Essay: Microwave Hybrid Sintering may extend the service life of dental ceramics

ABSTRACT:
INTRODUCTION: Dental Ceramics are required to withstand a complex environment with several mechanical and chemical challenges. Testing the mechanical properties of the material can provide a good insight of the compatibility of the material for dental function. However, due to the particular failure mode of ceramics, fatigue tests are more reliable to predict the service life of the material. Using a Step Stress Accelerated Life Testing method, we analyzed if Microwave Hybrid Sintering produced any difference in the service life of lithium disilicate dental ceramics samples when compared with conventionally sintered samples of the same material. METHODS: IPS e.max CAD beams were tested using a Step Stress test in a water environment, over 50,000 cycles in a range from 100 to 250 MPa of stress load, at a 37º C temperature. Data was collected using an Instron testing machine and software, and analyzed using ALTA and Weibull++ software.
RESULTS: Step Stress test suggests ceramic samples sintered using microwaves will have an approximately 20% larger service life than ceramic samples sintered using conventional sintering, with a 58,7% chance to survive longer than conventionally sintered ceramics.
CONCLUSION: Microwave Hybrid Sintering may extend the service life of dental ceramics.
INTRODUCTION
One of the important challenges in the manufacture of dental ceramics is the production of a compound with sufficient strength but keeping a natural translucency. When a significant reconstruction of teeth is required, the material used in the reconstruction should provide a mechanical performance compatible with the tooth function without compromising the appearance of the normal tooth. In these cases, ceramics are the preferred materials in dentistry. [Christensen 2007, Goznelli et al. 2013, Kidd et al. 2003, Qualtrough et al. 2005, Robertson et al. 2006, Summit et al. 2000]. Dental ceramics are generally hard and brittle, with a range of thermal and electrical conductivity compatible with oral conditions and are typically chemically stable [Carter et al. 2007, Chu et al. 2005, Heimann 2010, McLaren et al. 2009].
Changes in microstructure of the ceramics can be used to modify, and usually improve, the mechanical properties of dental ceramics [Gonzaga et al. 2011, Xie at al. 1999]. The simplest way to modify the microstructure of a ceramic is using a different sintering process, such as microwave hybrid sintering (MHS), a process in which sintering is achieved using microwaves instead of a radiating or convective heating, with several advantages. Dental ceramics are usually transparent to microwaves, and therefore MHS is achieved using external heating elements (in our experiments, silicon carbide susceptors), to heat the ceramic material up to a temperature at which the ceramics couple with the microwaves, starting a volumetric heating phenomena (thermal energy produces across the whole sample, not from the surface to the core as in conventional heating systems)[Almazdi et al. 2012, Kimrey et al. 1991, Krage et al. 1981, Kashi 2010, Menezes et al. 2012]. MHS allows shorter processing times, with reduced energy consumption and improvement of mechanical properties (bending strength, hardness) [Almazdi et al. 2012, Agrawal 1998, Agrawal 2006, Clark et al. 2000, Kashi et al. 2005, Kashi et al. 2008, Ma et al. 2007, Menezes et al. 2007, Menezes et al. 2012, Pendola and Saha 2015, Upadhyay et al. 2001].
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Although MHS seems a good alternative to improve the sintering process of dental ceramics [Kashi et al. 2005, Kashi et al 2008, Pendola and Saha 2015], the impact of MHS in the clinical life of dental ceramics is one of the most important elements to test whether MHS is a convenient method to produce dental ceramics.
For ceramic restorations, in vitro fatigue studies are good predictors of clinical survival. These studies are indicated for situations of repetitive subcritical loading. The fatigue lifetime distribution depends on the flaw distribution and it provides an insight of the materials’ quality. In these experiments we designed a Step-Stress Accelerated Life test to evaluate the difference in the service life of dental ceramics sintered using MHS and Conventional Sintering Furnaces (CFS).
MATERIALS AND METHODS
For these experiments the material used was lithium disilicate dental ceramic (IPS e-max CAD from Ivoclar Vivadent, Amherst, NY). This glass-ceramic is manufactured with a high degree of homogeneity, a convenient property for experimental work. The homogeneity of the material helps to reduce the errors that could be induced by variations in the samples [Dickerson 1999, Ivoclar 2005, Rahaman 2003].
