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Tooth Crown vs. Filling vs. Onlay: How a Dentist Decides What Your Tooth Needs

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You go to the dentist for a routine cleaning and exam. Your images get pulled up and there on the screen is a shadow. Your dentist takes a closer look, and sure enough there’s decay. So now what? Do you need a filling, an inlay or onlay, or a crown? What determines which one makes sense?

It ultimately comes down to how the decay has impacted the structure of the tooth. Once the decay has been removed, how structurally sound is the tooth? Can it withstand chewing forces on its own? And if it can’t, how much of it needs to be covered?

In this article, we’ll break down how these questions guide treatment and explain what your different options mean.


Defining the Four Options

What is a filling?

A filling is used as a repair for a tooth that is still able to hold itself together. The decay is removed and a dentist fills the hole that’s left behind with a tooth-colored composite that’s shaped by hand in a single visit.

‍What is a dental inlay or onlay?
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An inlay does the same job as a filling but for a bigger hole. Instead of being built by hand directly in your mouth, it is made as a single piece in a dental lab using a scan of your tooth. It’s then bonded into place by your dentist.

A dental onlay serves a slightly different function than an inlay. While still custom-made in a lab, it has a bigger structural role, because it has to go up and over the cusp of your tooth (the raised part of a tooth’s chewing surface). When you chew, the cusps of your teeth take the brunt of those forces. The job of the onlay is to take on those forces, protecting a cusp that no longer can.

‍What is a tooth crown?

A crown is for a tooth that is too far gone to be able to reliably anchor an inlay or onlay, and needs a significant amount of support in order to bring function back to the tooth. How it works is everything above the gumline is reduced to a core, and then capped in ceramic, gold, or porcelain over metal. This cap holds the tooth together and functions like a normal tooth would. 

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When a Filling Is Enough

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If the cavity is small to moderate, and the walls and cusps of the tooth are intact, then a filling can be the best option. In this case, the tooth is still structurally sound and can carry chewing forces on its own. The filling merely fills the hole that was left behind by removing the decay.

Dentists are held to certain standards of care, and when it comes to treating cavities, the American Dental Association’s guidelines call for keeping as much of the original tooth as possible when removing decay. The goal is conservative treatment, which also carries into the decision of whether to even do a filling in the first place. For example, sometimes a shadow on an X-ray can indicate the beginning of a problem that hasn't broken through the enamel yet. At this point, a filling may not be necessary, and fluoride varnish or sealant may be the better option. This is when your dentist might say “let’s watch it.”

For those early spots, Noble offers Curodont, which is a treatment that’s applied to the tooth and helps the enamel rebuild itself. A 2023 review in the Journal of the American Dental Association found it to be effective at stopping early cavities before they need a filling.

At Noble, if a filling is all you need, a filling is all that will be recommended.

When Dental Inlays and Onlays Are the Best Option


Inlays and onlays are the best option when the hole left behind from removing the decay is too big for a filling to reliably handle, but not so big that the tooth needs a full crown.

What does it mean for a hole to be too big for a filling? A filling isn’t designed to carry the bite on its own, so when the hole is sized and shaped in such a way that the repair needs to take on chewing forces, a sturdier, lab-made piece makes more sense.

The healthy walls of the tooth, which a crown would cut down, stay where they are, and the inlay or onlay gives the tooth the structure that it needs to absorb the forces of everyday activities like chewing. A dentist will choose an onlay for teeth that have lost a cusp but have healthy walls intact, and an inlay when the cusps are all still standing and the hole in the center of the chewing surface is too large for a filling.

Porcelain inlays and onlays are the most common, but gold and lab-processed composite are also used. At Noble, inlays and onlays are made in a lab using a digital scan of your teeth, and then bonded into place.

Onlays and inlays have a long track record of holding up over time. A 2016 review published in the Journal of Dental Research took the data from 14 studies and found the survival of ceramic inlays, onlays, and overlays to be 92 to 95 percent after five years, and 91 percent after ten. The most common way they failed was fracturing and chipping, which happened in only about 4 percent of cases.

When a Tooth Needs a Crown

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A crown is the best option when there isn’t enough healthy tooth left for an inlay or onlay to anchor to, or when the tooth needs support from all sides in order to stay together.

Here are a few common reasons why a tooth may need a crown:

  • A tooth has a large filling that’s failing and there isn’t enough tooth remaining to hold a new one. 
  • A tooth is cracked and a crown prevents the crack from getting worse. 
  • A root canal was performed on a back tooth.
  • A tooth has been worn down or broken. 

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In all of these cases, the problem with the tooth goes beyond just a weak cusp that could be remedied with an onlay. When a crown is needed, it means that the structure of the tooth is failing and can no longer handle chewing forces.

Cracks are some of the most clear-cut cases for crowns. The American Association of Endodontists puts it like this: “unlike a broken bone, the fracture in a cracked tooth will not heal.” A crown provides support by wrapping the tooth on all sides so the crack doesn’t worsen.

Crowns do come at a cost unrelated to money. When a tooth is prepared for a crown, most of the tooth’s structure above the gumline is removed and that structure never grows back. This is why a tooth crown is always the option that comes after a filling, inlay, and onlay have been ruled out.

What about a front tooth crown? A front tooth crown follows the same logic as one you’d get on a back tooth, but there’s just a bigger emphasis on aesthetics and making sure the porcelain matches the shade of the surrounding teeth.

At Noble, crowns are made by a dental lab using a digital scan of the tooth, and then bonded into place.


