There are many philosophies of how an artificial crown should meet the tooth. The place where the crown ends and the tooth begins is called the margin.
One decision to be made is how far down the tooth to locate the margin: above the gumline, at the gumline precisely, or below it just a bit. There are often considerations that cause us to place crown margins slightly below the gums. One is the need to go past decay or old fillings. (It's bad form to end a crown on a filling; its margin should always meet tooth). Another is esthetics--it's unseemly to see the seam. And since gums often recede over time, even just from the normal wear and tear of living, we want to hedge against that loss.
So we strive for natural-looking crowns that blend in with their neighboring teeth at the gumline, as well as along their more public surfaces. In this photo, the second tooth from the left is an older crown; the second from the right, a modern one. The new crown is impossible to tell apart from its neighbors when seen from a conversational distance, and part of that blending in is its margin.
What about when gum recession exposes the whole works, though? When should we replace such a crown?
This image shows gum recession past the crowns on the inside aspects of some upper teeth. There's crown-porcelain, then the metal substructure of these types of crowns, then exposed root:
This next picture was taken with the Soprolife cavity-detecting camera. With this device, green tooth structure is healthy, and red means a decayed area. This exposed root is free of decay:
So the question becomes: When whould we replace a crown when the gums have receded away from it?
A reasonable course of action is to do nothing unless one or more of the following conditions occurs:
-The patient is unhappy with the appearance
-A cavity has occurred
-There is excessive sensitivity
-There is uncomfortable food impaction
-Health of the gums is being negatively affected
If none of these issues exist, it is often perfectly fine to leave crowns in place when the gums have receded beyond their once-ideal margins. As always, careful monitoring over time is called for.
For more on the Sporolife cavity detection camera:
For more on the causes of gum recession, see:
http://rickwilsondmd.typepad.com/rick_wilson_dmds_blog/2010/08/bundle-bone-and-dental-recession.html
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