Inlays and onlays: repairing a damaged tooth without crowning it
An inlay and an onlay are indirect restorations: the piece is made outside the mouth, in a laboratory or by milling, and then bonded into a tooth that has been prepared for it. It repairs a tooth that is still in place, damaged by decay, by a fracture or by an old filling that has grown too large to be replaced like for like. A tooth that is no longer there calls for something else entirely, and that distinction runs through everything below.
What they are for becomes clear once you place them between the two treatments on either side. A direct filling, shaped in the mouth in one sitting, is quicker and cheaper. A crown, at the other end, covers the whole tooth and means preparing it all the way round. Inlays and onlays rebuild a large cavity while preparing only the part that has failed, which is why they come up when a filling would be stretched to its limit and a crown looks like more than the tooth needs.
This article sets out what separates an inlay from an onlay, when each is indicated, what the material changes, how long these restorations last according to published studies, what an NHS band 3 course of treatment costs, and the point at which a crown becomes the sensible choice again.
What an inlay or onlay actually is
The dentist begins by removing the decay and the old restoration, then shapes a cavity with slightly divergent walls and no undercuts, so that a solid piece can be seated in it. An impression follows, taken conventionally or with an intraoral scanner, and the restoration is layered or milled from a block, tried in, and finally bonded, usually under rubber dam.
That bonding step is not a procedural detail. A cemented crown holds mainly because of the shape of the prepared tooth, whereas an inlay or onlay relies on adhesion to enamel and dentine. How much sound tissue is left, how well moisture is controlled during bonding and how carefully the margins are finished therefore matter as much as the accuracy of the piece itself.
The materials fall into three groups: glass-ceramics, including lithium disilicate and feldspathic porcelain; laboratory composites and hybrid blocks, which combine a resin matrix with a ceramic network; and metal alloys, gold in particular. Spitznagel et al. (2018) describe how the field has moved towards bonded glass-ceramics used in monolithic form and milled by CAD/CAM for defect-oriented restorations, inlays and onlays among them.
Inlay, onlay, overlay: the cusp is what changes
An inlay stays inside the tooth. It fills a cavity lying between the cusps, the raised points that give the biting surface its shape, without covering them. It suits moderate to large cavities that are too wide for a comfortable filling but still surrounded by thick walls.
An onlay covers at least one cusp as well as the central cavity. That changes the nature of the job: instead of filling a space, the restoration protects a weakened wall by carrying the chewing load above it. When every cusp is covered the restoration is usually called an overlay, or full cuspal coverage.
The decision therefore turns on the state of the walls rather than the size of the cavity. A cusp that is cracked, badly thinned or already broken will be covered; a thick, sound wall will be left alone. Abu-Awwad (2019) showed that the boundary is still debated: of thirteen clinical situations put to a hundred and twenty dentists, six failed to reach 75% agreement on whether cuspal coverage was needed.
Why not simply a crown?
This is the comparison most UK practices lead with, and it comes down to how much tooth is sacrificed. Edelhoff and Sorensen (2002) weighed the amount of structure removed by different preparation designs on calibrated artificial teeth: adhesive and inlay preparations removed between 5.5% and 27.2% of the anatomical crown, whereas complete crowns required between 67.5% and 75.6%. The figures come from laboratory typodonts rather than from patients, but the order of magnitude explains why restorative dentistry has shifted towards partial designs.
Root-filled back teeth are where that shift matters most, because they were long crowned almost as a matter of routine. The systematic review by Lane et al. (2024) compared full veneer crowns with full cuspal coverage onlays on endodontically treated posterior teeth: onlay survival ranged from 95% to 100% at two years and from 90.7% to 100% at three years, while crowns were reported at 87.8% beyond two years and 95.1% at three years. The authors conclude that onlays are favourable in the short to medium term, and note that data at ten years is missing.
| What the tooth shows | Treatment usually discussed |
|---|---|
| Small cavity, sound walls | Direct filling |
| Wide cavity between cusps | Direct filling or inlay |
| A cracked or thinned cusp | Onlay |
| Root-filled tooth, thin walls | Onlay or crown |
| Very little tooth left, root sound | Crown over a core |
| Tooth missing, root included | Implant, bridge or denture |
And why not simply a filling?
The answer is less clear-cut than practice websites tend to suggest. The meta-analysis by Angeletaki et al. (2016), which pooled randomised trials comparing composite placed directly in the mouth with bonded composite inlays, found no significant difference in the risk of failure at five years or after eleven years of function. The only parameter that separated them was marginal discolouration, slightly in favour of direct fillings at eleven years. The authors are explicit about how few trials were available and about their risk of bias.
