In 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?
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.
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).
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.