A dental crown is a tooth-shaped cap placed over a damaged or weakened tooth to restore its shape, size, strength, and appearance. This restoration covers the entire visible portion of the tooth down to the gum line, protecting it from further deterioration. Crowns are used to support teeth with large fillings, hold dental bridges, cover implants, or improve the aesthetic quality of a tooth. The process, from initial preparation to final placement, involves precise clinical work and advanced laboratory technology.
Preparing the Tooth for the Crown
The process begins with the dentist preparing the existing tooth structure. This involves reshaping the tooth through reduction, where enamel and dentin are carefully removed from all surfaces. This reduction creates enough space for the crown material so the final restoration does not appear bulky or interfere with the patient’s bite. The amount of tooth structure removed depends on the chosen crown material, with all-metal crowns requiring less reduction than all-ceramic options.
Once the tooth is shaped, the dentist takes an accurate impression or digital scan of the prepared tooth and the surrounding teeth. Traditional impressions use a putty-like material to capture a mold of the mouth. Modern digital impressions use a handheld scanner to create a precise three-dimensional image. These detailed records are sent to a dental laboratory, providing the blueprint for the custom-made crown.
A temporary crown, typically made of acrylic resin, is then cemented onto the reduced tooth. This temporary restoration protects the exposed, sensitive dentin and prevents the prepared tooth from shifting. It also maintains the patient’s appearance and chewing function while the permanent crown is being fabricated. The temporary crown is held in place with a weaker cement that allows for easy removal during the next appointment.
Selection and Properties of Crown Materials
Crown material selection is influenced by the tooth’s location, aesthetic goals, and the need for durability against chewing forces. Primary categories include metal alloys, all-ceramic/porcelain, and combinations of the two. Metal crowns, often composed of gold alloys, palladium, or nickel-chromium, offer superior strength and longevity, making them ideal for out-of-sight molars.
Porcelain-fused-to-metal (PFM) crowns combine a metal substructure for strength with a tooth-colored porcelain layer for aesthetics. Although they balance durability and appearance, the metal edge can sometimes show near the gum line, and the porcelain surface may chip. For the most lifelike results, all-ceramic or all-porcelain crowns are preferred because they closely mimic the translucency of natural tooth enamel.
Zirconia is a popular modern ceramic material, which is a white crystalline oxide. Zirconia crowns offer exceptional strength, rivaling metal, while providing excellent aesthetics and biocompatibility. Lithium disilicate, often marketed as E-max, is another high-strength ceramic that allows for thinner crown designs, preserving more natural tooth structure.
Laboratory Fabrication Methods
The dental laboratory translates the impressions or digital scans into a physical crown using traditional and advanced methods. Traditional fabrication often uses the lost-wax technique. Here, a wax pattern of the crown is created, invested in a mold, and burned out, leaving a cavity into which molten metal alloy is cast to form the metal substructure.
Modern crown fabrication is dominated by digital workflows using Computer-Aided Design and Manufacturing (CAD/CAM) technology. Digital scan data is imported into CAD software, where a technician designs the precise contours and fit of the crown. This digital design is then sent to a high-speed milling machine, which carves the crown from a solid block of ceramic, zirconia, or resin.
The CAD/CAM milling process allows for exceptional precision, ensuring the crown fits snugly against the prepared tooth margin and preventing bacterial leakage. Once milled, ceramic crowns are sintered in a high-temperature oven to achieve maximum strength. They are then characterized with stains and glazes to match the patient’s natural tooth shade and translucency.
Seating and Final Placement
When the custom-fabricated crown returns from the laboratory, the patient returns for the final placement appointment. The dentist removes the temporary crown and thoroughly cleans the underlying prepared tooth. The permanent crown is then tried in to check its fit, color match, and aesthetic harmony with the adjacent teeth.
The dentist carefully assesses the crown’s marginal fit, which is the seal where the crown meets the tooth structure. They also verify the patient’s bite (occlusion) to ensure proper function and prevent excessive force on the new restoration. Minor adjustments to the crown’s surface may be made using specialized dental burs before the crown is permanently bonded to the tooth.
A specialized, strong dental cement or resin is applied to the interior of the crown and the prepared tooth structure. The crown is seated under firm pressure, and any excess cement is meticulously removed from the margins. Following permanent cementation, the final restoration functions like a natural tooth, and with consistent oral hygiene, a modern dental crown can last for many years.