Assessing Flexural Strength Degradation of New Cubic Containing Zirconia Materials

INTRODUCTION

Zirconia restorations have been widely used since their introduction in 2004 for fixed dental prosthetics applications.¹ With the increased utilization of all-ceramic crowns, zirconia has been a popular choice due to its superior strength and toughness when compared to all other dental ceramics, especially for multiunit fixed dental prostheses.² Although there is a lack of long-term clinical studies quantifying failure rates of monolithic zirconia, it is suggested that they fracture at a relatively low rate.³ Zirconia is undoubtedly strong, however, the limited translucency of these restorations have historically been a drawback when selecting a restorative material for esthetic cases.⁴ Thus, manufacturers have continually focused their efforts towards improving the esthetic properties of zirconia.

In its pure form, zirconia can exhibit three phases; monoclinic, tetragonal, and cubic. While the stability of these phases is dependent on increasing temperature, it is possible to achieve each phase at room temperature by adding stabilizing oxides. The most widely used form of zirconia for dental applications is in the tetragonal phase which is a high temperature phase stabilized by adding 3 mol% yttria and therefore known as 3% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP). In this phase, zirconia demonstrates transformation toughening which results in a high flexural strength (FS). Transformation toughening occurs when sufficient stress is placed onto an yttria-stabilized zirconia prosthesis. In the stressed area, the tetragonal phase zirconia crystals can transform to the monoclinic form which causes a 3% volume expansion at the site of stress and halts crack progression.⁵ This allows for the material to exhibit a biaxial FS of 900–1,200 MPa.⁶ To compare, the 3-point FS of lithium disilicate is around 250–400 MPa and as high as 500 MPa (biaxial FS) as recently reported by the manufacturer.^7,8

Although tetragonal zirconia has proven to be a very strong material, its limited translucency results in a less esthetic finished product.⁹ Manufacturers have improved the formulation of zirconia by lowering the alumina content and controlling the zirconia grain size and processing density to achieve an increase in translucency.¹⁰ Coloring is often utilized to further improve esthetics through the addition of metal oxides or tinting of milled restorations but either of these coloring methods may result in a further reduction of translucency.¹¹ Although there has been some success in increasing the translucency and esthetics using these methods, they have yet to provide a result equal to that of glass ceramics.¹²

Currently, focus has been to improve translucency of zirconia through the utilization of a significant cubic crystalline phase interspersed with the tetragonal phase.¹³ With the abundance of these newer “high-translucency” zirconia on the market, some confusion exists between the identification and utilization of cubic zirconia in these products. Products listed as “high-translucency” zirconia have been found to describe both 3Y-TZP as well as the cubic zirconia materials, which have been defined as 4 mol% and 5 mol% yttria partially-stabilized zirconia (4Y-PSZ and 5Y-PSZ). This critical differentiation is necessary as the newer zirconia products that utilize the cubic phase at a higher ratio do not exhibit the transformation toughening exhibited by the more readily available 3Y-TZP materials.⁹ This results in a significantly lower biaxial FS ranging from 500–700 MPa, or roughly 30–40% less FS of that found in 3Y-TZP.⁷ Despite the reduction in strength, many manufacturers advertise these cubic zirconia restorations to have both the translucency and strength to be used in single and multiunit restorations anywhere in the mouth.⁷ Although this has yet to be proven, it appears the major advantage of utilizing the strength of zirconia in a multiunit fixed dental prosthesis may be sacrificed for greater translucency in the material.

The manufacturer of Lava Esthetic (3M ESPE, St. Paul, MN), a 5Y-PSZ material, reports the highest biaxial FS of the cubic zirconia materials available at 800 MPa.¹⁴ Lava Esthetic is advertised to be utilized as crowns, 3-unit fixed dental prostheses, inlays, onlays, and veneers. Katana UTML, a 5Y-PSZ material (Kuraray Noritake Dental Inc.), with an advertised biaxial FS of 557 MPa, is presented to be most suitable for full-contour restorations in the anterior region, but can be considered for a posterior single unit crown.¹⁵ Katana STML (Kuraray Noritake Dental Inc.), a 4Y-PSZ material, reports an average biaxial FS of 748 MPa and is advertised for fabrication of FDP frameworks, FDPs, crowns, inlays, onlays, and veneers.

