A dental crown is a cap-like restoration used to cover a damaged tooth. Crowns can give support to misshapen or badly broken teeth and permanently replace missing teeth to complete a smile or improve a bite pattern. They may be molded from metal, ceramic, plastics, or combinations of all three. They are cemented in place and coated to make them more natural looking. Historically, a variety of materials have been used as tooth replacements. The ancient Egyptians used animal teeth and pieces of bone as primitive replacement materials. More recently, artificial teeth have been fabricated from substances such as ivory, porcelain, and even platinum. With modern technology, high quality tooth replacements can be made from synthetic plastic resins, ceramic composites, and lightweight metal alloys.
There are several key factors to consider in the design of dental crowns. First, appropriate raw materials with which to make the crown must be identified. These materials must be suitable for use in the oral cavity, which means they must be acceptable for long term contact with oral tissues and fluids. Crown components must have a good safety profile and must be non-allergenic and non-carcinogenic. The American Dental Association/ANSI specification #41 (Biological Evaluation of Dental Materials) lists materials which have been deemed safe for use. In addition to safety considerations, these materials must be able to withstand the conditions of high moisture and mechanical pressure, which are found in the mouth. They must be resistant to shrinkage and cracking, particularly in the presence of water. Metal is preferred for strength but acrylic resins and porcelain have a more natural appearance. Therefore the selection of crown material is, in part, dependent on the location of the tooth being covered. Acrylic and porcelain are preferred for front teeth, which have higher visibility. Gold and metal amalgams are most often used for back teeth where strength and durability are required for chewing but appearance is less critical.
The second factor to consider when designing a crown is the shape of the patient's mouth. Dental restorations must be designed to mimic the bite properties of the original tooth surface so the wearer does not feel discomfort. Since every individual's mouth is different each crown must be custom designed to fit perfectly. Successful crown design involves preparation of an accurate mold of the oral cavity.
There are four main types of materials used in crown construction: The plasters used to create the mold, the materials from which the crown itself is made (e.g., metal, ceramic, plastic), the adhesives used to cement the crown in place, and the coatings used to cover the crown and make it more aesthetically appealing.
Plaster molds are made from a mixture of water and gypsum powder. Used for dental applications since the 1700s, gypsum is finely divided calcium sulfate dihydate. Different types of plasters are used depending on application: impression plaster is used to record the shape of the teeth, model plaster is used to make durable models of the oral cavity, and investment plaster is used to make molds for shaping metal, ceramics and plastics. Waxes are also sometimes used in this regard.
Crown construction materials
Metals are frequently used in crown construction because they have good hardness, strength, stiffness, durability, corrosion resistance, and bio-compatibility. Metals formulated as mixtures of mercury have been historically used. In fact, one source notes that metal amalgam was used as a dental restorative as early as 1528. Common alloys used in crowns are based on mixtures of mercury with silver, chromium, titanium, and gold. These mixtures form a blend than can be easily shaped and molded, but which hardens in a few minutes.
Ceramics are well suited for use in crowns because they have good tissue compatibility, strength, durability and inertness. They can also be made to mimic the appearance of real teeth fairly closely. However, the tensile strength of ceramic is low enough to make it susceptible to stress cracking, especially in the presence of water. For this reason, ceramic is most often used as a coating for metal-structured crowns. The two primary types of ceramics used in crowns are made from potassium feldspar and glass-ceramic.
The first resin used in denture materials was vulcanized rubber in 1839. Since then, a number of other resins have been developed which are more suitable for dental applications. Today, acrylic polymer resins are commonly used in dentures and crowns. Specifically, polymethyl methacrylate is most often used. This type of resin is made by mixing together chemical entities known as monomers with activating chemicals which cause the monomers to react and link together to form long chains called polymers. Some of these resins harden at room temperature as this reaction progresses. Others require heat or ultraviolet light to catalyze the change.
Special dental adhesives, or dental cements, are used to hold the crown in place. These can be classified as either aqueous or nonaqueous. The aqueous type include zinc phosphates, polycarboxylate cements, glassionomer cements, and calcium phosphate cements. The nonaqueous type include zinc oxide-eugenol, calcium chelates, and acrylic resins such as polymethyl methyacrylate.
Coatings are used to make the crown appear more natural. Porcelain is used in this regard, but it is difficult to work with and hard to match to the tooth's natural color. Resins similar to the ones used in tooth construction are also used to create tooth-colored veneers on crowns. These resins have an advantage over other veneers in that they are inexpensive, easy to fabricate, and can be matched to the color of tooth structure. However, acrylic coatings may not adhere to the crown's surface as well as porcelain or other materials. Therefore, the prosethedontist may design the crown's surface with mechanical undercuts to give the coating a better grip. Resin coatings also have relatively low mechanical strength and color stability and poor abrasion and stain resistance as compared to porcelain veneers.
