a 'ledge' margin on zirconia frameworks

[sampson]

i recall seeing a video where it was recommended that a ledge be created on zirconia framework margins -to support the porcelain layered on it. Porcelain now will have resistance to failure from shearing forces as the porcelain is put in a compressive state --. is this a common design used ?

 

[rkm rdt]

Porcelain is always in a compressive state.

The ledge design would have to also apply to metal and emax would it not?

I don't ledge.

 

[JMN]

i recall seeing a video where it was recommended that a ledge be created on zirconia framework margins -to support the porcelain layered on it. Porcelain now will have resistance to failure from shearing forces as the porcelain is put in a compressive state --. is this a common design used ?

That's still used some, but it was really more because of still adapting to the new (at the time) techniques and bonding requirements between zirc/alumina/etc and layered porc so it wouldn't shear as easy. Chemistry has improved and the safety net is no longer really needed.


Now, if you are really sub-gingival, you may want a collar, or not, but that's a different discussion.

 

[Affinity]

Porcelain is mechanically bonded, so if you create undercuts, then yes it will be less likely to shear off like glaciers melting into the ocean. Theoretically.

 

[sampson]

thank you
i figured it was a bit of a safety net and is dependant on each case

 

[sampson]

That's still used some, but it was really more because of still adapting to the new (at the time) techniques and bonding requirements between zirc/alumina/etc and layered porc so it wouldn't shear as easy. Chemistry has improved and the safety net is no longer really needed.


Now, if you are really sub-gingival, you may want a collar, or not, but that's a different discussion.

thank you ,
is there a common name (apart from ledge) for this type of margin -that a milling lab would know/ understand?

 

[JMN]

thank you ,
is there a common name (apart from ledge) for this type of margin -that a milling lab would know/ understand?

A collar on the coping, a prep would be a more of a shoulder style ideally to make it most amenable to having a collar.
This is older, but gives a good example of prep styles.
http://ceotecdental.com/PrepGuide/PrepGuideMargin.html

 

[Affinity]

Maybe I read this wrong.. if a 'ledge' is a 'zirconia margin' or as some old skoolers call a '2 point' margin, (3 point, being the tooth, the metal, and the porcelain, I guess)


I was describing building up the incisal 1/3 so that the middle third is undercut when looking from occlusal.

 

[JMN]

Maybe I read this wrong.. if a 'ledge' is a 'zirconia margin' or as some old skoolers call a '2 point' margin, (3 point, being the tooth, the metal, and the porcelain, I guess)


I was describing building up the incisal 1/3 so that the middle third is undercut when looking from occlusal.

Gotcha, i went entirely the other direction. Never had heard of it done that way. Seems like the varying width of porc would introduce more stress

 

[JohnWilson]

Who the heck is layering zirconia copings today?

 

[user name]

Who the heck is layering zirconia copings today?

 

[Juko]

Porcelain is mechanically bonded, so if you create undercuts, then yes it will be less likely to shear off like glaciers melting into the ocean. Theoretically.

I was taught porcelain is a chemical bond.
The opaque chemically bonds to the framework and the porcelain chemically bonds to the opaque.


On the other hand if we were going to apply composite to framework we would then need to utilize some sort of mechanical retention for the opaque to bond to so then the composite could bond to the opaque. Other wise it would shear off.

 

[kimba]

Who the heck is layering zirconia copings today?

Me

 

[sampson]

A collar on the coping, a prep would be a more of a shoulder style ideally to make it most amenable to having a collar.
This is older, but gives a good example of prep styles.
http://ceotecdental.com/PrepGuide/PrepGuideMargin.html

i guess i was speaking to the beveled shoulder style of margin

 

[CatamountRob]

I was taught porcelain is a chemical bond.
The opaque chemically bonds to the framework and the porcelain chemically bonds to the opaque.

Yes, that is correct, except as I understand it, with zirconia.

 

[Juko]

Yes, that is correct, except as I understand it, with zirconia.

I believe with the crystal zirconia it is still chemical bond as the porcelain bonds to the crystal. Same with lithium disilicate, it bonds the the disilicate. Perhaps I'm wrong.

 

[CatamountRob]

I believe with the crystal zirconia it is still chemical bond as the porcelain bonds to the crystal. Same with lithium disilicate, it bonds the the disilicate. Perhaps I'm wrong.

I don't know, we need an expert.
Ohhhh, @disturbed where are you???

 

[2thm8kr]

I believe with the crystal zirconia it is still chemical bond as the porcelain bonds to the crystal. Same with lithium disilicate, it bonds the the disilicate. Perhaps I'm wrong.[/QUOTE

I haven't been able to find any credible source of a chemical bond between zirconia and porcelain. All data I have found/read points to mechanical bond only.
The mpa of veneered porcelain to zirconia is the weakest at approximately 90mpa.

 

[Affinity]

Yes porcelain to metal does have a bond between the metal oxides and the opaque/porcelain.. IF everything is compatible. Porcelain still shrinks and compresses around the metal, creating a mechanical bond, this is why the CTE has to be similar. A chemical bond only has so much 'strength', as you can see by porcelain chipping or even separating from the metal.

If porcelain didnt have a chemical bond to zirconia, or between zirconia/liner/porcelain.. then I dont think doing things like buccal veneers would stick becuase there is less undercut, its just a facing. Ive found that being able to do very very minimal layering gives a better effect and less prone to large amounts fracturing. Having said that, I just got back a emax central that the M corner broke off.. Maybe one of the first ever. It might have had 1mm of porcelain over the incisal edge.

 

[sampson]

For layering porcelain over zirconia - what system do you use?
Im looking at either Shofu vintage zr or GC initial zr -- any suggestions?