed sintering resulted in less 2BW of the zirconia than the control group ( p = 0.013). High-speed sintering ( p = 0.001–0.006) and an increase in layer thickness ( p < 0.001–0.012) resulted in higher FL values, while artificial aging led to a reduction of FL ( p < 0.001).

Significance

As high-speed sintering resulted in less two-body wear of the zirconia and comparable or even higher fracture load results than the control group, this cost- and time efficient alternative presents promising mechanical results.

Introduction

Due to ever-increasing esthetic demand’s, the dental market for ceramic restorations is continuously growing. While glass-ceramics such as leucite-reinforced- or lithium-disilicate ceramics provide excellent esthetic results for fixed prosthetic restorations in the anterior region, zirconia (zirconium dioxide, ZrO 2 ) is considered the material of choice in posterior regions exposed to high masticatory forces . Dental zirconia materials can be categorized according to their chemical composition, notably via their content of the stabilizer yttrium oxide . While early high-strength yttria stabilized-tetragonal zirconia polycrystal (3Y-TZP) provided superb strength and fracture toughness at the cost of esthetic demands, newer compositions have focused on improving translucency by raising the content of yttrium oxide: 5Y-TZP (5 mol% Y 2 O 3 ) and 4Y-TZP (4 mol% Y 2 O 3 ) .

Zirconia restorations are nowadays routinely manufactured employing computer-aided design and computer-aided manufacturing (CAD/CAM) technology. To avoid high material wear of the grinding tools and detrimental phase transformations, soft zirconia structures are milled from pre-sintered blocks, thus yielding white stage specimens in need of a final sintering process. As the sintering conditions dictate the grain size, they possess a strong impact on both the stability and mechanical properties of the final product, with an increase in firing temperature and holding time resulting in a larger grain size . This correlation however only holds until sintering temperatures of 1600 °C, above which a decrease in flexural strength has been reported . In addition to their influence on a zirconia’s mechanical properties, sintering conditions have been observed to affect optical characteristics such as translucency . With conventional sintering protocols taking 4 – 12 h to complete, ensuing a high expenditure in regard to cost and time, speed sintering has recently gained attention as a possible alternative. While speed sintering at ∼1510 °C takes 30–120 min, high-speed sintering at ∼1580 °C requires less than 30 min. First studies have reported high-speed sintering to lead to similar or even higher flexural strength than observed for standard sintering protocols .

To test a material’s mechanical properties and thus ensure the longevity of a restoration, various in vitro examinations have been established in the dental sciences.

One important factor is the wear of a material over time, closely paired with the wear it causes on its antagonist. Two-body wear (2BW) is routinely investigated by employing chewing simulators: while 1,200,000 chewing cycles are supposed to equal a restoration’s clinical condition after five years in vivo , an occlusal force of 50 N applied with a frequency of 1.1 Hz is aimed to match physiological parameters . As dental restorations are exposed to various elements when in situ, thermocycling should be included in the study set-up to approximate clinical conditions. To simulate natural antagonist-tooth relations , in which the vertical wear rate of human enamel has been reported to be about 20–40 μm per year for a posterior tooth , antagonists made of human enamel are considered the gold standard. When regarding zirconia specimens, low wear rates of both the zirconia and respective antagonist have been observed in previous studies .

Another important factor for assessing a material’s mechanical performance is its resistance to the application of an external force, which can be quantified by determining fracture load (FL). Due to its dense crystal structure, paired with the ability provided by a process called “fracture toughening”, where the zirconia structure transitions from the tetragonal- to the monoclinic phase in a martensitic transformation, thus expanding its volume by 3 – 5% to counteract internal stress , zirconia has been shown to provide excellent results regarding fracture toughness and flexural strength . Compared with 3Y-TZP, 5Y-TZP and 4Y-TZP possess a smaller amount of tetragonal phase in favor of the cubic phase, hereby reducing the ceramic’s ability for a martensitic transformation. Fracture load is of course highly dependent on a material’s layer thickness: while lithium-disilicate specimens usually call for a minimum layer thickness of 1.0–1.5 mm in the occlusal area to withstand masticatory stress , 3Y-TZP and 5Y-TZP restorations can be manufactured in only 0.5 mm layer thickness , thus enabling the realization of minimal invasive treatment concepts .

