porcelain to gold problem

[a14vmk]

Hi guys just wanting your opinions on a problem ive been experiencing in my lab over the past few months, for all my implant cases i use a 51% au alloy, i recently fabricated a 3 unit bridge on 2 implants and was really happy with it so i decided to glaze it and the bridge came out of the furnace with visable hairline cracks from the lingual of each abutment towards and through the lingual of the pontic, so i decided to strip the ceramic off and relayer it but this time i introduced a 8 min cooling cycle but this did not solve my problem as it happened again! The alloy i use has a corresponding cte to my ceramic and other labs around town also use this alloy with the same ceramic and have absolutly no problems. The interesting thing is that it doesnt happen on any single units i make just bridges!

 

[stt672]

It is nine times out of ten to much porcelain in ratio to metal. If your porcelain and metal are compatiable like you stated it is a design flaw usually. Try doing the opposite when cooling, get hemostats on both abutments as soon as you can and do a fast cool.

 

[stevenson]

Nowadays, there is not much of a difference between porcelain and gold inlays and onlays. Inlays are fillings cemented into place to fix a cavity. Onlays are much the same but extend to the whole cusp, which is why a crown is a type of onlay...

 

[Malcolm Graham]

It sounds as though you are getting tensile cracks. Usually a sign of the cte of the alloy is not high enough. I have had the same problem with implant work using an alloy that was in the correct cte range for the ceramic.
What I think the problem is with implant bridge work, there is a large amount of alloy used compared to normal crown and bridge work. Porcelain thickness is also maxed out (within the amount of support needed) to save some gold.
There is little in the way of cooling alloy exposed compared to normal crown work where you have the whole internal surface of the crown to cool the work.
Also there are usually a couple of extra fires in the furnace which can increase the lucite in the ceramic, which increases the cte of the ceramic.
I changed the alloy I use to one where the cte is a little higher, try and reduce the amount of fires I need to do, and keep the units seperate from each other when they are fired. Hope this helps.

 

[charles007]

Change alloys to a better match. There's a wide range in the CTE that will work with your porcelain, so they say ! Ask what that range is when inquiring about a new alloy. When your alloys CTE is on the top end or bottom end of your porcelain, you can get checking, and this checking more likely occurs on thick porcelain/heavy pontics, that's why you see checking on bridges only. Slow or fast cooling can help to a point. When your using an alloy that needs to be slow or fast cooled as an example, your playing with fire. You want to be more on the lower end , sweet spot, on the CTE so you can slow cool on large cases.
Example, Ivoclar make 2 cheaper priced alloys for design porcelain, design 59 and design 53. Using the 53, it has more silver than 59, and has a higher cte, and it's better to slow cool "BUT" can still cause checking, especially on bridges .
Not that I know what brand of porcelain your using, an alloy in the 14.1 -14.2 is in the sweet spot for many porcelains/not all.....I've always used alloys with ideal CTE so I wouldn't have to worry about checking on larger cases. If your alloy is on the high end range of the CTE, wax pontics closer to contour, slow cool and cross your fingers.... that's about the only insurance that helps....... If you need to fast cool, you can wax pontics with exposed metal band on linguals, which is not done anymore and will not solve basic expansion issues.

 

[PDC]

Change alloys to a better match. There's a wide range in the CTE that will work with your porcelain, so they say ! If you need to fast cool, you can wax pontics with exposed metal band on linguals, which is not done anymore and will not solve basic expansion issues.

These 2 observations mean a lot when trying to avoid checking issues. If you like the porcelain your using, I would take Charles' advice and expose some metal bands to help cool heavy pontic sections. If that doesn't work I would probably change porcelain rather than alloy if the CTE's match up and you are still having checking issues. Checking issues can be a real pain to solve sometimes, but I have found that having some silver (7-10%) in alloys helps to avoid checking while also improving casting, finishing, and bonding.
You didn't state what brand of porcelain you are using, but that might be a good idea so that other users on this forum who might be using the same porcelain could offer alloy advice.

Personally, I use Vita VMK Master. It has a CTE of 13.1 – 13.6 x 10-6 , ideal for veneering metal frameworks that have a (CTE) in the range of 13.8 – 15.2 x 10-6.

I"ve never had any checking issues with this porcelain and I can wrap it all the way around a pontic with no metal exposed.

Hope this helps.

 

[labmanmike]

Sounds a litttle like the porc is too thick on the ponic and/or it could be "Thick on the occ and buccal and thin on lingual. I have had this issue happen more than once. As long as all the CTE etc is correct, it could boil down to thick porc. and uneven cooling issues. Hope you have good luck my friend