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Every filling replaces missing tooth, but a filling does not make a tooth stronger. Past a certain size, the remaining walls become thin enough to flex and eventually split — often right down into the root, at which point the tooth cannot be saved.
A crown or an onlay solves that by covering the tooth instead of filling it, so chewing forces press the tooth together rather than wedging it apart.
The signs a tooth has reached that point
- The filling is wider than about half the biting surface
- One or more cusps have already broken off
- There is a crack line that hurts on release when you bite something hard
- The tooth has had root canal treatment
- The same filling keeps failing
Crown or onlay?
| What it covers | When it suits | |
|---|---|---|
| Onlay | The biting surface and one or more cusps | Enough healthy wall is left to keep |
| Crown | The whole tooth, down to the gum | Little sound tooth remains, or after root canal treatment |
An onlay removes less tooth than a crown, and where it is possible it is usually preferable. Twenty years ago a crown was the default; better adhesive materials have made the partial option far more viable.
What the appointments involve
First visit. Numbing, then the tooth is shaped — reduced by roughly a millimetre or two to make room for the new surface. A scan or impression is taken, the shade is matched, and a temporary is fitted.
In between. The temporary is deliberately weaker than the final one so it can be removed. Avoid anything very hard or sticky on that side, and clean around it carefully — slide floss out sideways rather than pulling it up.
Second visit. The temporary comes off, the fit and bite and shade are checked, and it is cemented. You will be asked to bite on coloured paper; say so if it feels high.
Materials
Most crowns now are all-ceramic, which look best and bond well. Zirconia is extremely strong and suits heavy biters and back teeth. Gold is less used cosmetically but is still, technically, excellent — it is kind to the opposing tooth and can be made very thin, so less tooth needs removing.
How long they last
Well-made crowns commonly last ten to fifteen years and often considerably longer. What ends them is rarely the crown itself. It is decay starting at the margin where the crown meets the tooth — which is a cleaning problem, not a materials problem.
That margin sits right at the gum line, which is exactly where interdental cleaning earns its keep.
The real decision is rarely crown versus filling. It is whether this tooth is being held together or merely filled in again.
