To test the impact of three varying step-stress protocols on the fatigue behavior of two 3Y-TZP, one 4Y-TZP and one 5Y-TZP zirconia materials.
Eight specimens per zirconia material (N = 32) were selected for static testing to determine the start load for dynamic tests (30% of the mean value of static fracture load). 45 specimens per material (N = 180) were used for dynamic load tests using three step-stress protocols: 1. 50 N/5000 cycles; 2. 5% of static load/5000 cycles, and 3. 10 N/1000 cycles. Following materials were tested: 3Y-TZP (<0.25 Al2O3) (O: opaque) 3Y-TZP (<0.05 Al2O3) (T: translucent), 4Y-TZP (<0.01 Al2O3) (ET: extra translucent) and 5Y-TZP (<0.01 Al2O3) (HT: high translucent). The specimens (4 ± 0.02 × 3 ± 0.02 × 45 mm) were placed centrally on the support rolls and the load was applied perpendicularly over the 4 mm specimen side (∼4-point flexural strength according to the DIN 6872:2019). Data was analyzed with Kolmogorov–Smirnov-test, t-test, one-way ANOVA with post-hoc Scheffé-test, Chi-square-test, Kaplan–Meier with Log-Rank-test and two-parametric Weibull analysis (p < 0.05).
The step-stress protocols showed no impact on the fracture load or Weibull modulus within one zirconia material. However, the zirconia materials T, ET and HT showed differences in cycle number to fracture between the step-stress protocols (T: 3 > 2 > 1; ET: 2 > 3 > 1; HT: 2, 3 > 1) with lowest cycle number to fracture for protocol 1. Within one step-stress protocol, the cycle number to fracture varied according to the zirconia material as follows: 1: T, O ≥ O, ET > HT; 2: ET > O, T, HT; 3: O, T, ET > HT. Cracking started at the tensile side of the specimens at all times. All specimens showed typical compression curls (single or double). Fragmentation patterns were similar for all materials with a lot of crack branching and fragmentation due to secondary cracks indicating high energy fractures.
Dynamic fatigue tests seem to provide important information on the long-term stability of zirconia materials. Zirconia materials with higher opacity seem to be more robust towards varying step-stress protocols than translucent zirconia materials. Regarding expenditure of time, a step-stress protocol with a load increase of 50 N every 5000 cycles seems favorable to gain information on the long-term stability of zirconia materials.
Zirconia has various properties which favor the material as dental restoration, such as good biocompatibility, high mechanical properties and adequate aesthetical properties [ ]. This applies especially for the most widely used tetragonal stabilized 3Y-TZP (yttrium-stabilized tetragonal zirconia polycrystal) zirconia with the highest flexural strength and fracture toughness, but high opacity [ ]. To achieve natural optical properties of zirconia, several polycrystalline zirconia materials were developed [ ]. In that respect subsequent zirconia generations, 4Y-TZP and 5Y-TZP [ ] were developed by increasing the yttrium oxide (Y 2 O 3 ) content from 3 mol% up to 4 mol% and 5 mol% and by decreasing the dopant alumina (Al 2 O 3 ) [ ]. Thereby a microstructure with more cubic (30% and 50% respectively [ , , ]) and less tetragonal phase was achieved, favoring less birefringence (light scattering) [ ]. Based on the increased translucency, the application of monolithic zirconia restorations is increasingly considered [ , , ]. This applies to 4Y-TZP and 5Y-TZP zirconia materials, while common 3Y-TZPs are established as framework material [ , ]. Previous investigations compared 3Y-TZP, 4Y-TZP and 5Y-TZP materials with respect to mechanical, optical, and microstructural properties, as well as aging resistance [ , , , ]. There was full agreement that higher cubic phase content leads to better translucency, but at the expense of strength and toughness [ , , , ]. A recent study summarized that a compromise was always inevitable between translucency and aging resistance on one side and mechanical properties on the other side [ ]. Another study consistently stated, that aging and dynamic load did not negatively affect the fracture resistance of monolithic four-unit FDPs made from 3Y-TZP and 4Y-TZP, while cracks were found in FDPs from 5Y-TZP after dynamic loading [ ]. Therefore, the impact of dynamic loading of zirconia materials is relevant to determine their long-term stability.
