Crowns are used when natural tooth strength is compromised. Crowns can be used on natural teeth, implants or as a functional unit on a bridge. They differ in a material that they are made from, and in design. In the past the most popular crown was porcelain fused to metal. Nowadays this type of crown is replaced by Lithium Disilicate or Zirconia by surpassing esthetics and strength of the old type. They are made by computerized scanners and milling units versus human hand. The accuracy of the porcelain restorations also improved and the fit is much better now.
If a tooth have large cavity but crown seems overly aggressive, a porcelain veneer, onlay, or inlay is recommended. Those replacements are made from porcelain outside of patient mouth in a lab. They are hybrid restorations that poses the properties of a crown in an area where composite filling would not be sufficient like in the place below the gum level or replacing a bigger structure of a tooth like a cusp. They fit on a tooth like a piece of a puzzle and they are permanently cemented with its wall.
Almost all root canal teeth need a crown. The tooth after root canal is structurally compromised and becoming brittle with time. The crown then is a device securing and protecting the root canal and make the whole tooth stronger. Number one reason why teeth with root canal fail is the lack of a protecting crown.
Implants are becoming more and more popular way of restoring missing teeth. They also need a crown as a final stage and coverage. The crown is cemented or screw into an abutment (intermediate) that is already screw into an implant body.
There are cases in which the tooth is healthy but size or shape deficient. Also a bad location can destined the tooth to need a full coverage like a crown. In those instances to improve patient’s bite or make teeth look well positioned, symmetrical and proportional a crown or a series of crowns is made to make up for a deficient esthetics.
There are also instances in which the teeth look normal but they are deficient in quality and thickness of enamel thus making dentin (the inside of a tooth) vulnerable to bacteria. A porcelain crown would be an example of such a solution.
Finally, crowns might replace missing teeth when they are fused together. It is the middle crown that replaces the missing tooth in a bridge. The shortest bridge is a 3-unit device, but they can be as long as 16 units in an arch. Those bridges are usually made from the hardest material like Zirconia to become resistant to fracture.