- Cusps are the points on top of your back teeth. They take the chewing force.
- An inlay sits inside the cusps. It fills the hole and leaves the walls as they are.
- An onlay covers the weak cusps. It works like a partial crown.
- A crown covers the whole tooth. Its preparation removes roughly 68 to 76% of the tooth above the gums.
- An onlay preparation removes roughly 36 to 47%.
- Ceramic onlays last. About 91% are still in place at 10 years.
- Once a tooth is cut for a crown, you can’t go back.
Inlay vs onlay vs crown at a glance
The difference between an inlay, an onlay and a crown is how much of the tooth each one covers. A lot of people, and a lot of dental websites, put inlays and onlays in the same bucket. They solve different problems.
| Filling | Inlay | Onlay | Crown | |
|---|---|---|---|---|
| What it covers | A small hole | The space between the cusps | One or more cusps | The entire tooth above the gums |
| Tooth removed | Only the decay | Decay plus a shaped cavity | About 36 to 47% of the crown of the tooth | About 68 to 76% of the crown of the tooth |
| Protects weak cusps? | No | No | Yes | Yes |
| How it stays on | Bonded | Bonded or cemented | Bonded | Usually cemented |
| Made where | In your mouth, same visit | In a lab or milled | In a lab or milled | In a lab or milled |
| How I use it | Small cavities | Rarely | Most of my back-tooth repairs | Teeth already prepped for a crown |
What is an inlay?
An inlay is a custom piece of porcelain, composite or gold that fits inside the top of a back tooth, between the cusps. It’s made outside your mouth, then bonded or cemented into place.
Think of a puzzle piece dropped into a hollow. It fills the space, and the walls around it stay exactly as they were.
That’s the strength of an inlay and also its weak spot. An inlay replaces what’s missing in the middle of the tooth and leaves the walls around it unprotected.
What is an onlay?
An onlay is a restoration that covers one or more cusps of a back tooth, which is why it’s often called a partial crown. It saves the parts of the tooth that don’t need to be drilled, and it gives strength back to the tooth.
Most people know what a crown is. You drill the tooth all the way around and put a cap over it. An onlay is the more conservative way to do it.
I only remove the tooth structure that’s damaged or too thin to survive chewing. Healthy enamel stays.
Why I do a lot of onlays and very few inlays
I do very few inlays because an inlay leaves weak cusps uncovered, and those cusps are the part most likely to crack. If a cavity is small enough that the walls are still strong, a bonded filling in one visit usually does the job. If it’s big enough to weaken the walls, I want those walls covered.
Hollowing out the middle of a back tooth makes it far less stiff. A classic study on tooth stiffness in the Journal of Endodontics measured how much stiffness teeth lost after different procedures:
- A small cavity on the biting surface: about 20% less stiff
- A cavity that also opens both side walls (an MOD): about 63% less stiff
- A root canal access on its own: about 5% less stiff
The biggest loss came from cutting through the ridges on the sides of the tooth. That’s the exact cavity shape a large inlay fills.
An inlay fills that space. The thin cusps standing on either side still flex with each bite. Over years, that flexing is how a chipped tooth or a cracked cusp happens.
An onlay sits over those cusps and holds the tooth together from the top. That’s why it’s the restoration I reach for most.
Onlay vs crown: why I almost never crown a tooth
I almost never crown a tooth unless it was already prepped as a crown, because a crown removes far more healthy tooth than an onlay does. In the 2002 study, full crown preparations removed 67.5 to 75.6% of the tooth above the gums. Onlay and partial crown preparations removed 35.5 to 46.7%.
Picture a thimble. To slide a thimble over a tooth, you have to shave down every wall, including the healthy ones. An onlay only covers the corners that need it.
More drilling, more risk to the nerve
Cutting a tooth down for a crown can be hard on the nerve inside it. In a long-term study of vital teeth under crowns in the International Endodontic Journal, 15.6% of teeth with a single crown went on to have nerve problems needing endodontic treatment. At 10 years, 84.4% still had a healthy nerve.
That’s about one in six teeth. Keeping more natural tooth gives the nerve a better chance.
Do onlays hold up as well as crowns?
The research so far shows no meaningful difference in survival between onlays and full crowns on back teeth. A 2022 meta-analysis comparing onlays and partial crowns with full crowns in Head & Face Medicine found no significant difference in survival at one and three years, or in fracture rates at three years.
That review pooled a small number of studies, so the evidence is still limited. But I don’t need an onlay to outperform a crown. If it lasts as well and saves a third or more of your tooth, I’ll choose the onlay.
Why “already prepped” changes everything
Once a tooth has been cut down for a crown, the enamel is gone, and I can’t put it back. There’s nothing left to bond an onlay to.
So when an old crown needs replacing, I replace it with a crown, and when a tooth comes in untouched, I protect as much of it as I can.
What biomimetic dentistry means for your back teeth
Biomimetic dentistry means restoring a tooth so it behaves like the natural tooth it replaces, while removing as little healthy structure as possible. “Biomimetic” means copying nature.
