A dental crown is a custom-made cap placed over a natural tooth that has been severely damaged, decayed, or weakened. This restoration protects the remaining structure, restores the tooth’s original shape and function, and improves its appearance. The success of a dental crown relies entirely on the foundation provided by the underlying natural tooth structure. If the remaining tooth cannot provide adequate support, the restoration will fail prematurely.
The Critical Requirements for Crown Success
The mechanical stability of a crown is governed by two fundamental principles of tooth preparation: retention and resistance. Retention form prevents the crown from being pulled off the tooth vertically, such as when biting into sticky foods, and is achieved by having parallel or nearly parallel prepared walls.
Resistance form prevents the crown from being dislodged by horizontal or rotational forces that occur during chewing. This form keeps the crown stable under heavy sideways pressure and is directly influenced by the height and diameter of the prepared tooth walls. A short, wide preparation offers less resistance and is more susceptible to failure than a tall, narrow one.
These two forms dictate the need for specific geometry. The walls must be tall enough to provide a sufficient gripping surface while also being minimally tapered to maximize friction between the tooth and the crown. Without adequate height and proper wall angle, the crown may loosen or rotate off the prepared tooth.
Minimum Dimensions for Tooth Preparation
To satisfy the mechanical requirements, specific dimensions must be achieved during tooth preparation. A minimum height of approximately 3 to 4 millimeters of prepared tooth wall is the accepted standard. This height provides the necessary surface area and leverage to effectively resist dislodgement from chewing forces.
The walls must have a minimal taper, or angle of convergence, ideally maintained between 4 and 10 degrees (with 6 degrees often cited as the clinical goal). This slight inward slope allows the crown to be seated while ensuring the frictional grip required for retention.
The tooth must also be reduced enough to accommodate the crown material thickness. For PFM or all-ceramic crowns, the biting surface typically requires a reduction of 1.5 to 2.0 millimeters. This clearance ensures the material has enough bulk to withstand biting forces without fracturing and to achieve acceptable aesthetics.
Solutions When Structure is Lacking
When a tooth is extensively damaged by decay, fracture, or large fillings, the remaining structure may be insufficient to meet minimum dimensional requirements. In these cases, a core build-up is employed to create a solid foundation. This involves using a strong filling material, such as composite resin or amalgam, to replace the missing dentin and enamel, rebuilding the tooth to the required height and shape.
If the tooth has undergone root canal therapy and lost significant coronal structure, a post and core procedure may be necessary. A small metal or fiber post is cemented into the root canal space for deep anchorage. The core material is then built up around this post, creating a stable, reinforced platform for the final crown.
A critical threshold for saving a tooth is the amount of sound structure extending above the gum line. If decay or fracture extends too far below the gum line and approaches the bone level, achieving a proper seal and margin becomes nearly impossible. When the remaining structure cannot be salvaged, the tooth is considered non-restorable, and extraction is the only viable option.