A dental crown is a common restorative treatment used to cover a damaged or weakened tooth, shielding it from further harm. Crown materials, such as porcelain, ceramic, or metal alloys, are impervious to the acid-producing bacteria that cause tooth decay. Despite this protection, a crowned tooth can definitively get a cavity, as the natural tooth structure underneath the restoration remains susceptible. The crown acts like armor for the visible part of the tooth, but it does not make the entire tooth immune to bacterial invasion. The underlying tooth tissue is still composed of dentin and enamel, which are susceptible to decay if exposed to the oral environment.
The Vulnerable Margin
The point of vulnerability for a crowned tooth is the narrow area known as the crown margin or junction. This is the precise location where the edge of the restoration meets the natural tooth, typically near or just below the gum line. Even with meticulous technique, this microscopic interface is the weakest point against decay-causing bacteria. The cement used to bond the crown acts as a sealant but can degrade over time, creating a minute space.
This tiny gap traps dental plaque and food debris, which are difficult to remove completely with routine brushing and flossing. Bacteria colonize this area and produce acids that dissolve the natural tooth structure beneath the crown’s edge. Once bacteria bypass the margin, they spread rapidly through the underlying dentin, which is softer than enamel. The decay then progresses unseen, compromising the foundation supporting the crown.
Signs and Underlying Causes of Decay
Decay under a crown occurs when the protective seal at the margin is compromised, allowing bacteria to infiltrate the remaining tooth structure. If the restoration does not perfectly hug the tooth preparation, a microscopic pathway may be present from the start. Over years of chewing and temperature changes, the bonding cement can dissolve or fracture, widening this entry point. Normal wear and tear can also lead to small cracks that expose the underlying tooth.
A significant cause is gum recession, which often happens naturally with age or due to periodontal disease. As the gum tissue pulls back, the root surface is exposed below the crown margin. This root surface lacks the protective enamel layer and is primarily composed of cementum and dentin, which are softer and more susceptible to decay. Bacteria can quickly attack this newly exposed area, initiating decay that travels inward underneath the crown.
The signs of decay can be subtle, making regular dental X-rays an important diagnostic tool since the crown material often hides the cavity from visual examination. Patients may notice increased sensitivity to hot or cold temperatures, indicating decay advancing toward the nerve. Pain when chewing or biting down is a common symptom, signaling pressure on the underlying tooth structure. A persistent bad taste or bad breath can also be a red flag, suggesting an active bacterial infection. Sometimes, a visible dark line or discoloration appears along the gum line, indicating a margin breach.
Treatment and Long-Term Care
The approach to treating decay under a crown is determined by the extent and location of the cavity. If the decay is small and confined only to the margin, a dentist may be able to access the area by carefully drilling through the crown and placing a small filling, effectively re-sealing the junction. However, if the decay is extensive, the existing crown must typically be removed to fully clean out the infected tooth structure and ensure the complete elimination of the cavity. Once the decay is removed, a new, well-fitting crown must be fabricated and placed to restore the tooth’s integrity and provide a fresh, tight seal.
If the decay has progressed deep enough to reach the tooth’s pulp chamber, which contains the nerve tissue, a root canal procedure will be required before a new crown can be seated. This procedure removes the infected tissue and seals the root canals to prevent further infection. In the most severe cases where the decay has destroyed too much of the remaining tooth structure, the tooth may not be salvageable, and extraction becomes the only option.
Long-Term Protection
Long-term protection relies heavily on diligent home care and professional oversight. This includes:
- Brushing thoroughly twice a day, paying close attention to the gum line area around the crowned tooth.
- Flossing daily to mechanically disrupt plaque accumulation directly at the crown’s edge.
- Regular dental checkups and professional cleanings, typically every six months.
These professional visits allow the dentist to monitor crown margins, assess surrounding gum tissue, and use diagnostic X-rays to detect decay before symptoms arise.