Inlays and onlays: the conservative step between a filling and a crown.

Both are custom-made, solid restorations bonded into a prepared back tooth — made in a lab (or milled) rather than built up by hand like a filling. An inlay sits within the cusps (the raised points) of the tooth, filling the hollow in the middle. An onlay extends further, covering one or more cusps as well. They're sometimes called "partial crowns" because they restore a damaged tooth while keeping more of it intact than a full crown.

It's a spectrum of how much tooth is damaged. A small-to-medium cavity → a filling. A cavity too large for a filling to be strong, but with healthy tooth still worth keeping → an inlay or onlay, which is stronger and more precise. A tooth that's heavily broken, cracked or root-treated → a crown, which caps the whole thing but removes the most tooth. Inlays and onlays are the conservative middle: more durable than a big filling, less destructive than a crown.

The dentist removes the decay or old filling and shapes the cavity, then takes an impression (or digital scan). The inlay/onlay is made to fit precisely and bonded into place on a second step, with a temporary in between — or, with an in-house lab, often within the same trip. Because it's made outside the mouth and then bonded in, the fit and the contact with the neighbouring tooth tend to be more accurate than a large hand-placed filling.

Inlays and onlays are usually made from ceramic/porcelain (tooth-coloured and strong, the common choice), composite, or occasionally gold (extremely durable and kind to the opposing tooth, though rarely chosen now for looks). A well-made bonded inlay or onlay is strong and long-lasting — commonly 10–15 years or more — and because it preserves more natural tooth than a crown, it's often the better long-term choice when the tooth still has enough healthy structure.