Introduction: there is something between a filling and a crown
When a cavity is too large for an ordinary filling, but the tooth does not yet require a full crown at all, indirect restorations come into play: the onlay and the overlay. These are solutions on the border between conservative dentistry and prosthetics — modern and tissue-preserving, and at the same time sometimes overlooked in conversation with the patient. An overlay and an onlay make it possible to rebuild a damaged tooth while preserving more healthy tissue than a crown and providing greater durability than an extensive filling.
In this article we explain how these restorations differ from a filling and from a crown, when it is worth choosing an onlay or an overlay instead of a large filling and when instead of a full crown, how the procedure itself works and — important for durability — how to care for such a restoration. At the end you will find a decision table that organises the choice according to the extent of damage to the tooth.
Key conclusions in brief
- Onlays and overlays are indirect restorations on the border between a filling and a crown — they preserve healthy tooth tissue.
- An onlay/overlay instead of a large filling: when the cavity is too extensive for a durable direct filling.
- An onlay/overlay instead of a crown: when enough healthy tissue remains and we want to avoid preparing the whole tooth.
- The procedure usually takes one to two visits; durability is determined by adhesive bonding, the material and the amount of tissue.
- Most often a restoration fails through secondary caries at the margin and through bruxism — margin hygiene and a protective splint help.
What onlays and overlays are — and how they differ from an inlay
All three are indirect restorations: they are made outside the mouth (in a laboratory or digitally, using CAD/CAM technology) and then bonded adhesively into the prepared tooth. They differ in how much of the biting surface they cover:
- Inlay — fills a cavity within the body of the tooth, without covering the cusps.
- Onlay — restores the cavity and covers at least one cusp, protecting it against fracture.
- Overlay (full onlay) — covers the entire biting surface and all the cusps, often raising or restoring the vertical dimension.
For comparison: a filling (direct restoration) is made straight away in the mouth from a material cured in the cavity, whereas a crown covers the tooth on all sides after it has been prepared. The onlay and the overlay sit exactly in between — hence their role as a tissue-preserving solution.
The philosophy of tissue preservation — onlay versus crown
The most important advantage of an onlay over a crown is biological. A crown requires the tooth to be prepared all the way round, which means removing some amount of healthy tissue. Onlays and overlays are prepared far more conservatively — mainly damaged tissue is removed, preserving the healthy walls of the tooth. This is in line with the modern, minimally invasive approach, which in research on the restoration of worn and damaged dentition is explicitly recommended (Fan et al., Clin Oral Investig 2025).
Preserving tissue matters in practice: the more healthy tooth that remains, the better the prognosis of the restoration and the easier any future treatment. That is why, when conditions allow, an onlay can be a wiser choice than going straight to a crown.
When an onlay instead of a large filling
A direct filling is excellent for small and medium-sized cavities. When, however, the cavity is extensive — involving the cusps, reaching between the teeth or leaving thin walls prone to fracture — a large filling can be less durable and distributes chewing forces less well. An onlay or overlay, by covering and protecting the cusps, then better protects the tooth against fracture.
This applies especially to teeth after root canal treatment, which are more brittle. Studies of molars and premolars after root canal treatment show that restorations with cuspal coverage, especially fibre-reinforced ones, withstand fatigue loading better than restorations without cuspal coverage (Fráter et al., Clin Oral Investig 2022). It is worth asking about this: with a large cavity, it is not only the onlay itself that matters but also the way the core (build-up) beneath it is restored — a well-designed, reinforced restoration “buys” the tooth more years.
When an onlay instead of a crown
If a tooth is damaged enough for a crown to be considered but has retained sufficient healthy tissue, an overlay can be an equivalent yet less invasive alternative. This also applies to many teeth after root canal treatment: it is not the fact of root canal treatment itself but the amount of tissue lost that determines the need for cuspal coverage, and an appropriately designed indirect restoration with cuspal coverage protects such a tooth without full preparation for a crown (Bhuva et al., Int Endod J 2021).
In prosthetic practice at Modern Dental & Orthodontics (Klinika MDO) we treat the overlay as a natural intermediate step: it allows function and aesthetics to be restored while at the same time leaving the way open for the future.
