A crown, or full-coverage restoration (sometimes incorrectly called a cap) is an prosthetic tooth designed by a dentist and usually created by a lab technician (or more recently, a CAD-CAM machine). Crowns can be either metal, metal bonded to procelain (PFM) or all porcelain/ceramic. Metals can either be noble metals or base metals, noble metals having higher content of gold, platinum or palladium. Noble metals are typically more accurate when they are cast by the lab technician and offer a better bond with porcelain. Nobel alloys in PFMs have a much lower incidence of allergic reactions as they do not contain nickel. All porcelain restorations can be made from feldspathic (traditional) porcelain, lithium disilicates, aluminous porcelains, or zirconia. All ceramic restorations typically are not as strong, as those with metal substructures and can cause increased wear of the opposing teeth.  Why use a crown?
A very badly damaged tooth may be difficult or impossible to restore to correct form and function using a direct dental restorative material such as amalgam or dental composite, since these materials are placed in the damaged tooth and carved to shape by the dentist in the mouth, and thus have limited strength. In such cases, the tooth may require an indirect restoration made outside the mouth by a dental technician, who can work to produce a customised tooth shape in the chosen material that will fit the patient's damaged tooth exactly, somewhat like a thimble fits over a finger to protect it. Crowns can also be used to support bridgework which replaces missing teeth adjacent to the crowned teeth and may be required in cases of very severe staining or where the visible form of teeth need to be realigned without the use of orthodontics. Makeover shows such as Extreme Makeover use crowns extensively, as the timeframe of the makeover is not long enough to allow up to 18 months for orthodontic treatment. Finally, crowns can also provide a suitable form for a removable partial coverage denture to link with for added denture retention.  Why not use a crown?
Preparation of a tooth for a crown involves removal of a significant amount of tooth structure, which is an irreversible procedure. Thus, if not necessary, crown preparation is contraindicated. Additionally, crowns do not last forever; the average lifespan of a crown is around 10 years, though this depends on the skill of the dentist and technician, the material used, and the oral hygiene of the patient. Placing a full coverage crown results in no tooth surfaces being present in the mouth for pulp testing, which is a vital part of endodontic diagnosis. Finally, crowns are expensive (ranging around US$750) and require several visits to the dentist.  Process of applying a crown
Note: many jurisdictions allow parts of this process to be done by a qualified dental assistant. All decay is removed from the tooth either by a normal dental drill or via newer methods such as a dental laser. If the tooth is heavily damaged, the dentist may "build up" the height of the tooth with a dental filling material, allowing the crown to attach to a surface that extends further into the crown, allowing the crown to better withstand the force of chewing and allowing a more accurate fit of the crown. The dentist prepares the tooth for the crown using a dental drill, removing some tooth and core to allow sufficient thickness of crown material to cover the damaged tooth and restore its strength. Different materials need different thickness. Gold generally requires the least amount of thickness for strength, porcelain the most. The dentist will then prepare the margins of the crown preparation, i.e. the join between the natural tooth and the crown. There are several designs in the literature, the common types being a chamfer (typically used for gold and other metal crowns), and a shoulder (used for porcelain where thickness is important for good strength and aesthetics). This margin must be on healthy tooth structure the entire way around; the crown may never end on the core. The dentist takes an impression of the teeth and records how the teeth meet. Typically a polyvinylsiloxane putty and a lightbody wash system are used to record the shape of the prepared tooth. Dentist chooses the shade and material of the crown, and writes a prescription for the dental lab technician to follow. If the shade is difficult to match either due to unique tooth coloration, or because of an area of the mouth where people may be able to easily notice defects (such as a single central, the middle front tooth) the dentist may have the patient go see the lab to get a custom shade match, after the temporary has been placed. A temporary crown is fabricated by the dentist and placed over the prepared tooth. Some systems exist to manufacture these temporaries partially ahead of time, but a custom make temporary generally has a better fit. Because a temporary is cemented with a temporary cement, they can come loose, even if treated with care. The eugenol used in many over the counter temporary cements or dental pain relievers can interfere with the cementation of the final crown (especially if the crown is to be bonded in, a must for most all porcelain systems). Call your dentist before using them. It is important that the temporary be placed back on the tooth as soon as possible. Any movement of the teeth can cause the final crown to have an improper fit. Teeth can move in a very short period of time, so it's important that the temprorary be on the tooth. The models of the teeth are sent to the dental laboratory where the technician makes the crown according to the prescription. The technician will not only try to make a shape that fits the shape of other teeth is the mouth, they will also pay attention to the fit of the teeth during chewing and biting. Any high spots, even if they are a fraction of a millimeter too high, can cause the tooth to become extremely sore. The finished crown is returned to the dentist for fitting. The temporary crown is removed and the fit of the final crown is checked, usually with an explorer. Crowns require extremely precise fits (under 100 micrometers or so to be acceptable, some materials such as gold can produce fits within 10 or 20 micrometers). At this time the crown's cosmetics can also be checked. The final crown is cemented over the damaged tooth, using a luting cement, restoring the tooth to a good strength and function.