Finding a black area on a tooth under a crown can be unsettling, but it is a common observation requiring professional assessment. A dental crown is a protective cap placed over a damaged tooth to restore its form, strength, and appearance. Discoloration signals that the restoration’s integrity has been compromised, potentially due to a structural problem or an aesthetic issue. Because causes range from simple stains to serious infection, timely evaluation is necessary.
The Primary Causes of Discoloration
The most concerning reason for blackness is recurrent decay, which occurs when oral bacteria infiltrate the space between the tooth and the crown’s margin. This bacterial invasion allows acid to demineralize the natural tooth structure, creating a cavity that darkens the underlying tooth to a black or deep brown color. Decay is a progressive disease that weakens the tooth’s foundation, potentially leading to pain, infection, and the failure of the entire crown.
A purely aesthetic cause of discoloration is the presence of metal, either from the crown itself or an old filling. Older crowns, specifically porcelain fused to metal (PFM), contain a dark metal base that becomes visible as a dark line at the gum margin if the gum tissue recedes. If the tooth was restored with a silver amalgam filling before the crown, metallic ions can leach into the dentin, causing a harmless but noticeable grey or black internal stain.
Another common reason for darkening is margin leakage and subsequent staining. The crown is secured to the tooth with dental cement, which can break down over time due to wear, poor fit, or chewing forces. Once the seal is compromised, dark liquids and food debris, such as coffee or wine, can seep into the micro-gap and stain the exposed dentin or cement line. This staining signals a potential pathway for bacteria to enter and should not be ignored.
Distinguishing Between Decay and Staining
Differentiating between a cosmetic stain and active decay relies on a thorough clinical examination. A dentist performs a visual inspection, looking for signs such as a visible gap at the margin or gum recession exposing the crown’s edge. The discoloration itself offers clues; decay often presents as a softer, more extensive dark area compared to the thin, hard line characteristic of a metal margin.
The definitive diagnostic tool is the dental radiograph, or X-ray, which allows the professional to see beneath the crown and into the tooth structure. Because a crown is opaque, a visual check is insufficient. The X-ray can reveal radiolucency—a dark shadow—that indicates bone loss or active decay under the restoration. The dentist may also use a dental explorer to gently probe the exposed margin, checking for a sticky or soft texture characteristic of decayed tissue.
The diagnosis guides the urgency of intervention. If the blackness is confirmed to be an amalgam stain or an exposed PFM margin, the tooth structure is sound, and treatment is elective, focusing on aesthetics. Conversely, a diagnosis of decay requires urgent crown removal and excavation of the infected tissue to prevent the decay from reaching the pulp, which would necessitate a root canal procedure.
Necessary Steps for Treatment and Resolution
If the black discoloration is confirmed to be recurrent decay, treatment involves removing the existing crown to access the compromised tooth structure. The decayed tissue must be completely excavated, and the remaining tooth prepared for a new restoration. If the decay is extensive, a core buildup procedure is often necessary to recreate a stable foundation for the replacement crown.
The core buildup uses a restorative material, often composite resin, to replace lost dentin and enamel, ensuring the tooth has sufficient height and bulk to retain a new crown. If the tooth has undergone a root canal and is hollow, a post may be cemented into the root canal space for additional internal support. Once the foundation is stable, a new, custom-fit crown is fabricated and cemented into place.
If the issue is purely aesthetic, such as an exposed metal margin or deep staining, resolution involves replacing the old crown with a modern, metal-free restoration. All-ceramic materials like lithium disilicate or high-strength ceramics like zirconia eliminate the risk of a dark line appearing at the gum line, even with gum recession. Zirconia is durable, making it a good choice for posterior teeth that withstand heavy chewing forces, while all-porcelain crowns are preferred for their superior translucency on visible front teeth.
Maintaining Crown Health and Preventing Recurrence
Protecting the crown margin is paramount for longevity, as this junction is the most vulnerable area for recurrent decay. Maintaining a strict oral hygiene routine focused on plaque removal at the gum line is the most effective preventive measure. Brushing twice daily with a soft-bristled brush, holding the bristles at a 45-degree angle toward the gum line, helps disrupt the bacterial film where the crown meets the natural tooth.
Daily flossing is necessary for cleaning the sides of the tooth and the crown margin; string floss should hug the tooth in a “C” shape, gently sliding beneath the gum tissue. Some patients find interdental cleaning tools, such as floss threaders or water flossers, more effective for navigating around the crown. Water flossers use a pulsating stream of water to flush out debris and plaque, supplementing string floss, especially for those with dexterity challenges.
Regular professional monitoring is necessary for crown maintenance. Dental checkups every six months allow the dentist to examine the margins visually and take new X-rays to screen for silent decay forming beneath the restoration. Limiting highly acidic and sugary foods reduces the fuel source for decay-causing bacteria, helping to preserve the integrity of the crown’s seal.