how to avoid undercutting and making the margin too big and just how to measure stufff and stuff like that....
need crown prep advice
- Thread starter Stephie3
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+1 on practice.
But if you're practicing incorrectly, that doesn't really do you any good now does it?
Understanding the concept of an undercut, really what it is, and how you create it will allow you to avoid making them in the first place. Focus on the angulation of your bur by looking at the back of the handpiece. Basically the part of the handpiece (if you're using a friction grip) that you press to release the bur is a flat surface that should be roughly parallel with the occlusal plane of the tooth you're cutting. That concept alone should minimize you're undercutting the preparation margins.
Measurement is done with the bur itself. You can measure with calipers, or read manufacturer specifications, the diameter of any bur you are using. If you have a course chamfer bur that is 1.6mm in diameter at the tip, using half of that bur to create a chamfer margin will yield a margin that is 0.8mm deep. Roughly prepare your crown prep with that, then go to a larger fine diamond bur with perhaps a 2.0mm tip and use that at a slower speed for finishing and creating your 1mm margin (assuming that is dictated by your restorative material).
These concepts are generally taught in every dental schools Pros 101 course. Really bone up on the information in your textbooks, and if necessary go watch youtube videos of crown preparations. Repetition is the only way you'll ever master theses techniques, and all others in dentistry.
Good luck!
But if you're practicing incorrectly, that doesn't really do you any good now does it?
Understanding the concept of an undercut, really what it is, and how you create it will allow you to avoid making them in the first place. Focus on the angulation of your bur by looking at the back of the handpiece. Basically the part of the handpiece (if you're using a friction grip) that you press to release the bur is a flat surface that should be roughly parallel with the occlusal plane of the tooth you're cutting. That concept alone should minimize you're undercutting the preparation margins.
Measurement is done with the bur itself. You can measure with calipers, or read manufacturer specifications, the diameter of any bur you are using. If you have a course chamfer bur that is 1.6mm in diameter at the tip, using half of that bur to create a chamfer margin will yield a margin that is 0.8mm deep. Roughly prepare your crown prep with that, then go to a larger fine diamond bur with perhaps a 2.0mm tip and use that at a slower speed for finishing and creating your 1mm margin (assuming that is dictated by your restorative material).
These concepts are generally taught in every dental schools Pros 101 course. Really bone up on the information in your textbooks, and if necessary go watch youtube videos of crown preparations. Repetition is the only way you'll ever master theses techniques, and all others in dentistry.
Good luck!
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Yup, practice is key!
One thing with respect to margin width, and also the undercut issue that I found/still do find helpfull, is to have a clear mental image of what the bur you're using to prep the tooth looks like shape wise, and as a result, what your prep *should* look like after you're done using that bur.
Everyone/every lab tech has their own preference/opinion about what margin shape/size/finishline location will get the best final result, and as long as you can deliver a consistent shape to the lab tech that allows them to give you a result you're happy with that's great. Once you find that combo between margin shape/width and lab tech ability, what you'll find is that realistically there's a bur out there with those contours and that will become your goto bur. For example, I know that the diamond bur I use for about 98% of my finishline prepping of my crowns has a tip diameter of 1.4mm, which is enough space to allow the lab tech that handles my cases to be able to stack enough porcelain to get just about any possible color matching that I need. Since that bur also has the taper that I desire for the axial walls of my prep, all I have to do is make sure that at the finishline, that bur goes 360 degrees around the prep, to full tip width, with myself keeping the handpiece in the proper angulation. If I do that, I've got a prep with adequate, consistent marginal width/shape and a prep that is undercut free.
Also, as to width of the margin, especially if you're prepping an endodontically treated tooth, inspite of what your d-school professors might tell you, just about any lab tech out there will appreciate if you give him/her a little more than "ideal" width than a little less 😉 What most lab techs will tell you is that there's a minimum width they need, an ideal width they want, but there isn't a maximum width they can't handle!
One thing with respect to margin width, and also the undercut issue that I found/still do find helpfull, is to have a clear mental image of what the bur you're using to prep the tooth looks like shape wise, and as a result, what your prep *should* look like after you're done using that bur.
Everyone/every lab tech has their own preference/opinion about what margin shape/size/finishline location will get the best final result, and as long as you can deliver a consistent shape to the lab tech that allows them to give you a result you're happy with that's great. Once you find that combo between margin shape/width and lab tech ability, what you'll find is that realistically there's a bur out there with those contours and that will become your goto bur. For example, I know that the diamond bur I use for about 98% of my finishline prepping of my crowns has a tip diameter of 1.4mm, which is enough space to allow the lab tech that handles my cases to be able to stack enough porcelain to get just about any possible color matching that I need. Since that bur also has the taper that I desire for the axial walls of my prep, all I have to do is make sure that at the finishline, that bur goes 360 degrees around the prep, to full tip width, with myself keeping the handpiece in the proper angulation. If I do that, I've got a prep with adequate, consistent marginal width/shape and a prep that is undercut free.
Also, as to width of the margin, especially if you're prepping an endodontically treated tooth, inspite of what your d-school professors might tell you, just about any lab tech out there will appreciate if you give him/her a little more than "ideal" width than a little less 😉 What most lab techs will tell you is that there's a minimum width they need, an ideal width they want, but there isn't a maximum width they can't handle!
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