Which tooth to start practicing on first?

The recommended sequence of the tooth to practice on is: (1) Lower Molar, (2) Upper Molar, (3) Upper Premolar, at the least. Doing a lower premolar will depend on the school that you will be in.

Regarding Lower Molar cavity preparation, it is the largest prep that you could possibly do. It is relatively easier to do based on its anatomy (like the outline form is not divided by an oblique ridge in between) compared to other molar teeth. Since the cavity is large, it will also be a starting ground to gauge where your hand skills stand as well. So if you have perfected the lower molar cavity prep, you may proceed on doing the lower second molar before proceeding to the next tooth (according to the list above)

Outline Form

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Proper Bur Angulations

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The direction of the handpiece should always be the same as the long axis of the clinical crown (not the tooth).

Can’t stress enough how important it is to maintain a proper angle throughout the preparation, and only changing it when you consciously decide to do so, not by mistake. This is achieved by giving a strong and steady finger rest, without that it’s very easy to not-knowingly tilt your handpiece.

Your bur must be parallel to the long axis of the clinical crown, not the tooth itself. The crowns of lower molars are lingually inclined about 15 degrees, and so should your bur. Check-out the differences below, but don’t mind that my handpiece is piercing through the patient’s cheek; that’s not the mistake (even though it probably is), but focus on the angle.

As you approach proximal walls, the bur is angulated in such a way that the walls in these are diverge occlusally. This strictly follows the configuration of the enamel rods in this region (which is also diverging) thus minimizing the presence of unsupported enamel.

However, overdoing the divergence just for the sake of doing a divergence is a bad habit because of the risk of thinning out the marginal ridge instead. Some would say, “Oh, I think my divergence in this are is not so clear, I think I should do more!” Please do not do this. If you did a slight angle of your bur in this area then that’s it. So, keep in mind that by doing a slight a divergence in this area is because of the anatomy and direction of the enamel rods and not just because of just doing so.

Check-out the explainer video below to see what happens when your bur is at an incorrect angulation.

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Cavity Depth

The depth of the cavity is where many people get thrown off a bit. It’s 1.5mm minimum thickness for the amalgam. This does NOT translate into 1.5mm walls.

Cavity depth video

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Proximal Box

The proximal box is complicated, easy to mess-up, and very nerve-wracking to work on.

Before starting on it (and probably messing a few up), let’s go through what we are trying to achieve first. It’s much easier with drawings.

Occlusal View

1. 90 degree exit angle
2. No curving or sloping of the B & L wall within the box
3. No enamel lips: didn’t discuss it in the video, more details on this coming tomorrow. This is achieved with hand cutting instruments.
4. Reverse S curve on the B = sloping around the MB triangular ridge = reducing tooth structure, often more than you think!

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Buccal or Lingual View

1.  The gingival floor is wider than you think Molar: 1.2-1.5mm
2. Premolar: 1-1.2mm
3. There is a bevel at the gingival floor, we’ll use a margin trimmer for that.

Mesial View

1. The lingual tilt of your handpiece is crucial to achieving the shape we want.
2. The facio-gingival line angle is acute!
3. The linguo-gingival line angle is close to 90 degrees.

Reverse S-Curve

The reverse S-curve has basically 2 main functions: 1. Preservation of the cusps near the proximal box. 2. Facilitates formation of 90 degree exit angle, creating no undermined enamel.

There are two terms that you need to know about how this curve exits towards the proximal surface of the tooth: (1) Exit point, (2) Exit angle.

EXIT POINT is the exact location at which the angles are exiting. It is a reference point in which you want to check if you have formed a perfect 90 degree exit angle in this area.

The EXIT ANGLE is the direction at which the exit point it is exiting from. To check the angle, you just focus on the exit point. There will be angles that you need to look at. One on the side of the tooth and the side on the restorative material. When looking at these, you have to achieve a 90 degree exit angle. Simply saying when you have this feature, you have created an equal amount of tooth and restorative material on this area.

Retention Locks

Cavity preparations for the bench tests are on intact teeth and its dimensions are small. Retention Locks are indicated only on very large cavities and are needed to increase retention of the restoration. Though it is pretty to look at on books, retention locks are not at all necessary in the bench test. But it also depends whether the school ask you to do it or not. So better check the guidelines first.

When it says Retention Locks are optional, we suggest that you DO NOT put it. Placing it would not add points to your score anyway. Also, putting it there will just lay you out a possible risk of making your preps look bad if it doesn’t go well.

Also, placing it on ideal preparations, when in the first place is unnecessary, will only invalidate the idea of preserving the tooth structure.

However, in UoP, basing on the guidelines, you need to perform retention grooves on the axiobuccal and axiolingal line angles. Now to prevent any risk of losing these line angles, use hand instruments instead rather than a bur. This will minimize any mistakes that may go along the way. We made a separate section discussing about how to prepare for UoP, you may want to go there and check for what to expect about their guidelines.