Welcome to another episode of AmericaSmiles, tips, and tricks. Today, we’re going to talk about handling your zirconia or managing your zirconia from mill to bill. Zirconia itself is very fragile, and one of the key things I’m going to focus on in this video is the cracks that may show up post sintering. So, when we’re dealing with machines or milling machines, these are technically CNC machines, which are new to the dental world, just for the record.
CNC machines have been around for a long time as well as old school manual milling machines and other automated machines. So, when we talk about other industries using milling machines, they tend to look at our machines as little bitty toys that are not that special. Our machines have an x-axis, a y-axis, a z-access, and some of them tilt. So, you’ve got anywhere from three to five-axis, but these are all vibrating parts. You’ve got spindles that are spinning that can carry a little vibration. You can have a collet that can get loose and have a little vibration. You also have what’s called your “ball screws”, sliding back and forth as they get old and sloppy.
With zirconia being very fragile you can be incorporating cracks into your zirconia and never even notice it until post centering. The zirconia is a real chalky white, and you really can’t see a crack if it gets incorporated in there. So you want to make sure that your milling machines are in good, healthy condition by making sure your collets and spindles are relatively fresh. We change our spindles every six months, whether they need them or not. You don’t want to be incorporating a vibration gate and cracks that may not show up until they go into the patient’s mouth with a break in the patient’s tooth six months to a year down the road.
Here at the AmericaSmiles milling center, we’re manufacturing about 200 to 250 teeth a day. We inspect every one of these cases underneath the microscope, looking for chips and cracks. A good indication of a bad spindle, collet, or cut is a long crack that runs along the margin straight up the access of the tooth. If you’ve ever dropped your handpiece down and you put a slight bend in that tool that you’re cutting out, you have vibration, and that tool you could be incorporating cracks into that as you’re cutting, and you don’t even know it.
Inspect all these things. Post centering, look at them under the microscope. Otherwise, you may deliver something with a fracture. Sometimes, you have to rotate and twist them. There may be a cack you can’t see on one angle, and then you see it on another. You don’t want to deliver that case and have a crack later down the road. Make sure you’re always using a good sharp bur as well. We like something that’s got a little taper, like a 5-6 tapered unit. It’s important to have a good technique for cutting those things out.
Know the Signs
Ensure sure you’re not incorporating any cracks into that as zirconia. Then when you’re inspecting under the microscope, look for chips around the margins. If you’re seeing your chips are increasing around the margins, it’s a sign that you have bad collards or spindles. The secret to your zirconia case going from mill to bill is good, healthy milling machines, good cutting technique, and a good inspection post centering.
And that’s it!