Zirconia framework fracture
Well this would certainly be a very bad day! A zirconia framework fracture would result in the entire prosthesis needing to be remade. A considerable amount of time and money would have gone into this prosthesis. One thing for sure is that the thickness of material around the screw access holes is important to keep in mind, as it seems that is the main spot these are breaking. When the labs are having these made the software should tell them the dimensions and should be adjusted to ensure not too thin anywhere. All of these seem thin where they broke to me but that is only evident to the dentist upon breaking. We need the labs to really watch this for us.
These photos are found on dentaltown. Some broke before the patient even used it. My guess is that cheap or inferior off brands are being used for the zirconia and/or made too thin or handled poorly.
Zirconia thickness requirements for pontics
The guidelines for a zirconia single pontic bridge (unable to find my original source of this one but heard a second time in Marinello Carlo digital dentistry lecture)
- 9 mm² in the posterior
- 7 mm² in the anterior
For double pontic the number is greater than 12.5 mm² and a free end pontic is 6 mm² in the anterior and 36 mm² in the posterior.
Glidewell has a Rule of 27 for their Bruxzir bridge. This is quite a bit more than others and I think is more realistic.
Tischler says need 12mm thickness and no more than 11mm cantilever. He also believes that using Zirkonzahn Prettau zirconia and slow heating and cooling makes a big difference, which I think few would not agree with that. They have a rather impressive study proving it does work.
Larsson found 4mm to be the minimum of the samples they tested in 2007.
Why is the zirconia breaking?
What I think is a very good explanation can be found in the thread itself. Almost all of these broke before even trying in so for some reason are clearly very weak.
“Without seeing these cases up close, I can say that the ones that fracture before they get off the cast are usually caused by manufacturing process errors. Zirconia is a wonderful material and almost indestructible, if all procedures and processes are done to exacting standards. Unfortunately, the procedures sometime have shortcuts that do not manifest error until later in the service life of a prosthesis. The weakness of Zirconia is that it is not a great thermal conductor and all firings take forever to accomplish. If you shorten this process, you may do this in many instances without short term consequences. Running the correct program in a firing furnace (to add color) takes twice as long as an Emax or PFM. If you layer porcelain then you can tie up that furnace for hours longer than processing other materials. If your business model is to provide these units at a low cost……time is money.”
Three year old zirconia from another dt thread