Inlays and Onlays Explained: Filling vs Inlay/Onlay vs Crown

When a back tooth has lost structure to decay or a fracture, there are usually three ways to rebuild it: a composite filling placed directly in the mouth, an inlay or onlay made outside the mouth and bonded in, or a crown that covers the whole visible tooth. The main deciding factor is how much healthy tooth remains, though your bite and any previous root canal treatment also matter. Below are the differences, when each option is considered and what research says about longevity. The right option for your tooth can only be decided after an examination and, where needed, X-rays.

Three options, depending on how much tooth is lost

Direct composite filling

Cleveland Clinic describes composite as a tooth-coloured resin containing glass or quartz filler. After the decay is removed, it is placed directly into the tooth in layers, hardened with a curing light and finished in a single visit. Cosmetic composite work on front teeth is covered on our composite bonding page.

Indirect restorations: inlays, onlays and overlays

Indirect restorations are made outside the mouth, on a digital or physical model, and then bonded to the tooth. Cleveland Clinic describes inlays and onlays as custom restorations for cavities too large for a filling but not damaged enough to need a crown. They can be made from ceramic or composite resin. Our inlays and onlays page has more detail.

Crown

A crown covers the entire visible part of the tooth. It is considered when little healthy structure remains or when the whole tooth needs protecting. Our crowns and bridges page covers the options.

Inlay vs onlay vs overlay

The chewing surface of a back tooth has raised points called cusps. Whether a restoration covers them determines its name:

• Inlay: sits within the cusps, filling the space in the centre of the tooth.
• Onlay: extends over at least one cusp, as Cleveland Clinic describes it.
• Overlay: covers the whole chewing surface, including all of the cusps.

By covering weakened cusps, onlays and overlays help support the tooth against biting forces, while usually removing less tooth from the sides than a crown does.

When a composite filling is enough

For small and medium cavities with sound walls, a composite filling is often all that is needed. It is done in one visit and usually removes the least tooth structure of the three options. Cleveland Clinic notes that fillings can last up to 10 to 20 years depending on the material and conditions, and the Oral Health Foundation says modern composites are much stronger than older white fillings and can last about as long as amalgam, depending on size, location and bite.

The evidence has limits too. A 2021 Cochrane review found low-certainty evidence that composite fillings in back teeth may fail about twice as often as amalgam, mainly because of new decay at the margins, with no higher risk of fracture. That is a reason to take isolation, careful technique and regular check-ups seriously, particularly for larger fillings.

When an inlay or onlay may fit better

An indirect restoration is often worth considering when:

• the cavity is too large to restore reliably with a filling, but enough healthy tooth remains to avoid a crown;
• one or more cusps are weakened or broken and need covering;
• a large old filling needs replacing anyway and the remaining tooth suits an indirect restoration;
• the contact with the neighbouring tooth and the chewing surface need to be shaped in a controlled way outside the mouth.

This does not make an inlay or onlay the superior choice in every case. A small cavity may be better served by a filling, and a heavily broken-down tooth by a crown. The decision is made tooth by tooth.

When a crown is needed

Cleveland Clinic lists weak, cracked, worn or root-canal-treated teeth, implant restorations and severe discolouration among the reasons for a crown. The American Dental Association's patient site adds large fillings with little tooth left and broken or weakened teeth.

Root-canal-treated teeth deserve a separate look. The NHS notes that a crown may be advised after root canal treatment if the tooth is weakened. In some teeth with enough sound structure, an onlay that covers the cusps can be an alternative. The remaining walls, any cracks and your bite decide which. Our root canal treatment page explains that procedure.

How ceramic inlays and onlays are made

Digital scan

The old filling and any decay are removed and the cavity is shaped for an indirect restoration. The tooth is then recorded with an intraoral scanner instead of a tray of impression putty. Cleveland Clinic notes that scanning is usually easier for people with a strong gag reflex, and a 2021 systematic review by Siqueira and colleagues found scanning faster than conventional impressions, generally preferred by patients and with similar prosthetic outcomes.

CAD/CAM design and production

The restoration is designed on the digital model against the neighbouring and opposing teeth, then made in one of two ways:

• Chairside: the design is milled from a ceramic block in the practice and can be bonded at the same appointment. Cleveland Clinic and the Oral Health Foundation describe this same-day workflow for crowns and point out that not every tooth or patient is suitable.
• Laboratory: the digital file is sent to a dental laboratory. This usually means two appointments, with a temporary restoration in between.

Materials include lithium disilicate glass-ceramic, feldspathic ceramic and composite resin blocks. Our digital dentistry page explains the workflow in more detail.

Adhesive bonding

Cleveland Clinic notes that inlays and onlays are bonded in place. During bonding the tooth is kept isolated from saliva, its surfaces are prepared, and the restoration is attached with an adhesive system and resin cement. The bite is then checked, adjusted if needed and the surfaces are polished.

Longevity: what studies show

A 2016 systematic review and meta-analysis by Morimoto and colleagues estimated survival of ceramic (glass-ceramic or feldspathic) inlays, onlays and overlays at about 92 to 95% after 5 years and about 91% after 10 years. The most common failure was fracture or chipping (about 4%), followed by complications needing root canal treatment (about 3%).

For a longer view, a practice-based case series by Otto and Mörmann, published in 2017 in the International Journal of Computerized Dentistry, reported Kaplan-Meier success of about 87.5% for chairside CAD/CAM ceramic inlays and onlays followed for up to 27 years. Most failures were fractures of the ceramic or of the tooth. It was not a randomised trial.

These figures are study averages, not a promise about any single restoration. The amount of remaining tooth, your bite, grinding habits and oral hygiene all influence how long a restoration lasts.

Care, check-ups and treatment from abroad

• Brush regularly with fluoride toothpaste and clean the margins with floss or interdental brushes.
• Keep up regular check-ups so the margins can be checked for new decay.
• If you clench or grind, mention it; a night guard may be discussed to reduce the risk of fracture.
• Avoid biting ice, pens or hard pits.
• Get in touch if you notice pain on biting, sensitivity or a rough edge.

If you are travelling for treatment, share your travel dates early: a chairside restoration may be completed in one visit when the tooth is suitable, while a lab-made one needs a second appointment. Take copies of your X-rays and scan files home so your local dentist can follow up. Records in Turkey use FDI two-digit numbering, whereas US dentists usually number teeth 1 to 32; the upper-right first molar, for example, is #3 in the US and 16 in FDI notation. Our tooth chart shows the FDI numbers.