Understanding tooth crowns process
Tooth Crowns 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.
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.
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.
The important 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.
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.
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.
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.
If existed crown failure due to biological factors 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 after can then easily be lifted from the underlying tooth without force.
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.
