A crown is a tooth-shaped “cap” that’s placed over a weak or damaged tooth to improve its shape, size, strength, or appearance. Most crowns last five to 15 years and can be made of metal, porcelain fused to metal, resin, or ceramic. Before a crown is seated, the existing tooth is filed down; then the crown is cemented over it, fully encasing the tooth. Onlays and three-quarter crowns cover the underlying tooth to a lesser extent.