For example, the class III composite preparation. Know, first of all, that if you are preparing a typodont tooth, you must have a clear picture of where the decay WOULD be. It's pretty much the same as for a class II, the decay is at the gingival portion of the contact area, but here we enter from a completely different direction. Now there is some question about how much of the contact needs to be involved. In reality, typically, very little of the contact is involved. The incisal margin is generally in contact with the adjacent tooth, but it is not placed too far into the contact - if it is IN contact, that is far enough incisally.
The facial extension is another issue - both for clinical exams and manikin exams. For the patients it is easy to see how far the facial margin will have to be extended, because the surface decalcification is generally found into the facial embrasure, at least far enough so that the facial clearance will allow a bevel to remove the last of it. For a typodont, various approaches may be mandated as to how far the facial margin is extended - there could be the directions "don't break facial contact" - which tells you exactly nothing. Generally, I council my students to extend the facial wall, angled outward, until there is just enough clearance facially so that a facial bevel can be placed all the way to the incisal wall. This most closely duplicates what we commonly find in patients.
Should there be a bevel on the class III margins? All the textbooks say yes. Regional clinical board exams, even for patient-based procedures, generally state that bevels are optional! What does this mean? It really means that you need to decide whether to place a bevel or not. I can picture situations where, just for retention, a bevel would be an absolute clinical requirement. And in all other cases I would still place a bevel for good marginal bonding, good esthetic gradient, and to allow a more conservative preparation that still removes all of the surface decalcification. But be careful about local beliefs here.
For the class II composite preparation it is clear that there should be a gingival bevel - and yet it is rarely done in practice. I sometimes think that the lack of requirements for a bevel on clinical board exams is more a reflection of what the examiners do in their practice - as opposed to what the schools teach. I admit, I have not been in any regional board calibration meetings for years, and never got the chance to ask this question, but that could be it. What will you choose to do in your practice? Personally, I won't place a class II composite when I do not have gingival enamel to bond to, and I will always take that extra 30 seconds to place the bevel - why not?
The other interesting thing about the class II composite preparation is the proximal wall exit angles. If they are flared - i.e. the angle of tooth structure is obtuse, the end of the enamel rods is exposed by the wall itself and no proximal wall bevel is necessary. On the other hand, a more conservative preparation can be made by underextending facially and lingually and then removing the last of the decalcification with the bevel. On the other hand, in the dental community, the burs that are most commonly used for class II preparations are too large to permit a conservative class II preparation anyway - whether amalgam or composite.
So we basically do the class II composite preparation just as we do the amalgam - the same technique exactly, but we flare the proximal walls a little, and bevel the gingival margin. The isthmus width can be smaller, the "axial depth" can be smaller, but otherwise it is the same in most respects.
I'd like to hear about what experiences you might have observing dentistry in the U.S. in the area of composite preparations. Many of my students are dental assistants, and they always have something interesting to say about how dentistry is practiced in their office. Generally, not how it is taught!