A dental crown is a custom-fitted cap placed over a damaged tooth to restore its shape, strength, size, and appearance. It is recommended when a tooth has lost significant structure due to severe decay, fracture, or a root canal procedure. The long-term success depends entirely on the remaining natural tooth structure, which must serve as a stable foundation. Without an adequate base, the crown cannot be securely cemented, risking failure.
The Critical Requirement for Crown Success
The primary structural requirement for a successful, long-lasting crown is the “ferrule effect.” The ferrule is a band of sound, vertical tooth structure that encircles the tooth preparation. This remaining structure must be strong enough to resist the lateral and rotational forces applied during chewing. A circumferential ferrule acts as a bracing mechanism, improving the fracture resistance of the tooth root.
This supportive band of dentin should ideally measure between 1.5 and 2 millimeters in vertical height above the preparation margin. Achieving this measurement is important for teeth that have undergone root canal treatment, which often weakens the overall structure. The ferrule helps distribute the stress from the crown deep into the tooth, preventing root fractures. While a complete 360-degree ferrule is optimal, preservation of the dentin on the cheek and tongue sides is important for stability.
If less than 1.5 millimeters of sound tooth structure remains, the risk of the tooth or root fracturing under normal biting forces increases. The ferrule provides the necessary retention and resistance form, keeping the crown cemented firmly in place. This measurement is a factor a dentist considers when determining if a tooth can be reliably saved with a crown.
Assessing Overall Tooth Viability
While the ferrule measurement is important for mechanical stability, a comprehensive assessment of the tooth’s overall health dictates its viability for a crown. The extent of decay or fracture must be evaluated, particularly if it extends below the gum line. If the damage reaches too far down, it can compromise periodontal health and make a stable restoration difficult to achieve.
The health of the root structure is another significant factor, especially in teeth that have had root canal therapy. Dentists check for evidence of root fractures or signs of infection at the tip of the root. A tooth with a severely compromised root or an unfavorable crown-to-root ratio may not be able to withstand the forces of chewing.
The position of the tooth within the patient’s bite, or occlusal forces, also plays a substantial role. Teeth in the back of the mouth, such as molars, must withstand significantly higher biting forces than front teeth. A patient who clenches or grinds their teeth, a condition known as bruxism, places greater stress on any restoration. These dynamic forces must be considered when planning the crown material and preparation design to ensure long-term survival.
Options When Insufficient Structure Remains
When a tooth lacks the minimum structural requirement, the dentist may first attempt to rebuild the foundation before placing the crown. For teeth that have lost less than half of their original structure, a core buildup is often performed using restorative materials like composite resin or amalgam. This material is bonded to the remaining dentin to create a solid base that supports the crown preparation.
If the tooth has had a root canal and lost a large amount of internal and external structure, a post-and-core technique may be necessary. This involves placing a small metal or fiber post into the root canal space to act as an anchor. The core material is then built up around this post to create the required foundation for the crown. The post itself helps retain the core material but does not strengthen the root.
If the tooth structure is severely lacking, or if the root health is compromised, the tooth may be deemed unsalvageable for a crown. In these cases, the dentist may recommend extraction. Following extraction, the space can be restored with a fixed dental bridge, which uses the adjacent teeth for support, or a dental implant, which replaces the missing root and supports a new crown.