An “onlay” is a form of dental restoration used to repair a decayed, chipped, cracked, or otherwise damaged tooth. An onlay corrects damage extending to a “cusp” (the raised points on the biting surface), in contrast to an inlay, which is used to repair only that portion of the tooth between these cusps.
Generally speaking, there are two different types of dental restorations: “direct” and “indirect.” For example, a “filling” is a type of direct restoration, because the material used to repair the tooth undergoes its hardening process while in “direct” contact with the tooth, and *inside* the mouth.
Unlike a filling, where a malleable material is placed into the tooth, and allowed to harden, an onlay is an *already* hardened material (like a crown) that is joined to the tooth by bonding or cement. Because the material used to repair the tooth is created *outside the mouth*, either in a lab by a dental technician, or via a computer-controlled milling machine, the process is considered an “indirect” restoration.
In all, there are five surfaces of the tooth eligible for restoration: the distal, occlusal, buccal, mesial, and lingual/palatal surfaces. With this dental procedure code, an onlay is is made from either porcelain or ceramic, and placed on three of those surfaces. An onlay constructed of either of these materials yields positive cosmetic results, with porcelain creating a more natural look than the “heavier” look of full ceramic. For this reason, an all porcelain onlay is typically used to repair decay on teeth within the “smile-zone,” whereas a fully ceramic onlay is most often used in the rear of the mouth.
To prepare for a three-surface onlay, a dentist will first remove any decayed or weakened areas of the tooth. Then, depending on the process used by your dentist, either a physical impression of the tooth will be made, or 3D imaging will be used to render a digital impression. Next, you may be fitted with a temporary onlay until the custom version is completed, or if your onlay can be fabricated on-site your dentist will proceed with the installation.
Once the onlay has been prepared and is ready for installation, it will be laid into the excavated area of the tooth and bonded or cemented (luted) in place.