A crown or “cap” covers a tooth or a dental implant. Dr. Hawryluk often recommends crowns to support broken, weak, or misshapen teeth.
One of the main types of dental crowns made at St. Lawrence Dentistry is zirconia crowns. They have quickly become one of the most requested types of dental crowns for good reasons.
Zirconium dioxide, a white powdered ceramic material, is the principal ingredient of zirconia crowns. Its ceramic properties and its milling from a singular block make it a trustworthy dental prosthetic. Zirconia crowns have fundamental advantages in terms of toughness, durability, longevity. In addition, Dr. Hawryluk often must remove less tooth to place a zirconia crown. Zirconia is one of Dr. Hawryluk’s favorite materials due to its biocompatibility. It’s less likely to prompt the body into producing a reaction or immunological response such as inflammation.
One of the drawbacks of zirconia dental crowns is that its traditional “bond strength” to a tooth is not as good as other crowns. Consequently, there is a higher chance of this type of crown falling off from a tooth on which the dentist placed it. Fortunately, new research has uncovered ways of increasing the bond zirconia has to teeth, circumventing this legacy problem of zirconia crowns.
Traditionally dentists were taught that zirconia could not be reinforced by bonding it with resin cement. Bonding to zirconia means forming some chemical bond between zirconia and our resin cement. However, studies have shown the opposite-this legacy teaching has contributed to most dentists still only “luting” zirconia crowns with conventional resin-modified glass ionomer cement- this is the crux of the issue regarding the inadequate retentiveness of these crowns. The most common time when you need to bond zirconia crowns is for short second molars. In these cases, your dentist is limited in the interocclusal space distance between upper and lower teeth when “preparing” the tooth for a crown. It translates into minimal material thickness possible. In these situations, Dr. Hawryluk wants to use the strongest material we have, which besides metal, is zirconia.
Dr. Hawryluk makes sure his Mississauga patients benefit from the latest research in dentistry. A study from the journal of dental research looked at 144 different laboratory studies examining whether various chemical primers could improve the bond strength between resin and zirconia. They found that if you sandblast the zirconia surface with regular alumina or silica-coated alumina and then used an MDP-based primer, you get an improved bond to zirconia.
When Dr. Hawryluk’s ceramist makes our zirconia crowns, the protocol used is: sandblast with 50-micron alumina at 1-2 bars of pressure, with the blaster aimed from about 10 mm away from the crown. Most of the time, we use alumina for sandblasting, but there are different particles you can use. Rocatec, which is alumina covered in silica particles, is a suitable alternative and is mentioned further in this article.
The concept of sandblasting is when you roughen up the internal surface of the crown, the cement can lock into the microgrooves. For roughening up zirconia, you have to use alumina particles, and we can think of this as “little rocks”. The roughening up provides micromechanical interlocking with cement. Dr. Hawryluk explains he must know what type of zirconia he’s dealing with before sandblasting. Studies have looked at sandblasting zirconia at different pressures and how it will allow bonding to zirconia. The pressure must not be too high to affect the strength of zirconia. “3Y” zirconia is the stronger form of zirconia, whereas “5Y” is the more translucent form. When you sandblast “3Y”, zirconia gets a tiny bit stronger, and when you increase the pressure, it does not affect the strength of zirconia. The “5Y” zirconia gets a little bit weaker as you sandblast it with more pressure.
The sweet spot for sandblasting zirconia is at 1 bar of pressure. Between 1-2 bars is acceptable as it allows a good bond, but it does not significantly affect the strength of the material. Researchers did these tests with 15-micron alumina particles. One bar of pressure is the same as about 15psi, and 2 bars are 30psi. The blasting should be for about 10 seconds from about 10mm away.
We use a silane primer when using glass-ceramics like lithium disilicate (another type of crown material Dr. Hawryluk commonly uses). Saline is a chemical “linker”. On one end, it has a silanol group that bonds to glass, and on the other, it has a methacrylate group that bonds to resin types of cement. The problem with silane is it cannot bond to zirconia since there is no glass in zirconia. Zirconia is a polycrystalline material with no glass phase inside of it – this means we need a different linker molecule called 10MDP. On one end, it has a phosphate functional group that bonds to zirconia (and some metals as well), and on the other is a methacrylate that bonds to resin cement. Some call 10MDP the “silane of zirconia”.
There are different companies offering products with 10MDP. Z prime is Dr. Hawryluk’s favorite and from Bisco Corp. Monoband plus and Clearfill ceramic primer are combination products that have both silane and 10MDP in them. “All bond” or “scotch bond universal” also contain it. When applying 10MDP to a surface, its phosphate functional group bonds to zirconia and its methacrylate group bonds to resin.
We have many great dental products in Canada allowing zirconia bonding. Monobond plus and Clearfill ceramic primers are universal primers that have 10MDP in them. “Scotch bond” and “All Bond” universal are classed as bonding agents for tooth structure but can also be used for zirconia priming since they have 10MDP in them. “Z-prime” is a pure MDP product from Bisco, which Dr. Hawryluk uses for zirconia bonding. It can also bond to different metals, including titanium.
As mentioned above, your dentist cannot use silane on its own for zirconia bonding, as it only bonds glass-based materials. However, there is an exception to this rule if the sandblasting material used is Rocatec. It is an alumina particle surrounded by a layer of silica. When you use it, it sprays silica all over the zirconia specimen. Silica is the same thing as glass, and once you have glass on the surface of zirconia, you can bond to it with silane primer. At our Mississauga dental office, we, of course, ensure the correct priming of your dental crown.
When you try a zirconia crown in the mouth, saliva phospholipids stick to the MDP bonding sites. If your dentist tries to clean this off with phosphoric acid, phosphate-based contaminants in phosphoric acid will stick to the MDP bonding sites. Then there would be nothing for the MDP to bond – therefore you cannot use phosphoric acid to clean zirconia. Solutions available to clean zirconia are Ivoclean, Katana Clean, Bisco makes Zir-clean which is also another zirconia cleaner. Interestingly researchers also found Clorox bleach cleans saliva off zirconia, however not as good as the commercial products. Also, re-sandblasting is another way of obtaining a reasonable bond.
In review, Dr. Hawryluk’s steps to bond zirconia are:
- Sandblasting between 1-2 bar pressure with 50-micron alumina from a distance of about 10mm away.
- The crown is then ‘tried in in the patient’s mouth, at which time contamination occurs with salivary phospholipids.
- Dr. Hawryluk then applies cleaning solutions to the crown, such as Zir-Clean.
- The crown is rinsed out after 20 seconds, and a 10MDP primer is applied.
- Finally, the crown is cemented with the widely acclaimed ‘Link-Force’ resin cement.
We hope you have enjoyed this article. If you live in the Mississauga area and are looking for a dentist, please give us a call!
Viva Learning: Bonding to Zirconia: Nathaniel Lawson, 2021