One of the golden rules of dentistry is to preserve tooth structure when restoring a broken or decayed tooth. 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. In the past onlays were made of gold and although they preserved tooth structure compared to doing full crowns they still required significant tooth reduction. This was due to reliance on friction and weak cements to retain the onlays.
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. So we wind up removing less tooth structure and have a stronger tooth as a result. And of course as a side benefit we have a beautiful cosmetic outcome without showing gold when we smile.
Dr. H.Kopel, DDS