Chamfer Prep for Zirconia Crowns

[MushMaster]

Recently I have gotten more and more chamfer preps for zirconia crown and bridge requests, to the point where the prep in reality is more like a knife edge with chipping (post sintering) as a result. From my understanding, shoulder preps are fine. Chamfer also fine as long as the prep is +/- 0.5 to 1.0 mm in depth. I just want to rule out that I am missing some new study/method/material that indicate knife edge preps to be fine to use for zirconia restorations. Or, whether the operator well intended to prep a chamfer but ended up with a knife edge prep, chipping the crown and then say, "But chamfer preps should be no problem". And thereby, before we start using less aesthetic zirconia by simply solving this issue by choosing a bruxer type zirconia for strength (and thinking knife edge preps are then no longer a problem)?

What is your knowledge on this?

 

[Brett Hansen CDT]

Knife edge margins for zirconia crowns, or any crowns for that matter, are not ideal. Having said that, we see them from certain accounts all the time. We don't have any issues with chipping, but that could be due to the zirconia we use(Ivoclar Prime Multilayer) and the milling parameters we have set up in 3Shape. We don't change anything due to the prep design because we just haven't encountered any problems with margins chipping.

 

[MushMaster]

What is your emergence profile like on those knife edge preps if you don't mind me asking? Do you taper from margin to ideal rather quickly getting a more 'bulbous' design. Because when there is a knife edge, the other area to consider is the cement space. So coming up from the margin, there will 'again' be a thinner area where the cement space starts. (This is the reason for asking about your emergence profile when knife edge preps are present).

 

[Brett Hansen CDT]

Cement Gap .020 mm
Extra Cement Gap .060 mm
Dist to margin line .1 mm
Smooth Dist .2 mm

These are the settings we use for our zirc on every prepped tooth case. minimum thickness is set to .5 mm