As 4Y-TZP materials have only recently entered the market, studies investigating this novel composition’s mechanical and optical properties are warranted. New manufacturing processes, such as the implementation and critical evaluation of high-speed sintering, could provide valuable insights into the future use of these materials.

The aim of this study was to examine the impact of high-speed sintering, layer thickness and artificial aging in a chewing simulator on the fracture load and two-body wear of 4Y-TZP crowns. The first hypothesis stated, that the sintering protocol did not present an influence on the wear of the zirconia ceramic or its enamel antagonist. The second hypothesis stated, that neither the sintering protocol, nor the choice of different layer thicknesses nor the pretreatment with or without artificial aging showed an impact on the fracture load of the zirconia ceramic.

Materials and methods

Zirconia crowns (4Y-TZP, Ceramill Zolid HT+, Amann Girrbach AG, Koblach, Austria) in different layer thicknesses were sintered using different sintering protocols and examined after artificial aging ( Fig. 1 and Table 1 ).

Fig. 1
Study design.

Table 1
Material, manufacturer, composition and Lot.no. used.
Material Manufacturer Composition Lot.no.
Ceramill Zolid HT+ Amann Girrbach AG, Koblach, Austria ZrO 2 + HfO 2 + Y 2 O 3 : ≥99.0
Y 2 O 3 : 6.0 – 7.0
HfO 2 : ≤5
Al 2 O 3 : ≤0.5
Other oxides: ≤1
1712002-107-08-105-93-133-123-135-132-95-134-107

Specimen preparation

Molar crowns in different occlusal layer thicknesses (0.5, 1.0, 1.5 mm; N = 192, n = 64/group) were designed with CAD/CAM software ( Fig. 2 , Ceramill mind and Ceramill match 2, Amann Girrbach AG) and manufactured employing a milling machine (Ceramill motion, Amann Girrbach AG). Connectors were cut with a coated diamond disk (918PB; Komet) and a round end taper diamond bur (7351; Komet Dental, Gebr. Brasseler GmbH & Co. KG, Lemgo, Germany). Specimens were sintered at either 1580 °C (high-speed sintering, exp. Ceramill Therm RS) or 1450 °C (control group, Ceramill Therm 2, Amann Girrbach AG; n = 32/subgroup; Table 2 ). Occlusal surfaces were polished in two-steps with a ceramic polisher (94003M 104 260, Komet Dental) followed by a goat hair brush (9638 900 190, Komet Dental) using diamond polishing paste (Yeti Dia Glace, Yeti Dental, Engen, Germany). Prior to bonding, the inner surface of each specimen was air-particle abraded with alumina powder at 50 μm/0.1 MPa. The zirconia crowns were bonded to standardized abutments with a 360° chamfer preparation of 1 mm cast from a CoCr alloy (ZENOTEC NP; Wieland+Dental) with Multilink Automix (Ivoclar Vivadent, Schaan, Liechtenstein), strictly following the manufacturer’s instructions. Specimens were then stored for 24 h in distilled water at 37 °C in an incubator (HERAcell 150, Thermo Scientific, Waltham, USA).

Fig. 2
CAD/CAM design of the tested specimens (Ceramill mind and Ceramill match 2, Amann Girrbach AG).

Table 2
Sinter parameters for the high-speed sintering and control group.
Heating time Heating rate Firing temperature Cooling time
°C h °C/min °C h °C h
High-speed sintering 20 –1580 0:05 300 1580 0:10 1580 –950 0:10
Control group 20 –1450 2:58 8 1450 2:00 1450 –20 1:20

Two-body wear measurement

Three-dimensional measurements of the occlusal surface of the zirconia crowns and respective antagonists were performed to determine material loss for half the specimens that underwent artificial aging. Specimens were scanned with a 3D laser scanner (LAS-20D, SD Mechatronik, Feldkirchen-Westerham, Germany) before aging and after 1,200,000 chewing cycles. Data were superimposed to calculate the volume loss from two-body wear with a three-dimensional data measuring software ( Fig. 3 , GOM Inspect, GOM, Braunschweig, Germany) according to the best fit method.

