Hello fellow dentists,
I used to tutor international applicants for advanced studies at Marquette University when I was resident so I now would like to share what I had done for others. Before we look into the details, you need to know the following:
1. Understand the type of margin for that particular type of preparation. Full gold coverage preparation is different from porcelain fused to metal (PFM) is different from all ceramic (AC) is different from veneers (V) etc
That is because of the type of material being used. Gold can be made very thin (0.5mm) in thickness and you will be OK. However, if you make your porcelain that thin, it will be brittle like rice paper and fail even before it is seated in the mouth! So PFM and AC require more reduction to allow for the materials and also for strength. If you prepare a tooth like a FGC and ask for porcelain, the material will either break as you attempt to seat in the mouth or the technician will call you and inform you that it cant be done.
2. Understand the type of finish that the bur you are using will produce. For example, a rounded tip diamond bur will give you a chamfer finish and a blunt or flat ended tip will give you a shoulder finish. Those burs have different shapes because they are designed to serve a particular purpose so use them intelligently!
3. Understand the amount of occlusal reduction for the type of preparation. Gold as mentioned before can be made thin so you do not need to have reduction like you are preparing for PFM or AC coverage. Excessive occlusal reduction is one of major errors that will lower your grade.
4. Take your time and go slow. As you become competent, you can increase your speed. If you end up with a heavy shoulder for a FGC prep, how are you going to correct that?
5. Know what you want to achieve BEFORE you start!
Now, lets look at the FGC criteria for tooth preparation.
a. FGC ( example #19):
Occlusal Reduction for Functional Cusp: 1.5 mm
Occlusal Reduction for Non-Functional Cusp: 1.0 mm
Axial Reduction: at least 1.0 mm but less than 3 mm.
Chamfer Margin of 0.5 mm. That means there are no sharp corners.
Functional Cusp bevel 1.0 mm in width.
b. PFM (example #19)
Occlusal Reduction for Functional Cusp: 2.0 mm
Occlusal Reduction for Non-Functional Cusp: 1.5 mm
Axial Reduction: At least 1.5 mm.
Shoulder Margin ( Radial Shoulder like a stair ) of 1.5 mm.
Chamfer Margin for the lingual since this part is in metal. Treat it
like a FGC here.
Functional bevel of 1.0 mm in width.
c. All Ceramic (example # 30)
Think of this as an all around PFM except the lingual is in porcelain.
Just copy what you did on the facial and apply to the lingual. The
reason why porcelain margin require a shoulder is for the support of
the material. Chamfer margin will result in a very thin porcelain layer
that will fail quickly!
TIPS:
1. Make your preparation smooth and free of jagged edges. Use your SLOW SPEED to refine your margin. First, the slow rotation of the bur will smooth out the surface out rather than cutting into the surface. Second, you do not have to fear about slipping and cause a gouge in the prep just before you turn the prep in! Start your prep on a high speed and finish your prep on a SLOW speed is my motto!
2. Avoid sharp corners or edges. Use a diamond bur on the SLOW speed will smooth these edges out and give it a professional look!
3. Avoid Flat Top preparation. In other words, you should be able to see facial and lingual cusps regardless of the type of preparation. Why? That is because you want to maximize the surface area for cement bonding and to prevent rotation of the crown. Plus, it simply looks better!
4. Follow the contour of the gingival in the mesial and distal areas. The margins of the preparation should NOT be straight across but should be higher occlusally in these areas because of the bone morphology and the attachment of soft tissues. What if you ignore this and make it straight across? You will GOUGE the plastic ( a major point reduction) and get into the biological space ( in real life application). Not too nice here.
5. Avoid excessive taper. Your prep should have a slight taper when viewed facial lingually. How do you prevent overtapering ( another major point killer)? ALWAYS look at your bur on the side when you do the axial reduction. If you look at your bur from the top down (occlusal-gingivally), you may end up with excessive taper and not knowing it until too late!
6. How do you break contact without damaging the adjacent teeth? First use the wedge to push the teeth out. Second, use a matrix band to protect them. Finally, use a smallest diamond bur as these do less damage than a big one and gives you much less risk of over-tapering. Overtapering during breaking contact is a very common rookie mistake!
7. How do you correct minor nicks on the adjacent teeth? Swap them out when nobody is watching? No, use composite sandpaper strips to smooth them out.
8. Avoid "knife edge" margin where you can't physically see a definitive margin as this type of preparation has no place for any type of restoration. Guarantee to get your score lowered!
Well, here are the basic reviews of tooth preparation. You need to check what exactly the examiners want for a particular preparation such as bevel on the margin or so forth. Remember that once you know what you are looking for in a preparation, you can make what you want regardless of what type of burs they give you. Dont get fixated on what burs do I need for this and that but rather know the proper criterias that are needed for that type of preparation . Good luck and let me know if I can be of some help. DP[Thanks a lot for the tips yoy gave doctor]
