Most people have heard of crowns. Far fewer have heard of onlays, and that gap matters, because for a lot of broken and heavily filled back teeth an onlay does the same job while leaving far more of the natural tooth in place.
What an onlay is
A crown is a cap that covers the whole tooth above the gum. To fit one, the tooth is shaped down on every side. An onlay covers only the part of the tooth that needs it, usually the biting surface and one or more cusps, and is bonded onto the sound tooth that remains. Think of it as a precise patch rather than a full cover.
Onlays are usually made from ceramic, often lithium disilicate (known by the trade name e.max), or from composite. Because they're bonded rather than simply cemented, they rely on good enamel to stick to and a dry working area while they go on.
How much tooth each option removes
This is the core of the argument. In comparisons of preparation designs, full crown preparations removed roughly two thirds to three quarters of the tooth structure, against roughly a third to a half for onlays and partial crowns.
Tooth structure is the one material in dentistry that can't be replaced like for like. Every restoration eventually needs redoing, and each time it's redone a little more tooth is lost. Starting with an onlay leaves more to work with later. A crown can always follow an onlay. It doesn't work the other way round.
Cracked teeth: the evidence is genuinely mixed
Cracked teeth are one of the commonest reasons for needing either. They often show up as a sharp pain on biting something, or on releasing a bite, that's hard to pin to one tooth. Covering the cusps holds the tooth together and stops the crack flexing.
Which covering is better is less clear than either camp would like. A 2025 trial of root-filled cracked teeth found no meaningful difference between ceramic onlays and crowns at twelve months, though it was small and short. A long-term retrospective study found crowns did better overall, but also that the extent of the crack, particularly one running down onto the root, mattered more than the restoration.
In practice that means the crack decides. A crack confined to the crown of the tooth is often well served by an onlay. A deeper one may need a crown, and one that runs well down the root may mean the tooth can't be saved at all. That's why cracks are assessed with magnification, transillumination and sometimes by removing an old filling to look, before anyone promises an outcome.
After root canal treatment
A back tooth that has had root canal treatment has usually lost a lot of its internal structure and is more likely to fracture. Covering the cusps afterwards protects it, and that's well established. What has changed is that the covering doesn't automatically have to be a full crown. Where enough sound tooth remains, a bonded onlay can provide the cusp protection while keeping more tooth. Where the walls are thin or largely missing, a crown is still the better choice.
When a crown is still the right answer
- The tooth is already crowned. Once the tooth has been prepared for a crown, there's nothing left for an onlay to sit on.
- Decay or a fracture extends below the gum. Bonding needs a clean, dry surface, which isn't possible deep under the gum line.
- Very little enamel remains. Onlays depend on a strong bond to enamel. Mostly dentine with thin walls favours a crown.
- Heavy grinding with lots of missing tooth. Sometimes the extra strength of full coverage outweighs the cost in tooth structure. Tooth wear changes the whole plan, not just the single tooth.
What to ask if you've been told you need a crown
- Is an onlay possible on this tooth, and if not, why not?
- How much of the tooth is sound, and where is the crack or decay?
- What material is proposed, and why that one for this tooth?
- If the tooth has had root canal treatment, is the root filling sound before we cover it?
Resources
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