Patients often enquire about dark lines that appear at the gumline of crowns. Sometimes it can be a wide area and there may be darkness appearing around the gums of a tooth that has a crown too. The reasons are usually related to how and a crown was fabricated and how the light that lets you see your teeth affects your teeth and gums. A little bit of background information will help explain all of this.
When you look at teeth (or anything you can see) what you are actually seeing is the light that bounces off each tooth. But light doesn’t just bounce off a tooth; it also bounces around inside the tooth, and comes out of different areas of the tooth including from below the gums. This means a tooth has translucency (think light passing through frosted glass). Today when a crown is fabricated we try to mimic these characteristics by using porcelain, layered with translucent as well as opaque layers to mimic the enamel and dentine layers in natural teeth. Porcelain is fragile and in many of the ways we use it for teeth it needs support. Traditionally a thin layer of precious metal including gold and palladium (because or their biocompatibility and strength) as thin as ½ mm is used. So you may now start seeing the difficulty in making a natural looking crown. A thin layer of gold at the edge or a crown will show if the crown edge is not below the gumline (where a dentist often places it). In the back of the mouth a line of metal is not an aesthetic problem as no one can see it, but in the front of your mouth it is definitely unattractive. And if the gums recede the line will appear. So today we very often design crowns that need metal support with only porcelain at the edge so there is no possibility of seeing metal at all. These types of crowns supported by metal are mostly used in the back of the mouth because they give you a very strong and long lasting crown with good appearance, but we can do better in the front where it counts.
As I mentioned above, light needs to bounce around inside the tooth and obviously metal won’t allow this and so any tooth root exposed by receding gums will look darker than nearby teeth and so can the gums at the edge of a crown (if they are thin enough that you can see a darkening of the underlying tooth root).
Newer technologies allowing all porcelain crowns can solve a lot of these problems of light “illuminating” the crown and gums. But some teeth are crowned because they are dark or discoloured and we want the light to be blocked so we can improve the appearance of the teeth. Examples of this are teeth that have had root canal treatment, some of which have metal posts that can also darken the tooth root. Here we can use either the traditional metal supported crowns in the posterior of the mouth or special newer materials like Zirconium (think artificial diamonds- Zircons) to act as metal by being strong, blocking the darkness underneath but having the advantage of being white so adding porcelain on top of it give us a very natural appearing crown.
Dentistry is getting much better at mimicking nature but it’s a tough job!
Treating Black lines and Dark Roots
This post on black at the gumline of crowns has been very popular. So I thought many of you might be interested in how the patient in the photo in the above post was treated.
This man wasn't really too unhappy with his smile ( his front teeth didn't show completely whenhe smiled), but in a routine check up I found a problem that meant he would loose one of his center teeth ( central incisor). Since finances were an issue together we decided to replace the two center teeth, so they would match in his smile. I replaced one tooth with a dental implant, on which a white ceramic post was attached and that tooth and the other central incisor were both finished with identically made ceramic crowns. Here is what the result was:
Now I know some of you would like to know more, so here are some photos of what I did. The center tooth on the left had to be removed and replace by a dental implant, that can present all kinds of esthetic challenges especially in the center of your smile. All four of the teeth, as you can see suffered from black at the edges of the old crowns and the exposed part of the teeth shows how dark the tooth under each of the crowns is, due to the old posts placed in the teeth years back (we don't place posts like that anymore). The dark roots also make the gums look dark too. By using the latest white ceramic post attached to the implant ( you can see it in the photo below) the gums take on the natural bright pink colour of good health! Finishing with two porcelain crowns results in natural looking teeth.
In the future we will tackle the other crowns. ( and maybe he will become interested in tooth whitening too!)