Creating the mold
- 1 Before beginning, the dentist may need to prepare the area where the crown is to be installed. This may require the removal of 2-3 millimeters of tooth structure from the four sides and the biting edge. Then, an impression of the tooth is taken to record its shape. This step uses impression plaster which is the softest and fastest setting type of dental plaster. The impression plaster is mixed with a small amount of water until it is fluid. This slurry is placed in a tray that is fitted over the teeth. The tray is held still in place until the plaster hardens. When the tray is removed from the mouth, it retains a three dimensional impression of the tooth that is to be covered. This impression is a negative, or reverse, image of the tooth.
- 2 The next step is to prepare another type of plaster, known as model plaster. This type of plaster is harder than the impression plaster. Once again the plaster is mixed with the appropriate quantity of water. Then the slurry is poured into the impression mold. In this way a positive model of the tooth can be made. This positive model made from the negative impression mold is called a cast. The cast is used by the dentist for study purposes.
- 3 The impression is also used to make a mold, called an investment, which is capable of withstanding high temperatures. This is an important consideration because some metals and ceramics require temperatures higher than 2,372° F (1300° C) for molding. These investments are made from calcium phosphate mixed with silica and other modifying agents.
- 4 Fabrication of the crown is done by filling the investment with the appropriate material. In the case of metals, this is done at a high temperature so the metal is molten. For ceramics and plastics, the mixture is initially fluid but may require the addition of heat to cause the materials to cure and harden. A vertical vise may be used to help pack the casting investment tightly. The process also requires the mold first be treated with a release agent to ensure the crown can be easily removed after it has hardened. Some acrylic resins must be heated for up to eight hours to make sure they are fully cured. After the processing is done and the investment has cooled, the mold is broken apart and the crown is removed.
- 5 After the crown has been successfully completed, it is ready for installation. The prosethedontist applies cement to the inside of the crown surface and then fits it into place over the tooth. Because of the number of processing steps there may be a slight discrepancy in the fit and the crown may require minor grinding and smoothing of its surface to ensure it fits correctly.
- 6 The crown may require a finishing coat to seal it and improve its natural appearance. Such coatings are typically acrylic polymers. The polymer can be painted on as a thin film, which hardens to a durable finish. Some polymers require a dose of ultraviolet light to properly cure.
Good quality control is critical to ensure the crown fits and looks natural in the patient's mouth. Every crown is unique because every person's mouth is different and every crown is custom molded to fit. To ensure appropriate fit and feel, fine details can be added to the crown by hand after the molding process is completed. Even with minor adjustments, quality problems and failures in crowns are likely to occur. Key quality control issues include failures due to biological factors (such as caries, recurrent decay, sensitivity problems, and periodontal diseases), mechanical reasons (including fracture of the crown surface, and poor cementation), aesthetic problems (discoloration of the surface), and damage due to traumatic accidents. In such situations it may become necessary to reposition or remove a crown to allow for either replacement or other dental operations. There are special crown and bridge removal systems that have been developed for easy removal of these prosthedontics. This is accomplished by placing a precision vertical channel in the surface of the crown, then threading the surface until the cement layer has been broken. The crown can then easily be lifted from the underlying tooth without force.
Denture manufacture generates little waste other than minimal amount resulting from the gypsum and plaster materials used in mold making and the excess acrylic resins used in crafting the teeth and mounts. These materials are not generally in large quantities since crowns are crafted by hand and are not mass produced on a production line.
Dental technology is constantly advancing and these improvements are already finding application in dental crown manufacturing. State of the art crowns can be made with an industrially produced core made of densesintered ceramic, and an outer layer of porcelain is added by hand. This futuristic crown material is made by an advanced Computer Aided Design (CAD) process, known as Procera process, which was introduced in the mid-1990s in Switzerland. This process results in crowns with improved strength and optimal fit. Unlike other crown materials, crowns made by the Procera process can be used anywhere in the mouth due to the strength of its core material and its more natural appearance. Another advance in crown technology involves pre-made and pre-sized stainless steel crowns, which are designed as generic tooth replacements. Usage of this new type of crown is very simple: first the tooth surface is prepared then the selected crown is cemented in place with a standard stainless steel crown adhesive. The crown can be crimped or cut to fit and the epoxy finish will not chip or peel. While this new technology offers increased simplicity, it does not give the same appearance as a custom made crown. Other future advancements are likely to come from new resins, which have improved adhesion in the high moisture environment of the oral cavity.
Where to Learn More
Geering, Alfred H., Martin Kundert, and Charles Kelsey, ed. Complete Denture & Overdenture Prosthetics. Thieme Medical Publishers, Inc., 1993.
Goldstein, Ronald. Change Your Smile. Chicago: Quintessence Publishing Co., 1997.
Ellison, Robin E. "Developing the Wax Pattern for Removable Partial Dentures." VHS tape distributed by the National Audiovisual Center, 1980.