Crowns are used to restore teeth that are either broken or decayed so badly that fillings would not be strong enough to last a reasonable length of time. They are also used to change the shape and/or color of teeth for cosmetic reasons. Crowns can either be all porcelain, porcelain fused to a metal substructure or gold. Gold crowns are the strongest and longest lasting, but are generally only used in back teeth where they will not be noticed. Porcelain crowns are nearly always used in teeth that are visible when smiling. Crowns cover most or all of a tooth after decayed or weakened structure is removed.