Fig. 3
Volume loss of an enamel antagonist after two-body wear (GOM Inspect, GOM).

Artificial aging

Half the specimens were artificially aged in a chewing simulator (Chewing Simulator CS-4.10, SD Mechatronik). The mesiobuccal cusps of maxillary human molars fixed in amalgam (Dispersalloy, Dentsply Sirona) were used as enamel antagonists after the tips of the cusps were adjusted to a spherical shape with a 40 μm bur. Specimens were mechanically loaded with 50 N for 1,200,000 times at 1.1 Hz. Simultaneous thermocycling in distilled water was performed for 6,000 cycles by changing the ambient water temperature every 30 s from 5 to 55 °C. Additionally, a sliding movement of 0.7 mm from the central fissure toward the buccal cusps was performed.

Fracture load test

Fracture load was tested for specimens treated without and with artificial aging using the universal testing machine (Zwick 1445, Zwick/Roell, Ulm, Germany) employing a testing stamp with a diameter of 6 mm (chrome-nickel steel, Deutsche Edelstahlwerke GmbH, Witten, Germany) at a crosshead speed of 1 mm/min ( Fig. 4 ). The stamp was positioned on the occlusal surface of each crown. To avoid force peaks, a 0.1 mm tin foil (Dentaurum, Ispringen, Germany) was placed between the crown and stamp. The fracture load test was stopped as soon as the maximum FL decreased by 10%.

Fig. 4
Specimen mounted for the fracture load test.

Statistical analysis

Statistical evaluation of the results was performed with descriptive analysis followed by Kolmogorov–Smirnov for testing the violation of normal distribution. For global analysis, univariate ANOVAs, post hoc Scheffé, partial eta squared ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='ηp2′>𝜂2𝑝ηp2
η p 2
) and unpaired t -test were calculated. Furthermore, Kruskal–Wallis- and Mann–Whitney- U -test were computed to analyze significant differences between the tested groups. All p -values below 0.05 were construed as statistically significant. Data were analyzed with SPSS version 25.0 (IBM, Armonk, NY, USA).

Results

The results of the descriptive analyses are presented in Tables 3 and 4 . While all 2BW groups were normally distributed, the Kolmogorov–Smirnov test indicated a violation of the assumption of normality for 33% of the tested FL groups. Separate statistical computations employing non-parametric and parametric tests were thus performed.

Table 3
Descriptive statistics for two-body wear of the zirconia and respective antagonist.
Two-body wear (10 −3 mm 3 ) High-speed sintering Control group
Mean (±SD) 95% CI Mean (±SD) 95% CI
Zirconia −23.8 ± 11.3 a [−18.8; −28.6] −35.2 ± 18.7 b [−27.2; −43.1]
Antagonist −404 ± 198 a [−319; −488] −380 ± 171 a [−306; −453]

abc Different letters present significant differences between sintering protocols.

Table 4
Descriptive statistics for the fracture load of the zirconia.
Fracture load (N) High-speed sintering Control group
0.5 mm 1 mm 1.5 mm 0.5 mm 1 mm 1.5 mmm
Mean (±SD) 95% CI Mean (±SD) 95% CI Mean (±SD) 95% CI Mean (±SD) 95% CI Mean (±SD) 95% CI Mean (±SD) 95% CI
Initial 3099 ± 1443 aAβ [2328; 3868] 4499 ± 1266 * bBβ [3823; 5174] 5156 ± 1254 * bAα [4486; 5824] 3949 ± 2067 * aAβ [2847; 5051] 3099 ± 826.0 aAα [2657; 3539] 4507 ± 923.5 bAβ [4013; 4999]
Chewing simulator 1638 ± 683.1 aBα [1272; 2002] 2161 ± 563.7 bAα [1859; 2462] 4456 ± 1084 cBα [3876; 5034] 1073 ± 293.8 aAα [916.7; 1230] 2930 ± 1382 bAα [2192; 3666] 3178 ± 748.0 * bAα [2778; 3577]
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Aug 9, 2020 | Posted by in Dental Materials | Comments Off on Impact of high-speed sintering, layer thickness and artificial aging on the fracture load and two-body wear of zirconia crowns
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