With the loss of transformational toughening, the mechanical properties of cubic containing zirconia materials have changed substantially.^9,16 Fatigue behavior and strength degradation of the newer cubic containing zirconia materials may have a greater significance in clinician’s decisions to use them in scenarios where traditionally, the strength of zirconia was not in question. This is particularly true for multiunit posterior prostheses.

Limited research has been published evaluating FS degradation of these newer cubic zirconia materials. The purpose of this study is to evaluate the FS and the flexural fatigue strength and degradation of two 5Y-PSZ materials, one 4Y-PSZ material, two 3Y-TZP materials, and one lithium disilicate glass ceramic material. The null hypothesis is that there will be no difference in properties based on ceramic material.

MATERIALS AND METHODS

The study was conducted in the United States Air Force Dental Research and Consultation Service at Joint Base San Antonio Fort Sam Houston, Texas, USA. The following materials were tested: two 3Y-TZP materials (Lava Plus with A2 dyeing liquid, 3M ESPE; Katana ML Shade A Light, Kuraray Noritake Dental Inc.); one 4Y-PSZ (Katana STML Shade A2, Kuraray Noritake Dental Inc), two 5Y-PSZ (Lava Esthetic Shade A2, 3M ESPE; Katana UTML Shade A2, Kuraray Noritake Dental Inc.); and one lithium disilicate material (IPS e.max CAD LT, Shade A2, Ivoclar Vivadent).

RESULTS

DISCUSSION

The null hypothesis was rejected because the results of this study demonstrate that there are significant differences in properties based on ceramic materials. The data shows similar trends between FS as those from previous studies by Zhang et al. and Kwon et al.^9,18 In this study, the 3-point bend FS data of the 4Y-PSZ and 5Y-PSZ groups (534 MPa, 470 MPa, 485 MPa) was 38–46% less than that of the 3Y-TZP groups (778 MPa, 870 MPa). It should be noted that the Katana ML zirconia has a multilayer shade gradient incorporated into the material, whereas the Lava Plus zirconia is uniform and stained prior to sintering. Despite the differences in coloring technique of the 3Y-TZP groups, there was no statistical differences found between the FS of the 3Y-TZP groups in this study.

The flexural fatigue strength was material dependent, but showed similar distribution as the FS decreased with the increase in the cubic phase contained in the materials. This was especially true for the Katana products, however, the exception to this finding was the 5Y-PSZ Lava Esthetic group which showed a higher flexural fatigue strength than 4Y-PSZ Katana STML group. The 5Y-PSZ Lava Esthetic group showed statistically similar FS to 4Y-PSZ Katana STML and 5Y-PSZ Katana UTML groups and statistically similar flexural fatigue strength to the 4Y-PSZ Katana STML group. However, testing showed strength degradation of the 5Y-PSZ Lava Esthetic percentages equal to those of the 3Y-TZP Katana ML and Lava Plus groups. It is unclear why this group diverged from the trend, but interestingly the manufacturers of Lava Esthetic report the highest biaxial FS of all the cubic zirconia materials currently available.¹⁴ The data in this study did not substantiate this claim and found the 3-point bend FS to have no statistical difference to the other 5Y-PSZ group tested. However, the flexural fatigue strength did result in a higher value than that found in the other cubic containing groups. Obviously, between companies there is a difference in material chemical composition and sintering process. These proprietary formulas and fabrication methods have been shown to affect the material properties of zirconia.¹⁹ Regarding the 3Y-TZP materials, the significant difference in flexural fatigue strength between Katana ML and Lava Plus groups may be associated with the shaded layering of the Katana ML zirconia which may have decreased strength properties between layers. Or, it may simply be the difference in proprietary formulation of these products.

