What is an inlay?
Dentists use inlays when the material of the filling will be primarily on the inside of the tooth with only a small portion making up the tooth wall. Inlays fill a hollow that has developed as a result of injury or decay and strengthen the integrity of the tooth. Such hollows can create openings between the peaks of the tooth or in the side of the tooth. The placement of the opening on the tooth is inconsequential; a filling is considered an inlay if it fills an interior hollow or pocket.
What is an onlay?
Onlay is used when the exterior of a tooth has been damaged or decayed to such a degree that a complete or partial filling is necessary to restore the tooth’s shape. This type of crown is shaped from an impression of the tooth, then glued into place. Onlays fit outside and around the tooth. A common misconception is that onlays are used on more seriously damaged teeth than inlays. But a dentist decides which technique to use based on where the tooth is decaying or damaged. Exterior problems results in an onlay, while interior repairs call for an inlay.