Inlays and onlays: what they are, when to choose one over a filling or a crown, how long they last and what they cost

What are an inlay, an onlay and an overlay?

Inlays, onlays and overlays are indirect restorations, meaning they are made outside the patient’s mouth, fitted to the prepared cavity and permanently fixed in the tooth. MeSH, the medical vocabulary of the US National Library of Medicine, describes an inlay as a restoration of metal, porcelain or plastic made to the shape of a cavity and cemented into the tooth, and an onlay as a restoration that also covers the chewing surface. In Polish, all three are called wkłady koronowe.

The difference between them lies in how much of the chewing surface they cover. The choice is not a matter of taste but of how much sound tissue remains and whether the cusps are strong enough to do without being covered.

• An inlay fills a cavity inside the tooth and does not cover the cusps.
• An onlay covers the cavity and one or more cusps, which it replaces or protects.
• An overlay covers the whole chewing surface of the tooth but, unlike a crown, does not wrap around its side walls.

Inlay or filling: when does an inlay have the advantage?

An inlay or onlay has the advantage over a filling for a large cavity in a back tooth, especially when the cusps are weakened and an ordinary filling would have to rebuild several surfaces at once. For a small cavity an inlay is not needed, because a good composite filling removes only diseased tissue and is done in one visit.

Data on fillings support an inlay for large cavities. In a meta-analysis of patient data from 12 studies of composite fillings in back teeth, each additional surface of the filling increased the risk of failure by 30 to 40% (Opdam et al., 2014). An older review also found that indirect restorations in back teeth had a lower mean annual failure rate than direct fillings (Manhart et al., 2004), although composites have improved since then.

Not every comparison favours an inlay. In a review of randomised trials, laboratory-made composite inlays did not differ significantly from composite fillings after 5 or 11 years, although there were only 3 studies (Angeletaki et al., 2016).

Inlay, onlay or crown: a comparison

Inlays and onlays preserve more tissue than a crown, which requires the tooth to be ground down all the way round. In a laboratory study on models of front teeth, crown preparation removed 63 to 72% of the tooth’s crown tissue (Edelhoff and Sorensen, 2002), which is why an inlay is considered when enough sound tissue remains to put off a crown.

In studies, both options last about equally long. A review of 9 studies reported mean survival of 90.89% for inlays, 93.50% for onlays and 95.38% for crowns, with 5-year survival above 90% in all groups; a meta-analysis was not possible (Vagropoulou et al., 2018).

How long do inlays and onlays last?

In studies, ceramic inlays and onlays usually last many years: in a review of 14 studies, survival of glass ceramic and feldspathic ceramic restorations was 92 to 95% after 5 years and 91% after 10 years (Morimoto et al., 2016). The most common reasons for failure were fracture or chipping (4%), complications requiring root canal treatment (3%), secondary decay (1%) and debonding (1%).

In the same review, restorations in non-vital teeth, that is, after root canal treatment, did worse than in teeth with a living pulp. An older review reported mean annual failure rates in back teeth of 1.4% for gold inlays, 1.7% for CAD/CAM milled ceramic, 1.9% for ceramic and 2.9% for composite inlays (Manhart et al., 2004).

Figures from studies describe groups of patients, not a particular tooth. How long an inlay lasts also depends on hygiene, the risk of decay, biting forces and regular check-ups, at which the dentist checks the seal at the margins.

Composite, ceramic or gold inlay: materials and prices

An inlay or onlay can be made of composite, ceramic or gold, and the material affects the appearance, how it is fixed and the price. Lithium disilicate is a type of glass ceramic; in the latest review of crowns, from 2026, monolithic crowns made of this material had the highest survival after 5 years, 98.5% (Pjetursson et al., 2026), although these data concern crowns, not inlays.

In the clinic’s price list, “Inlay/Onlay kompozytowy” (composite inlay/onlay) costs 1 200 zł, “Inlay/Onlay porcelanowy” (porcelain inlay/onlay) 1 800 zł and “Inlay/Onlay/Overlay dwukrzemian litu” (lithium disilicate inlay/onlay/overlay) 1 900 zł. The dentist sets the final scope and cost at the consultation, and prices may change.

How is an inlay or onlay fitted?

Fitting an inlay or onlay usually takes two visits, because in between the dental laboratory makes the restoration from an impression or scan. It can also be made with CAD/CAM technology, that is, designed by computer and milled from a ceramic block.

The usual order of stages is:

• An examination and an X-ray, on which the dentist assesses the depth of the cavity, the condition of the pulp and the thickness of the tooth walls.
• Removing decay or an old filling and shaping the cavity so that the inlay can slide in.
• An impression or intraoral scan, and choosing the colour of the material.
• Placing a temporary filling, which protects the tooth until the next visit.
• A try-in of the inlay, bonding or cementing it, and adjusting the bite.

When is an inlay or onlay not a good choice?

An inlay or onlay is not a good choice when the cavity is small, because a filling then removes less tissue and is done in one visit. Nor will it work when the tooth has lost most of its walls or a crack runs deep, because a crown or another type of restoration is then needed.

A tooth after root canal treatment needs a separate assessment. Onlays or overlays that cover the cusps can be an intermediate option between a filling and a crown for back teeth after root canal treatment, but inlays and onlays in non-vital teeth did worse in studies (Morimoto et al., 2016). The details are covered in a separate article on restoring a tooth after root canal treatment.

When the risk of decay is high, its causes are best brought under control first, because secondary decay at the margins is among the reasons both fillings and inlays fail.

How inlays and onlays are made at DentalBlue

At DentalBlue in Białystok, fillings, inlays and onlays are among the clinic’s services, and prosthodontics works in the same building, so with a large cavity the dentist can compare a filling, an inlay and a crown in a single plan. Intraoral and panoramic X-rays are taken in the clinic’s own radiology unit. A patient without pain can start with a “Wizyta bez bólu” (visit without pain, 300 zł), which ends with a written treatment plan, and a patient in pain with a “Wizyta z bólem” (visit in pain, 340 zł). The dentist sets the final scope and cost, and prices may change.