cracking porcelain on long span bridge

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labguy5381

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I have a porcelain cracking question to pose. Attached are photos of a large anterior case. The porcelain on this case cracks horizontally across the entire lingual and some interproximal and buccal. It is not a crack across the incisal edge indicating too much unsupported porcelain at the incisal edge. It is multiple cracks across the middle third of all of the pontics. While these cracks were somewhat evident in the initial firings, the real problem occurs in the glaze. The case continues to crack over time. Under the microscope there does not appear to be contamination in the metal. The surface of the metal looks quite good. The opaque was (and still is) well bonded to the metal. Notice that this is a large bridge with only 4 abutments. The whole anterior section is edentulous. This is 7 pontics supported by only two abutments on each end. I don't like it, but it is the hand we were dealt. So, the question is...how would you do this case to prevent the cracking? I have a theory, but I would love to hear your experience on this problem.

Thanks for your help,
Jim
 

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dabster

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Coefficiency of expansion? Is this alloy correct for the powder you are using? Was this bridge cooled over several hours?
 
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grantoz

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Its to do with the cooling try slow cool as there is a lot of metal .you may need to start again if the metal is contaminated with the wrong alloy.also check you havent used your zi porcelain on metal i have done this.
 
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grantoz

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PS i would do it in zirconia
 
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charles007

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How/what did you use to support the bridge on the firing tray ? pillow ?
Its going to take a long time for the pontics to cool, what was your slow cooling time and temp while still in the oven before taking out....
On this mass of metal I hope the porcelain to metal CTE's were no less than ideal........
Are you sure your porcelain is ok to use with the alloy ?.......... anything less than ideal and your going to see checking on the pontics...
 
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dmonwaxa

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Sometimes though the CTE of matches between materials there is a mismatch in cooling rate between fmk and porcelain. Porcelain isccioling faster than the fmk because the fmk is too bulky causing it to cool much slower than the porcelain. Reduce fmk bulk, build porcelain a but bulkier and sloooooooooooow cool. Its cracking because the previously veneered porcelain I was too thin.
 
Baobabtree

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I've had this before and I didn't have enough exposed alloy to allow cooling. Drove me nuts, and this was well before DLN. Where possible I design in metal backings if bite doesn't allow porcelain coverage and collars in the cingulum areas or intersticially of the pontics. If your doc doesn't like exposed alloy then perhaps suggest doing it in Zr.
 
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charles007

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Looking at this today it looks like the metal is kissing-touching the tissue, how thick is porcelain under the metal ?
Looks like there is lack of reduction on the bi and molar ? it that what I'm seeing ?
Very uneven layers of porcelain is definitely a problem in the design... thin porcelain and heavy-bulky in metal design........
 
BobCDT

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If you have well matched CTE this should not happen. I figure you are probably up on this and have been using the combination of the two for some time.
Could someone have mixed up a button from another alloy when casting this case??
 
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labguy5381

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I appreciate all of the great replies. I agree that it is probably a cooling issue. The metal and porcelain are matched from the same manufacturer. Contamination is a possible issue, but in my experience if the metal is contaminated, it shows up visually under the microscope in some funky(dental term) way. You are correct that there is not a lot of reduction on the abutments, creating some of the issue. Grantoz...yes we have fired ZI porcelain before too, but not on this one.

In response to the porcelain under the pontics, there is an exposed implant and cap in the middle of that span that we decided to rest metal on for support. Would you have done this? This may be the reason for your comment about the metal close to the tissue. The current metal design definitely has a metal island underneath touching the implant cap.

What I am considering is an old style metal band on the lingual to stiffen the arch and allow for more even cooling. What do you think? While I hate changing more than one variable when there is a problem, we will also use new metal and crucible in recast.

Bob...would you have done this case in Zirconia? Our CAD guy was worried about those joints at the abutments because of the limited space.

Advice?

Thanks,

Jim
 
rkm rdt

rkm rdt

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always a lingual band,always!
 
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adl

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I appreciate all of the great replies. I agree that it is probably a cooling issue. The metal and porcelain are matched from the same manufacturer. Contamination is a possible issue, but in my experience if the metal is contaminated, it shows up visually under the microscope in some funky(dental term) way. You are correct that there is not a lot of reduction on the abutments, creating some of the issue. Grantoz...yes we have fired ZI porcelain before too, but not on this one.

In response to the porcelain under the pontics, there is an exposed implant and cap in the middle of that span that we decided to rest metal on for support. Would you have done this? This may be the reason for your comment about the metal close to the tissue. The current metal design definitely has a metal island underneath touching the implant cap.

What I am considering is an old style metal band on the lingual to stiffen the arch and allow for more even cooling. What do you think? While I hate changing more than one variable when there is a problem, we will also use new metal and crucible in recast.

Bob...would you have done this case in Zirconia? Our CAD guy was worried about those joints at the abutments because of the limited space.

Advice?

Thanks,

Jim
CTE issues can happen even when the manufacturer tells you they are a match.
 
Car 54

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With some porcelains the cte can also change over multiple firings. On a single that you just do your normal opaque, build and glaze, probably won't be a issue. But if you have to do multiple firings on a bridge, the cte can change and need to do a slow cool. That is why I like Vintage Halo, it's stable over multiple firings, I used to have more cracking issues with Vintage, when I had to run it multiple times.
 
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Tayebdental

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Many issues are involved when this happen, it needs complete re evaluation of the process imo.
 
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charles007

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Still think the CTE is off just enough to create checking with the long mass of pontics and maybe not long enough of slow cooling........
Metal bands might help but if its a cooling problem it still leads back to cte issue
 
BobCDT

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when there is a problem, we will also use new metal and crucible in recast.

Bob...would you have done this case in Zirconia? Our CAD guy was worried about those joints at the abutments because of the limited space.

Advice?

Thanks,

Jim
I don't think I would have done this case without implants. Absolutely would NOT do this in zirconia. Probably would recommended NP for strength. My concerns are keeping the case cemented. I don't know if the bond strength of the cement will be adequate. Fracture of the connectors and or loss of the bi's due to overload all are of concern.
Don't offer any warranty on this one.
 
Tayebdental

Tayebdental

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I would agree that if l would fabricate such a case it would be with no warranty. SEVEN pontics ??!!Ahhhhh
 
ps2thtec

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I agree with Bob, similar thoughts on this recent case, small abutments, long pontic span.
Had it milled in CoCr. Metal linguals . SLOW cool.
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ztech

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A common problem to address in a case of this size is the heating and cooling of the alloy and the porcelain maturation as mentioned above. On a case of this size I routinely increase the high temp by 5-10 C and increase the high temp hold by up to +75% on the first bake to overcome the mass of material that is heated. Under matured porcelain close to the alloy interface will wreak havoc on subsequent layers that will easily mature over them. I would also add a slow cooling time of 6-10 min. As for the design, I almost never expose metal unless needed due to lack of space. I also agree with Bob, the span of this bridge exceeds what I would feel comfortable making. It would go out with a lot of disclaimers.
 
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