Dental materials are primary investigated under standardized static tests, which unfortunately do not consider the clinical factors such as induced stress while mechanical-cyclic loading, parafunctional habits and oral environment (humidity, pH, temperature) [ , ]. However, the primary reason for fractures is the weakening of the material as a result of crack growth under the repeated cyclic or dynamic stress [ ]. In clinical use, the natural dentition biting forces range between 30 N and 640 N [ ]. At the same time 800–1400 cycles per day [ ] are achieved. It must be taken into consideration that the chewing frequency and the maximum pressure vary according to the food eaten and the patient’s dental status [ ]. To determine fatigue the material is loaded dynamically or cyclically with values far below its critical load [ ]. There are only very few exceptions in study cases where the tests were conducted under cyclic loading until fatigue to reproduce clinically characteristic failure. Mostly, the investigations are performed under cyclic loading for a certain number of cycles (e.g. often used chewing simulation), the specimens are then subsequently exposed to standardized single loading test [ ]. The only exceptions are the standardized tests with regard to “ceramic materials made of yttrium-stabilized tetragonal zirconium oxide (Y-TZP)” (DIN EN ISO 13356:2016-02) and the standard for “dynamic fatigue testing for endosseous dental implants” (DIN EN ISO 14801:2008-02), which only apply for testing of zirconia used in manufacturing of surgical implants. But mostly on not standardized specimen geometries, for example crowns. In a large number of studies, dental ceramics have already been subjected to classical dynamic tests, with the goal of imitating the real stress in the patient’s mouth [ , ]. In this case, accelerated lifetime tests where specimens are subjected to stress levels lower than those used in fast fracture (static) test but at the same time higher than those found during mastication to achieve a decrease in test duration to an acceptable period of time [ ]. Literature also shows that cyclic loading propagates cracks in a similar manner to dynamic fatigue, making it an important test to perform additional to the static test [ ]. However, none of the already conducted tests used either a standardized machine or a standardized method, as there is no existing experimentally approved clinical simulation with regard to applied load and number of cycles. Consequently, the varying influencing variables (applied load, cycle numbers, test geometries, machines) of each conducted dynamic test method exclude a direct comparison of the results. This leads to the desire, to implement a standardized dynamic testing [ , ].
The aim of this study was to test the impact of three varying step-stress protocols on the fatigue behavior of two 3-TZP, one 4Y-TZP and one 5Y-TZP zirconia materials. The null hypotheses tested that there are:
no differences between the step-stress protocols within one zirconia material with respect to cycle number to fracture, fracture load and Weibull modulus and
no differences in cycle number to fracture between zirconia materials within one step-stress protocol
Materials and methods
The study design and the tested materials are presented in Fig. 1 and Table 1 . In summary, 51 specimens per material (N = 204) with rectangular cross-section (4.0 ± 0.2 mm × 3.0 ± 0.2 mm × 45.0 mm) were milled (Ceramill Motion 2, Amann Girrbach, Koblach, Austria), ground (SiC abrasive paper P1200 and P2500, Struers, Ballerup, Denmark), chamfered and sintered in a sintering shell on sinter sand according to manufacturer’s instructions: from room temperature to 1450 °C at 10 °C/min, holding time of 2 h at sintering temperature and cooling back to room temperature at 10 °C/min (LHT 02/16, Nabertherm, Lilienthal/Bremen, Germany). The surface roughness was measured (Mahr Perthometer SD26, Mahr, Göttingen, Germany) and showed following mean values: Ra: 0.104 ± 0.001 μm and Rz: 0.925 ± 0.037 μm. Each specimen’s cross-section dimension was measured to ±0.01 mm accuracy (electronic micrometer Holex 421490, 0–25 mm, ±0.001 mm, Munich, Hoffmann, Germany). In all test methods, the specimen was placed centrally on the support rolls of the testing machine. The load was applied perpendicularly over the 4 mm specimen side until fracture. All tests were conducted under dry conditions at room temperature.
|Zirconia||Material||Abbreviation||Shade||Y 2 O 3 content (wt%)||Al 2 O 3 content (wt%)||Batch number|
|4Y-TZP||Extra translucent||ET||White||6.65−7.95%||<0.01||W035 16ET|
|5Y-TZP||High translucent||HT||White||8.55−10.11%||<0.01||Y189 10HT|