Dental Crown
Background
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.
Design
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.
Raw Materials
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.
Quality Control
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.
The Future
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. Other future advancements are likely to come from new resins, which have improved adhesion in the high moisture environment of the oral cavity.
