A dental crown is a custom-made cap placed over a damaged or weakened tooth to restore its shape, size, strength, and appearance. Although a crown covers the entire visible portion of the tooth, a cavity can definitely form underneath. The natural tooth structure beneath the crown remains susceptible to decay, known as secondary caries. A crown is not a permanent shield against cavity-causing bacteria.
Why Decay Still Occurs
The belief that a crown completely seals the tooth is a common misconception, as the junction where the crown meets the tooth is the weakest point in the restoration. This meeting point, known as the crown margin, is typically located near the gum line and is highly susceptible to plaque accumulation.
Even a perfectly fitted crown can experience degradation of the cement that holds it in place over time, creating a microscopic gap. This tiny space allows for a process called microleakage, where oral fluids, bacteria, and food debris seep into the area between the crown and the natural tooth structure.
Once bacteria infiltrate this margin, they produce acid that attacks the remaining tooth material, leading to secondary caries. Decay can also result from pre-existing decay not fully removed during the initial tooth preparation. Furthermore, gum recession can expose the root surface below the crown margin, which lacks enamel and is more vulnerable to decay.
Recognizing the Symptoms
Decay under a crown is difficult to detect early because the crown hides the cavity. The most common sign is new or worsening sensitivity to hot, cold, or sweet substances, which occurs when decay affects the dentin layer.
A more advanced cavity may cause sharp pain when chewing or biting down, indicating significant structural compromise or inflammation of the dental pulp.
Localized symptoms can also include swelling, redness, or tenderness of the gum tissue immediately surrounding the crowned tooth. Patients might also notice a persistent foul taste or odor emanating from the area, which is a byproduct of bacterial activity and decay.
Confirming the Diagnosis and Treatment Steps
Dentists must employ specialized tools to confirm secondary caries, as visual inspection is often insufficient. The most reliable method is bitewing X-rays, which reveal radiolucencies—dark areas indicating decay—beneath the crown margin. Physical probing around the crown’s edges can also detect soft spots or margins that are no longer tightly sealed.
Treatment depends on the extent of decay beneath the crown. If decay is mild and localized near the margin, the dentist may remove the crown, clean the tissue, and place a large filling or a new crown, provided enough healthy tooth structure remains for support.
If decay reaches the pulp, or nerve, a root canal procedure is necessary to save the tooth. The root canal is performed through the crown or after its removal, followed by placing a new crown to protect the treated tooth. Extraction is a last resort when the remaining tooth structure below the crown margin is too compromised to be restored.
Preventing Future Sub-Crown Decay
The longevity of a crowned tooth relies on meticulous daily oral hygiene, focusing particularly on the area where the crown meets the gum. Thorough brushing twice daily with fluoride toothpaste is necessary, paying special attention to the crown margin to disrupt plaque buildup. Flossing or using interdental cleaners, like floss threaders or water flossers, is important to clean the space just under the crown’s edge.
Regular dental check-ups, typically every six months, are a proactive measure that allows a dentist to monitor the crown margins for subtle signs of leakage or wear. Routine X-rays are also an important part of these visits, enabling the detection of decay long before a patient experiences any noticeable symptoms.
Limiting the consumption of highly acidic and sugary foods and drinks can also help maintain a neutral oral environment, which reduces the risk of demineralization and subsequent decay around the crown.