Inlays and onlays are types of what are commonly known as “fillings.” They can be made of gold or porcelain. The typical filling, however, is made of composite materials.
Sometimes a tooth needs to be restored but there is so much decay or damage, more than a typical filling is needed. This kind of restoration calls for a stronger solution that will reinforce the entire tooth structure and provide positive occlusal (or biting) forces against other teeth.
Inlays are stronger than composite fillings. They are made of a solid substance such as gold or porcelain. The tooth is prepared by removing the decay, leaving a clean space or cavity. The inlay is fitted into the prepared cavity and cemented in place.
Inlays resist occlusal forces and protect against recurring decay because the edges that meet with the surrounding tooth structure are finished and polished to a nearly seamless surface, making it nearly impossible for decay to happen again!
When a tooth is damaged to the point where the one or more cusps of the tooth (for example, the ridges on the surface of our molars that help grind food) are compromised, onlays are the fillings of choice.
Onlays are similar to inlays, except that rather than being inserted into a prepared cavity, they cover the missing cusps, forming new a new surface that covers more area of the restoration, replacing and strengthening the surface and avoiding the need for a crown.