They are used to reinforce and rebuild severely damaged/weakened teeth or improve the appearance of heavily filled or broken down teeth. They are also often used on top of dental implants.
Crowns can be made of a variety of different materials and new materials are continually being introduced. However a tooth has to be drilled down to receive the crown hence there are risks associated with this.
Onlays are used for back teeth and have a similar function to crowns but they don’t cover the whole tooth. This reduces the risks to the health of the tooth as there is less drilling. Often the margin of onlays finishes high up the tooth so a join line can be visible hence they are more used in less visible situations.
They are advised following root canal treatment, when a tooth is very heavily filled or when we are rebuilding the bite of several teeth.