The short answer
It usually comes down to how much healthy tooth is left. A filling repairs a small to mid-size cavity and relies on the surrounding tooth for strength. An onlay rebuilds one or more chewing cusps when a filling would be too large. A crown covers the whole tooth when the walls are thin, cracked or weakened by a root canal. Your dentist will check the tooth, your bite and X-rays before recommending one.
General information, not a diagnosis. Your dentist will examine your mouth before recommending treatment.
Start with how much tooth is left
You have been told a tooth needs work, and you want to know whether it is a quick filling or something bigger. Most of the answer is in one measurement: how much healthy tooth structure will remain after the decay or the crack is cleaned out.
The ADA's patient site describes a crown as a cover that restores a tooth to its normal shape and size, used when there is not enough tooth remaining to hold a filling, to protect a weak tooth from breaking, or to restore one that has already broken. A filling, by contrast, is shaped directly into the cleaned cavity and is suited to small and mid-size repairs. An onlay sits in between: a piece made outside the mouth that covers one or more chewing cusps without wrapping the entire tooth.
The three options are not grades of quality. They are matched to how much tooth is missing. A filling on a tooth that has lost a cusp is a repair waiting to fail, and a crown on a tooth with a small cavity removes healthy enamel for no benefit.
When a filling is usually enough
A composite filling (the tooth-colored kind, which is what this mercury-free practice places) is bonded directly into the tooth in one visit. The ADA notes that composite provides good durability and resistance to fracture in small to mid-size fillings that need to stand up to moderate chewing pressure, on front or back teeth.
Fillings are the usual answer when:
- the cavity is small or moderate and the walls around it are thick and sound
- the decay is caught at a checkup, before it has spread under a cusp
- a chip or worn edge needs rebuilding and the bite force on that spot is moderate
- an older filling has worn or chipped but the tooth around it is still healthy (see when old fillings need replacing)
When an onlay fits better
Once a cavity or an old filling has undermined a cusp, packing more composite into the tooth asks the thin remaining walls to carry chewing force they are not built for. The ADA's glossary defines an onlay as a restoration made outside the mouth that covers one or more cusp tips and the adjoining chewing surface, held in place with cement. It replaces the lost cusp with a solid piece of ceramic, resin or gold while leaving the healthy walls and the rest of the tooth alone.
Onlays are typically considered for a back tooth with a cavity or old filling that is too large for a direct filling but has not lost enough structure to need a crown. The what is an onlay guide walks through how one is made. Not every tooth has a clean onlay option, and your dentist may find after cleaning out the decay that the tooth needs more coverage than planned.
| Situation | Filling | Onlay | Crown |
|---|---|---|---|
| Small or mid-size cavity, thick walls | Usually | Rarely needed | Rarely needed |
| Large old filling, one cusp undermined | Often too large | Often considered | If the walls are thin |
| Cusp broken off around a filling | Sometimes, if small | Sometimes | Often |
| Crack running from the chewing surface toward the root | No | No | Usually, often with a root canal |
| Back tooth after a root canal | Rarely | Sometimes | Usually |
| Crack extending below the gumline | No | No | May not be savable; ask about options |
When a crown is the safer choice
A crown (what many people call a cap) surrounds the remaining tooth above the gumline. The ADA lists the usual reasons: strengthening a tooth with a large filling when there is not enough tooth remaining to hold a filling, protecting a weak tooth from breaking, restoring a broken tooth, anchoring a bridge, and covering a tooth that is badly shaped or discolored.
Two situations push a tooth toward a crown even when the cavity looks modest. The first is a crack. The American Association of Endodontists describes a cracked tooth as one where the crack runs from the chewing surface toward the root; it is treated with a root canal and a crown to keep the crack from spreading, and if the crack extends below the gumline the tooth can no longer be saved. The second is a tooth that has had a root canal. The AAE says a treated tooth is restored with a crown or filling for protection, and the practice's root canal page explains that a treated tooth can become more brittle, so a crown is usually placed.
If you want the warning signs in plain language, the post on signs you might need a dental crown covers them. Crowns can be made in a lab over two visits or milled in the office the same day with CEREC; the same-day vs. lab crown guide compares the two routes.
What waiting usually does
It is tempting to postpone a crown and ask for a filling instead. Sometimes that is reasonable, and your dentist will tell you if a filling can hold the tooth for now. More often the smaller repair is only available early. Decay that reaches the pulp turns a filling into a root canal plus a crown. A crack that was treatable with a crown can extend below the gumline, and the AAE is direct that a cracked tooth left untreated progressively worsens, eventually resulting in the loss of the tooth.
Some situations are not a wait-and-see decision at all. Facial swelling with fever, trouble breathing or swallowing, or bleeding you cannot stop needs an emergency room or 911 first, not a dental office. MedlinePlus advises contacting a medical provider for sudden, painful or severe facial swelling or difficulty breathing. A severe toothache without those signs is a dental problem; the practice offers same-day appointments when possible.
How to use this at your appointment
Ask your dentist to show you the X-ray and point to what is left of the tooth. Ask whether a cusp is already undermined, whether there is any sign of a crack, and whether the tooth has had or will need a root canal. Those three answers explain almost every filling-versus-crown recommendation.
If the plan is a crown or onlay, ask what material is proposed and why. The ADA notes that all-ceramic materials are used for inlays, onlays, crowns and veneers, that metal alloys have a long record for durability, and that back teeth fracture restorations more often than front teeth, which is why the choice depends on the tooth's location and your bite. The questions to ask before a crown, veneer or root canal guide gives you a short list to bring.
Coverage differs by plan. The practice accepts all PPO plans and does not accept Medi-Cal or Denti-Cal, and the office can review your benefits before treatment, as its insurance and payment page explains. The dental crowns page describes how a crown is placed here.