CDOCS Staff Author: Dr. Michael Snider
Sponsored by 3M
In our last article, we discussed a very brief history of the CEREC system, as well as a limited history of the materials that are available for use in the CEREC System. In this article, we are going to discuss one of the biggest influencing materials in the market today: Zirconia.
The zirconia that we all know in our restorative cases, actually starts off quite different. Zircon (ZrSiO4) is the initial silicate mineral that is mined from deposits in Africa and Australia.1 After the zircon is purified to produce zirconia powder, metal oxides are added to the powder.
Aluminum is then added to prevent water corrosion, yttrium is added to stabilize the tetragonal phase of zirconium.
Figure 1. Classic “3Y”, High Strength Zirconia
The benefits of zirconia were huge for us as clinicians. We had incredible strength in a monolithic restoration. The necessity to bond as with other high strength glass ceramics was removed. Due to the unique characteristics of how the zirconia restorations are milled and sintered, we had a multitude of options in regard to margin design, prep design, and even conventional cementation. The one thing that was missing was the ability to produce this restoration chairside. Sure, the restoration could be milled in our office, but the sintering times required for zirconia did not make them suitable for chairside restorations.
All of that changed with the release of the CEREC SpeedFire oven from Dentsply Sirona partnered with the ability to dry mill. The hours previously needed to sinter zirconia restorations was cut down to under 30 minutes. With the technological advances made by Dentsply Sirona, zirconia was now ready for chairside application
Figure 2. Dentsply Sirona PrimeMill and Figure 3. CEREC SpeedFire Oven
The ability to now produce these restorations chairside was exciting for many of us. But, I still found myself utilizing the high strength glass ceramics because of their superior esthetics. The initial zirconias that most of us were using were 3Y zirconias. They were extremely strong materials, but they had little translucency and looked quite opaque in the mouth.
So what’s the real story with of the “Y’s” in today’s zirconias? Since we have so many options now available to us, let’s go through what the real differences are. I’m sure you’ve by now heard the terms 3Y, 4Y, and 5Y in reference to zirconia, but what do these terms actually mean?
Let’s begin by discussing why yttria is added to zirconia. We stated early that it helps to stabilize the tetragonal phase of the zirconium. The amount of yttria in the composition of the zirconia directly affects the strength and translucency by adjusting the phase composition. In other words, the more yttria in the composition, the more cubic phase of zirconia in the product.
Figure 4. Comparison of Monoclinic, Tetragonal, and Cubic zirconias
The tetragonal phase of zirconia is responsible for the strength we all love with zirconia. The tetragonal phase increases strength by hindering crack propagation through phase transformation.2
Figure 5. Tetragonal Phase, Strength
The cubic phase of zirconia is responsible for the translucency of the material. The cubic phase increases translucency by reducing light scattering and deflection. As the material has less tetragonal phase, there is less refraction of the light at tetragonal boundaries.
Figure 6. Cubic Phase, Translucency
The 3Y zirconia is going to be similar to the CEREC Zirconia that many of you may have used in the past. With only 3% yttria composition, 90% of the material is in the tetragonal phase and only 10% in the cubic phase. These materials are extremely strong, but not very esthetic.
The 5Y zirconia acts like a completely different material. At 5% yttria composition, we have only 45% of the zirconia in the tetragonal phase, and we have 55% of the zirconia in the cubic phase. This material has markedly better esthetics. But, by gaining the increased esthetics, we have lost strength. We have moved from nearly 1400MPa with our 3Y zirconias and are now around 800MPa with our 5Y zirconias.
This is why 3M™ Chairside Zirconia has been unique for me in my practice. This zirconia is a 4Y Zirconia, or we have 4% yttria composition. This composition makes 3M™ Chairside Zirconia the best of both worlds in my practice. I still get the strength that I want from a zirconia restoration being at 1000MPa, but with the increase in yttria, I also get the translucency that I like compared to the 3Y zirconias. 3M™ Chairside Zirconia is a perfect blend of beauty and esthetics.
Figure 7. 3Y vs 4Y vs 5Y
Figure 8. Strength Comparison amongst materials with varying yttria compositions.
Figure 9. Translucency Comparison amongst materials with varying yttria compositions
In our next article, we move back into more clinical applications of 3M™ Chairside Zirconia. We will review the effects of different milling speeds on the restoration. We will also discuss the effects of polishing and glazing zirconia restorations; is one preferable over another?
See you soon!
References:
1. Burgess, J. Zirconia: The Material, Its Evolution, and Composition. CompendiOKum. 2018.
2. 3M Internal Data.
