You’ve been told a tooth needs a crown. Maybe it already has a filling, and it doesn’t hurt. It’s a fair question whether another filling would do. Sometimes it would. Here’s how we decide, and the rule of thumb we use.
Key points
- A crown can’t be undone, so the first question is always whether something smaller will hold.
- Our rule of thumb: a new filling shouldn’t take up more than about half of the tooth’s biting surface.
- When decay comes back around an old filling, the new filling is bigger than the old one. That’s often when a filling turns into a crown.
- A crack running across the tooth from front to back needs a crown to hold it together.
- In our practice, back teeth get a crown after a root canal. Front teeth don’t always.
Start with the smallest repair that will hold
A filling replaces the part of a tooth that decay took and leaves the rest standing. A crown covers the whole tooth, and preparing a tooth for one removes structure that doesn’t grow back. Crowns are one of the most reliable things in dentistry, but that trade is worth making on a cracked molar. It isn’t worth making on a tooth that just needs a good filling.
Every filling we place is tooth-colored composite, which bonds to the tooth. That means we only remove what’s actually decayed. When decay is caught early, a filling is the whole treatment. More on fillings.
The half rule
A filling restores a tooth. It doesn’t strengthen it. The bigger it gets, the thinner the walls of tooth left around it, and thin walls can flex or crack when you bite.
So we use a rule of thumb: a new filling shouldn’t be more than about half of the biting surface of the tooth. Under that, a filling is usually the right answer. Past it, we start talking about an onlay or a crown.
A back tooth seen from above. The shaded area is filling. It’s a rule of thumb, not a measurement we take with a ruler, and the walls and any cracks matter too.
Why an old filling often becomes a crown
What usually ends a filling is new decay starting at the edge, where the filling meets the tooth. To fix it, we have to remove the old filling and the new decay around it, so the replacement is always bigger than the filling it replaces.
That’s how a tooth that’s been filled two or three times over the years ends up past the half rule. Nothing went wrong with the work. Each repair simply had less tooth to work with than the last. Old silver fillings, and when they should be replaced.
What the research shows, and what we’ve seen
No study we know of has tested the half line itself. The research that does exist measures size in other ways, and it points the same direction. A 2014 analysis pooled 12 long-term studies covering 2,816 composite fillings in back teeth. Fillings that covered more surfaces of the tooth were significantly more likely to fail, and decay and fracture were the main reasons.
A study in Belgium followed large restorations in back teeth, all placed by one dentist between 1982 and 1999. Half of the large composite fillings had needed redoing by 7.8 years, and half of the large amalgams by 12.8 years. More than half of the crowns were still going at 14.6 years, and how extensive a restoration was affected how long it lasted. Composites have improved since then, so we read those numbers as a direction, not a forecast.
It matches what we’ve seen. Watching restorations perform over close to 30 years, we’ve found that large fillings don’t last as long as small ones, and a crown will often outlast a large filling.
Opdam NJ, van de Sande FH, Bronkhorst E, et al. Longevity of posterior composite restorations: a systematic review and meta-analysis. J Dent Res. 2014;93(10):943-949. PubMed Van Nieuwenhuysen JP, D’Hoore W, Carvalho J, Qvist V. Long-term evaluation of extensive restorations in permanent teeth. J Dent. 2003;31(6):395-405. PubMed
Cracks change the answer
A crack that runs across the tooth from front to back, from the side that touches the tooth in front of it to the side that touches the tooth behind, is one we crown. A filling can patch over a crack, but it can’t hold the tooth together. A crown wraps the whole tooth and does.
A tooth that hurts on one spot when you bite, or when you let go, is one of the signs. Why does my tooth hurt when I bite?
After a root canal
In our practice, a back tooth that has had a root canal gets a full crown. A root canal hollows out the inside of the tooth, and the tooth has usually already lost structure to the decay or crack that made the root canal necessary. What’s left is more brittle, and back teeth do the heavy chewing.
Front teeth don’t always need one. It depends on how much of the tooth is left, and we’ll tell you which yours is. Either way, we plan it as part of the treatment from the start, so it isn’t a surprise later. Root canal or pull it?
Where an onlay fits
An onlay sits between the two. It rebuilds the part of the tooth that’s missing, including one or more of the cusps, and leaves the healthy walls standing instead of shaping down the whole tooth. On a younger tooth with a failing filling and good structure around it, that’s exactly what an onlay is for, and we still place them.
On teeth that have been wearing down for years, there often isn’t enough solid structure left for an onlay to be the conservative choice it was designed to be, and a full crown does the job better. We decide tooth by tooth. Why we moved toward full crowns for most of our patients.
Side by side
| Filling | Onlay | Crown | |
|---|---|---|---|
| What it rebuilds | The spot decay took | The missing part, including cusps | The whole tooth above the gum |
| Tooth removed | Only what’s decayed | Less than a crown | The whole tooth is shaped down |
| Best for | Under about half the biting surface | A younger tooth with good walls left | Past half, a crack, a back tooth after a root canal, a worn bite |
| Visits | One | Two, like a crown | Two, a couple of weeks apart |
We don’t do same-day crowns. We did for years, starting in 2000, and went back to a lab because of the bite. The full story is on our crowns page.
Worth asking if you’ve been told you need a crown
- How much of the biting surface is filling now, and how much will it be after the decay is out?
- Is there a crack, and which way does it run?
- Has the tooth had a root canal?
- Would an onlay work on this tooth?
- Can you show me the photo and the X-ray?
Not sure what your tooth needs?
We’ll show you what we see before we talk about treatment, and if a filling will hold, we’ll tell you. Call 702-734-0776 or tell us what’s going on.
General education, and not a substitute for an exam.