When a back tooth has a cavity or a broken filling that is too large for a simple filling but not damaged enough to need a full crown, a dentist may mention an inlay or an onlay. The two words sound almost the same, and many patients leave the chair unsure what the difference is.
This guide explains what inlays and onlays are, how they differ from each other, how they compare with fillings and crowns, and what questions are worth asking if one has been recommended for you.
What is an inlay?
An inlay is a restoration that is made outside the mouth to match the shape of the prepared area of a tooth, then cemented into place. The American Dental Association (ADA) glossary describes it as a fixed restoration that sits within the tooth and is made to correspond to the form of the prepared cavity before being luted, or cemented, to the tooth.
In practical terms, an inlay fills a space in the chewing surface of a back tooth, inside the raised points of the tooth called cusps. It does not cover those cusps.
What is an onlay?
An onlay is also made outside the mouth, but it covers more of the tooth. The ADA glossary describes an onlay as a restoration that covers one or more cusp tips and the adjoining chewing surface, but not the entire outer surface of the tooth.
Because an onlay reaches over one or more cusps, it is sometimes called a partial crown. It is often considered when a cusp is cracked, weakened or already missing.
Inlay vs onlay: what is the main difference?
The key difference is how much of the tooth the restoration covers:
- Inlay: fits within the cusps and restores the center of the chewing surface.
- Onlay: extends over one or more cusps to protect and rebuild them.
- Crown: surrounds the remaining visible portion of the tooth. The ADA glossary describes a crown as restoring missing tooth structure by surrounding the remaining part of the tooth above the gums.
You can think of these as steps along a range. The more of the tooth that is damaged or weakened, the more coverage a dentist may suggest. Which option fits depends on how much healthy tooth remains and where the damage is.
How are inlays and onlays different from fillings?
A regular filling is a direct restoration. The material is placed and shaped inside your mouth during a single visit. Inlays and onlays are indirect restorations. According to the ADA, indirect restorations are fabricated outside the mouth, usually in a dental laboratory, based on impressions of the prepared tooth. The ADA notes that this process usually takes more than one visit, while direct restorations can generally be completed in one visit.
The ADA also notes that some laboratory steps can take place in the dental office, and that computer aided design and milling (CAD/CAM) technology has expanded the options for ceramic restorations. Whether your restoration is made in the office or in a lab, and how many visits it takes, depends on the material and the workflow your dentist uses.
When might an onlay be recommended instead of a crown?
The ADA explains that a crown can strengthen a tooth with a large filling when there is not enough tooth left to hold the filling, protect a weak tooth from breaking, or restore a tooth that is already broken. An onlay may be considered when a tooth needs more support than a filling can provide, but still has enough strong structure that it does not need to be fully covered.
A crown may still be the better choice when damage is extensive, when a crack runs deep, or when there is not enough sound tooth left to support partial coverage. Only an exam and, when needed, X-rays can show which situation applies to a particular tooth.
What are inlays and onlays made of?
The ADA describes several groups of materials used for indirect restorations, including metals such as gold, ceramics and resin composites. Each has tradeoffs:
- Gold: The ADA notes that gold has long been considered among the most durable options for indirect restorations. MouthHealthy, the ADA's patient site, says gold inlays and onlays can last 20 years or more, though they are more costly and do not match tooth color.
- Ceramic: Ceramic restorations are tooth colored. The ADA notes that ceramics can be prone to chipping or fracture.
- Resin composite: The ADA notes that composite materials are easier to work with and are used as an alternative for inlays, onlays and single crowns, though they are generally less strong.
The right material depends on where the tooth is, how much biting force it handles, your habits such as clenching or grinding, and your preferences about appearance.
How long do inlays and onlays last?
There is no single answer. How long any restoration lasts depends on the material, the size of the restoration, how much healthy tooth supports it, your bite, and how well the area is kept clean. Gold has the longest track record, and the ADA cites reported average longevity of about 18 to 20 years for gold restorations. Tooth colored materials can also last many years, but results vary from person to person.
Daily brushing with fluoride toothpaste, cleaning between teeth and regular checkups help your dentist spot early wear, chipping or new decay around the edges before a small problem becomes a bigger one.
Questions to ask your dentist
- Why do you recommend an inlay, onlay or crown for this tooth rather than a filling?
- How much healthy tooth structure will remain?
- Which material do you suggest, and why?
- How many visits will it take, and will I have a temporary in between?
- Is there anything about my bite or grinding that could affect how long it lasts?
Wondering if an inlay or onlay is right for you in Palo Alto?
If you have a large or broken filling, a chipped cusp or a tooth that feels sensitive when you bite, an exam is the best way to understand your options. You can learn more about inlays and onlays at our Palo Alto office. If a tooth is painful, cracked or getting worse, please do not wait to have it checked.
Book an appointment to talk through whether an inlay, onlay or crown makes sense for your tooth.
Sources
American Dental Association: Glossary of Dental Terms
American Dental Association: Materials for Indirect Restorations
MouthHealthy (ADA): Fillings (Gold Inlays)
This article provides general education and does not replace an individual dental examination or diagnosis.
