Crowns constructed with modern, natural-looking porcelain can restore function, appearance, and the proportion of your smiling face.
If more than half of your tooth has been restored with a filling, a crown is necessary. Without adequate support, the tooth may break away around the filling. As a rule, when fillings get larger, teeth become weaker. See more about porcelain crowns here including a movie: http://www.stonesmiles.com/porcelain-crowns/