Christopher Mazzola, DDS 

The purpose of looking at recent research data is to see how we can improve our clinical success. Different scholarly journals inform us of ways to decrease risk and hopefully increase the longevity of the services that we’re providing to our patients. In this article, I’ll summarize what I’ve learned from recent research literature about the various types of zirconia we receive from our dental laboratories and properties of zirconia that influence the quality and longevity of the dentistry we do. I found these facts fascinating from the perspective of wanting to know how to use zirconia optimally.  

Composition: There are three primary compositions of zirconia on the market: 3y, 4y, and 5y. The Y in the composition descriptor refers to Yitria or Yttrium Oxide (Y2O3). The more Yitria the zirconia contains, the more translucent the crown will be. Adversely, the more Yitria the crown contains, the less flexural strength the crown has. The 3y zirconia can handle adjustments better than 4y and 5y in terms of both strength and optical properties. 

Color: The marketplace offers zirconia that is prestained and zirconia that is not prestained. In the case of prestained zirconia, we have about a tenth of a millimeter in reduction capability before we notice a color change. Knowing this is important when we need to adjust a restoration, so we can manage adjustments to deliver the shade of restoration our patient expects. 

Wear on Other Materials: If zirconia is rubbing against other zirconia there is less wear than if zirconia is rubbing against lithium disilicate and softer ceramics or composites. When you’re setting up an occlusion or you are looking at a hybrid denture versus a normal denture, you must take this into account. If a patient has an upper fixed-hybrid denture, zirconia is going to wear those denture teeth relatively fast. Knowing this, we can best prepare our patient for the wear that is likely to occur. 

Zirconia Removal Burs Vs. Restorative Diamond Burs: In an in vitro study, the burs that are marketed specifically for adjusting zirconia restorations did not perform better on 3y, 4y, or 5y zirconia, and they did not generate less heat. 

Note About Polishing Zirconia: When we are polishing high-strength ceramics like zirconia and lithium disilicate, we must be careful not to push too hard, and we must use a continuous cooling spray of water. Otherwise, we risk nerve damage due to thermal irritation. Also, if we generate too much heat, we will overheat the glue that bonds the fine diamond particles on the bur and are likely to render the bur useless. 

Restorative Notes on Bonding to Sclerotic Dentin and Removing All-Ceramic Crowns 

By Lee Ann Brady, DMD 

Bonding to Sclerotic Dentin 

Bonding to sclerotic dentin is difficult, if not close to impossible. If the lion’s share of the tooth’s surface is sclerotic, you may not have the longevity that you’re hoping for. I’m specifically thinking of some lower anterior restorative cases I’ve seen over the years, where the veneers just haven’t held up and we’ve had to go to full coverage. 

I don’t trust some of the self-etching adhesives to result in a strong bond on sclerotic dentin, even the newer ones in the eighth generation. Fortunately, one thing we don’t need to worry about is sensitivity because the dental tubules are closed. Since I’m not worried about sensitivity, I can apply the same techniques I would with enamel with the intent of improving the probability of a strong bond. I can do a light prep, get rid of the sclerotic surface, and etch it with phosphoric acid for 25 or 30 seconds. Alternatively, I can use 30- to 50-micron aluminum oxide in an abrasion unit.  

Removing All-Ceramic Crowns 

Removing dental crowns can be a delicate and time-consuming procedure. In a world of so many different materials, it’s helpful to have an idea of which bur to use and how long removing the crown could take. One of the biggest challenges is determining whether a crown is a lithium disilicate or zirconia restoration. The radiograph and visual inspection will give us clues but afterwards, we must go through a process to understand what may be involved. 

Our First Clue: Zirconia looks like metal on a radiograph, and lithium disilicate looks radiolucent like natural tooth structure.   

Our Second Clue: If the crown is partial coverage, it’s much more likely to be bonded and I plan to prep down the entire restoration.  

Lithium disilicate restorations are often easier to cut through or section but they could be bonded and impossible to remove in pieces. Even if we can cut four pieces, we may have extensive prepping to do.  

On the other hand, zirconia can be harder to cut through, especially the 3y or 4y variety. But at least once you get to the cement layer, you can normally break it into pieces and remove them instead of having to extensively prep the entire tooth.  

If the restoration is full coverage, I can easily remove it in sections. In this case, I attempt to make my cuts all the way from buccal to lingual across the occlusal surface without bothering to stop. At this stage, I can pick up a crown remover and apply some general pressure to crack it off. If the crown is not budging at all, I assume it is bonded to the tooth, and the next thing I do is pick up a big flat-top diamond to do my occlusal reduction as if I were prepping a natural tooth. Once all the occlusal is off the glass, the pieces on the buccal, lingual, and interproximal fall off.