These are custom made in lab restorations. They are made from gold, silver, porcelain or composite and glued to teeth with combatable cement. They can be good alternatives to fillings where retention is poor to hold the fillings or the cusps are lost such as in broken teeth.
They are also good alternatives to crowns because they need minimum or no tooth preparation and thus they are conservatives and their main function is to repair and replace the lost part of the tooth. Onlays are strong and withstand occlusal forces and suitable for large repairs.