Inlays & Onlays

An inlay or onlay repairs a tooth that has more damage than a filling should handle, but not enough to justify the full coverage of a crown — a middle option that replaces exactly the compromised part of the tooth.

Where they fit between fillings and crowns

A standard filling is placed directly into a cavity in one step, shaped and hardened right in the mouth. That works well when there’s enough healthy tooth structure left to support a filling reliably. Once decay or an old, failed filling has taken out a larger portion of the tooth — particularly a wide area between the cusps, or a cusp itself — a direct filling of that size becomes more prone to fracturing under normal chewing force. A crown solves this by covering the entire tooth, but that also means reducing and covering healthy structure that doesn’t strictly need it. An inlay or onlay is fabricated to fit the specific area that needs replacing, restoring strength without removing more of the natural tooth than necessary.

What separates the two

An inlay sits within the tooth’s grooves, filling the space between the cusps without extending over them — essentially a precision-fit filling made outside the mouth rather than shaped directly inside it. An onlay covers that same central area but extends up and over one or more cusps, used when the damage or the original cavity reaches further across the tooth’s chewing surface. The choice between the two, and whether either is appropriate versus a crown, comes down to exactly how much of the tooth structure remains once decay or a failing filling is removed.

How they’re made and placed

Both are fabricated outside the mouth — from a dental lab or an in-office milling unit — typically from ceramic (matching tooth color) or gold, then bonded or cemented into the prepared tooth. Because they’re made from a solid block rather than packed in layer by layer, they can achieve a denser, more precisely shaped fit than a large direct filling, which is one reason they’re often favored for molars carrying heavy bite force. The process otherwise resembles a crown: a scan or impression, either a same-day mill or a lab turnaround with a temporary in between, and a cementation visit to finish.

When a dentist recommends one over the other options

The decision usually comes down to how much sound tooth structure survives after decay or a failed filling is removed. Enough structure for a direct filling to hold reliably means a filling is the simplest and most conservative option. A larger defect that a direct filling would struggle to support, but that still leaves the tooth’s outer walls intact, points toward an inlay or onlay. Once too little of the original tooth remains to support even that, a full crown becomes the more reliable choice.