A dental crown is a custom-made, tooth-shaped cap placed over a natural tooth to restore its form, size, and strength. Its primary function is to protect a damaged tooth from fracturing or to restore a broken or severely worn tooth. The long-term success of a crown depends almost entirely on the quality and quantity of the remaining natural tooth structure beneath it. This foundation must provide mechanical stability and enough space for the restorative material to have adequate thickness, requiring a precise balance of conserving healthy material while creating the ideal geometric shape.
The Geometry Required for Crown Stability
The prepared tooth structure must provide two distinct mechanical properties for the crown to remain secure: retention and resistance. Retention is the ability of the preparation to prevent the crown from being dislodged along its path of insertion (e.g., by sticky food). Resistance prevents the crown from dislodging due to horizontal or oblique forces common during chewing.
These mechanical requirements are primarily governed by the height of the prepared tooth walls and their slight inward angle, known as taper. Taller prepared walls offer a greater surface area for the cement to bond, increasing retention. A minimum wall height of 3 to 4 millimeters is generally desired, particularly on molars where chewing forces are highest.
The taper is the convergence angle between two opposing prepared walls, which allows the crown to slide onto the tooth. The ideal total occlusal convergence (TOC) for a crown preparation is generally considered to be between 10 and 22 degrees. If the taper is too steep, exceeding 25 degrees, the crown’s retention is compromised because it is too easy to dislodge. Conversely, a taper that is too close to parallel creates physical undercuts that prevent the crown from being fully seated.
Creating Space for Crown Material
Beyond mechanical stability, the remaining tooth structure must be reduced to provide clearance for the restorative material. This reduction is necessary to ensure the crown has sufficient bulk to resist fracture and to allow for a natural appearance. The required reduction varies significantly based on the chosen material, because different materials possess different inherent strengths.
Occlusal reduction, the reduction on the chewing surface, requires the greatest material thickness to withstand biting forces. For a full-metal crown, which is exceptionally strong, only about 1.0 to 1.5 millimeters of reduction may be necessary. In contrast, all-ceramic and porcelain-fused-to-metal (PFM) crowns require more space, often 1.5 to 2.0 millimeters, to accommodate the weaker porcelain layer.
Axial reduction, along the sides of the tooth, also has specific requirements. PFM crowns, for example, require 1.2 to 1.5 millimeters of axial reduction on the cheek-facing side to allow for the metal substructure and aesthetic porcelain layer. Newer materials like monolithic zirconia may permit less aggressive preparation, sometimes requiring only 0.6 to 1.0 millimeter of reduction, which helps conserve more natural tooth structure. The preparation also ends with a clear, smooth boundary called the margin, which is where the crown meets the tooth, sealing the restoration against the oral environment.
Restoring Teeth with Minimal Structure
When decay, trauma, or previous large fillings have removed a substantial amount of the tooth, special procedures are needed to rebuild the necessary foundation for a crown. The most important structural element for a severely damaged tooth, particularly one that has had a root canal, is the ferrule effect. This effect requires a circumferential band of healthy tooth structure, typically 1.5 to 2.0 millimeters high and at least 1 millimeter thick, surrounding the base of the prepared tooth.
The ferrule acts like a metal ring on a wooden hammer handle, bracing the remaining tooth structure and root to prevent catastrophic root fracture under chewing load. Without this protective band, forces applied to the crown can act as a wedge, leading to failure and often requiring the tooth’s extraction. Achieving this 360-degree band of sound tissue is frequently the deciding factor in whether a tooth can be saved.
If the internal bulk of the tooth is insufficient to support the crown, a core build-up is performed using a strong filling material to replace the missing dentin. If the core build-up lacks sufficient retention, a post may be placed into the root canal space to anchor the core. The post’s primary function is to secure the core build-up, not to reinforce the root; the ferrule provides the crucial resistance against fracture. If there is not enough natural tooth structure above the gum line to create the ferrule, procedures like crown lengthening may be performed to expose more root surface and ensure the long-term survival of the restoration.