Exposing 3 Myths About Zirconia

How do myths form? When a subject or topic is understood to a point…but also misunderstood to a point. When Source A says one thing, and Source B says something different about the same topic? When an opinion is put forth as truth, but no one really has enough information to say it isn’t true? In these scenarios, these topics enter the famed gray area of myth. Some believe it, some don’t believe it and most are stuck in the middle, not sure what to believe! Zirconia is a good example in dentistry. It’s a fascinating material with much potential, but it’s also unlike anything we’ve seen in dentistry before. And, because we tend to apply attributes to the unfamiliar based on the familiar, much has been said about zirconia, both truth and myth. Keep reading to debunk three of the more persistent zirconia myths once and for all!

Myth #1: You Can’t Bond to Zirconia.

There are myths that you can’t bond to zirconia. In fact, bonding of zirconia restorations is easy, but different to glass ceramics. Important to know: The zirconia surface is chemically very different to silica based glass ceramics. Silanes do not bond to zirconia, but phosphate monomers or polyacids bond very strong to zirconia (and do not to glass ceramics!). Such monomers are used in cement or bonding systems developed for zirconia. Further, a key to getting strong zirconia bonding is proper preparation of the intaglio surface. Initially, with sandblasting but also important after try-in to clean saliva. Why is this so crucial? Zirconia has oxide groups which bond to phosphate groups in your dental cement; However, when you try-in the crown, that process brings the zirconia into contact with saliva, which just so happens to contain a good deal of phosphate as well. These phosphate groups from the saliva will grab on to the oxide groups on the zirconia and make it impossible for bonding to take place. That’s why it’s vital to clean the intaglio after try-in either by sandblasting or by wiping the inside of the restoration with gauze soaked in 5% sodium hypochlorite, rinse with water and air dry. For additional content on this topic, give this roundtable discussion a read. Remember: Zirconia can be seated conventional for many clinical situations. This is ideal when moisture tolerance is needed.

Myth #2: When prepping zirconia, you can use phosphoric acid to etch the intaglio surface.

Another myth: you can use phosphoric acid to etch the intaglio surface when prepping zirconia. Don’t do this! Sandblasting is the way to prepping zirconia; etching does not work. And more importantly: phosphoric acid grabs and blocks the oxide groups needed for bonding. You might remember the discussion of phosphate groups from Myth #1 when we explored how those interact with the oxide groups in zirconia and end up grabbing on to the bonding interface. Dr. Alec Ganci does a nice job of explaining bonding to zirconia in this Dental Economics article. The bottom line is that zirconia will attract phosphate groups, which ruin the bonding surface. You already need to be conscious of any saliva contaminating the restoration, and adding in Phosphoric Acid (which, yes, is full of phosphate groups) will seriously compromise the clinical outcome of your zirconia restoration.

Myth #3: All zirconia materials are the same.

There is a reason why not all zirconias are alike! Composition, grain size, shape of the zirconia particles, type and amount of the stabilizing oxides like yttria content (which determine crystal phases) and the processing are all factors that can vary in dental zirconia. Y-TZP stands for Yttria-stabilized Zirconia Polycrystals, but most dentists simply know it as zirconia. In dentistry today we have already seen 3Y, 4Y and 5Y blends of zirconia – with the ‘Y’ value denoting what mol% of yttria has been added. A 3Y-TZP is going to be strong, but less translucent. A 5Y-TZP is not going to be as strong but will be more translucent because it has more cubic phase polycrystals. A 4Y-TZP is going to have a very balanced blend of strength and translucency. To further explore the variations of zirconia, download this whitepaper.

Conclusion

When it comes to dental materials, trusting in the science is the surest path to clear understanding. If you’re unsure about zirconia, contact an expert at 3M. They’ve been studying this material for 30+ years and can help you separate the myth from the fact!