Onlays and partial crowns: partial coverage that keeps more tooth

When a back tooth breaks, the conversation often jumps straight to a crown. But many broken teeth are only partly broken. One cusp has cracked off, or a large old filling has failed on one side, while the rest of the tooth is sound. Covering the whole tooth to fix one corner means drilling away healthy walls that were doing their job. An onlay or partial crown is designed for exactly that in-between situation.

What onlays and partial crowns are

The names overlap, and dentists use them loosely, so here is the plain version:

• Inlay: sits inside the tooth, like a filling made outside the mouth. It does not cover the cusps.
• Onlay: replaces one or more cusps as well as filling the cavity.
• Overlay or occlusal veneer: a thin layer covering the whole biting surface, often for worn teeth.
• Partial crown: covers most of the tooth but leaves at least one healthy wall uncovered.

All of them are made to fit from a scan or impression, usually in ceramic (such as lithium disilicate, the material family of E-max) or a resin-based material. They are bonded to the tooth with an adhesive cement, which is why the surface of the tooth matters so much. Our article on how crowns are cemented explains bonding versus conventional cement.

How much tooth they save

A laboratory study on premolars and molars measured tooth structure removed for different preparation designs, from inlays and onlays through partial crowns to full crowns (Edelhoff and Sorensen, 2002). Bonded and inlay designs removed roughly 5.5% to 27% of the tooth by weight, while full crowns removed about 67.5% to 75.6%, the most of any design tested. The authors concluded that the less invasive designs kept significant amounts of tooth.

Two things follow from keeping tooth. First, there is more natural tooth left for the future. If the onlay fails in fifteen years, a crown is still an option. Second, keeping walls of the tooth often means the edge of the restoration can sit above the gum, where it is easier to clean and easier for your dentist to check.

What the research says about how long they last

A systematic review and meta-analysis of ceramic and resin inlays, onlays and overlays with at least five years of follow-up estimated survival of about 92% to 95% at five years and 91% at ten years for glass-ceramic and porcelain restorations (Morimoto and colleagues, 2016). Failures were mostly fractures or chipping (about 4%), then root canal problems (3%), with new decay and debonding at around 1% each.

Another review compared inlays and onlays with full crowns. Pooled survival was about 91% for inlays, 93.5% for onlays and 95.4% for crowns, but the studies were too varied for a true head-to-head comparison (Vagropoulou and colleagues, 2018). The authors concluded that survival at five years was above 90% for all of them, with decay and ceramic fracture the main problems.

A more recent review of randomised trials on partial coverage for back teeth found lithium disilicate restorations had a three-year survival of about 94%, slightly ahead of resin-matrix ceramics at about 89%, and noted that long-term performance of these restorations is still uncertain (Prott and colleagues, 2025).

The honest summary: partial coverage holds up close to full crowns over the periods studied, while keeping more tooth. The evidence beyond ten years is thinner, as it is for most modern ceramics.

Who suits an onlay

Good candidates

• A back tooth with one or two cusps broken or undermined, and the other walls thick and sound.
• A large old filling that needs replacing, where the cusps around it are weak.
• A cracked cusp that needs covering, provided the crack does not run deep into the root.
• Some root-filled back teeth with enough tooth left to cover the cusps without a full crown.
• Worn back teeth that need their biting surfaces built up, using thin overlays.

When a full crown is usually better

• When most of the tooth is already gone and there are no sound walls to keep.
• When the edges of the damage run well below the gum, where it is hard to keep the area dry for bonding.
• When there is little enamel left to bond to.
• When an existing crown is being replaced. The tooth has already been prepared for full coverage.
• For heavy grinders in some cases, where the dentist judges that a different design or material is safer. Our article on grinding, clenching and crowns covers that.

Bonding is the part that decides success

A full crown gets much of its hold from the shape of the preparation: tall walls that grip. An onlay gets its hold mostly from adhesion. That makes the technique sensitive. The tooth surface must be clean, the area must be kept dry from saliva and blood while bonding, and the ceramic inside surface is treated to accept the cement. This is why the position of the edges matters: an edge well below the gum line is harder to keep dry.

