I keep brushing, but this black line around my cap won’t go away. What can I do?

There could be several reasons for this appearance. The first of which is the material of the crown. Many caps or crowns are made of porcelain fused to a metal base. The porcelain is designed to mimic natural teeth, however it is layered on top of metal for better strength and support. When there is gum recession, some of the metal layer can be exposed and appears as a dark line along the gum line.
 
Another reason for the dark line could be if the tooth previously had trauma or a root canal done before getting the crown done and gum recession exposed some of the natural tooth. The natural tooth under the crown may be discolored, however this does not always indicate the presence of decay or a cavity. A clinical exam would need to be done to determine the health of the tooth structure under the crown. 
 
Lastly, the black line could be decay or a cavity. Although the cap is a false tooth to cover the natural tooth, it does not mean the natural tooth will be immune to cavities. With proper home care (brushing twice a day, flossing once a day) and regular check up visits (at least every 6 months), patients with crowns can ensure their longevity. Some patients will complain of bad breath or halitosis around their old crowns. Indeed bacteria and decay trapped under crowns needing to be replaced will cause an odorous breath.
 
At our office, while porcelain/metal crowns are sometimes indicated, almost 99% of our crowns are made of zirconia, porcelain, or ceramic. With today’s technology, these materials can be very strong, eliminating the need for a metal base. This will also make sure you won’t get the dark metal color showing through around the gum line.
 
With our office’s 3D scan technology (our office loves to keep up with the newest technology), when indicated, we can match a new crown to a previous tooth’s look so your retainer will still fit and your bite will feel just like your old tooth!

Take a look at the pictures below to see what may be hiding under old crowns that are due to be replaced. 

This particular crown (porcelain fused to metal base) served our patient almost 20 years, which is about 20,000 meals! It was finally time for an upgrade to a brand new zirconia crown. Hopefully our patient can get another 20,000 meals out of the new one.

After the old cap is removed, all the decay is removed, and adequately built back up with a build-up filling to fill in all the gaps where decay was. The build-up filling will also protect the nerve inside the tooth if it is alive, or protect the root canal treatment which was done. Then we do a 3D scan of the natural tooth so a brand new custom made crown can be made.

Our dental lab was given instructions to match the patient's new tooth to their old tooth (to fit their retainer) using the 3D scan seen above.

The final crown restoration will be uploaded after our patient comes back in for its cementation!