In dentistry, an inlay is an indirect restoration (filling) consisting of a solid substance (as gold or porcelain) fitted to a cavity in a tooth and cemented into place. An onlay is the same as an inlay, except that it extends to replace a cusp. Crowns are onlays which completely cover all surfaces of a tooth.
Sometimes, a tooth is treatment planned to be restored with an intracoronal restoration, but the decay or fracture is so extensive that a direct restoration, such as amalgam or composite, would compromise the structural integrity of the restored tooth by possibly undermining the remaining tooth structure or providing substandard opposition to occlusal (i.e. biting) forces. In such situations, an indirect gold or porcelain inlay restoration may be indicated. The following documents the indirect (out of the mouth) fabrication of a gold inlay.
When an inlay is used, the tooth-to-restoration margin may be finished and polished to such a super-fine line of contact that recurrent decay will be all but impossible. It is for this reason that some dentists recommend inlays as the restoration of choice for pretty much any and all filling situations. While these restorations might be ten times the price of direct restorations, the superiority of an inlay as a restoration in terms of resistance to occlusal forces, protection against recurrent decay, precision of fabrication, marginal integrity, proper contouring for gingival (tissue) health, ease of cleansing and many other aspects of restorative quality offers an excellent alternative to the direct restoration. For this reason, some patients request inlay restorations so they can benefit from its wide range of advantages even when an amalgam or composite will suffice. The only true disadvantage of an inlay is the higher cost.
Additionally, when decay or fracture incorporate areas of a tooth that make amalgam or composite restorations essentially inadequate, such as cuspal fracture or remaining tooth structure that undermines perimeter walls of a tooth, an "onlay" might be indicated. Similar to an inlay, an onlay is an indirect restoration which incorporates a cusp or cusps by covering or onlaying the missing cusps.
All of the benefits of an inlay are present in the onlay restoration. The onlay allows for conservation of tooth structure when the only other alternative is to totally eliminate cusps and perimeter walls for restoration with a crown. Just as inlays, onlays are fabricated outside of the mouth and are typically made out of gold or porcelain.
Gold restorations have been around for many years and have an excellent track record. In recent years, newer types of porcelains have been developed that seem to rival the longevity of the gold. Either way, if the onlay or inlay is made in a dental laboratory, a temporary is fabricated while the restoration is custom made for the patient. A return visit is then required to deliver the final prosthesis. Inlays and onlays may also be fabricated out of porcelain and delivered the same day utilizing techniques and technologies relating to CAD/CAM Dentistry.