Zirconia crown fracture
Zirconia crown fracture does occur. When it does it is often due to poor prep design, inappropriate material selection, or poor handling of the material. If you want to see some zirconia framework fractures click here. To learn more about zirconia crowns check out page on zirconia crowns.
Zirconia crown fracture due to poor prep design
The main prep design issue is not giving the laboratory enough space. Technically the minimum occlusal reduction is 0.5 mm, but 1 mm is ideal. Personally we aim for 2mm knowing we will end up short somewhere. Another issue is dentists prepping for a high quality brand zirconia but using some other zirconia product. Not all zirconia is the same and the prep clearance requirements will vary. For instance, BruxZir anterior restorations require a minimum occlusal reduction of 0.8 mm and 1.25 mm, but other less esthetic zirconia is only .5mm. Abdulmajeed 2020 J Pros study shows 3Y zirconia minimum is .7mm while 4Y and 5Y is 1.2mm. These numbers are close to what Bruxzir asks for as well.
Another prep design issue is leaving sharp corners or edges. This can induce a point of contact where crack prorogation can begin. Stress concentration occurs at the point where the sharp edge of the tooth meets the zirconia.
Zirconia crown fracture due to poor material handling
The are many ways the lab that manufactures the material can cause fracture. There are also many ways the manufacturer of the product can as well. However we will focus just on the dentist’s issues. As dentists, the way we can cause a zirconia crown fracture is by drilling on the crown, especially without copious water spray. This is done while adjusting a crown to fit on the tooth preparation. Adjustments that cause Bruxzir crown fracture can be inside the crown, interproximal, or occlusal. Of course an endodontic access also requires significant drilling of the zirconia crown. The most common time we have seen zirconia fracture is with an endodontic access of the tooth. This is the cause of one of the two zirconia products we have recently seen fracture.
The most common reason zirconia is cracking and breaking is simply that it is too thin.
As you can see in this graph the thickness of zirconia has s big impact on it’s strength. Even if dentists supply 1.5mm of clearance, which is difficult to verify in every location, then the milling procedure will chew up some of that.