When a crown is nonetheless the better choice
In all honesty: an onlay is not a solution for everything. When the damage is very extensive, little healthy tissue remains, the so-called ferrule effect (a healthy ring of tissue) is lacking, or the tooth is heavily loaded and cracked, a full crown provides better encasement and protection. The decision is made individually, after assessing the amount of tissue, the occlusal load and the condition of the tooth.
Overlay and onlay — decision table
| Extent of the cavity / tooth condition | Recommended restoration | Notes / material |
| Small–medium cavity, healthy cusps | Filling (direct restoration) | Composite; quick, conservative |
| Cavity involving 1 cusp | Onlay | Ceramic or composite; protection of the cusp |
| Extensive cavity, whole biting surface | Overlay | Ceramic (lithium disilicate, zirconia); coverage of all cusps |
| Tooth after root canal treatment, enough tissue | Overlay (cuspal coverage) | Protection against fracture, less invasive than a crown |
| Very little tissue, no ferrule | Crown | Full encasement of the tooth |
The table is a starting point for a conversation with the dentist, not a ready-made prescription — the final choice depends on individual clinical assessment.
Materials and durability — what the research says
Onlays and overlays are most often made of ceramic (lithium disilicate, zirconia) or of (indirect) composite. They differ in durability. In a long-term (mean 7.8 years) retrospective study of partial posterior restorations — inlays, onlays and overlays — the survival of lithium disilicate ceramic was 96.8% and was higher than that of indirect composite (84.9%); among the factors significantly increasing the risk of complications were bruxism, hygiene and the type of material (Lempel et al., Dent Mater 2023).
The durability of the tooth foundation itself beneath ceramic restorations is also good: a review of monolithic zirconia crowns on natural teeth showed survival in the range of 91–100% over short-term observation, with a marked decline in patients with bruxism (Leitão et al., J Prosthodont Res 2022). The practical conclusion: the choice of material (often ceramic under greater loads) and control of bruxism — with a protective splint where needed — genuinely affect how long an onlay will last.
How placing an onlay works — step by step
Placing an onlay is a tissue-preserving and usually painless procedure (under local anaesthesia). It proceeds in several steps: removal of damaged tissue and the old filling, conservative preparation of the tooth for the onlay, taking an impression or an intraoral scan, fabrication of the restoration in a laboratory or digitally (CAD/CAM), and finally adhesive bonding — cementing the onlay to the tooth, which requires very good isolation from saliva.
In the classic scheme two visits are needed: at the first, the tooth is prepared and an impression or scan is taken (while the restoration is being made, a temporary restoration protects the tooth); at the second, the finished onlay is fitted. With CAD/CAM technology the whole process can be shortened to a single visit. It is precisely the adhesive-bonding stage that is crucial for durability: it is the quality of the bond, rather than the material itself, that largely determines how long the onlay will last.
How to make an onlay last
The most common causes of problems with indirect restorations are secondary caries at the margin of the onlay, fractures and — less often — debonding; in studies, hygiene, the type of material and bruxism had a significant impact on durability (Lempel et al., Dent Mater 2023). For the patient, three practical conclusions follow.
- Hygiene of the onlay margin: thorough cleaning of the restoration margin and the interdental spaces (floss, interdental brushes) protects against secondary caries — the most common reason for needing a replacement.
- Control of bruxism: if you clench or grind your teeth, a protective splint worn at night genuinely prolongs the life of the restoration, because bruxism significantly reduces its durability.
- The first few days and regular check-ups: immediately after placement it is worth avoiding very hard and sticky foods, and periodic visits allow minor problems to be caught early, before they become serious.
Frequently asked questions
How does an onlay differ from an overlay?
An onlay restores the cavity and covers at least one cusp of the tooth, protecting it against fracture. An overlay covers the entire biting surface and all the cusps, often restoring the vertical dimension. Both are indirect restorations, made outside the mouth and bonded adhesively; they differ mainly in how much of the biting surface they cover.
When an onlay instead of a filling?
