This is a question I often get asked and it came up again today.
There are a number of reasons for it, the first is recurrent decay at the margin of the crown (see image left). Sadly placing a crown does not confer immunity from decay and the ravages of daily living still apply, it is even more important to keep your teeth clean especially at the gum margin and to see your hygienist regularly to prevent breakdown and further damage to your tooth.
Over time your gums can recede leading to the margin of your crown being exposed,
everyones gums recede at different rates and some are more prone than others. In the image to the right the metal margin of the crown has become visible. Modern all porcelain crowns such as emax can lessen the impact of this phenomenon. Some crowns do not transmit light well and this can make the root appear darker than it actually is.
A further reason for black lines appearing is that the root of the underlying tooth is dark and this can make the join between the tooth and crown obvious this can be offset by whitening the tooth prior to placing the crown. There are solutions to all of these problems, if you're worried about your crowns drop us a line to see if we can help, send us a photo and we can give you an idea whats going on.