Your natural tooth is enamel bonded to dentin. The two layers flex and share load together. A bonded onlay is designed to work the same way.
This is where adhesive dentistry comes in. A traditional crown holds on mostly through its shape. The tooth is tapered so the cap can grip it. A bonded onlay relies on adhesion to your own enamel and dentin instead.
That’s what lets me skip the cuts a crown needs. The bond does the holding, so I don’t need to shape the whole tooth to give it grip.
How long ceramic onlays last
Ceramic inlays, onlays and overlays have high survival rates, around 92 to 95% at five years and 91% at 10 years. That comes from a meta-analysis of ceramic and resin inlays and onlays in the Journal of Dental Research covering over 5,800 restorations.
The most common reason they failed:
- Fracture or chipping (about 4%)
- Nerve problems needing a root canal (about 3%)
- New decay at the edge (about 1%)
- Coming unbonded (about 1%)
Most of those risks go down with a well-fit restoration, a night guard if you grind, and regular cleanings.
When a crown is still the right call
A crown is still the right choice when there isn’t enough healthy tooth left to bond an onlay to. In my practice that comes up in a few situations:
- The tooth was already prepped for a crown by a previous dentist
- An old crown needs replacing, often because of decay under the crown
- Very little natural tooth remains after decay or a fracture
- The edges of the damage sit too far below the gums to keep dry for bonding
A back tooth that’s had a root canal still needs its cusps covered. For a lot of those teeth an onlay covers them well. When too little tooth is left, I crown it.
What getting an onlay is like
An onlay is usually done in two visits: one to prepare the tooth and one to bond the final piece.
- First visit: I numb the tooth, remove the decay or old filling, and shape only what needs shaping. Then I take a digital scan.
- In between: a temporary protects the tooth while the onlay is made.
- Second visit: I try in the onlay, check the fit and bite, and bond it to the tooth.
Before any of this, I sit down with you and go through your X-rays tooth by tooth. I want you to see what I see and understand why I’m recommending what I’m recommending.
A patient who has been with me for years came in to fix a back tooth. We planned an onlay instead of a crown, covered the weak cusps and left the healthy side of the tooth alone.
Questions to ask your dentist
If you’ve been told you need a crown on a back tooth, these questions help you understand your options:
- “Can I see the X-ray and photos of the tooth?”
- “Which cusps are weak, and which parts are healthy?”
- “Is an onlay an option for this tooth?”
- “Will the restoration be bonded or cemented?”
- “What will my insurance cover?” Ask for a pre-estimate, since plans code onlays and crowns differently.
Red flags
- Pain when you bite down on one tooth
- A sharp edge or a piece of tooth missing
- Lingering sensitivity to cold on one tooth
- A tooth that feels different when you chew
If a tooth hurts when you put pressure on it, a cracked cusp is one of the common causes. Catching it early is what makes an onlay possible instead of a crown.
FAQ
Is an onlay better than a crown?
An onlay is the more conservative choice when enough healthy tooth remains, because it removes far less tooth structure. Research so far shows similar survival for onlays and crowns on back teeth. A crown is better when the tooth was already prepped for one or too little tooth is left.
Is an onlay the same as a partial crown?
Yes, an onlay is often called a partial crown. It covers one or more cusps of a back tooth without wrapping around the whole tooth.
What is the main difference between an inlay and an onlay?
An inlay sits inside the cusps of a back tooth, and an onlay covers one or more of them. That makes an onlay the better option when the cusps are weak or thin.
How long does an onlay last?
Ceramic onlays have a survival rate of about 91% at 10 years in a large meta-analysis. How long yours lasts depends on your bite, grinding habits and home care.
Does getting an onlay hurt?
Getting an onlay feels much like getting a filling. The tooth is numbed for the preparation, and most people feel some mild sensitivity for a few days after.
Can an old crown be replaced with an onlay?
Usually not. Once a tooth has been prepped for a crown, the enamel an onlay bonds to is gone, so an old crown is normally replaced with a new crown.
Is an onlay more expensive than a filling?
Yes. An onlay is custom-made outside the mouth, so it costs more than a filling. The cost compared with a crown depends on the material and your insurance, so ask your office for a pre-estimate.
Bera edits this library at 1flowww: the question each article answers, the sources it cites, and the update history that shows what changed. Every clinical claim is reviewed by Dr. Kahng before it publishes. About the editor
Dr. Joyce Kahng is a cosmetic dentist, veneer expert, and the founder of Orange + Magnolia Dental Studio in Costa Mesa, CA. With 16+ years in practice and a following of 1.5M+ across Instagram, TikTok, and YouTube, she's known for making dentistry actually make sense. She's been featured in Forbes, Vogue, Allure, Glamour and GQ, and is the creator of the VeneerLIFT™.
Our editorial process · written by the editorial team, medically reviewed by a dentist, corrected in the open.
- 7 Oct 2026 Published. Clinical review by Dr. Joyce Kahng, DDS.
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