Educate your teeth!
The crown margin is the edge of the crown where it meets the tooth. This is the area where the seal is created. Precise fit of the crown margin is critical to long term success of the crown.
Crown margins can be made in a variety of forms as follows:
METAL MARGIN: All metal crowns by their nature have metal margins. A skilled laboratory technician and dentist can create crowns with highly precise and tightly fitting metal margins. Unfortunately, all metal crowns are not cosmetic, so they are generally reserved for use on back teeth.
METAL COLLAR: Many crowns made of porcelain and metal have a thin metal collar along the gum line edge of the porcelain (indicated by the black arrows). This often makes the porcelain a little stronger. Generally when a metal collar is used on a crown, it is positioned sufficiently below the gum line as not to be seen. However, as patients age, it is common for the gum line to recede. Frequently this leads to exposure of the metal collar leading to a less than desirable cosmetic situation. The solution for this problem is to have the offending crown remade with the metal collar below the gum line again, or preferably to have the crown made without the metal collar at all (indicated by the white arrow).
PORCELAIN SHOULDER: Crowns without metal collars are generally the preferred choice for cosmetic areas today. These types of crowns are made so that the porcelain extends all the way down to meet the tooth (white arrow). Even when the margin is all porcelain, it needs to meet the tooth slightly under the gum line so that none of the root is visible.
If the crown margin stops before it gets under the gum line, there will be tooth root exposed between the crown margin and the gum line (indicated by the yellow arrow). The porcelain on a crown almost never matches the root color; therefore, the exposed root surface may look dark compared to the crown. Your crown may have been made like this, or the crown margin may have been under the gum originally and subsequent gum recession has led to part of the root being exposed. If this presents a cosmetic problem, the only good solution is to replace the existing crown with a new one.