When the cavity is too extensive for a durable direct filling — involving the cusps, reaching between the teeth or leaving thin walls prone to fracture. An onlay or overlay distributes chewing forces better and protects the cusps against fracture, which is especially important in teeth after root canal treatment.
When an onlay instead of a crown?
When the tooth is significantly damaged but has retained enough healthy tissue. An overlay then makes it possible to rebuild the tooth without preparing it all the way round, as with a crown. This is a less invasive, tissue-preserving solution. When, however, there is very little tissue, a full crown remains the better choice.
How many visits does placing an onlay take?
Most often two: at the first the tooth is prepared and an impression or scan is taken, at the second the finished onlay is fitted; between the visits a temporary restoration protects the tooth. With CAD/CAM technology the onlay can be fabricated and fitted during a single visit. The procedure is usually carried out under local anaesthesia.
What are onlays made of, and are they durable?
Most often of ceramic (lithium disilicate, zirconia) or indirect composite. Long-term studies show high survival, especially for ceramic (around 96.8% for lithium disilicate over nearly 8 years of observation). Durability depends on the material, the amount of tissue, the quality of the bonding and control of bruxism.
Can an onlay come unstuck or fall out?
This happens rarely and is usually linked to the quality of the adhesive bond or to overloading. If an onlay loosens, comes unstuck or fractures, you should contact the practice — it can often be re-bonded or repaired. It is not worth delaying, because an exposed margin promotes secondary caries.
Key takeaways
- Onlays and overlays are tissue-preserving indirect restorations between a filling and a crown.
- An onlay instead of a large filling protects the cusps and distributes forces better — important after root canal treatment.
- An onlay instead of a crown makes sense when enough healthy tissue remains; when it is lacking, a crown is better.
- The procedure usually takes one to two visits; durability is largely determined by the quality of adhesive bonding.
- Most often a restoration fails through secondary caries at the margin and through bruxism — margin hygiene, a protective splint and check-ups help.
Read more:
- Dental crowns (prosthetics): klinikamdo.pl/en/offer/prosthetics-warsaw/dental-crowns/
- How long does All-on-4 last? https://klinikamdo.pl/en/blog/how-long-does-all-on-4-last/
- Dental implant vs bridge — which is better? https://klinikamdo.pl/en/blog/dental-implant-vs-bridge/
- Zirconia, all-ceramic or metal-based crown? A comparison of modern prosthetic solutions https://klinikamdo.pl/en/blog/zirconia-ceramic-or-metal-crown-comparison-of-prosthetic-solutions/
- Treatment fees: https://klinikamdo.pl/en/treatment-fees/
Sources
Source 1
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Description Lempel E, et al. „Clinical evaluation of lithium disilicate versus indirect resin composite partial posterior restorations – A 7.8-year retrospective study.” Dent Mater. 2023;39(12):1095-1104.
Source 2
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Description Fan J, et al. „Clinical performance of minimally invasive full-mouth rehabilitation using different materials and techniques for patients with moderate to severe tooth wear: a systematic review and meta-analysis.” Clin Oral Investig. 2025;29(2):96.
Source 3
Links https://doi.org/10.1007/s00784-021-04319-3 │ https://pubmed.ncbi.nlm.nih.gov/34846558/
Description Fráter M, et al. „Fatigue performance of endodontically treated premolars restored with direct and indirect cuspal coverage restorations utilizing fiber-reinforced cores.” Clin Oral Investig. 2022;26(4):3501-3513.
Source 4
Links https://doi.org/10.1111/iej.13438 │ https://pubmed.ncbi.nlm.nih.gov/33128279/
Description Bhuva B, et al. „The restoration of root filled teeth: a review of the clinical literature.” Int Endod J. 2021;54(4):509-535.
Source 5
Links https://doi.org/10.2186/jpr.JPR_D_21_00081 │ https://pubmed.ncbi.nlm.nih.gov/34615842/
Description Leitão CIMB, et al. „Clinical performance of monolithic CAD/CAM tooth-supported zirconia restorations: systematic review and meta-analysis.” J Prosthodont Res. 2022;66(3):374-384.