Tooth Crown vs. Filling: What the Dentist Is Looking at on Your X-ray


Here are the things that your dentist is looking at and weighing when deciding on the best way to treat your tooth:

‍How deep and wide the decay is relative to the tooth

When a cavity stays inside the cusps of the tooth, and it hasn’t impacted the walls of the tooth, a filling is the likely answer. However, when the decay has hollowed out the tooth underneath a cusp or runs across most of the chewing surface, that’s when an onlay, inlay, or crown comes into the picture, depending on how much of the tooth’s structure has been compromised by decay.

‍How thick the remaining walls are

When the walls of the tooth are thin, a filling doesn’t provide the needed structural support, and the walls crack. If a cusp has been hollowed out or has broken, then it needs to be covered, whether that’s by an onlay if the rest of the tooth is sound or by a crown if it’s not.

‍Whether there’s a crack, and if there is, how deep it runs

A fractured cusp, a crack that has reached the nerve, and a crack below the gumline all require different treatments. For a fractured cusp, depending on the severity, a filling or a crown is used. A crack that’s reached the nerve, on the other hand, needs a root canal followed by a crown. Then finally, for a crack below the gumline, the tooth usually can’t be saved. The depth of the crack is a big part of determining treatment.

‍Whether the nerve is involved

If decay or a crack has reached the nerve, then a root canal is needed first, and on a back tooth a crown usually follows. A 2002 study published in the Journal of Prosthetic Dentistry found that teeth that got a root canal and didn’t get a crown after were lost at six times the rate of the ones that did get a crown.

‍The tooth’s history

Whenever a filling is replaced, a little bit more tooth comes out with it, which can start to impact the structure of the tooth. A 2014 review published in the Journal of Dental Research covered 2,816 fillings in back teeth and found that the more surfaces a filling covered, the more likely it was to fail, and that fillings mainly failed through new decay and fracture.

‍How much force the tooth takes

Molars take more force than premolars, and people who clench and grind their teeth put more force on their teeth than people who don’t. This means that the same cavity can mean a filling on one tooth, but an onlay on another. It really depends on the conditions and forces the tooth will be put through.

It’s important that your dentist gets the treatment right, because if they don’t, it comes with consequences. For example, a filling where a crown was needed can end in a cusp fracturing, sometimes even below the gumline, making it so the tooth can’t be saved. Or in the other direction, maybe a crown is placed where a filling would have done the trick, and now healthy tooth structure has been removed for nothing and you’re committed to replacing that crown for the rest of your life.
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How Long Fillings, Inlays, Onlays, and Crowns Last


The research shows that fillings, inlays, onlays, and crowns can all last a decade or more.

Fillings

According to a review published in 2014 in the Journal of Dental Research, back tooth composite fillings fail at about 1.8 percent per year at the five year mark and 2.4 percent per year at the ten year mark.

‍Onlays and Inlays

The 2016 review in the Journal of Dental Research that we covered previously found the survival rate of onlays, inlays, and overlays to be 91 percent after ten years.

Crowns

A 2015 review published in Dental Materials found that at the five-year mark, the survival rate of crowns made from porcelain fused to metal was 94.7 percent, lithium disilicate was 96.6 percent, and zirconia was 92.1 percent. Cleveland Clinic states it plainly: crowns that are properly taken care of last anywhere from 5 to 15 years.

The things that can shorten the life of any of these include the following: new decay at the edge of the restoration, grinding your teeth, and biting down in a way that the restoration material wasn’t built for. Good habits and getting regular cleanings and exams are key for extending the life of restoration work.
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Common Questions


‍Can I get a filling instead of a crown?

It depends. If there is enough healthy tooth remaining to hold a filling and there isn’t a crack, a filling or an onlay may be the better choice. An X-ray and a proper exam from a dentist will determine if it’s possible.

‍How much tooth is needed for a crown?

Less than you might think on top, because the tooth is shaved down to a core anyway. However, for a crown to work as it should, it does need to sit on healthy tooth structure, so the tooth right above the gumline needs to be healthy all the way around. If decay or a crack runs below the gumline, there’s nothing for the crown to seal against, and is where the tooth may not be saveable.

‍What are tooth crowns made of?

Most are ceramic. A porcelain tooth crown, which can be either porcelain that sits over a metal base or an all-ceramic material like lithium disilicate, is the usual choice when aesthetics matter. A zirconia dental crown is the common option when strength is most important. Gold is also still used sometimes on back teeth, but has become much less common.

‍Do I need a crown after a root canal?

For back teeth, the answer is usually yes. Published in the Journal of Prosthetic Dentistry, a 2002 study found that root-canal-treated teeth without crowns were lost at six times the rate of crowned teeth.

How many visits does a crown or onlay take?

Usually two. One to prepare the tooth, take the necessary scans, and place a temporary. Then a second visit to fit and bond the lab-made piece to your tooth. 

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How The Decision of Tooth Crown vs. Filling vs. Onlay Is Made at Noble Dentistry of Chicago


At Noble, we take the necessary scans and images of your mouth to get a comprehensive understanding of what’s going on, and then Dr. Jared Siegel walks you through what he sees. He’ll show you where the decay is, explain how much wall is left, and tell you if there’s a crack. The most important thing to him is that you fully understand the why behind a treatment recommendation. We will always take the most conservative approach: a filling before a crown, a crown before an extraction, and nothing before it’s needed.

If you have any questions about a treatment you’ve already been told you need, or you want to know where your teeth stand, give us a call or book a consultation through our site. If it’s been a while since you’ve been to the dentist, no worries, we’ve seen it all, and we’ll never make you feel bad about how long it’s been. After all, you’re here now!

Noble Dentistry of Chicago is located at 3052 N Lincoln Ave in Lakeview, a short walk from the Southport Corridor and Roscoe Village.

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