All tested ceramic materials showed significant strength degradation after 6,000 loading cycles. The severity of the degradation shows a high dependency on the differences in the materials’ microstructure which can explain the observed variances. As discussed, the higher content of ytrria stabilizes the zirconia materials with a greater amount of cubic crystals in the microstructure which eliminates the transformation toughening mechanism. The lack of this mechanism is the primary factor in decreased FS and increased FS degradation of these materials.²⁰ The higher grain size found in the cubic containing zirconia materials may also contribute to the decreased FS and flexural fatigue strength values. Larger grain sizes can contribute to lower mechanical behaviors under static and fatigue assessments as smaller grain sizes require a higher applied stress to induce fracture. Smaller grains can limit the size of the dislocations on the crystal grain boundaries, thus increasing mechanical properties.^16,21,22

Based on the FS values, ISO standard 6872-2015 would grade the 5Y-PSZ groups as class 4 materials to be utilized as single-unit anterior or posterior prostheses or as a 3-unit anterior prosthesis.²³ After fatigue testing, Katana UTML, would be re-classified as a class 2 ceramic, with recommendation to be utilized as an adhesively cemented anterior or posterior single-unit crown owing to its significant decrease in strength. The 4Y-PSZ Katana STML, based on FS values, would be categorized as a class 5 ceramic material, able to be utilized as a substructure ceramic for three unit prostheses involving molar restoration. However, after fatigue degradation, the flexural fatigue strength value found in this study would reclassify it to a class 4 ceramic material, making 4Y-PSZ Katana STML not recommended for a substructure ceramic for a three-unit prosthesis not involving molar restoration. This is contrary to manufacturer’s recommendations. As a result of fatigue testing and expected increased degradation in the cubic containing products, care must be taken with the utilization of these cubic-containing materials in multiunit fixed dental prostheses.

Limited studies have looked at fatigue degradation of cubic containing materials. Pereira et al. performed biaxial flexural fatigue testing using a step stress fatigue approach for Katana ML, Katana STML, and Katana UTML.¹⁶ Similar degradation percentages were found for Katana ML (32%, 31%), but significantly lower were found for Katana STML (27%, 43%) and Katana UTML (36%, 51%). Different testing methods may account for differences in results, but overall degradation should be expected, and in weaker materials, may have a more profound effect on the chosen clinical application.

The thickness of a restoration can be significant for both translucent and mechanical properties of ceramics. In this study, the data is related to a specimen thickness of 1.3 mm and is not bonded to a substrate. A thinner specimen could alter the performance of each zirconia material.²⁴ Further, bonding to a substrate would likely result in higher stresses to fracture, as shown by Campos et al.^24,25

CONCLUSION

Based on our findings, the amount of strength degradation in zirconia materials was material dependent, with the 4Y-PSZ or 5Y-PSZ cubic containing zirconia materials demonstrating greater or similar strength degradation compared to the primarily tetragonal 3Y-TZP materials.

DISCLAIMER

The views expressed in this article are those of the authors and do not reflect the official policy of the US Air Force, the Department of Defense, Uniformed Services University of the Health Sciences, or the US government. The authors do not have any financial interest in the companies whose materials are discussed in this article.

REFERENCES

1. Anusavice KJ. Phillips’ Science of Dental Materials. Saunders; 2013.

2. Miyazaki T, Nakamura T, Matsumura H, et al. Current status of zirconia restoration. J Prosthodont Res 2013;57(4):236–261. DOI: 10.1016/j.jpor.2013.09.001.

3. Sulaiman TA, Abdulmajeed AA, Donovan TE, et al. Fracture rate of monolithic zirconia restorations up to 5 years: a dental laboratory survey. J Prosthet Dent 2016;116(3):436–439. DOI: 10.1016/j.prosdent.2016.01.033.

4. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, et al. Relative translucency of six all-ceramic systems. Part II: core and veneer materials. J Prosthet Dent 2002;88(1):10–15. DOI: 10.1067/mpr.2002.126795.

5. Hannink RHJ, Kelly PM, Muddle BC. Transformation toughening in zirconia-containing ceramics. J Am Ceram Soc 2004;83(3):461–487. DOI: 10.1111/j.1151-2916.2000.tb01221.x.

6. Guess PC, Kulis A, Witkowski S, et al. Shear bond strengths between different zirconia cores and veneering ceramics and their susceptibility to thermocycling. Dent Mater 2008;24(11):1556–1567. DOI: 10.1016/j.dental.2008.03.028.

7. Ceramic CAD/CAM Materials: An Overview of Clinical Uses and Considerations [Internet]. American Dental Association. 2017.

