Black line at gumline
This Chicago Northshore resident complained as a new patient that her recently done crowns did not match her natural teeth nor each other. The floss would shred when used around her dentistry and the gums would bleed. Knowing that bleeding is not normal, she felt that her dentistry did not fit well. Her gums were receeding from around the crowns one year after placement. Some of her new crowns had black lines at the gumline. She dreamed of having a pretty white smile and wanted her dentistry to be easy to clean.
Customized Empress Porcelain Selection
The patient elected to have new dentistry (veneers, crowns or bridges) done to replace or cover over and reshape all of her upper teeth. Our patients have the opportunity to decide how they want their porcelain to look. This patient wanted a bright and white look with less variation in color going from the gum to the edges of her teeth. Translucency in natural teeth often is what creates this variation. These restorations were done without translucency at the patient's request.
Black lines at the gumline of crowns
Nothing is more unesthetic than a dark area showing at the margin of your anterior crowns. This is especially true if you have a big smile and the dark lines show up in pictures. These dark gray or black lines are more likely to be present whenever metal is used as a substructure for the crown (see PFM or porcelain fused to metal / gold crowns). PFM crowns have an opaque “non-precious metal” or “semi-precious metal” or “high noble metal” alloy substructure (often called the coping) that seals the tooth and offers support for the tooth colored ceramic that is stacked and baked on top. Low gold content metal is a dark gray and is easily seen if not covered over. Even 14 carat gold alloys have a gray color. PFM crowns can be made beautifully by a skilled dentist-technician team but this type of crown is much harder to make attractive long term than all porcelain crowns or veneers. Metal is used to give the crown strength and often dentists use this type of restoration when the patient grinds their teeth. PFM crowns are also used by dentists that have not been trained on the newer bonding procedures necessary for the all porcelain restorations. Black areas at the gumline can also be present if the border or margin of the crown is exposed and there is a difference in brightness between the restoration and the root. All porcelain crowns made with high translucency are more difficult to make and deliver in the mouth but they do not have any metal and can be made to have a contact lens effect at the margin along the gumline.
Most common cause of dark crown margins or dark lines at the gumline
The metal of the crown is exposed. The average porcelain fused to metal crown made by a below average or average dentist will have metal extending to its border / edge, called the margin. A small amount of metal extends beyond the porcelain. Porcelain masks most of the underlying metal but not at the edge. A higher quality dentist will pay extra for the dental technician (who actually makes the crown) to cut back the metal margin and bake a special “margin porcelain” to seal the crown. This is called a “porcelain butt joint margin”. The gray metal is never visible on the parts of the crown that could be seen in a smile.
Other causes of black lines on teeth
Can high quality crowns have dark gum lines?
Yes, there are 4 possible scenarios.
- Teeth are surprisingly translucent and when light hits part of a tooth, the light is bounced around within the entire tooth including the root. The gum is actually illuminated by the lightened root and will appear a lighter shade of pink. If one of the teeth in your smile has a PFM crown (still the most commonly done type of crown), then when illuminated, the opaque metallic coping will prevent light from going up the root. The gum (not the crown) will appear darker. Adding a porcelain butt joint margin allows more light to be transmitted to the root to keep the gums from darkening but there is still less light going down the root.
- When a patient grinds their teeth or bruxes, large lateral forces are exerted causing flexure of the teeth. When there is a crown on a tooth, the flexure point will be at the crown margin. Over time, the cement used to seal the margins will slowly break down and begin to leak. Even with the highest quality crowns with porcelain butt margins, if there is leakage, there will eventually be some staining.
- One of the benefits of all porcelain crowns and veneers is that translucent porcelain can assume a contact lens effect at the margin, virtually making the margins invisible; even with gum recession. Some types of well-made all porcelain crowns can have a darkness at the gumline. This can occur if the crown/s were originally made to make the teeth brighter for cosmetic reasons. Porcelain crowns and veneers that are used to make teeth brighter are created using more opaque porcelain to mask the dark tooth below. Opaque porcelain margins do not have a contact lens blending effect and will be visible after gum recession occurs.
- It takes more effort on the part of dentists but they can tuck the margin of a crown beneath the gumline thus hiding the visual interface between the crown and the tooth root. As we age, our gums often /usually recede. This occurs even if we have healthy gums. 5-20 years after the crown is installed, gum recession exposes the margin. Planning for the future, the margin of the crown is stained by the lab technician to match an assumed shade of the root that the dentist cannot see. Often the optical properties of the crown and root are mismatched.