A dental crown is a custom-made, tooth-shaped cap placed over a damaged tooth to restore its original shape, size, and strength. Crowns are used to support teeth with large fillings, hold a dental bridge in place, cover a dental implant, or improve the tooth’s appearance. The journey of creating this precision prosthetic involves several detailed steps, beginning with capturing the exact dimensions of the patient’s mouth.
Capturing the Tooth’s Blueprint
The initial stage of crown fabrication requires capturing a precise three-dimensional map of the prepared tooth and the surrounding oral structures. This blueprint allows the dental laboratory to create a crown that fits perfectly against the patient’s existing dentition.
One common method involves traditional impressions, where a gooey, putty-like material is placed in a tray and seated over the prepared tooth. This elastomeric material hardens around the tooth, creating a negative mold that is then poured with gypsum to produce a positive stone model. This physical model serves as the working cast for the lab technician.
A modern alternative is the use of digital intraoral scanners, which construct a precise three-dimensional digital file. This technology bypasses the need for messy impression material, generating a virtual model that is immediately sent to the lab. The digital file is the direct input for computer-aided design (CAD) software.
Primary Crown Materials
The selection of material is dictated by the crown’s location, the patient’s aesthetic needs, and the required durability for chewing forces. Porcelain and ceramic materials are popular choices due to their ability to mimic the light-reflecting properties of natural enamel.
All-ceramic crowns, such as those made from lithium disilicate, offer superior aesthetics and translucency, making them ideal for visible front teeth. While they possess good strength for single-unit restorations, they are moderately less fracture-resistant than other options under heavy biting forces.
Zirconia has become a standard material due to its exceptional strength and durability. Monolithic zirconia crowns can withstand significant occlusal load, making them suitable for molars or for patients who grind their teeth. Newer generations of zirconia have been engineered with increased translucency, balancing strength with improved aesthetics.
Porcelain Fused to Metal (PFM) crowns combine a metal alloy core for structural support with a porcelain layer baked onto the exterior for a tooth-colored appearance. The metal coping provides high resilience, but the porcelain can sometimes chip, and the metal may create a dark line near the gumline. Full metal crowns offer the greatest strength and longevity with minimal wear on opposing teeth, and are generally reserved for back teeth where aesthetics are less of a concern.
Fabrication Techniques
Once the digital or physical blueprint is received, the dental laboratory begins transforming the chosen material into the final crown using specialized manufacturing techniques. The method used depends on the material selected and whether a digital or analog workflow is employed.
Digital workflows often utilize Computer-Aided Design and Computer-Aided Manufacturing (CAD/CAM) technology. The technician designs the crown’s shape, contacts, and occlusion using the CAD software, and this file is then sent to a high-speed milling machine. The CAM unit carves the precise crown shape from a pre-sintered ceramic or zirconia block, ensuring a high degree of dimensional accuracy.
For materials like zirconia, the milled crown is in a porous, “green” state and must undergo a high-temperature firing process called sintering. Sintering shrinks the crown to its final size and densifies the material, achieving its maximum strength. This structural core may then be further refined with a thin layer of aesthetic porcelain.
The traditional method for metal and PFM crowns is the Lost Wax Technique, which is a meticulous casting process. A wax pattern of the crown is either sculpted by hand or milled digitally, and this pattern is encased in a plaster-like investment material. The investment is heated in a furnace, burning out the wax and leaving a precisely shaped void. Molten metal is then forced into this cavity, forming the metal coping or full metal crown.
Porcelain layering is used to achieve the natural look of teeth, particularly with PFM or layered zirconia crowns. The porcelain material, supplied as a powder, is mixed with a liquid to create a slurry that is applied in successive layers onto the metal or zirconia coping. After each application, the crown is fired in a porcelain furnace, a process that fuses the ceramic particles together, giving the crown its strength and lifelike color depth.
Finalizing and Quality Check
The final phase involves a series of aesthetic and mechanical refinements to ensure the crown is ready for clinical placement.
Aesthetic refinement involves staining and glazing the external surface of the crown to mimic natural tooth characteristics. Stains are brushed onto the surface to replicate features like subtle color variations or crack lines. A final layer of glaze, a glass-like material, is then applied and fired, creating a smooth, shiny surface that simulates natural enamel and protects the underlying material.
Before the crown leaves the lab, it is subjected to a quality control check to verify its fit against the working model. Technicians check the marginal integrity, ensuring the crown’s edge precisely meets the preparation line without any gaps. The interproximal contacts with adjacent teeth and the occlusal relationship with the opposing arch are also verified.
The internal surface of the crown is prepared to ensure maximum retention before cementation. For all-ceramic crowns, this often involves etching the interior to create microscopic roughness, while metal-based crowns are typically sandblasted. These steps create a micro-mechanical surface texture that allows the dental cement to bond securely to the crown.