A tooth too damaged for a filling is not automatically a tooth that needs a crown. In between sits a pair of repairs that rarely come up until you need one.
Dental inlays and onlays are both custom-made pieces that rebuild a back tooth, and the choice between them turns on one thing: how much of the biting surface is still sound.
This guide sets out what separates the two, what a dentist weighs up before choosing, and how the fitting appointment actually runs.
What Are Dental Inlays and Onlays?
Both are indirect restorations. They are made outside the mouth to fit one specific tooth, then bonded into place, which is what separates them from a filling built up inside the tooth in a single sitting.
The difference between dental inlays and onlays is a matter of coverage.
An inlay sits inside the tooth. It fills the hollow between the cusps, the raised points at the corners of a back tooth, and does not extend over any of them. A UK dental school’s patient leaflet puts it plainly: inlays are used “to fill cavities within the tooth”.
An onlay does that same job and then continues over one or more cusps, rebuilding part of the biting surface itself. That extra coverage is why an onlay is sometimes called a partial crown.
So dental inlays and onlays are not two names for one thing. Which of them you are offered is settled by the state of the tooth rather than by preference.
Onlay or Inlay: Which One Will You Need?
Choosing between an onlay or inlay is an assessment rather than a preference, and four things drive it.
What the Dentist Is Looking At
How many cusps are involved. If the damage stops short of the cusps, an inlay is enough. Once a cusp is cracked, undermined or missing, an onlay covers and protects it.
Whether the tooth is cracked. A cusp with a crack running through it keeps flexing under a filling. Covering it stops that movement.
Whether it has had root canal treatment. A root-treated tooth is more brittle: a 2021 systematic review found the risk of failure of these restorations is higher on root-treated teeth than on living ones. That usually points to an onlay.
What is already in the tooth. A large filling that has failed often leaves too little sound structure to hold a replacement.
Expert Tip: Ask which cusps are involved. That single question is what separates an onlay or inlay, and it can be answered at the chair.
Whether you need a dental inlay or onlay comes down to those four answers together.
Inlay vs Onlay vs Crown: Where the Line Is
Set inlay vs onlay vs crown side by side and they form a ladder, each step covering more of the tooth than the one below it.
An inlay replaces the middle. An onlay replaces the middle and at least one cusp. A crown covers the entire tooth down to the gum, and the sound tooth underneath has to be reduced first to make room for it.
That is the whole argument for stopping lower down the ladder where the tooth allows it. A crown is the more thorough repair, but it is paid for by removing healthy structure that cannot be put back.
Expert Tip: If a crown is proposed, ask whether an onlay would do instead. The honest answer depends on how much sound tooth is left, not on preference.
The line falls where the remaining tooth can no longer hold a restoration on its own. Too little sound wall, and a crown becomes the right answer, the case our guide to when a crown is the better answer covers in full.
What Are Inlays and Onlays Made Of?
Four materials come up in any discussion of these restorations, and two of them are the routes offered here.
| Material | How it looks | Where it fits |
|---|---|---|
| Porcelain | Shade-matched to the teeth around it | The usual choice for a porcelain onlay or inlay |
| Zirconia | Tooth-coloured, a little more opaque | The tougher ceramic, for the heaviest bite forces |
| Composite resin | Tooth-coloured, less lifelike | Softer and quicker to wear, so it suits smaller direct repairs |
| Gold | Unmistakably visible | Long valued for durability, rarely chosen now for a tooth on show |
A ceramic inlay or onlay is the default for a reason. It resists staining, it takes a shade match well, and it can be milled on site, which is what makes a single-visit fitting possible. Porcelain and zirconia are the two materials listed for dental inlays and onlays in our published treatment schedule.
What Happens When You Have One Fitted
Almost every description of how dental inlays and onlays are fitted, including that dental school leaflet, says the restoration “will take 2 appointments”, with a temporary filling in between. That is the standard laboratory process. It is not how the work is done here.
- Schedule an appointment. A single visit is booked rather than a first and a second.
- Consultation and assessment. The tooth is examined and a digital 3D scan takes the place of a putty impression.
- Same-day restoration. The inlay or onlay is designed, milled on site, then bonded to the tooth. From 3D scan to final placement takes under two hours.
