If you have been told your tooth needs an inlay or an onlay, you may be wondering what either actually is, and why a filling will not do. The difference between an inlay and an onlay is simpler than the names suggest: it is entirely about how much of the tooth each one covers.
On this page
- The difference, in plain terms
- Why not just a filling?
- Why not go straight to a crown?
- What they are made of
- What treatment involves
- How long do they last?
- Is one right for my tooth?
- In summary
The difference, in plain terms
Picture a back tooth. It has a dip in the middle where you chew, and raised corners around the edge. Those corners are called cusps, and they do the heavy work.
An inlay sits within that central area, between the cusps. It restores damage contained inside the biting surface without covering the corners.
An onlay extends over one or more of those cusps, covering and protecting them as well as the middle. That is why onlays are sometimes called partial crowns.
Same material, same laboratory craftsmanship, same appointment process. The only real difference is territory, and the tooth decides which it needs.
Why not just a filling?
A filling is shaped soft in the mouth and set in place. An inlay or onlay is made outside your mouth, to precise measurements, then bonded in as one solid, fully hardened piece.
That manufacturing difference matters. Beyond a certain size, fillings flex under chewing forces, and flexing leads to leaking margins, decay underneath, and eventual replacement, with each replacement removing a little more natural tooth. A laboratory-made piece is engineered as a single unit, seals tightly, and reinforces the tooth rather than simply occupying space in it.
For a genuinely small cavity, a white filling remains the right and cheaper answer, and you should be told when that is the case.
Why not go straight to a crown?
Because a crown costs you healthy tooth. Preparing a tooth for a crown means reducing it all the way around, including sound parts that an onlay would leave completely untouched.
Since teeth tend to be restored several times across a lifetime, every millimetre of natural tooth preserved now keeps more options open later. Where an onlay will genuinely last, it is the better custodian of your tooth’s future.
Where it will not, a crown is honestly the right recommendation, and you should be shown the reasoning either way.
What they are made of
Usually tooth-coloured ceramic or a hardened composite, shade-matched to the tooth so the restoration effectively disappears. Ceramic is strong, hard-wearing and lifelike; composite is a little kinder to opposing teeth and generally more economical.
Gold is still used occasionally and is, from a purely engineering standpoint, an excellent material for back teeth, though most patients now prefer tooth-coloured options. The right choice depends on your bite and the position of the tooth, and it is worth discussing rather than assuming.
What treatment involves
- Examination, usually with an X-ray, to confirm which restoration the tooth actually needs.
- Preparation. With the tooth comfortably numbed, decay or the old failed filling is removed and the tooth shaped to receive its new piece.
- Impressions, capturing the tooth’s exact contours, with the shade matched at the same time.
- A temporary filling protects the tooth while the laboratory does its work.
- Fitting. At the second visit, the finished piece is tried in, checked for fit and bite, and bonded permanently in place.
Two comfortable visits, typically a fortnight apart. It should not hurt at any stage, and any sensitivity afterwards is usually mild and short-lived.
How long do they last?
Commonly ten years or more, and often well beyond, which is precisely the advantage over the large fillings they replace.
What extends them is unglamorous but effective: brushing twice daily, cleaning between your teeth, keeping your check-ups so the margins are monitored, and wearing a night guard if you grind. Clenching is hard on every restoration, and our post on why teeth wear down explains why.
Is one right for my tooth?
Broadly, an inlay or onlay tends to suit when a back tooth has damage too large for a reliable filling, but enough healthy structure that a crown would be overkill. A broken cusp, a cracked tooth needing bracing, or a tooth on its third failed filling are all classic cases.
The honest test is always the same: the most conservative restoration that will genuinely last. Under-treating wastes money on a repair built to fail; over-treating spends tooth you cannot get back. Our page on inlays and onlays explains where they sit in that ladder, and the Oral Health Foundation has useful general background on restorative options.
In summary
An inlay restores the middle of a back tooth; an onlay extends over the raised corners as well. Both are custom-made in a laboratory and bonded in as one solid piece, which makes them stronger than a large filling while preserving more natural tooth than a crown.
If a tooth of yours is on its third filling, or you have been told it needs a crown and would like a second opinion, ring us on 01255 422506. Our post on choosing between a crown and a filling explains the same decision from the other direction.
This article is general information rather than personal dental advice. Every mouth is different, so please speak to your dentist about your own teeth.