Refractory onlays cracking

[Aldad]

Ok, I know the first question will be. Why are you still doing refractory onlays when we have emax?
The answer is one of my Dr's loves the way they look compared to emax. I have to agree a miultipowder build up on refractory looks fantastic! She will pay extra for them to make it worth my time. The problem is half the time they crack before I get them off the die. Veneers are no problem. the onlays are often very large and should really be full crowns were talking 2.5-4mm thick in some areas. I have fast cooled, slow cooled both crack. I have used Creation CC, Creation ZI-CT, Wieland Reflex, and e-ceram all have the same problem. I use GC orbitvest. 1, Wash bake 2, 1/2 buildup, cut for shrinkage 3, 3/4 to full couture 4, final build and then glaze.
So 3,4 bakes and a glaze bake.

Any ideas?

 

[Dentalmike]

Many times, carefully devesting area that's cracking and refire for glaze with a peg putty material placed in same area to hold ceramic in place has worked for me. Also have increased thickness of a thin area that's cracking next to thick and finished back after devesting.

 

[kimba]

I also see this problem, especially in onlays of uneven thickness. i agree with dental mike's solution, but sometimes this will take off any stains you may have. Another solution is to thin the refractory die from underneath the thin area, but be careful as they can break. what also works is to devest while they are still quite warm, and by quite warm I mean almost uncomfortable to pick up. Be careful to aim the glass beads only on the die , as this seems to insulates the still warm onlay from the thermal shock of the blast . Like lots of fixes I only discovered this method in desperation on a case that kept cracking , with the courier tapping his foot at the door

 

[rkm rdt]

Instead of slicing the porcelain for shrinkage,try building up in layers.

Porcelain shrinks to it's mass , so I used to build up the deep areas and then the walls.Finalizing in the center.

You are getting stress fractures from slicing.That's why it's happening with different porcelains.

 

[Tayebdental]

I think it is a matter of incompatible materials. Find out the rate of expansion of the refractory investment material and the compatibility of the ceramic, this way you solve the problem once and for all. I used to use Noritake ( nurivest) for onlay /inlay with Noritake porcelain with no problems.( the procedure taken for maturing the investmant by baking before porcelain build up is also important). I hope this help.

 

[TheLabGuy]

What about Empress? Look up Dr. Jason Smithson from the UK, he'll blow your mind with his resin...try with some PMMA!!!

 

[ilsbadly]

have you tried pressing a thin e.max and then layering that with the additional porcelains? its just a case of getting used to removing the restoration from the model without it breaking but it gets easier the more you do it, its just way better with emax these days, especially if the restoration needs any further adjustment after trying in, best of both worlds.

PMMA can look like chewing gum after a year or so in the mouth, especially if there have been any slight occlusal adjustments, just my opinion

 

[Aldad]

Thanks for all of the suggestions. I think I have tried most of then. Some times I am able to repair with refiring after partial devesting. I also used to bake in layers from the center out, finishing with the outermost skin and margin. This is a very natural look. Still 2 or three out of 5 will crack. I really think it has the most to do with the size and shape of the prep. Unfortunately we can't control this and it is difficult to evaluate every prep, call the Dr. and reject the case.
I used to have problems with my solid dental porcelain art sculptures cracking in half when cooling. I started dropping the temp 100 degrees after final temp and holding for a couple of min. then slow cooling. this tempers the glass. art glass blowers do this to keep the glass from cracking. I started doing this on my sculptures and it works. maybe it will work on the onlays.
I think maybe I will revisit the idea of a presses emax base, then layer. Still won't look as good but sometimes the real world forces compromise!
Cheer!

 

[sonic65]

Hi

 

[Acrylic Innovation]

on thin restorations, cut apart the prep and invest the prep with your wax pattern still sealed down... press. saw it in JDT or some other mag... works!