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Question:
New Crown now shows metal: I had a crown installed recently, 2nd from back tooth on the
bottom left side.
It feels fine but I am concerned that the dentist ground off so much of the porcelain that the metal base of the crown is exposed. He stated that this would not affect the performance or longevity of the crown, but I am a bit skeptical.
This crown wasn't cheap and I want it to last as long as possible. ...Visitor from MN
Answer:
It is frustrating for us to deliver a beautiful crown restoration only to have to adjust the bite to the point that the undercasting shows through. Sometimes the bite is so tight that the dental lab does not have enough room for the metal base and the porcelain on top.
How does this occur? For a tooth to require a full protective crown restoration, the tooth probably had been filled a few different times. Each time the tooth decayed or the old filling fractured, it required taking more tooth structure away creating a bigger filling. You mentioned that the crown was done on a bottom tooth, which in my experience is the crown that always seems to need more adjustments to get the bite right.
The reason this is usually the case is that the bottom tooth with the large silver or white filling wears a bit each year as you chew, and subconsciously grind your teeth together. As the bottom tooth gets worn down the top tooth above it starts to extrude or drops down to keep in contact with it as it wears. Then after many years and more than a few fillings the tooth finally is in such weak shape that a full protective crown is needed.
As the dentist prepares the tooth on the bottom it's already pre-shortened, and the dentist can only grind it down so far before it gets too close to the nerve, or before the clinical crown gets too short for retention of the new crown that will be placed. A dentist certainly does not want to prepare the tooth down so far to create space for the new crown that he gets too close to the nerve or creates a pulp exposure which would require root canal treatment.
When a porcelain to metal crown is planned it requires more space to be created between the prepared tooth and the tooth above, which may have extruded down below the occlusal plane thus limiting the inter-occlusal space. In cases where a dentist has limited inter-occlusal space in the back of the mouth a full gold or full porcelain crown may be needed where the restoration can be kept thinner than the space needed for a porcelain to metal crown.
So, why did my dentist have to grind my new crown down to the metal and will it lessen the life of the crown?
It's unfortunate that your crown had to be adjusted down to the metal, but a small metal spot on the biting surface of your new crown shouldn't be a big problem. If the porcelain fractured in the next few years I'm sure an ethical dentist would replace that crown for you at no charge. We would redo it at no expense to the patient in our practice.
Sometimes when the bite is very tight, a porcelain to metal crown can be made with a built in metal stop in the biting surface, where it can be very thin. In the back of the mouth this should not be a problem cosmetically because it really does not show when you talk or smile.
Another thing your dentist may have done to prevent having to grind your tooth so much, was to trim back the upper tooth at the time of the crown preparation appointment. It could then be buffed and polished smooth before the final crown impression was taken. This creates instant new room for a crown, and since the upper tooth most likely dropped down a bit over the years, it really is just getting it back in it's normal space. Also, when a new crown is being seated and adjusted, the upper tooth can be smoothed a bit to prevent grinding through the new crown restoration.
ComfortableDental.com
Ron Briglia, DMD
Chester County Medical Building
600 East Marshall Street, Suite 201
West Chester PA 19380
(610) 692-4440
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