Zirconia dental crown

Zirconia dental crowns are a kind of ceramic crown. They are a CAD CAM technology and a dental lab will mill the crown from a puck of material. They are popular due to the fact they are very strong, cost effective, and are similar to natural tooth color.  The most popular brand is Bruxzir

What is dental zirconia?

Zirconium is a metal that we can find in Australia and Africa as the mineral zircon (ZrSiO4). We primarily process zircon in Japan, where we also add the metal oxides yttrium, aluminum, and hafnium oxide. Yttrium stabilizes polycrystals, aluminum prevents water corrosion and hafnium oxide reduces pore development. By the time it leaves the factories it is in block form or disk form and is the ceramic Zirconia.

Zirconia exists in the monoclinic, tetragonal, and cubic phases. The monoclinic phase is the most stable but we want it in the tetragonal or cubic phase so we must add dopants to stabilize it. Common dopants are yttrium oxide (Y₂O₃), calcium oxide (CaO), magnesium oxide (MgO) that we mix into ZrO₂ to stabilize the zirconia.

What do the dental crowns consist of?

Zirconia crown = ceramic oxides with CaO, MgO, Y₂O₃, CeO₂ as stabilizers. The stabilizer, or dopants, prevent phase transformations.

More zirconia dopants by brand

yttrium-stabilized tetragonal zirconia polycrystal (3Y-TZP) – strong because as tension induces tetragonal to monoclinic at area of crack, compression stress is then generated around the crack by surrounding tetragonal particles reducing crack propagation Garbelotto 2013

Not all zirconia is the same!

The main three types of zirconia we use for dental crowns are 3Y-TZP, 4Y-TZP, and 5Y-TZP. 3Y-TZP is the strongest and has phase transformation toughening properties. The 5Y-TZP lacks any phase transformation toughening. Furthermore, not all zirconia quality is the same from company to company. A couple names that appear to be very trustworthy are Nacera, Katana and Zirkonzahn .

Esthetic zirconia crowns

The dopant yttria (Y2O3) is increased from about 3mol% to 5mol% or more to make zirconia more esthetic (translucent).  Dopants, like Y2O3, stabilize the tetragonal zirconia phase causing a higher amount of cubic phase particles.  Cubic zirconia, think fake diamond, is much more translucent but is lower in strength.

1. Ceramil Zolid FX Multilayer – translucent and multi-shade
2. Prettau Anterior
3. BruxZir Anterior 650 MPa
4. Kantana has UTML and STML in multi-layer.  Impressive stuff.

Katana ST > Katana UT > IPS e.max Press Conejo J Dent Res. 2016

Translucency of zirconia types are listed below.  ST and Prettau anterior about same and HT and Bruxzir about same.  UT nearly as good emax.

In office millable zirconia

In office mills use post sintered zirconia. Pre sintered milled zirconia has a better intaglio fit because the shrinking that occurs of the zirconia post milling also shrinks the areas the bur overmills. Remember in milling the burs MUST overmill to make the crown fit. No matter the size of bur that the lab uses, it’s overmilling area shrinks with the shrinking of the crown resulting in better fit. Essentially it is like using a smaller bur for the milling. When milling it is better to use a smaller bur but those wear out faster so it is higher cost and more maintenance. Blatz has a nice lecture on zirconia and discusses many issues with milling.

1. BruxZir NOW – 800 MPa
2. IPS emax ZirCAD MT 4Y-TZP

3D printing of zirconia crowns

The Ceramaker 900 by 3DCeram can 3D print zirconia and are physically capable of making crowns that are as true as crowns from milling. Wang 2019 JPD

New Trend – Partial or Hybrid Zirconia dental crown

Zirconia-reinforced lithium silicates (ZLSs) are #2-3

1. Schutz Dental has a new zironia reinforced composite for its Tizian milling machine
2. Dentsply has Zir reinforced lithium silicate called CELTRA CAD for their Sirona MCXL
3. Vident had same as Dentsply called Suprinity but looks like Dentsply bought that under VITA Zahnfabrik

Cement or bond zirconia?  How bond zirconia?

Cementing with RMGI is the best even for short preps.  Ivoclean then RMGI.

Gordon’s tips

To bond zirconia the gold standard is tribochemical.  Can also Sandblast with 50-60um alumina or silica coated alumina particles below 2 bar, Z prime, Panavia 21.   tooth=sandblast, Panavia 21 primer A and B.  This is a version of the APC, air abrade, prime, dual-cure or self-cure cement. Nice little video.

• Tribochemically 30µm CoJet silicatized alumina sand with Microetcher then silane.  This will produce most stable bond for SA resin da Silva JPD 2014 Sept
• MDP (methacryloxydecyl phosphate=phosphate monomer) eg best=Z-Prime Plus Chen JPD 2013, Monobond Plus, Clearfil Ceramic Primer
• Sandblast does increase strength with MDP Bunek 2013 JCD
• Will work but aging decreases bond strength significantly.  Baltz 2004 JPD          Baltz Quit 2007
• Particle size differences from 25-125μm and pressure differences between .1-.4 MPa do not result in clinical differences. Shimoe JPD 2019

Xie 2016 Dent Materials shows CoJet is the best bond so maybe just do that.

Some same zirconia does not bond but I can assure anyone that these 6 PFZ crowns were very well bonded on when I had to remove them.

Wear of dental zirconia

Wear of polished and glazed Zr against enamel and ceramic Best to worst 1

• Against enamel polished Zr>enamel>polished then glazed Zr>glazed Zr>veneering ceramic
• Against ceramic polished Zr>veneering ceramic>polished then glazed Zr>glazed Zr>enamel
• Basically polished Zr is best 

Zirconia wear enamel less than PFM porcelain Mundhe 2015 JPD Sept

CR Report showing that zirconia does not wear opposing BUT it appears to be wearing more heavily.

The newest CR research (graph above) has me a little (with a little l) concerned that there is some phase transitions happening and this wear will accelerate.  The article makes mention that phase changes were seen in hips up to five years so we are likely entering a critical time period with this data. CR June 2014

Although I am far from an expert on medical zirconia it seems that based on this article (http://biomed.brown.edu/Courses/BI108/BI108_2007_Groups/group05/pages/saint_gobain_desmarquest.html) that zirconia joints got a bad name from a manufacturing error that resulted in one bad batch.  They are still in use today and the issue with phase transition doesn’t seem to exist though.

Can zirconia fracture?

Absolutely! Crowns and frames break and most of the time the reasoning is obvious in retrospect. Usually we use the wrong zirconia trying to get more esthetics or the product was too thin.

Experimental zirconia products – Glass-zirconia composite dental crown

Ceria-stabilized tetragonal zirconia/alumina is Ce-TZP/A is meh.

Work continues to improve the chemical make-up of crowns.  It is amazing how fast these changes are coming.  We are already to a point where the majority of dentists don’t understand the differences between the available products.  The research being done by Dr.Zhang appears to be similar to Lava Ultimate but the changes in density as get deeper would be nice improvement.  The ultimate goal is to produce a lifelike material that can be very thin and hold up.  This idea of mixing materials and changing the percentage of different material at different depths has promise.