If your dentist suggests a crown instead of an onlay, it is fair to ask whether that is because of how much tooth is left, where the edges are, or simply habit. All three are real reasons, but only the first two are about your tooth.

What having an onlay usually involves

The steps are similar to a crown, with less drilling. Under local anaesthetic, the old filling and any decay are removed, along with any weak, undermined enamel that would break off later. The dentist then shapes the cavity so the onlay can slide into place, with smooth walls and no sharp internal corners. The prepared tooth is scanned or an impression is taken, and the onlay is made to fit that shape.

When it is ready, the fit and bite are checked first, because a thin ceramic piece needs to sit exactly before it is bonded. The tooth is isolated from saliva, both surfaces are prepared, and the onlay is bonded in place. Any excess cement is cleaned away and the bite is checked again.

Afterwards, an onlay is looked after like the rest of your teeth: brushing, cleaning between teeth and regular check-ups. The edge where onlay meets tooth is where new decay can start, as it can around a crown, so keeping that area clean is what makes it last. If you grind, ask about a night guard, since fracture was the most common reason ceramic onlays failed in the research above.

Onlay or crown: questions to ask

• Which walls of my tooth are sound, and could they be kept?
• Where would the edges of the restoration sit relative to the gum?
• What material would you use, and would it be bonded?
• If the onlay failed later, could the tooth still be crowned?
• Is the nerve healthy, or does it need treatment first?

How this fits with a crown plan

Many treatment plans mix restorations. A patient might need crowns on two heavily broken molars and onlays or fillings on teeth that are only partly damaged. That mix is normal and usually keeps more of your own teeth than crowning everything. Our crown types page compares the full-coverage materials, and do you really need a crown? sets out the wider options.

At Precious Dental we prefer to protect a healthy tooth rather than crown it. Ask us whether partial coverage is right for your tooth; your written plan says what we recommend and why. For prices on the treatments we publish, see crown prices.

If you have a broken back tooth and want to know whether it needs a full crown, send photos and a recent X-ray through our contact page and we will reply with a free written plan for that tooth.

Common questions

Is an onlay as strong as a crown?

Research suggests survival is similar over five to ten years. An onlay depends more on good bonding, so it suits teeth with enough enamel and a dry working area. A crown relies more on the shape of the preparation.

Can a front tooth have an onlay?

Onlays are mostly for back teeth. For front teeth the partial-coverage options are bonding and veneers. See crowns vs veneers.

Can an onlay be replaced by a crown later?

Usually yes. Because less tooth is removed, a crown remains possible if the onlay ever fails. That is one of the main reasons to choose partial coverage first.

Is an onlay suitable for a root-filled tooth?

Sometimes. Covering the cusps is what protects a root-filled back tooth, and an onlay can do that if enough tooth remains. Read root-filled teeth, posts and crowns.

Do you make onlays?

Ask us whether partial coverage is right for your tooth. Your free written plan says what we recommend for each tooth and why.

Sources

1. Morimoto S et al. (2016). Survival Rate of Resin and Ceramic Inlays, Onlays, and Overlays: A Systematic Review and Meta-analysis. J Dent Res
2. Vagropoulou GI et al. (2018). Complications and survival rates of inlays and onlays vs complete coverage restorations: A systematic review and analysis of studies. J Oral Rehabil
3. Prott LS et al. (2025). Survival and Complications of Partial Coverage Restorations on Posterior Teeth: A Systematic Review and Meta-Analysis. J Esthet Restor Dent
4. Edelhoff D, Sorensen JA (2002). Tooth structure removal associated with various preparation designs for posterior teeth. Int J Periodontics Restorative Dent
5. Oral Health Foundation. Crowns

Checked October 2026. Where we give our own view as a clinic, we say so.