FCZ Bridge Failures and Solutions

[charles007]

All of us at one time or another has had at least one failure using zirconia on bridges and single FCZ. Let's talk about solutions to lower the failure rate using the ceramic steel that growing like a weed.
Lets also include the new lower MPa zirconia(615-769) that's more translucent zirconia in this discussion..

My only bridge failure was on a case I really shouldn't have used zirconia when I saw the connector dimensions. The connector height was a tad less than 3mm on one connector. I bulked up the surrounding connections and ended up just over 9mm sq.. but this case had really short preps.. 17x19 and broke on #19 and occlusal . Doc adjusted the occlusal over the connector to add more grief.
fyi... I learned my lesson and never again..

 

[2thm8kr]

Tell your clients that if they make occlusally adjustments, NOT to put new secondary anatomy back in with a carbide. Tetragonal structure of zirconia is supposed to stop/resist crack propagation, but I don't believe it makes it indestructible.

 

[ChrisBWJ]

So the Law of Beams referenced in the connector thread runs true here as well.

Double the vertical dimension reduce flex by a factor of 8. Double the horizontal dimension reduce the flex by a factor of 2. Give yourself as much vertical connector dimension as you can for maximum strength.

Zirconia is a thermal insulator.

It is slow to heat up and slow to cool, right? That's why your porcelain firing cycle for PFZs is different than that of PFM. Cycles needed to be adjusted to make sure the porcelain near the zirconia core properly vitrified and so that the porcelain wasn't unnecessarily stressed during the cool down cycle.

With full contour bridges, there are isn't any porcelain, so why should it matter? Zirconia is still an insulator whether it has porcelain on it or not.

So imagine a FCZ bridge being stained and glazed. It's in your oven and the hood has finally gone up. You pull the tray out and set it aside to cool. The outside surface of the bridge comes down to room temperature within a few minutes. The intaglio surfaces of the abutment teeth have cooled down as well. What hasn't cooled down is the center of that big honky mass of zirconia pontic of your bridge. It may still be at 300 degrees while the outside surface of the pontic is at 30 degrees. CTE goes out the window if the materials aren't at the same (or very close) temperatures.

Slow that cool-down cycle down!!!! Take an extra 5-10 minutes, or maybe more. Especially on the larger bridges. Otherwise you can introduce at thermal gradient induced stress internal to the pontic or near a connector. If it happens you'll likely be hearing from your doctor soon after you deliver the case. They don't usually last too long if they've got an internal stress fracture.

I kid you not. I once had a full contour bridge right out of the porcelain oven, cool enough to hold in the palm of my hand, fracture while we were admiring the technician's design skills. It wasn't dropped or being flexed... it just popped on its own.

Once we slowed down cooling cycles and had our customers do the same, no more FCZ fractures.

I really wanted to do a formal study on this but identifying internal stress fractures on zirconia pontics wasn't easy to do with the tools I had at hand... once the changes were made, the fractures went away!

 

[2thm8kr]

So typically, what is the cool down rate/time on something as large in mass as a full coutour implant supported bridge?

 

[ts4341]

They should not have to touch the Occlusal surface if designed correctly.
Connectors are stronger when designed with more vertical height

 

[2thm8kr]

They should not have to touch the Occlusal surface if designed correctly.
Connectors are stronger when designed with more vertical height

If temporaries didn't break, wear or come out and the mouth wasn't a dynamic environment I would agree completely.

 

[JohnWilson]

As you guys know from my posts I have been doing a ton of Full arch Hybrids, my case selection has been stringent and while I have produced stuff inside spec with a disclaimer "written/signed" I have had 2 to date that have fractured in the mouth and I am now around 200 arches

The vast majority of our issues are due to clients missing bites and wanting to play catch up when the case is completed. While our protocol is to mill a beta appliance in PMMA, some clients are not doing their due diligence and really working out the bite. I have gone to a new disclaimer on these items that state "ANY" adjustment to the zi Voids the warranty, of course this is just a scare tactic but it opens their eyes to the fact that the lab feels there is a chance of damage and forces them to be more diligent in nailing down the bite in the plastic.

