Hey everyone. I'm a doctor in a big group practice with an internal lab, and I'm the liaison between the doctors and the lab. I was hoping you could help me out with this. Our lab mills zirconia crowns, the after they're sintered, they trim back the margins manually. Whenever there's a margin issue, my doctors say that the error is caused by the manual trimming, and that other labs don't do that. Our lab says that of course everyone does that - that's the only way it can be done. So who's right? Does everyone trim back their margins manually, and does that tend to be a big source of margin error?
You know what you need is EXACTLY the same as We require. PROPER tissue management is imperative. If you can't see your margin either can the camera or us. "Ring around the collar" is still the goal for IOS. (Full retraction and pic up with impression material) This us critical for a proper restoration. Have heard too many times with cerec type docs," if it fits past the lips" it fits. IOS is NOT a Magic Wand!! It's like any software, no matter how complex. What you feed in is what matters. A prep with zero defined margins, just a fliud sulcus does no way near suffice. I have operated for so many years on the Dr., post insertion, Not finding my margins with a Piano Wire Explorer, these are the tolerances of my life. No way this works without us working past the sintering cycle. That's where the real talent comes into play. Blending in the margin, maintaining at least 2 mm of blend of contour with emergence profile of underlying preparation. "Ring around the collar" (Tanaka) impressions give you that Really required anatomic reference to blend with. This is Very important with regards to Periodontal stability and enhancement post delivery.
Not EVER why I chose this to be my profession and working life. Please understand that " Machining" requires set properties to be observed. "Marginal offset" is one of about 20 settings we observe and utilize for the Best outcome of our products. Marginal offset thickens the margin directly lateraly in order to have less complications post milling ie, a chipped margin area. IF you don't reduce that marginal offset either in green state or post sinter, you Will have a marginal Overhang of whatever the offset was programmed at. I currently use 0.12 as my offset, for sure if left you could easily detect with your finger, as JMN said.
Biggest thing for you to consider and manage is preps being the best you can, non- sub-G as best as you can. Way sub-G means the camera can't see it nor will you or I. Best to use whatever retraction methods you like, I prefer you use a double cord technique with a bufferred Aluminum Chloride cord.
There are many things coming down the road, probably soon, that will make IOS alot better. We are Not there yet and you need to consider seriously what you're output scan looks like. Can you Easily find the margin you prepared? If not then you're asking for a guess. We Don't like to guess and furthermore are Not licensed to do so. If I get a horrible scan I advise the Dr. to take a PVS as I realize there's so much going on soft tissue-wise that an IOS is Completely Contra-indicated.