What that leaves is a narrower and more honest case for going indirect: rebuilding a contact point between two teeth with a piece shaped outside the mouth, obtaining stable anatomy across a very wide cavity, and above all covering a cusp, which a direct filling handles less well as the span increases.
Porcelain, composite or gold
Material does affect longevity, and Bustamante-Hernández et al. (2020) put a figure on it. Across twenty-seven studies, the estimated survival of posterior onlays reached 94.2%, with two predictors: length of follow-up and the material used, composite onlays surviving less well than reinforced ceramics.
The head-to-head comparison between composite and ceramic is nonetheless poorly established in randomised trials. Fron Chabouis et al. (2013) found only two eligible trials, covering 138 inlays in 80 patients, both at high risk of bias. The three-year failure risk ratio leaned towards ceramic without reaching significance, and the authors describe their own level of evidence as very limited. Clinical preference for porcelain rests more on its surface properties and colour stability than on a solid comparative demonstration.
Gold alloys retain excellent mechanical behaviour and wear at a rate close to enamel, though their colour confines them to back teeth that barely show. Hybrid blocks, described by Spitznagel et al. (2018), were introduced partly for single-visit work, their edge stability allowing thin margins to be milled.
Two appointments, or one with chairside CAD/CAM
The conventional route takes two visits, as Bristol Dental School sets out in its patient information. The first covers local anaesthetic, removal of the old filling and the decay, preparation of the cavity, the impression and a temporary restoration. The second is given over to the fit, checking the contact with the neighbouring tooth and the way the teeth meet, then bonding the restoration in place. Allow one to two weeks in between, depending on the laboratory.
Chairside CAD/CAM shortens that path. A digital scan feeds a milling unit in the practice, so preparing, milling and bonding can happen in a single appointment. Spitznagel et al. (2018) tie this possibility specifically to glass-ceramic and hybrid blocks. Not every practice is equipped for it, which makes it worth asking about when the number of visits matters to you.
Sensitivity to hot, cold or pressure may follow the fit and usually settles within days or weeks. Discomfort that persists, floss that catches at the margin or a feeling that the tooth meets first when you bite are reasons to go back rather than to wait. Bristol Dental School also lists decay developing underneath the restoration and, less commonly, loss of pulp vitality among the drawbacks to be aware of.
How long they last, and why they fail
Morimoto et al. (2016) remains the reference on this point. Across 5,811 restorations followed for five years, estimated survival for glass-ceramic and feldspathic porcelain lies between 92% and 95%; at ten years it stands at 91% across 2,154 restorations. Neither the type of ceramic, nor a university or private practice setting, nor the length of follow-up significantly altered those rates.
The breakdown of failures is more useful than the headline figure. Fracture and chipping accounted for 4% of restorations, ahead of endodontic complications (3%), secondary caries (1%), debonding (1%) and severe marginal staining. The odds ratio associated with pulp vitality was 0.19, pointing towards a better outlook on a vital tooth.
Vagropoulou et al. (2018) set partial restorations against full coverage, while warning that the heterogeneity of the nine included studies rules out any direct statistical comparison. The pooled means, gathered for illustration only, read as follows:
| Type of restoration | Mean reported survival |
|---|---|
| Inlays | 90.9% |
| Onlays | 93.5% |
| Full coverage crowns | 95.4% |
| Studies pooling inlays and onlays | 99.4% |
Caries emerges in that review as the leading biological complication whatever the restoration, followed by tooth and root fracture, while ceramic fracture leads the technical complications ahead of loss of retention. Two practical consequences follow. Cleaning between the teeth, where secondary decay starts at the margins, bears directly on how long the restoration lasts. And unprotected bruxism subjects the piece to repeated loading that its design does not compensate for.
What it costs on the NHS, and privately
In England, restoring a damaged tooth with an inlay or an onlay falls within band 3, the same band as crowns, bridges and dentures, and the patient charge for a band 3 course of treatment is £332.10. That is a flat charge covering everything clinically necessary in the course, however many appointments it takes, and it does not change with the material used (NHS, 2026).
Privately, fees are quoted per tooth and vary with the material, the laboratory and the practice, which is why a written treatment plan matters more than a headline figure. Two points are worth checking on any quotation: whether the temporary restoration and the fit appointment are included, and what the practice does if the restoration debonds within the first year.
When a tooth is beyond restoring this way
Two situations fall outside the scope of inlays and onlays, and it helps to recognise them early.
The first is extensive breakdown. Where too little of the tooth remains for bonding to be relied on, a crown becomes the reasonable option, sometimes after a core has been built up first. Our page on dental crowns covers the materials and the sequence involved.
