A dental crown is a custom-made cap designed to cover a tooth entirely, restoring its shape, size, strength, and appearance. Dentists use crowns to protect teeth weakened by extensive decay, large fractures, or root canal procedures. The success and longevity of any crown depend entirely on the remaining natural tooth structure, which serves as the foundation. The primary concern is how much sound, healthy tooth remains to support the final restoration. This foundation must meet specific mechanical requirements to ensure the crown stays in place and withstands chewing forces.
The Essential Requirements for Crown Retention
The stability of a crown relies on two mechanical principles: retention and resistance. Retention prevents the crown from being pulled off (e.g., by sticky foods). Resistance prevents the crown from dislodging due to oblique or horizontal forces, such as those experienced during chewing. For a mechanically retentive preparation, the vertical walls of the prepared tooth must have a minimum height, typically 3.0 millimeters for anterior teeth and 4.0 millimeters for molars.
The walls must also have minimal taper, with an ideal convergence angle between opposing walls of about five to six degrees. A preparation that is too short or too tapered lacks the necessary surface area and geometry to resist forces, leading to crown failure. The width of the tooth also plays a role, as wider teeth provide a larger surface area for the cement to bond to, increasing overall retention. The relationship between the height and width of the remaining tooth structure is a determining factor in a crown’s long-term survival.
A specific structural requirement, especially for teeth that have undergone root canal treatment, is the ferrule effect. This describes the mechanical bracing of the crown margin around a band of sound tooth structure above the bone level. For optimal fracture resistance, dentists aim for a ferrule of 1.5 to 2.0 millimeters of vertical tooth structure. This band of dentin must also be at least 1.0 millimeter thick circumferentially to resist the wedging forces or stress transferred from the crown. The ferrule effect reduces the risk of the tooth root fracturing under heavy chewing loads.
Restorative Techniques for Minimal Structure
When a tooth is severely damaged, the natural structure may not meet the minimum retention and resistance requirements. Restorative techniques are used to rebuild the tooth’s core before crown preparation begins. The choice of technique depends on how much tooth structure is missing and whether the tooth has had root canal treatment.
Core Buildup
If a tooth has lost a significant amount of its center but retains healthy surrounding walls, a simple core buildup is often performed. This involves using a strong restorative material, such as composite resin or dental amalgam, to fill the void and replace the missing dentin. The core buildup creates a solid, uniform foundation that can then be shaped to the correct dimensions for the final crown. This procedure is favored when less than half of the coronal tooth structure is missing.
Post-and-Core Restoration
When the tooth lacks enough coronal structure to retain the core material, or if it has been severely compromised by a root canal procedure, a post-and-core restoration is necessary. A post is a metallic or fiber-reinforced rod placed into the strongest, widest root canal space to act as an anchor. The post is cemented into the root, and the core material is built up around the part of the post that protrudes from the root.
The post’s function is purely to retain the core material, which supports the crown; it does not strengthen the root itself. To ensure the root remains sealed, dentists must leave a minimum of 4 to 5 millimeters of the original root canal filling material. The post-and-core technique allows the dentist to create the necessary vertical height (3.0 to 4.0 mm) and ferrule (1.5 to 2.0 mm) required for successful crown placement.
When a Tooth Cannot Be Saved
Despite advanced rebuilding techniques, there are limits to what dentistry can restore. When a tooth is deemed non-restorable, it means no procedure can return the tooth to adequate health and function. A major limiting factor is the extent of a fracture or decay that extends deep below the gum line and bone level.
A tooth is often considered non-restorable if it meets one or more of the following criteria:
- Greater than 75% loss of the clinical crown.
- A fracture line extending vertically into the root.
- Less than 50% bone support or significant mobility due to advanced gum disease.
- A poor crown-to-root ratio, where the portion outside the bone exceeds the portion encased in bone.
Periodontal health is a deciding factor. Placing a crown on a tooth with these issues is likely to result in long-term failure. In these cases, extraction and replacement with an alternative, such as a dental implant, is the recommended course of action.