Dark Line at the Crown-Gum Border — Three Causes and How to Tell Them Apart

Hello, I'm Dr. Junghoon Choi, director of Seoul Crown Dental Clinic.

"My crown has been in place for several years, but a dark line has appeared right at the gumline. Is it a cavity, or is something wrong with the crown itself?"

"Everything looked fine when it was first fitted, but one day I noticed the gum area around my front tooth was starting to look dark. Would replacing the crown get rid of it?"

Short answer: A dark line at the crown-gum border comes from one of three distinct causes — metal show-through, gum recession, or secondary (recurrent) decay at the crown margin. Although they look similar from the outside, the treatment direction differs for each: sometimes changing the material alone resolves it, sometimes gum treatment has to come first, and if secondary decay has formed, the condition of the tooth itself needs to be rechecked, not just the crown remade. Identifying which case it is comes first.

Why does that dark line appear? — Three causes

1. Metal show-through — a material issue

Many older crowns are built with a metal inner framework covered by a ceramic outer layer. If the gum recedes even slightly, the metal margin (the edge where the metal framework ends) becomes visible from the outside and appears as a dark line. Nothing has actually gone wrong with the crown itself — this is simply a structural characteristic of that type of material becoming apparent over time.

In this situation, replacing the crown with one made from zirconia or an all-ceramic material can reduce the chance of the same thing happening again. That said, the condition of the gum and the tooth root should be assessed before any remake is planned.

2. Gum recession — when gum treatment comes first

When the gum is not maintained in good health, the margin — the edge where the crown begins — gradually becomes exposed. As the part of the crown that was originally tucked under the gum emerges, the color difference makes it appear dark, and in some cases the color of the tooth root itself shows through directly.

Simply making a new crown will not remove the dark line in this situation. Unless the underlying causes of the recession — inflammation, brushing technique, bite problems — are addressed first, a new crown will follow the same course. This is one of the clearest examples of gum treatment needing to come before any prosthetic remake.

3. Secondary cavity — when the tooth itself needs to be re-evaluated

A new cavity forming around the edge of an existing crown is called a secondary cavity (recurrent decay). It often begins when bacteria accumulate in the microscopic gap between the crown and the tooth, or when the fit at the margin gradually breaks down over time. Darkening or browning at the gum margin, sensitivity to cold, or discomfort when pressing on the gum around the tooth are all signs that a secondary cavity should be ruled out.

If a secondary cavity is found, the crown must be removed so the condition of the tooth underneath can be assessed directly. Depending on how much healthy tooth structure remains, a new crown may be made, or additional treatment to preserve the tooth may be needed. The same decision-making principle is discussed in When a Tooth Chips — Filling or Crown? It Depends on How Much Tooth Is Left.

Are there situations where a remake is not urgent?

A dark line does not automatically mean the crown needs to be replaced right away. If the gum is stable, the crown's fit is still well maintained, and there is no sign of a secondary cavity, then replacing the crown — even with a visible dark line — is not necessarily the better choice at that moment.

The remake process itself can place stress on the tooth, and removing a well-fitted crown can introduce unexpected variables. When the main concern is aesthetic, monitoring the situation at regular check-ups is a reasonable approach.

How to work through it — the order of evaluation

1. Start with an X-ray to check for secondary decay and root condition

Color changes visible to the eye alone are not enough to identify the cause. An X-ray is the starting point — it reveals whether decay is forming near the crown margin and how the tooth root is doing.

2. Assess the gum — inflammation and recession

If there is gum inflammation or a general trend of recession, a gum treatment plan should be drawn up before any crown remake is considered. Evaluating the remake only after the gum has stabilized makes the outcome of a new crown easier to predict.

3. Discuss material options and remake design

If the evaluation confirms the issue is the crown material rather than decay or gum disease, the next conversation is about which material is suitable for the remake. When the Color or Shape of a Front Crown Is Not Right — What to Check Before a Remake covers the decision criteria for front-tooth remakes in more detail.

• A dark line or color change has appeared at the crown-gum border
• Sensitivity to cold or discomfort around the gum is also present
• The crown has been in place for seven years or more and has not been checked recently
• The gum feels as though it has generally receded
• A remake is being considered for aesthetic reasons, but the cause is not yet known

When treatment keeps repeating, the redo has to be designed right. If you have questions, feel free to ask.