A dental crown is a custom-made cap designed to completely cover a damaged tooth, restoring its shape, strength, and appearance. This restoration fits over the entire visible portion of the tooth above the gum line. The tooth structure underneath the crown is fundamentally changed to create a stable foundation for the cap. This underlying material is often called the abutment.
Why Preparation is Necessary
The natural tooth must be precisely modified to ensure the crown can fit securely and function correctly within the mouth. This preparation is performed for two primary reasons that affect the crown’s longevity and the patient’s bite. The first involves creating sufficient space for the crown material, typically requiring the removal of 1 to 2 millimeters of tooth structure from all surfaces. If insufficient space is created, the crown will be too bulky, interfere with the bite, or be too thin and prone to fracture.
The second reason is to achieve retention and resistance form. Retention prevents the crown from being pulled off, such as when biting into sticky foods. Resistance stops the crown from dislodging due to horizontal forces experienced during chewing. These forms are achieved by shaping the sides of the tooth to be slightly tapered and ensuring the remaining structure has adequate height relative to its width.
The Specific Shape of the Prepared Tooth
Once the necessary tooth structure has been removed, the underlying tooth resembles a miniature version of a natural tooth. This prepared structure, or abutment, is smaller in all dimensions, having been reduced on the chewing surface and all four sides. The overall height is reduced to create clearance, and the sides are made to converge slightly toward the biting surface, which is the taper necessary for the crown to slide on and lock into place.
The margins, where the prepared tooth meets the surrounding gum tissue, are precisely cut to a specific design. Common margin types include a chamfer, which is a smooth, rounded cut, or a shoulder, which is a right-angle or beveled ledge. The chosen margin design is specific to the type of crown material being used, such as a deep shoulder for an all-ceramic crown, to ensure the crown material has enough bulk to prevent fracture and create a tight seal.
The color of the prepared tooth is also notably different because the preparation removes the outer layer of white enamel, exposing the underlying dentin. Dentin is a yellower, more opaque, and less uniform tissue than enamel. If the tooth had a large filling or was treated with a root canal, the exposed dentin may also show internal discoloration or the color of old restorative materials. This darker color of the abutment can sometimes influence the final shade of a highly translucent ceramic crown placed over it.
Long-Term Integrity of the Tooth Structure
The crown is secured to the prepared tooth with dental cement, creating a seal intended to protect the underlying structure. Despite this protection, the natural tooth remains susceptible to issues that affect its long-term health. One significant concern is recurrent decay, which forms at the margin where the crown meets the tooth structure, often near the gum line. Bacteria can seep into this junction if the seal is compromised due to poor fit, wear, or gum recession.
Once decay begins at the margin, it can progress silently underneath the crown until the damage is extensive. Symptoms like localized sensitivity, a dark line around the crown, or a change in the crown’s fit can indicate this underlying problem. When a crown is removed for replacement, the underlying tooth often requires cleaning and filling before a new crown can be placed.
A core buildup is often necessary when the original tooth was severely damaged. If a tooth has extensive decay, a large old filling, or has had a root canal, the dentist uses a restorative material to rebuild the structure first. This core buildup creates a stable, solid base, reinforcing the foundation before it is shaped into the abutment.
Post-operative sensitivity can be a short-term issue if the preparation was deep and came close to the nerve tissue. However, persistent sensitivity to hot or cold stimuli years later may signal a problem with the crown’s seal or the formation of recurrent decay. The longevity of the entire restoration relies heavily on maintaining the marginal seal and consistent oral hygiene.