- Follow-up and long-term care. The bite is checked and the restoration is reviewed at routine appointments from then on.
No temporary is fitted, nothing is sent away, and there is no second appointment. More detail on how the restoration is made.
How Long an Inlay or Onlay Lasts
Longevity depends more on the tooth and the habits around it than on the piece itself. A 2024 study of ceramic inlays and onlays put cumulative survival at 93.9% after a mean service time of 15.2 years, and 91.7% at 23.5 years. Those are measured figures rather than the round numbers usually quoted.
What you do afterwards matters more than the material:
- Keep the margin clean. The join between restoration and tooth is where new decay starts.
- Avoid ice and hard sweets on that side. Ceramic is strong under chewing pressure and less forgiving of a sudden impact.
- Wear a night guard if one has been recommended. It takes the load off while you sleep.
- Keep routine check-ups. The margin is inspected at every visit, and problems caught there stay small.
Helpful Tip: Grinding is the single biggest threat to a bonded restoration. If you grind your teeth, say so before the fitting rather than after it.
What to Remember Before You Book
The distinction between dental inlays and onlays sounds technical, but the decision behind it is not. It comes down to how much of your tooth is still sound, and whether the repair can stop short of covering all of it. Everything else follows from that one judgement.
The short version:
- Coverage is the whole difference. An inlay sits inside the cusps; an onlay extends over at least one of them.
- The tooth decides, not preference. Cusps involved, cracks, root canal history and the existing filling settle it.
- Stopping below a crown is worth asking about. A crown means reducing healthy tooth that cannot be replaced later.
- The fitting is one appointment. Same-day milling removes the temporary and the second visit that most guides still describe.
Frequently Asked Questions
The questions below are the ones that come up most often once the difference between the two is clear.
What are the downsides of inlays and onlays?
Dental inlays and onlays cost more than a filling and take longer to fit. They also cannot be patched the way a composite filling can, so a failed restoration is replaced rather than repaired. The bond matters too: the tooth has to be clean and dry when the piece is fitted.
Is an onlay the same as a partial crown?
Close enough that the two terms are used interchangeably. An onlay covers one or more cusps but never the whole tooth, which is what “partial crown” describes. A full crown covers everything down to the gum line, so the difference is how much natural tooth is kept.
Can you have an inlay or onlay on a front tooth?
Rarely. Both are designed for the chewing surfaces of premolars and molars, where the forces are vertical and there are cusps to protect. Damage to a front tooth is usually handled with composite bonding, a veneer or a crown.
What happens if an inlay or onlay comes loose?
Keep the piece if you can find it and call the practice. A loose restoration usually means the bond has failed rather than the restoration itself, and it can often be cleaned and re-bonded. Avoid chewing on that side until it is seen.
Do dentists still do onlays?
Yes, and more readily than before. Digital scanning and in-house milling removed the two-visit laboratory step that made them awkward to offer. An onlay is now a routine way to rebuild a badly broken back tooth without reducing it for a crown.
Is an onlay better than a crown?
Neither is better in the abstract; they answer different amounts of damage. An onlay keeps more of the natural tooth, and that advantage lasts as long as the tooth does. A crown gives more coverage, which matters once too little sound wall is left.
Does having an inlay or onlay fitted hurt?
The tooth is numbed, so the appointment itself should not be painful. Mild sensitivity to cold for a few days afterwards is common while the tooth settles. Sensitivity that worsens over a week rather than easing is worth reporting.
Can a porcelain inlay or onlay stain over time?
Porcelain does not take up stain the way enamel and composite do, so the restoration holds its colour. What can change is everything around it: if your natural teeth darken over the years, a piece matched to today’s shade can start to look lighter.
Book a Consultation at Dental Artistry
The only way to settle whether your tooth needs an inlay, an onlay or a crown is to have someone look at what is left of it. Book a consultation at Dental Artistry and you will be told which of the three the tooth actually needs, and why the others were ruled out.
Information Sources
Inlays and onlays (Bristol Dental School)
Long-term performance of ceramic in-/onlays versus cast gold partial crowns (via PMC)
Clinical outcome of bonded partial indirect posterior restorations on vital and non-vital teeth (via PMC)