Samples were obtained from commercially available monolithic blocks of IPS e.max CAD from the same production batch, using a low-speed rotatory saw (IsoMet 3000, Buehler, Lake Bluff, IL) with a diamond coated wafering blade (15HC, Buehler, Lake Bluff, IL) under alcohol irrigation, to obtain beams of 12x2x1 mm. All samples were polished to correct imperfections in shape using Soflex discs (3M ESPE, Saint Paul, MN)
Ceramic samples were divided into two groups for the experiments. One set of samples was sintered using a research microwave furnace (Microwave Research Application Inc), at 2000 W input power at a frequency of 2.45 GHz, as previously described [Pendola and Saha, 2015]. Temperature profiles were measured and recorded with the infrared pyrometer, using measurement software (Infrawin 5.1, Lumasence). Conventional Furnace Sintering (CFS) was performed following the manufacturer’s guidelines in two professional dental labs under the student supervision (Marotta’s Lab in Farmingdale, LI, and Tech Square Lab, Manhattan, New York), using a conventional dental furnace (Programat E5000, Ivoclar Vivadent, Amherst, NY).
A Step Stress Test of 50,000 cycles with a 10Hz frequency was designed based on previous data, assuming a failure load of 162 N [Pendola and Saha 2014, Pendola et al. 2015]. The testing profile is showed in Figure 14 and their parameters are outlined in Table 1.
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This test was performed following the profile presented in Figure 2, using an Instron testing machine and data collected using its custom software (Bluehill, Instron Inc, Norwood MA):
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Water plays a crucial role in subcritical crack growth, the most important failure mode in ceramics[Bergmann et al. 2013, Benaqqa et al. 2005, Borba et al. 2013, Carter et al. 2007, Martinez 2007, Mohsen et al. 2011, Park et al. 2008] and therefore, the Step-Stress test with cyclic loading was performed with the samples submerged in deionized water at a constant temperature (37º C). This to mimic the oral environment (Figure 3)[Braem et al. 1994, Morena et al.1986, Reid et al .1990]
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The analysis was performed using a Weibull prediction model using ALTA Reliasoft software, on right-censored data. [Belli et al. 2014, Borba et al. 2013, Joshi et al. 2013, ReliaSoft Corporation 2015, Tucson AZ].
RESULTS
All the samples were tested in 5 steps of 10,000 cycles of load, to produce stress levels of 100, 125, 150, 200 and 250 MPa. The samples were loaded starting with a cyclic load of 100 MPa and continued to the next step until failure. These samples were registered as “failures” and the number of cycles was noted. When the sample survived to the 50,000 cycles, it was labeled as “suspended”.
The registered timelines of failure vs. suspension for CFS samples and MHS samples shows a higher rate of survival for samples sintered using MHS (Figure 4).
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A simple analysis of the results shows an average number of 39,822 cycles for the samples sintered using CFS, and 41,974 cycles for the samples sintered using MHS. There was no significant difference between the two groups (P= 2.851, 2-samples T test) in this comparison. However, this results does not take into account the larger number of suspensions, distribution or predictive values within both sets of samples.
ALTA software was used to analyze the distribution of the two set of samples. This software is designed to calculate models and results of SSALT experiments. Weibull model analysis is more suitable for this type of experiments allowing better predictive values. Weibull models have been used previously in the literature with reliable results [Borba et al. 2013, Balakrishnan et al. 2012, Denning 2012].
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Weibull analysis required a recalculation of the distribution of the sample sets, as it represents a different predictive model. Based on the presented results, the CFS samples set kept their log-normal distribution, but the MHS sample set has a Gamma distribution (Figure 5).
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The results showed different results for the reliability of IPS e.max CAD for CFS and MHS samples. MHS samples reliability achieves the 90% reliability around 31,000 cycles while the CFS samples reached the same level of confidence at approximately 29,000 cycles (Figure 6). However, the contour plot of the same analysis shows that MHS samples have a higher probability of 90% survival (area of the graphic) at a higher number of cycles (Figure 7). These results suggest that the reliability of the IPS e.max CAD is improved when sintered using MHS.
