Why do crowns fail?

There are several different reasons that a crown can fail. A crown relies on two main things to stay in the mouth. The first is the shape that the dentist has created of the actual tooth structure. A well prepared tooth will have nearly parallel sides and will have enough tooth structure left to support the crown. The parallel sides help the crown to stay in by providing retentive friction to the inside of the crown. In other words you want the tooth and the crown to touch each other over as much surface area as possible. If a tooth is cut down too much and if it looks too much like a tepee then it will have a harder time staying in. The second thing that holds a crown in is the cement. A good strong cement is needed to resist all of the things that are trying to make the crown come out. Like, the chewing forces that compress the cement 1000' s of times over the life of the crown and the expansion and contraction that happens as we eat and drink things that are hot and then cold. We also want to have a little of the cement exposed around the edge of the crown to prevent it from being washed away.

How do crowns work?

A crown works by covering the natural tooth to restore it's proper form, function, and look. If we have had a large cavity or if we have had a portion of the tooth break off, a crown may be the best treatment to restore and preserve that tooth. One great benefit of having a crown over having an excessively large filling is, that the crown encircles the entire chewing portion of the tooth and reinforces it against the pressure that we generate as we chew food, swallow, and grit our teeth together.