Crown

a tooth crown is a single tooth prosthesis made by a lab technician and set in mouth by a dentist.
The goal is to replace shape, aesthetics, and function of the affected tooth.
treatment decision
It is usually the damage to a tooth that dictates the need for a single crown, when alternative treatments are usually less effective. Always assuming that the tooth can be saved.
When considering the replacement of a missing or severely destroyed tooth, then there are more alternatives (removable partial denture, bridge or implant).
Sometimes a crown adds extra support and retention to a tooth that is going to be used as abutment for a removable partial denture, normally by means of external crown attachments. This are sort of single axis insertion connectors that provide more stability to the removable prosthesis. This support crowns are known as surveyed crowns.
A crown placement is many times crucial on non-vital teeth, that is, teeth whose root canal has been treated previously. It has been proved that crowns on endodontic treated teeth suffer much fewer fractures as those restored with intracoronal restorations, such as fillings.
Why?
The vitality of a tooth is remarkable in its ability to provide the tooth with the strength and durability it needs to function in mastication. First, inner blood support keeps the living tooth structure is surprisingly resilient and can sustain considerable abuse without fracturing. Consequently, after root canal therapy is performed, a tooth becomes extremely brittle and is significantly weaker than its vital neighbors.
The average person can exert 150–200 lbs (70-90 kg). of muscular force on his or her posterior teeth, which is approximately nine times the amount of force that can be exerted in the anterior. If the effective posterior contact area on a restoration is 0.1 mm², over 1 million PSI of stress is placed on the restoration. Therefore, posterior teeth (i.e. molars and premolars) should in almost all situations be crowned after undergoing root canal therapy to provide for proper protection against fracture (mandibular premolars, being very similar in crown morphology to canines, may in some cases be protected with intracoronal restorations). Should an endodontically treated tooth not be properly protected, there is a chance of it succumbing to breakage from normal functional forces. This fracture may well be difficult to treat, such as a “vertical root fracture” . Anterior teeth (i.e. incisors and canines), which are exposed to significantly lower functional forces, may effectively be treated with intracoronal restorations following root canal therapy if there is enough tooth structure remaining after the procedure.
Fractures of endodontically treated teeth increase considerably in the posterior dentition when cuspal protection is not provided by a crown.[4]