How to scan and mill retention groove

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chohk246

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Hi Guys
I have a big question to ask.
One of my Dr prep pretty good amount of retention groove and when I scan with 3shape and mill that crown, retention groove comes out very sharp and narrow but Dr wants to have the way he prepped so is there any way can make it the way he wants.
I really don't know what to do.
 
boboyrobert

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It depends on what type of mill you have and what you have the milling diameter set to in the crown settings...
 
npdynamite

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The reason that your result is sharp and narrow will mostly be do to drill compensation and could be influenced by the cement spacing. You do need these things turned on to ensure the crown fits. Basically, the corners that would need to be milled to get an intimate fit on the retention groove are tighter than the bur can fit into, so it has to plan to overmill to ensure a good fit. Because the length burs need to be to mill the interior aspect of the crown, you won't be able to address this from the milling aspect. The most direct way I can think of to improve this is to see if the Dr. will round the edges of the groove off a little bit to make the tightness of the corner able to be reached by the bur. In 3Shape you should be able to see at the step where you set your drill compensation and cement gap, about how well the retention groove will be filled in. I would show your Dr. how 3Shape does this and explain that it is necessary to be able to mill a crown that fits and see if he will adjust his preps to be more ideal for digital dentistry.
 
Brett Hansen CDT

Brett Hansen CDT

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Hi Guys
I have a big question to ask.
One of my Dr prep pretty good amount of retention groove and when I scan with 3shape and mill that crown, retention groove comes out very sharp and narrow but Dr wants to have the way he prepped so is there any way can make it the way he wants.
I really don't know what to do.
your cement gap settings are causing the effect you are seeing. There has to be room for the cement so the grooves the doctor is putting in his preps won't be perfectly captured, nor should they. Are they having a problem with your crowns debonding? This really is a non issue.
 
HonestAbe

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I don't know if this idea is a red herring and maybe the more experienced techs could weigh in on it, but if the doc absolutely didn't want to modify the groove but wanted a more intimate fit, would a PFM or other cast restoration give that (or emax press)?
 
npdynamite

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I don't know if this idea is a red herring and maybe the more experienced techs could weigh in on it, but if the doc absolutely didn't want to modify the groove but wanted a more intimate fit, would a PFM or other cast restoration give that (or emax press)?
Yes and no... Yes, a handwaxed PFM or pressed emax will achieve this. But who is handwaxing if they have a mill? and if the wax is milled, you will encounter the same thing because it's the digital process that creates the issue. You could in theory mill the wax and the plunge your waxer through it to fill the groove, but that's getting rather silly and overcomplicated.

Another over complicated solution is pour a check die, modify the grooves to make it more friendly to the milling process and just know that you are going to have to fiddle with the fit in that area more when you seat it to the unmodified die.

I would stick with having a conversation with the Doctor about the reasons that his grooves aren't great for digital dentistry and see if he will make a slight modification to make everything work better
 
Car 54

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If it's regarding zirconia, just tell the Dr to use Z-Prime plus and be done with it.
 
Brett Hansen CDT

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I guess that is the second part of this, convincing the Dr. that his grooves really aren't necessary
I feel like one of the most under rated aspect of our jobs is developing a relationship with our doctors so that they trust us with all the new advances in our industry. I'm going to assume this doctor is older and is relying on a technique that used to be the "gold standard." With better cements and the widespread use of mills, techniques must evolve. A milled crown will give a very precise fit with a very even gap for the cement to do it's job. We become more and more of a resource for our doctors to help educate them with how the new materials and processes can achieve a great final result. Communication wasn't always the strongest characteristic for many dental techs, but I find it to be more and more critical in my work.
 
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tyjthomas

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Here is another perspective that might be worth considering. I personally want the grooves in some of the teeth I prep. It increases resistance/retention form, particularly on a short clinical crown or a crown with no distal tooth to help stabilize it. I'm not a huge fan of universal use of resin cements. I would prefer to lute a crown with a modern RMGI cement and let the prep form do the heavy lifting for resistance and retention. They are also certainly easier to cut off if the crown ever needs to be remade! The resin cements are costly, technique sensitive, and with a crown, you are bonding to dentin which hasn't been generally shown for great long term effectiveness with the MMPs and other proteins causing loss of adhesion in the long term. Doesn't mean I don't use resin cements! They are great and have their indications and certainly necessary for veneers and such. Hope this at least gives some food for thought from the clinical side of things.
 
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Here is another perspective that might be worth considering. I personally want the grooves in some of the teeth I prep. It increases resistance/retention form, particularly on a short clinical crown or a crown with no distal tooth to help stabilize it. I'm not a huge fan of universal use of resin cements. I would prefer to lute a crown with a modern RMGI cement and let the prep form do the heavy lifting for resistance and retention. They are also certainly easier to cut off if the crown ever needs to be remade! The resin cements are costly, technique sensitive, and with a crown, you are bonding to dentin which hasn't been generally shown for great long term effectiveness with the MMPs and other proteins causing loss of adhesion in the long term. Doesn't mean I don't use resin cements! They are great and have their indications and certainly necessary for veneers and such. Hope this at least gives some food for thought from the clinical side of things.
Appreciate your perspective, we do a lot of crowns with axial grooves and very short clinical crowns. Having a discussion with the doctor on expectations with those preps once and saving it to the doctor preferences prevents a lot of disappointment and remakes.
 
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