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Using the data generated for the Step-Stress test using the Weibull model, a new test was performed to compare the service life of both sets of samples. The results show that MHS samples have a probability of 58.7% of having a longer service life than CFS samples (Figure 8). The estimated life of CFS samples was 42,938 hours, approximately 4.9 years. This life span is consistent with the clinical estimation of ceramic crowns for dental treatments. Samples sintered using MHS showed an estimated life of 45,668 hours, approximately 5.2 years.
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DISCUSSION
Step-stress tests were performed on beam shaped ceramic samples instead of the usual crown / sliding cyclic load as previously described. Coehlo et al. [2009] analyzed the fatigue of simulated dental crowns formed from a zirconia core layered with LAVA and VITA. Silva et al. [2012] tested lithium disilicate crowns in a conventional dynamic fatigue test. Beams are a more standardized testing method for comparison between samples eliminating the confounding variables of crown design or tooth shape. These variables were simulated by Rekow et al. [2006], where seven factors involved in the mechanical performance of a dental crown were analyzed. Some of those variables were the material type, cementing agent, and tooth element. They generated 128 possible combinations of effects from these 7 variables, which are closely related. Beams, on the other hand, allowed a simpler approach, with the material as the majority factor in the test, with MHS as the independent variable. Another important factor in step stress is the form of loading. The literature suggests that cyclic loading does not increase the rate of crack growth in ceramics. Therefore, using a testing frequency (10 Hz), higher than rates of normal physiological activity, are not likely to influence the results. This was suggested by Joshi et al. [2014], in their study of cyclic fatigue on fluorapatite glass-ceramics.
Step-stress test results showed an increase in the reliability for MHS samples when compared to CFS samples. This is indirectly reflected by the higher ratio of Suspension to Failures (number of samples surviving the test vs number of samples failing the test) for samples sintered with MHS (Figure ). Previous studies have shown that step-stress testing provides a more conservative estimate than dynamic fatigue testing while maintaining predictive significance. Additionally, the step stress test is based on fewer assumptions (the same failure mechanisms will be present at the higher stress levels and will act in the same manner as at normal stress levels)[Ministry of Defense UK 2009] than a dynamic fatigue test, and therefore, has stronger predictive value. [Balakrishnan et al. 2012, Borba et al. 2013, Huh et al. 2011, Kamal et al. 2013, Yoshikawa et al. 2007]. The step-stress test results predict MHS samples have a probability of 58% of surviving longer than CFS samples. The results show a predicted performance of around 47,000 cycles for CFS samples, against over 59,000 cycles for MHS samples (Figure 64). This is a 20% increase in the predicted 90% reliability for the MHS processed material. Since the standard service life of a dental crown is 5 years, this adds another year of life to a dental crown. Although comparing these results to other tests is complicated, due to the use of crown or tooth shaped samples, the results obtained in this test are consistent with the results published in the review on dental ceramics by Li et al. [2014], which report lithium disilicate ceramics survival rates of 97-100% after two years. These results are compatible with the findings of Silva et al. [2012], mentioned before, whose fatigue experiments showed lithium disilicate crowns may be as reliable as zirconia crowns, usually considered the material with the best mechanical performance. In a recent review of survival rates of dental crowns by Lekesiz et al [2014], using another glass ceramic (IPS Empress, Ivoclar Vivadent, the predecessor to IPS e.max CAD) showed a predicted survival rate of more than 95% after 5 years. This result was confirmed in a review by Della Bona et al. [2008], where the glass ceramics system exhibited an excellent survival rate compared to the previous all-ceramics systems. Belli et al. [2015], in their own review on the reliability of dental ceramics, noted that the difference between Empress and previous materials was the presence of larger crystals within the materials. We observed a similar effect in experiments with samples produced used MHS (unpublished data). This may be the reason for the improvement in the reliability and longer service life for IPS e.max CAD samples sintered using microwaves.
CONCLUSIONS
The use of Microwaves in the sintering of dental ceramics it is likely to improve the reliability over time of the material, with an increased probability of survival when compared with ceramics sintered using conventional sintering. The use of beams in a water environment allowed a more standardized sample within conditions similar to the natural failure mode of ceramics.