When we refer to crowns, onlays and inlays, we are referring to whether we need to restore the cusp, or cusps, of the tooth. The cusp of the tooth is the pointed or rounded projection on its chewing surface. Molars typically have four cusps and bicuspids, also called pre-molars, typically have two.
When a tooth has been severely weakened or fractured, we will usually place a crown, which covers all of the cusps. When a tooth has one or more cusps that are weakened or fractured but other cusps that are still strong, we can elect to do an onlay, which can cover one or more cusps while retaining some of the tooth above the gumline.
When the cusps of the tooth are all healthy and structurally sound, we can elect to restore the surrounding structure but leave the cusps intact. This is called an inlay.
We strive to be conservative in the restorations chosen, so as to leave as much natural tooth remaining as possible.