Does everyone trim back their zirconia margins manually?

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VAdoc

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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?
 
JMN

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We've all been manually doing margins for over a century. FCG, PFM, Emax, Zirc, and anything else. The marginal thickness will always need to be reduced or you'll feel it with your finger, forget fingernail or explorer.

Ever make your own gold crown? When it came back from casting you still have to adjust/remove overextentions at a minimum? Same thing.

Machines are perfect at trying to do exactly what they are told to do, but that doesn't have anything close to the same meaning as the mill can perfectly output your perfect design.
 
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JMN

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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?
OH! And Greetings Earthling! Welcome!
 
JMN

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The general rule is that if it fits the model, we did our part correctly as it fits what we were given with the impression.

You can make the worlds worst crown on a wonderfully beautiful impresion, but a lackadaisical impression lowers the crown quality before the prescription is even written.
 
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Lots of reasons for trimming back.
Usually has to do with design settings and the milling tool life. Also milling strategy.

Milling Centers that have to deal with a quick turn around so their margin settings are usually thicker. Then you need to trim back.

You can always adjust them in the green state so you wouldn't haven't as much when it's sintered and hard. That can be risky because it's pretty delicate in the green state and can chip very easily.

Then of course comes the tool life. If it's near the end of its life your going to get some chips.

I would say it's extremely common to adjust the margin with a rubber wheel. If you just accept what your mills spits out. I don't think you would be very happy. There is a reason why you need to do some contouring in the green state and after it's sintered.

You can definitely make a mistake trimming it back and perhaps over trim..so if your looking to pick a winner. They're both right.
 
Jack_the_dentureman

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the machine will have less fit, less quality than manual correction. It is very important that this correction is accurate, preferably under a microscope, with some pencil marking the inner part of the edge.

Set margins to a minimum. then i use a hard rubber wheel which will last longer. there are no chipping on correct tooth preparations, not even on the knife edge. I change the drill in the machine when I feel stupid that I have not changed for over half a year.
 
TheLabGuy

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Your lab technician is correct. Everyone thins margins after sintering their zirconia. Trust your lab tech, they know what's going on!!!
As for source of error, if done with the right bur and under magnification scope shouldn't be an issue. Most margin errors are introduced at the margin marking step.
 
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ps2thtec

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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?

just wondering if these are “modelless“ crowns. Nothing to double check with.
I finish margins (finetune) down with rubber wheels after sintering.
 
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bet you anything its cerec and the doctors have been drinking the coolade about cerec bein so accurate they dont need to be adjusted, that or they have been talking to their cerec mates. of course you polish the margins down.
 
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Also depends on the milling machine. With a cerec mill forget it, its never going to happen unless you have a huge shoulder on your prep. Even then, the software has fail safes margin settings built in for their mills. With our versamill and exocad its a zillion times better than it used to be.
 
rkm rdt

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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?
Good preps and isolation required.
 
Sda36

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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.
 
bigj1972

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Think about who told Dentists that IO was better than analog. It wasn't the crown makers....it was scanner makers. Why not call and ask your scanner/ software tech support for help?
Because they know about real world lab work as much as your other doctors in the group practice.

We're just a group of technicians like yours that already gave you the answer.

Rant over.....welcome to forum. You gotta expect some sarcasm around here.
 
sidesh0wb0b

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We thin the margins in the green state, and fine-tune them after sintering, so that you get a margin that should be almost imperceptable when seated on the tooth.
that goes with a big IF....IF we have gotten a proper preparation and impression....the margins should be a nice smooth and even transition from prep to crown.
 
Car 54

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I "trim" my margin according to the prep in the 3Shape settings (offset) and then do any minor adjusting post sinter.
 
JMN

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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?
Yes labs thin down margins and adjust our cad setting to mill as thin as possible whatever material we're using. Go back and look at the IOS scan and or impression sent to your lab, then crack the whip........ ;)
 
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ps2thtec

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About techniques used... not sure how the rest of you approach it but here’s what I do.
Follow pics.
First shows the White margin I see that needs finishing. I go at it with wheel spinning in reverse looking inside crown so I can see whats happening.
Second I do a light smoothing finish in reverse from buccal , wheel spinning away from margin.
Third shows buccal margin is finished.
that’s the way I skin this cat.
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