Tooth reconstruction use tooth crowns
A dental crown is a cap-like restoration used to cover a damaged tooth. 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.
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 Tooth Crowns. First, appropriate raw materials with which to make the crown must be identified.
The materials of Tooth Crowns 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. Materials also 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.
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.
Depending on application different types of plasters are used: 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.
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.
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.
Polymethyl methacrylate is most often used for dental applications. 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.
The prosethedontist may design the crown''s surface with mechanical undercuts to give the coating a better grip. Resin coatings have relatively low mechanical strength and color stability and poor abrasion and stain resistance as compared to porcelain.
Before beginning dental crown process, 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.
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.
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.
Future advancements for Tooth Crowns are likely to come from new resins, which have improved adhesion in the high moisture environment of the oral cavity.