What is the Difference Between an Inlay and Onlay?

When it comes to restorations there are so many different options available. There are fillings, inlays, onlays, crowns, bridges, and more. Two of the more confusing restorations are inlays and onlays. Similar in name and function, it’s easy to mix them up when billing dental claims. With a little guidance, you can learn the difference between an inlay and an onlay. 

What is an inlay? 

According to the ADA, an inlay is “a fixed intracoronal restoration; a fixed dental restoration made outside of a tooth to correspond to the form of the prepared cavity, which is then luted to the tooth.” It’s essentially a restoration placed within the crown of the tooth. 

What is an onlay?

The ADA states an onlay is, “A dental restoration made outside the oral cavity that covers one or more cusp tips and adjoining occlusal surfaces, but not the entire external surface.” It’s a restoration that involves the cusps of the tooth but does not cover its entire surface.

How do I code inlays and onlays? 

There are 18 different onlay and inlay CDT codes ranging from D2510 to D2664. The codes vary based on the surfaces involved and the materials used. There are three different surface options for inlays (one, two, or three) and four for onays (one, two, three, and four). Both inlay and onlay codes have metallic, porcelain/ceramic, and resin-based composite options. 

When billing as yourself 3 questions to choose the correct CDT code. 

1. What treatment was completed; an inlay or an onlay? 
2. How many surfaces were involved; one, two, three, or four? 
3. What material was used; metal, ceramic/porcelain, or resin-based composite? 

If you need further assistance with coding, consider using a coding guide like Dental Coding with Confidence, from Practice Booster.  

How do I bill an inlay? 

When billing an inlay include the chart notes and ensure they include the surfaces involved in the restoration. Also, include pre and post-treatment x-rays and intraoral photos of the area involved. If the insurance carrier pays on the seat date, don’t bill until the inlay has been seated. If the inlay is a replacement, include the age (month/year) of the prior inlay and the reason for replacement. 

How do I bill an onlay? 

When billing an onlay include the chart notes and ensure they include the cusps involved in the restoration. If the cusps are not included on the chart note, the insurance carrier may downgrade the treatment to an inlay or filling. Also, include pre and post-treatment x-rays and intraoral photos of the area involved. If the insurance carrier pays on the seat date, don’t bill until the onlay has been seated. If the onlay is a replacement, include the age (month/year) of the prior onlay and the reason for replacement.  If your office utilizes 3D imaging, it helps to include it with the claim. A 3D image is often the difference between a full payment and a downgrade or denial.

Do inlays and onlays have frequency limits? 

Both inlays and onlays have the same frequency limits as crowns. Before completing treatment it’s essential to determine the insurance carrier’s frequency limit as it can range from 5-10 years, depending on the patient’s plan. If you are replacing an existing restoration make sure to find out the age (month and year) of prior placement so that your patient doesn’t receive an unexpected bill if frequency limits apply. 

Why did the insurance carrier downgrade an inlay to a filling? 

Most dental insurance plans do not cover inlays and they are almost always paid at an alternate benefit downgrade of a filling. It’s best to be aware of this ahead of time and bill accordingly so that you don’t need to adjust the account later. 

Inlays and onlay are very similar yet are completely different dental treatments. If you need help with differentiating between the two when billing, consider partnering with eAssist. Our Success Consultants are knowledgeable about CDT codes and will ensure the correct code is used each time. To learn more, schedule a consultation here.

Sandy Odle

Co-Founder and CXO, eAssist

Sandy Odle is the Co-Founder and CXO of eAssist Dental Solutions, where she has played a key role in shaping the company’s business and marketing strategies, leading to record growth and national recognition on the Inc. 500 and Utah Top 100 lists. A passionate social entrepreneur, Sandy believes that business is about building meaningful connections and creating personalized solutions that truly meet customer needs. Her relentless commitment to delivering exceptional client experiences is at the heart of eAssist’s mission. Drawing on the discipline and attention to detail honed during her early career as a ballet dancer, Sandy brings a unique blend of creativity, precision, and work ethic to everything she does.