Onlays are less invasive than crowns as they replace what is missing without sacrificing more tooth substance.
Onlays are made of durable materials such as Zirconia (a porcelain as strong as metal with high aesthetic qualities), E-Max (another high aesthetic porcelain) or metals containing gold, palladium and cobalt. These materials are more durable than ordinary fillings.
The purpose of the onlay is to replace the missing portion of tooth and to protect what is left behind from further fracture. Onlays are commonly placed on molars and premolars and protect the corners of the teeth where chewing forces are highest. If a back tooth has had root canal treatment then it is more brittle and susceptible to fracture. Likewise, a tooth which has had a large filling replaced a few times is also weaker. In both cases placing an onlay instead of a filling will result in a predictable and strong repair.
Previously a crown would have been prescribed but this is more invasive than an onlay as the edges of the crown are extended to the gum line. For an onlay, only 1-2 mm of the biting surface needs to be reduced so the edge of the onlay is above the gum line. As porcelain is aesthetic, the colour blends in well with the tooth and the join is barely visible.
More importantly, more of the tooth is preserved and so it is stronger than if a crown had been placed on it. Advances in bonding materials and porcelains have allowed us to make strong materials which stick reliably to teeth. Not only can we make beautiful restorations but can also preserve teeth at the same time resulting in less drilling and happier patients!