The short answer
An onlay is a restoration made outside the mouth that covers one or more of a tooth's chewing cusps and is cemented in place. It sits between a filling, which is shaped directly inside the tooth, and a crown, which surrounds the whole tooth. Dentists often suggest one when a cavity or old filling is too large for a filling but the tooth still has enough sound walls to avoid a crown.
General information, not a diagnosis. Your dentist will examine your mouth before recommending treatment.
Where an onlay sits among the options
If your dentist mentioned an onlay, you probably heard the word for the first time in the chair. The idea is simple. A tooth that has lost part of its chewing surface needs that part rebuilt, and there are three ways to do it depending on how much is gone.
The ADA's glossary separates restorations into two groups. A direct restoration, such as a filling, is made inside the mouth: the dentist shapes the material into the cleaned cavity and hardens it there. An indirect restoration is made outside the mouth, from an impression or a digital scan, and then cemented to the tooth. Inlays, onlays and crowns are all indirect.
Among the indirect options, the difference is coverage. The ADA defines an inlay as a restoration that fits within the tooth, an onlay as one that covers one or more cusp tips and the adjoining chewing surface without covering the entire outside of the tooth, and a crown as a replacement that surrounds the remaining tooth structure. Patients sometimes hear onlays called partial crowns, which is a fair description.
When a dentist suggests one
Cusps are the raised points on a back tooth that do the chewing. When decay or an old filling hollows out the tooth under a cusp, that cusp is left standing on thin walls. Packing more filling material into the space does not strengthen the walls; it asks them to carry chewing force on their own.
An onlay solves that by replacing the weak cusp with a solid piece that spans the gap. The usual reasons a dentist reaches for one:
- a cavity or worn filling on a back tooth that is too large for a direct filling; the ADA lists large restorations as a contraindication for direct composite
- a cusp that has broken off around an old filling (the AAE notes a fractured cusp is repaired with a new filling or a crown; an onlay is the middle option when enough wall remains)
- a tooth with enough healthy walls that a full crown would remove sound enamel for no benefit
- an older filling that has failed, where the tooth has lost more structure with each repair (see when old fillings need replacing)
How an onlay is made
Both routes start the same way. The dentist numbs the tooth with local anesthetic, removes the decay or the old filling, and shapes what remains so the onlay will seat cleanly. What happens next depends on where the onlay is made.
With a lab-made onlay, the dentist takes an impression or a digital scan, sends it to a dental laboratory, and places a temporary. You return once the finished piece is back to have it tried in, adjusted and cemented.
With an in-office onlay, a digital scan goes to design software, and a milling unit carves the piece from a ceramic block while you wait, so it can be bonded the same visit. The ADA has described chairside CAD/CAM systems that pair an intraoral scanner with a milling machine and furnace to make a ceramic inlay, onlay or crown in one appointment. The practice's CEREC page lists inlays and onlays among the restorations it can make in a single visit; whether your tooth suits that route is something the dentist decides after looking at it. The same-day vs. lab crown guide compares the two routes in more detail.
| Restoration | What it covers | Made where | Usually chosen when |
|---|---|---|---|
| Filling | The cleaned cavity only | Inside the mouth, one visit | Small to mid-size decay with sound walls |
| Inlay | Inside the cusps, no cusp tips | Outside the mouth (lab or in-office) | A cavity too large for a filling, cusps intact |
| Onlay | One or more cusp tips plus the chewing surface | Outside the mouth (lab or in-office) | A cusp is lost or undermined, walls still sound |
| Crown | The whole tooth above the gum | Outside the mouth (lab or in-office) | Thin walls, a crack, or after a root canal |
What onlays are made of
The ADA's review of materials for indirect restorations lists all-ceramic materials as suited to inlays, onlays, crowns and veneers, with resin-matrix composites as an alternative for inlays, onlays and single crowns. Metal alloys, gold in particular, have the longest track record for durability, and the ADA's patient site lists ceramic, zirconia or gold inlays and onlays among the filling options.
Ceramic is tooth-colored and is what in-office milling uses. The same ADA review notes that ceramics can chip or fracture and that back teeth, especially molars, see a higher rate of restoration fracture than front teeth. That is one reason the dentist weighs the tooth's position, your bite and whether you grind before choosing a material. If you clench or grind, ask about a night guard to protect the finished work.
What to expect afterward
An onlay is cared for like a natural tooth: brush twice a day with fluoride toothpaste, clean between the teeth daily, and keep up with checkups so the dentist can check the edges where the onlay meets the tooth. Avoid chewing ice, unpopped popcorn kernels or pens; the AAE lists these habits among the things to avoid to prevent cracked teeth.
The practice's crowns page notes that mild temperature sensitivity for a few days after a restoration is common and typically fades. Sensitivity that lingers, a bite that feels high, or an edge your tongue keeps finding are reasons to call rather than wait. The restorations page covers the general process, and a routine checkup is where most worn or failing restorations are caught before they hurt.
Onlay, filling or crown: a quick way to think about it
If you are weighing all three, the question is how much sound tooth will be left after the problem is cleaned out. Small to mid-size: a composite filling. One or more cusps lost but good walls: often an onlay. Thin walls, a crack running toward the root, or a tooth that has had a root canal: usually a crown.
The crown vs. filling vs. onlay guide lays this out as a decision table. Either way, the recommendation should come with a look at the X-ray and a plain explanation of what is left of the tooth, and you can ask for exactly that.