Crowns Not Staying Seated!

There's a Chinese idiom, 一朝被蛇咬，十年怕井绳

"Once bit by snake, for ten years afraid of ropes."

If you have ever had a patient report to you that their crown had fallen off, you might have become "Snake Bit". There is a danger of overcompensating on the shape of your subsequent tooth preparations. The concern for retention can become the source of seating issues. Instead of worrisome conical shapes, we begin making parallel walls, grooves, and box cuts, only to cause other problems.

Unfortunately, sometimes crowns will refuse to be seated. The accuracy of the Lab's digitally determined cement gap measurements and undercut compensation can be created to a fault. To overcome these types of hazards, Doctor's that think ahead are the most successful.

The good news is that exhaustive studies have determined a 20° taper is sufficient for good retention on a crown prep of only 2mm in height! Seldom do we ever see the need for a prep that short. So, if we use that 20° measure as our standard, we should feel confident that our work will be safe.

I have outlined some easy instructions to provide a quick solution for preparing a "Suitable for Seating" preparation.

The first thing we need to know is that for every one millimeter of height, 0.36mm is required to achieve the 20° taper.

If your tooth preparation is 3mm tall, a one mm taper is sufficient. Approximate the height of your prep using this formula: 3H = 1mm, 6H= 2mm, 9 mm= 3mm, etc. You will need to measure inward from the outer edge of the occlusal surface, toward the center of the prep, the millimeters required.

You can easily score the occlusal surface on its perimeter to determine each mm needed and then angle your straight diamond bur from the cervical of the prep wall to the surface area marked. This will ensure that the new conical shape will not be too drastic as we follow these simple rules. We can also feel confident that our cement will be able to do its job.

Information used from studies at the National Science Monitor & the National Institute of Health.