Variobase rotational stability

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tyjthomas

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Hey guys - I'm pretty new to in-house production of zirconia crowns. I am trying to do a zirconia crown on a straumann RN variobase abutment. It is scanned using the scan body, designed in exocad, milled using millbox and Roland DWX-52D mill out of Argen HT+ and sintered according to their program specifications. The fit is good but there is a little rotational movement. I have milled it now probably 7 times and they all have a little rotational play in them:

1. Milled out according to regular screw retained implant protocol in exocad.
2. Tightened the crown +.01 in millbox
3. Tightened the crown +.02 in millbox

Since you can't adjust the fit/crown bottom of an implant from the library, I scanned in the variobase abutment itself as a crown prep and designed on top of that.

5. Milled out according to regular workflow
6. Set cement space to 0
7. Dropped Anticipate milling from 1.2 to 1.0

With each one, the milling preview looks clean. Each fully seats on the titanium base. It looks like Straumann's variobase abutments don't have tons of anti-rotational features so I'm wondering if that may not be the culprit? Anyone else have experience with this? The play probably allows it to rotate 10 degrees. I don't want to just rely on the cement but is that normal?

(The mill is calibrated and the milling seems clean/not chipping so I don't think burs are the issue)
 
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Thats not acceptable.
Cement gap and drill comp are your tools,and if thats not fixing it, I wonder if you used a thick scan spray on the abut? Sloppy/worn mill? Incorrect expansion set for the puck?
The variobase has plenty of anti-rotation built in. Does it look accurate in the Zr, or is it over cut?
If you have cement set at 0 and drill comp at 1.1, it shouldnt even fully seat on the base.
 
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tyjthomas

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I agree - it is not acceptable.

I don't think it is the scan spray since I have the same result when using a scan body and the internal ti-base is already set and provided by the mfg. I checked the expansion and it is correct and entered to the 1/10,000 decimal. I think it looks accurate after milling (see photos). I can see the cams clearly which makes me think maybe it is in the sintering process? Does that make any sense though? It is a newer sintering furnace and it says it is getting to the hold temp for 2 hrs (I believe its 1550). Occlusion and contacts look fine so I am slightly at a loss.
 

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Sda36

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I agree - it is not acceptable.

I don't think it is the scan spray since I have the same result when using a scan body and the internal ti-base is already set and provided by the mfg. I checked the expansion and it is correct and entered to the 1/10,000 decimal. I think it looks accurate after milling (see photos). I can see the cams clearly which makes me think maybe it is in the sintering process? Does that make any sense though? It is a newer sintering furnace and it says it is getting to the hold temp for 2 hrs (I believe its 1550). Occlusion and contacts look fine so I am slightly at a loss.
I believe 2thm8ker had some comments on this, actual Ti bases vary by lot. I Always bond to Ti base on the model as some rotation always exists. Prep your proximal contacts post sintering and bond accordingly. I would Never bond off of the model, can't realistically be achieved within our tolerances, ever, Period
 
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What about flat burs ?
Do you set position of screw holes in your cam ?
 
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When you modify your cement gap, are you doing it in DB or during the design process with crown bottoms?
Dont forget to click apply.
I always select 'additional spacing' and drop it back a bit to snug things up.
Not all Zr is created equal. When entering a new puck with Katana, I bump up the expansion .002. It runs tight.
 
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tyjthomas

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I ran an update for millbox. New tests are sintering. Hoping it helps but not optimistic. I’ve changed the crown bottom in the software and applied it. I’ve tried tightening it in the software as well.

I do have the flat burs which are used when the piece is milled as a ti base instead of as an anatomical crown in millbox. The screw holes are set in the cam. I feel like we’re doing it all correct, just frustrating since we don’t have enough experience to compare to!
 

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