Why do I need a crown and not just a filling?

84u0JgEIn 2o years as a practicing restorative dentist, I must have told thousands of patients that they needed a crown. Aside from special situations like implants or teeth with root canals, why are crowns necessary?

amalgam-fillings
All 3 of these teeth have fillings of varying widths which are failing and are in need of replacement. Do they get fillings or do they get crowns?

Take a look at the picture of the three silver fillings. All 3 are in some degree of breakdown and of different widths. If the hole left from the removal of the filling is more than 50% of the width of the tooth, it means that there is more filling than supportive tooth structure, and there’s a decent chance that the tooth will split when put under pressure. That’s when we cover the tooth with a crown.

Think of a telephone pole that is splitting from the top. If one covers it with a metal bucket, it can be struck as hard as possible and it won’t split further. That’s what a crown does. After preparing the tooth into a shape that allows a crown to fit over it, it covers the tooth, replaces missing tooth structure, and keeps it from splitting.

crown-on-a-tooth
A crown is a cover over the prepared tooth structure and looks like a tooth, replacing lost tooth structure.

Now look at the original image of the three fillings again. The blue area represents the area taken up by the removed filling and any decayed/undermined tooth structure. The left tooth clearly will have enough tooth structure remaining to perform just a filling while the middle one will be over 50% gone, requiring a crown. The tooth on the right is a mix between the first two teeth and might be a great candidate for an onlay (a partial crown).

amalgam-fillings
The red line represents the overall width of the tooth and the blue represents the ultimate shape of the restoration. Notice how the middle tooth would have a hole more than 50% of the width of the tooth and without a crown to cover it, it might split.

The decision to make a crown instead of a filling is a subjective one and up to the discretion of the dentist. However, when the clinician feels that the tooth is in danger of fracture because of a filling that would be too large, a crown can be a fantastic option.

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