Crowns are a cosmetic restoration used to strengthen a tooth or improve its shape. Crowns are most often used for teeth that are broken, worn, or partially destroyed by tooth decay.
Crowns are "cemented" onto an existing tooth and fully cover the portion of your tooth above the gum line. In effect, the crown becomes your tooth's new outer surface. Crowns can be made of porcelain, metal, or both. Porcelain crowns are most often preferred because they mimic the translucency of natural teeth and are very strong.
Crowns or onlays (partial crowns) are needed when there is insufficient tooth strength remaining to hold a filling. Unlike fillings which apply the restorative material directly into your mouth, a crown is fabricated away from your mouth.