The second is a missing tooth. An inlay or an onlay never fills the space left by an extraction: there is no tooth left to repair and no walls to bond anything to. Replacement means a dental implant, which takes the place of the root before receiving a crown, or a bridge or denture where an implant has been ruled out; our review of alternatives to dental implants compares those options.
The two families are often confused, online comparisons included, because the word restoration covers both. The test is straightforward: an inlay or an onlay restores an existing tooth crown, whereas an implant replaces a missing root. Everything else follows from that, including the indications, the length of treatment and what recovery involves.
Having inlays or onlays fitted in Turkey
For a single tooth, travelling rarely adds up. The treatment falls within one NHS band, it needs the bite checked again a few weeks later, and a dentist close to home is better placed to do that.
The arithmetic changes when several teeth need work at once, or when these restorations form part of a wider plan involving crowns, root canal treatment or the replacement of missing teeth. Dental treatment in Turkey then offers up to 70% savings against UK private fees, provided you compare quotations covering the same work: how many teeth, exactly which restoration on each, which material, what has to be treated first, and which review appointments are included.
With Body Expert, the stay is built around your five-star hotel with breakfast, VIP transfers between the airport, the hotel and our partner clinic in Istanbul, a dedicated English-speaking patient coordinator who stays with you throughout, and 12 months of post-treatment follow-up. Flights remain your own cost.
If your plans go beyond repairing one tooth, our guide to dental treatment abroad sets out the treatments available and what separates them. A free quotation within 24 hours, with no obligation, gives you a costed basis for comparison.
Sources
Abu-Awwad, M. (2019). Dentists’ decisions regarding the need for cuspal coverage for endodontically treated and vital posterior teeth. Clinical and Experimental Dental Research, 5(4), 326–335. https://doi.org/10.1002/cre2.185
Angeletaki, F., Gkogkos, A., Papazoglou, E., & Kloukos, D. (2016). Direct versus indirect inlay/onlay composite restorations in posterior teeth: A systematic review and meta-analysis. Journal of Dentistry, 53, 12–21. https://doi.org/10.1016/j.jdent.2016.07.011
Bristol Dental School. (n.d.). Inlays and onlays: Patient information. University of Bristol. Read the page
Bustamante-Hernández, N., Montiel-Company, J. M., Bellot-Arcís, C., Mañes-Ferrer, J. F., Solá-Ruíz, M. F., Agustín-Panadero, R., & Fernández-Estevan, L. (2020). Clinical behavior of ceramic, hybrid and composite onlays: A systematic review and meta-analysis. International Journal of Environmental Research and Public Health, 17(20), 7582. https://doi.org/10.3390/ijerph17207582
Edelhoff, D., & Sorensen, J. A. (2002). Tooth structure removal associated with various preparation designs for posterior teeth. The International Journal of Periodontics & Restorative Dentistry, 22(3), 241–249. View the record
Fron Chabouis, H., Smail Faugeron, V., & Attal, J. P. (2013). Clinical efficacy of composite versus ceramic inlays and onlays: A systematic review. Dental Materials, 29(12), 1209–1218. https://doi.org/10.1016/j.dental.2013.09.009
Lane, J., Sadeghzadeh-Araghi, A., Jackson, G., & Bonsor, S. (2024). Survival and success rates of endodontically treated teeth restored with full veneer crowns or full cuspal coverage onlays: A systematic review. The European Journal of Prosthodontics and Restorative Dentistry, 32(1), 45–55. https://doi.org/10.1922/EJPRD_2547Lane11
Morimoto, S., Rebello de Sampaio, F. B. W., Braga, M. M., Sesma, N., & Özcan, M. (2016). Survival rate of resin and ceramic inlays, onlays, and overlays: A systematic review and meta-analysis. Journal of Dental Research, 95(9), 985–994. https://doi.org/10.1177/0022034516652848
NHS. (2026). Understanding NHS dental charges. Read the page
Spitznagel, F. A., Boldt, J., & Gierthmuehlen, P. C. (2018). CAD/CAM ceramic restorative materials for natural teeth. Journal of Dental Research, 97(10), 1082–1091. https://doi.org/10.1177/0022034518779759
Vagropoulou, G. I., Klifopoulou, G. L., Vlahou, S. G., Hirayama, H., & Michalakis, K. (2018). Complications and survival rates of inlays and onlays vs complete coverage restorations: A systematic review and analysis of studies. Journal of Oral Rehabilitation, 45(11), 903–920. https://doi.org/10.1111/joor.12695