Tooth compensation via dental crowns
A dental crown is a cap-like restoration used to cover a damaged tooth. Dental crowns can give support to misshapen or badly broken teeth and permanently replace missing teeth to complete a smile or improve a bite pattern.
Dental 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.
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.
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.
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 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.
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.
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.
Special dental adhesives, or dental cements, are used to hold the crown in place. Dental adhesives 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.
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.
The process of installation of dental crown begin after the crown has been successfully completed and created into prepared area. 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.
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.
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.
Future advancements for dental crowns are likely to come from new resins, which have improved adhesion in the high moisture environment of the oral cavity.
