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Inlays and onlays are laboratory-made partial crowns, stronger than a filling and smaller than a crown, cemented into a back tooth that has a large old filling or a damaged chewing surface. They are a popular treatment, and many good dentists recommend them. We do not offer them, for the reasons explained below, and for a tooth that has outgrown a filling we recommend a crown instead.
What they are
An inlay fits within the chewing surface of a tooth, between the cusps, like a filling made outside the mouth. An onlay extends over one or more cusps, replacing a corner of the tooth as well. Both are indirect restorations: the tooth is prepared, an impression is taken, a laboratory makes the piece, and it is bonded into place at a second visit. Today most are ceramic; traditionally many were gold.
The difference from a filling is where the work happens. A filling is built directly in the tooth in one visit and hardened in place. An inlay or onlay is made outside the mouth from an impression, which allows a harder, more wear-resistant material and a precisely finished shape, at the cost of a second visit, a temporary in between, and a higher fee. The difference from a crown is coverage: an onlay leaves the undamaged walls of the tooth untouched, where a crown wraps the whole tooth.
What a well-made onlay involves
The quality of an onlay depends heavily on the details.
- Step 1 of 4 Preparation and impression
The damaged part of the tooth is removed and the tooth shaped for the restoration. An accurate impression is taken, either a silicone impression or a digital scan; our own preference for indirect work is a silicone impression, though both are valid.
- Step 2 of 4 Temporary
A temporary filling material protects the tooth while the laboratory works. It is more fragile than a temporary crown and can come out with sticky food, so some care is needed.
- Step 3 of 4 Laboratory fabrication
The onlay is made to fit the preparation exactly. A laboratory-made restoration is, in our view, more precise than one milled beside the chair, though both approaches exist.
- Step 4 of 4 Bonding
The tooth is numbed again, isolated with a rubber dam so it is completely dry, cleaned, and the onlay is bonded with a resin cement. Moisture control decides whether the bond holds.
Note the difference from a crown at the second visit. A crown is usually cemented without anesthesia and often without a dam; an onlay needs both, because resin bonding fails if the tooth is not perfectly dry. The onlay has an easier first visit and a more demanding second one.
Materials
Most inlays and onlays made today are ceramic:
- Feldspathic or leucite-reinforced porcelains, which mimic enamel well
- Lithium disilicate, which is stronger
- Zirconia, which is stronger still and more opaque
- Composite onlays, made in the laboratory from the same family of material as a filling
- Gold, the traditional material and in many ways still the best for wear and longevity
Each has trade-offs in appearance, strength and how it bonds, and a dentist who offers onlays will have a preference among them.
Questions worth asking about any onlay
If a dentist recommends an inlay or onlay for one of your teeth, these questions will tell you a lot about how it will go:
- How much sound tooth is left, and is there a crack?
- How will the tooth be kept dry during bonding, and will a rubber dam be used?
- Will the restoration be made by a laboratory or milled in the office the same day?
- What cement will be used, and what has their experience been with it over the years?
- What would a crown on this tooth cost, and why is an onlay better here?
Good answers exist to all of them, and an onlay done with that care by a dentist who believes in it is a legitimate choice.
Why we do not offer them
Two things in our experience, and in what we have seen of other dentists’ work, have kept us from recommending ceramic inlays and onlays.
The first is the cement. Partial restorations depend on resin cements that bond them to the tooth. Those cements have excellent properties on paper, but over the medium and long term we have found them less reliable than the cements used for full crowns, and a loose or leaking onlay is a failed onlay.
The second is cracks. Teeth with large failing restorations often have cracks as well. A crown wraps around the whole tooth and holds it together; an onlay covers part of the tooth and cannot stop a crack from running underneath it. We have seen cracks travel beneath onlays that a crown would have contained.
So for a tooth that needs more than a filling, we usually recommend a crown, and for a tooth that does not yet need a crown, we keep it on a composite filling, done under rubber dam isolation, until it does. We skip the middle step. Research comparing onlays and crowns shows similar results in the short term and limited evidence over the long term; what we describe here is our experience, and dentists who use onlays carefully report good results with them. It is a legitimate treatment that we have decided not to provide, because we think our patients are better served by the other two options.
Gold
Gold inlays and onlays deserve their own mention. They are among the most durable restorations in dentistry and can last the rest of your life. But very few patients want a gold restoration today, and we do not offer them either.
What we do instead
- A tooth-colored filling when the damage is modest and the filling can be kept clean and dry, placed under a rubber dam.
- A crown when the tooth has lost too much to rely on a filling, or when a crack needs to be contained, with as little reduction as the crown allows.
- Sometimes a large filling as a deliberate interim step, with a crown planned for when it is needed, so that a tooth is not over-treated early.
If you have onlays already, they need no special attention beyond your regular checkups. If you would still prefer an onlay after reading this, say so; we explain what we would do, and you are welcome to have the onlay done elsewhere.
Frequently asked questions
What is the difference between an inlay, an onlay and a crown?
All three are made outside the mouth and cemented in. An inlay sits within the chewing surface, between the cusps. An onlay covers one or more cusps as well. A crown covers the whole tooth above the gum. The more of the tooth is damaged, the further along that scale the restoration goes.
Are onlays better than crowns because they remove less tooth?
They remove less tooth, which is a genuine advantage. Whether that makes them better depends on how long they last, and that is where our experience and other dentists' experience differ. Short-term results are similar in the research; long-term comparisons are limited. We have found crowns more durable and easier to maintain, and we plan around that.
Why do you not offer them?
Because in our experience the trade-offs have not been worth it compared with a well-placed filling or a crown: the resin cements that partial restorations depend on have been less reliable over the medium and long term, and partial coverage does not hold a cracked tooth together the way a crown does. That is our judgment, not a verdict on dentists who use them well.
What about gold onlays?
Gold inlays and onlays are among the most durable restorations ever made and can last a lifetime. We respect them. Very few patients want gold in their mouths today, so we do not offer them either.
I already have onlays. Should I worry?
No. A well-made onlay that is sealed and comfortable should be left alone and checked at your regular visits like any restoration. If one leaks, chips or the tooth cracks, we will discuss what to replace it with at that point.
What if I would still prefer an onlay?
Tell us, and we will explain what we would do instead and why. If you decide an onlay is what you want, you are welcome to seek it from an office that provides them; we would rather you make an informed choice than an uninformed one.
