When more than half of the tooth's biting surface is damaged, we may use an inlay or onlay. Inlays and onlays can be made of porcelain, gold, or composite resin.

The inlay or onlay pieces are bonded to the damaged area of the tooth. An inlay, which is similar to a filling, is used inside the cusp tips of the tooth. An onlay is a more substantial reconstruction, similar to the inlay but extending out over one or more of the cusps of the tooth.
Porcelain is the material of choice due to its color and strength, which matches the natural color of your teeth.

How are they applied?
Inlays and onlays require two appointments to complete the procedure. During the first visit, the filling being replaced or the damaged or decaying area of the tooth is removed, and the tooth is prepared for the inlay or onlay. To ensure proper fit and bite, an impression of the tooth is made by the dentist, and sent to a lab for fabrication. The dentist will then apply a temporary sealant on the tooth and schedule the next appointment.

At the second appointment, the temporary sealant is removed. The inlay or onlay is then bonded to the tooth with a strong resin and polished to a smooth finish.

Considerations
Traditional fillings can reduce the strength of a natural tooth by up to 50 percent. Inlays and onlays are bonded directly to the tooth using special high-strength resins can actually increase the strength of a tooth by up to 75 percent. As a result, they can potentially last from 10 to 30 years. In some cases, where the damage to the tooth is not extensive enough to merit an entire crown, onlays can provide a very good alternative.