An onlay is a partial coverage crown that restores and protects the chewing surface (the top) of the tooth but does not require the tooth to be entirely reduced and covered.  An onlay can be made of porcelain, tooth colored resin, or gold.  Whenever a tooth requires a larger restoration than a filling in order to avoid fracturing a cusp or wall, but we have enough healthy tooth structure to save part of the tooth, we like to place an onlay to avoid excessive destruction of healthy enamel.

Often, these restorations are healthier for the gingival tissues than a full coverage crown.  They are a more conservative restoration, often requiring less drilling.  When we place a porcelain (tooth colored) onlay, it usually blends seamlessly into the tooth structure, resulting in a very esthetic and invisible restoration.  On back molars for patients who clench and grind, sometimes we use gold to reduce the chance of a future fracture.