A dental crown is a restoration placed over a damaged or weakened tooth, restoring its shape, size, strength, and appearance. While the crown material itself (typically porcelain, ceramic, or metal) cannot decay, the natural tooth structure underneath remains vulnerable. The concern is not a cavity in the crown, but decay forming in the remaining tooth structure covered by the restoration. This secondary decay can compromise the tooth’s integrity and threaten the crown’s long-term success.
Where Decay Occurs Relative to the Crown
The most susceptible area for decay is the crown margin, which is the precise junction where the edge of the crown meets the natural tooth structure, usually near or slightly below the gum line. This margin forms a seal meant to prevent bacteria and food particles from entering the space beneath the crown.
Even a perfectly fitted crown can develop microscopic gaps at this margin over time due to wear, chewing forces, or the gradual breakdown of the dental cement. Once this seal is compromised, acid-producing bacteria from plaque can infiltrate the space. The decay begins on the exposed tooth material, often the softer dentin or cementum on the root surface, accelerating the damage. This process is known as recurrent decay, and it is a common reason crowns eventually fail.
Causes of Decay Under Dental Crowns
The primary factor contributing to this type of decay is insufficient oral hygiene, which allows plaque and bacteria to accumulate along the crown margin at the gum line. If this area is not cleaned, the bacterial colonies produce acids that erode the cement and tooth structure. A crown placed with a poor fit, leaving a minute gap between the restoration and the tooth, provides an immediate entry point for bacteria, often due to technical errors during fabrication or seating.
Changes to the gum tissue also increase the risk, particularly gum recession, where the gums pull back and expose more of the natural root surface beneath the crown’s edge. Root surfaces are significantly more susceptible to decay because they lack the protection of hard enamel. A diet high in fermentable carbohydrates and acidic beverages accelerates bacterial activity and acid production. Additionally, habits like teeth grinding (bruxism) place excessive pressure on the crown, which can weaken or crack the cement seal over time, opening a pathway for micro-leakage.
Detection and Repairing Crown Decay
Detecting decay under a crown can be difficult because the crown hides the damage from both the patient and the dentist’s direct view. Patients may first notice symptoms such as increased sensitivity to hot or cold temperatures, a dull ache, or sharp pain when biting down. A dark line or discoloration around the crown’s gum line can indicate underlying decay or a compromised margin.
Dentists rely on visual inspection, probing around the margin with a dental instrument, and dental X-rays to diagnose the issue. X-rays are essential because they reveal radiolucencies (dark spots) that signify decay beneath the crown’s edge or deep within the tooth structure. Treatment depends on the extent of the damage. If the decay is minor and accessible, the dentist may attempt to remove the decayed tissue and place a filling beneath the existing crown. If the decay is extensive, the entire crown must be removed, the infected tissue cleaned out, and the remaining tooth structure rebuilt, often with a core buildup. If the decay has reached the pulp, a root canal procedure may be necessary before a new crown is fabricated and placed.
Maintaining Oral Health for Crown Longevity
Oral hygiene focused on the crown margin is essential for extending the life of the restoration and preventing recurrent decay. Brushing twice daily with a soft-bristled toothbrush and fluoride toothpaste is important, paying special attention to cleaning the area where the crown meets the gum. Daily flossing is equally important to remove plaque and food debris from between the teeth and around the crown’s edge.
Targeted cleaning tools, such as interdental brushes, floss threaders, or water flossers, can be highly effective at cleaning the vulnerable area beneath the crown margin. Regular professional care requires semi-annual dental checkups and cleanings. During these visits, the dentist monitors the crown’s seal and takes X-rays every one to two years, depending on the patient’s risk profile, to detect hidden decay before it progresses. Limiting the intake of sugary and acidic foods and drinks reduces the fuel source for decay-causing bacteria, providing protection for the crowned tooth.