We protect the incisal edge of all ant with Zi and layer a very minimal amount .3 to .6, if the client missed the bite and the case slides to a more end to end fashion nothing is going to happen too the incisal UNLESS dr big bur try ands adjust it.

Like anything else with joint height we can get a visual feeling after designing hundreds of bridges but you really must implement your 2d cross section tools and like others have stated look at the abutment tooth B/L wall thickness as well.

 

[dentalartslab]

Reset...

Hello All.

So I'm familiar with longer cool down settings for firing on zirzonia substructures. VM9 in particular. So, I'm looking through the Emax Eceram and can't find anything about cooling. Maybe that's why there is a high failure on sheering?.. I haven't had that issue but question is that eMax has a lower firing temp approx. 750C . Is that low enough to where fracture/stresses are not incorporated. The Liner that is used to aid in bonding fires at 960C I could understand a longer cool period for that temp.

Also, does anybody run a heal program on frameworks before they start veneer process? If so... what paramaters....


THanks!

 

[rkm rdt]

Mine heals during the bake cycle.
I make full contour connectors and design my cutbacks similar to John's microlayering technique.
I don't make my zirconia bridges to look like tooth coloured pfm bridges. Completely different animal.

 

[Andrew Priddy]

Rule 27
Height squared X width= 27 or greater

or maybe i'm wrong... it's been one of those weeks where I feel like I don't know anything anymore

 

[2thm8kr]

Rule 27
Height squared X width= 27 or greater

or maybe i'm wrong... it's been one of those weeks where I feel like I don't know anything anymore

The week just started, I suggest you take the rest of the week off.

 

[Andrew Priddy]

The week just started, I suggest you take the rest of the week off.

you would assume my week actually stopped, to be able to have one "start"

 

[Car 54]

So the Law of Beams referenced in the connector thread runs true here as well.

Double the vertical dimension reduce flex by a factor of 8. Double the horizontal dimension reduce the flex by a factor of 2. Give yourself as much vertical connector dimension as you can for maximum strength.

Does this apply to anteriors, also? Should the formula be fudged slightly for a little more thickness, compared to height, as the force is from the lingual to the labial in centric occlusion as well as anterior guidance?

Rule 27
Height squared X width= 27 or greater

or maybe i'm wrong... it's been one of those weeks where I feel like I don't know anything anymore

What, you want me to do math after a long holiday weekend, now that make me want to I guess it's time for me to find my dads old slide rule

 

[lcmlabforum]

Chris, I like the beam theory you shared here, much like the text books stuffs.
Do bear in mind, Beam theory is based on a uniform cross-section, isotropic structure.
A dental prosthesis is neither isotropic in material construction, nor uniform in geometry,
so any dental prosthesis has areas of stress concentration, and vulnerable to multiple
direction of stresses, and in a bruxer or parafunctional masticatory system. all bets are offl
As such, always over-engineer, as long as esthetic and biological health will allow (no overhang).
FWIW . . .
LCM

 

[lcmlabforum]

I think dental structures, either natural or fabricated, would be perfect example of
anisotropy, which manufacturers try to overcome with milled blanks. But no one has
a perfectly produced blank, or perfectly milled, sintered/stained prosthesis . . . if only.
Even cast gold may have imperfections.
Always know your limitations.
LCM

 

[dentalartslab]

Mine heals during the bake cycle.
I make full contour connectors and design my cutbacks similar to John's microlayering technique.
I don't make my zirconia bridges to look like tooth coloured pfm bridges. Completely different animal.

Not familiar with John's technique... is there a link.

What material is everyone using to layer? GCs..

I am using eMax ceram and getting tons of bubbles... seems like material doesn't 'let go' of the bubbles like most porcelains. I've been using a metal-occlusal type technique. Trying to get this new high trans ZrO2 to look right... comes out too opalescent. Testing three different ones atm.

 

[rkm rdt]

no link but I have posted my designs somewhere on here.