Common Pitfalls for Reverse S-curve and Contact Opening

A lot of people are stressed about dealing reverse s-curves. A simple way of making a perfect reverse-s-curve is to first break down misconceptions surrounding about it. Some people would assume that the 90 degree exit angle and the reverse s-curve are the same, so they make the curve as “90 degree looking curve” as possible. This idea is totally WRONG. In fact, this 90 degree angle is talking about the the angles at which you will form when you exit towards the proximal surface of the tooth. The reverse s-curve, on the other hand, is a product at which you will curve around avoiding the mesiobuccal cusp. This curve just happens to be that way because you want to preserve the cusp and the triangular ridge in this area.

Let’s review the 3 main parts of this area: The exit point, exit angle and the reverse s-curve. Basing on this picture, it summarizes those 3 parts. They are separate features (NOT a single entity) that you need to keep in mind while making the proximal box. These features following certain rationale why these are formed:

Reverse S-Curve: formed because of wrapping of the mesiobuccal cusp, maintain the integrity of this cusp.
Exit Point: Plotting this exit point will get you the possible location at which you will break the contact area. This the landmark at which your buccal and lingual wall could be at its most extent.
Exit angle: This angle will depend on how it will parallel the enamel rods in this area.

Comparing all of these three, the most complicated thing to do is how to do the exit angles because you have no concrete basis (e.g. anatomy of the tooth: Mesiobuccal cusp for the reverse s-curve; contact area for the exit points). So it solely depends on how you imagine the enamel rods are directed in this area. The following is how to a good approach in this matter.

Exit Angle Mistakes

If you focus the term reverse s-curve before you learn the more important aspects down (like exit angles and exit points), these are the type of mistakes you could possible make:

A small exit angle
large exit angle. Either of which will create unsupported enamel because it doesn’t completely follow the direction of the enamel rods in this area.

Either of which will create unsupported enamel because it doesn’t completely follow the direction of the enamel rods in this area.

Small Exit Angle

This is the most common mistake. This is the result of having the idea that the reverse s-curve and the 90 degree exit angle are the same (notice that in the picture, the curve has this 90 degree corner, when in fact it should not be).

ERRORS

The problem here is that 1.) the mesiobuccal cusp is compromised and 2.) presence of unsupported enamel rods (highlighted as yellow); more likely there will be fracture of tooth at the margin.

TIPS

Visualize the cusp, and be sure to avoid cutting through it.
The walls on this area is rather boxy, if you look at it occlusally.
Same thing, in relation to the central groove, the buccal wall has a slight flare towards the buccal. It is because the enamel rods starts to flare in this area.

TROUBLESHOOTING

This can be solved by using hand instruments to define the axiobuccal line angle and the buccal wall. If you see that it needs more reduction, you may use the handpiece to define it more with hand instruments after.