by Dr. H.Kopel, DDS
An onlay is a restoration that does just that and is recommended instead of a crown procedure which requires drilling down all aspects of the tooth. Dentists always preserve the tooth structure when restoring a broken or decayed tooth.
Today we use porcelain instead of gold. Porcelain onlays rely only on bond strength in the same way the space shuttles heat shield tiles are glued or bonded in place.