Dental onlays and inlays are conservative, long lasting restorations that are fabricated outside of your mouth. This type of restoration is preferred over direct fillings (fillings fabricated in your mouth, chair side) when there is a lack of healthy tooth structure to support a filling.
Onlays and inlays are much more conservative than crowns. When a tooth is prepared for a crown, all of the protective outer surface (enamel) is removed. When a tooth is prepared for an onlay, only enough tooth is removed to make room for ceramic. This conservative process preserves enamel and is kinder to the tooth.