For  crowns, we’ll be working on 2 types of margins; chamfer and shoulder. There are several other types of margins out there, such as knife edge, shoulder with bevel, heavy chamfer etc, but those are not relevant for bench test purposes.

When working on reducing tooth structure axially, we’re also working on our margin at the same time. This means there’s 2 things to focus on at once; location of the tip & inclination of your bur. When working interproximally, add we another thing gets added on; not nicking the adjacent tooth.

The way I like to think of it is; as long as I keep my handpiece upright, I do not need to worry about the inclination anymore. This is because the taper of the bur itself will provide the taper for the crown. So, there’s one less thing to worry about. There will be at time where you need to tilt your handpiece into different directions, but do it consciously and don’t bring it in any more axially; think 3 dimensional and tilt your handpiece without affecting the convergence of the preparation.

Of creating depth grooves: There is a slight modification that we do when creating depth grooves because most of the time there is a risk of deepening it further when you start reducing, and it is sometimes very hard to remove the mark it leaves. To prevent this leave your depth grooves far away from your margins and make them shallow. With this, I think the more appropriate way of calling it may be guide grooves (or orientation grooves).

We want to demonstrate these techniques as consistent as possible in every student so that it is much easier to teach and performed by the students. So, instead of making depth grooves, we want to teach students to do guide grooves instead.

These guide grooves are placed in order for us not to get lost with the angulation of burs. As you reduce the axial wall, you will be reminded that the bur should be maintained parallel to the long axis of the tooth. They are used to aid in angulation and degree of taper but not so much about depth.

Exception to the Rule (LOWER MOLARS)

Remember when you had your practice on Class II of lower molars, you have to keep your bur angulation slightly lingual? You also need to apply that principle when reducing the axial walls of a lower molar. Since the tooth is lingually inclined, your walls should follow on suit so that you will create even reductions on all walls. Refer to this video on how to carefully analyze a tooth’s angulation and effectively replicate that on your bur angulation.

Making Perfect Tapered Walls

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Making Both at the Same Time

Making margins and axial reduction should be done at the same time. As you reduce, make sure that you also establish the form of the margins as well. And when making margins, you also need to touch the walls, and make both of them at the same time. An often seen mistake is that when widening margins (or refining it), they always focus only on the margin itself and while doing so they loose track of the angulation of the bur, creating a very huge undercut near the margin. The correct way of doing this is that you have to always retrieve the bur angle that you have done before, making sure that the whole surface of the bur touches the wall. And then, you may proceed on making the margins and walls at the same time.

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Full Gold Crowns

Drawing Video on Full Gold Crowns about axial reductions and margins

Metal-ceramic Crowns

Metal ceramic crowns require 1.2mm reduction on the buccal (shoulder margins) and 0.5mm on the lingual (chamfer margins) unless otherwise indicated by your program guidelines.

Occlusal View
Mesial View
Buccal View

Full Ceramic Crowns

1mm-1.2mm Shoulder margin all throughout unless it is otherwise indicated by your program guidelines.