8. IPS e.max: Ivoclar Vivodent, Inc., 2019, available from: https://www.ivoclarvivadent.us/explore/ips-emax-system-technicians.

9. Zhang F, Inokoshi M, Batuk M, et al. Strength, toughness and aging stability of highly-translucent Y-TZP ceramics for dental restorations. Dent Mater 2016;32(12):e327–e337. DOI: 10.1016/j.dental.2016.09.025.

10. Zhang Y. Making yttria-stabilized tetragonal zirconia translucent. Dent Mater 2014;30(10):1195–1203. DOI: 10.1016/j.dental.2014.08.375.

11. Pecho OE, Ghinea R, Ionescu AM, et al. Color and translucency of zirconia ceramics, human dentine and bovine dentine. J Dent 2012;40 (Suppl 2):e34–e40. DOI: 10.1016/j.jdent.2012.08.018.

12. Baldissara P, Llukacej A, Ciocca L, et al. Translucency of zirconia copings made with different CAD/CAM systems. J Prosthet Dent 2010;104(1):6–12. DOI: 10.1016/S0022-3913(10)60086-8.

13. McLaren EA, Lawson N, Choi J, et al. New high-translucent cubic-phase-containing zirconia: clinical and laboratory considerations and the effect of air abrasion on strength. Compend Contin Educ Dent 2017;38(6):e13–e16.

14. 3M™ Lava™ Esthetic Fluorescent Full-Contour Zirconia Disc: 3M; 2019, available from: https://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Lava-Esthetic-Fluorescent-Full-Contour-Zirconia-Disc?N=5002385+3291669973andrt=rud.

15. KATANA Zirconia UTML/STML: Kuraray Noritake; 2019, available from: https://kuraraydental.com/product/katana-zirconia-stml/.

16. Pereira GKR, Guilardi LF, Dapieve KS, et al. Mechanical reliability, fatigue strength and survival analysis of new polycrystalline translucent zirconia ceramics for monolithic restorations. J Mech Behav Biomed Mater 2018;85:57–65. DOI: 10.1016/j.jmbbm.2018.05.029.

17. Belli R, Geinzer E, Muschweck A, et al. Mechanical fatigue degradation of ceramics versus resin composites for dental restorations. Dent Mater 2014;30(4):424–432. DOI: 10.1016/j.dental.2014.01.003.

18. Kwon SJ, Lawson NC, McLaren EE, et al. Comparison of the mechanical properties of translucent zirconia and lithium disilicate. J Prosthet Dent 2018;120(1):132–137. DOI: 10.1016/j.prosdent.2017.08.004.

19. Chevalier J, Gremillard L, Deville S. Low-temperature degradation of zirconia and implications for biomedical implants. Annu Rev Mater Res 2007;37(1):1–32. DOI: 10.1146/annurev.matsci.37.052506.084250.

20. Sulaiman TA, Abdulmajeed AA, Shahramian K, et al. Effect of different treatments on the flexural strength of fully versus partially stabilized monolithic zirconia. J Prosthet Dent 2017;118(2):216–220. DOI: 10.1016/j.prosdent.2016.10.031.

21. Palmero P. Structural ceramic nanocomposites: a review of properties and powders’ synthesis methods. Nanomaterials (Basel) 2015;5(2):656–696. DOI: 10.3390/nano5020656.

22. Pande CS, Cooper KP. Nanomechanics of Hall–Petch relationship in nanocrystalline materials. Prog Mater Sci 2009;54(6):689–706. DOI: 10.1016/j.pmatsci.2009.03.008.

23. International Organization for Standardization TCIT, Dentistry. Dentistry: Ceramic Materials (ISO 6872:2015): European Committee for Standardization, 2015;2015.

24. Nakamura K, Harada A, Inagaki R, et al. Fracture resistance of monolithic zirconia molar crowns with reduced thickness. Acta Odontol Scand 2015;73(8):602–608. DOI: 10.3109/00016357.2015.1007479.

25. Campos F, Valandro LF, Feitosa SA, et al. Adhesive cementation promotes higher fatigue resistance to zirconia crowns. Oper Dent 2016;42(2):215–224. DOI: 10.2341/16-002-L.