Dental crown complications: The unexpected need for root canal treatment.

Why might a tooth that has had a crown placed (or is having a crown made for it) require root canal treatment?

It's true that a tooth's need for having a dental crown placed and its need for root canal treatment are two separate issues. The reasoning associated with placing a crown has to do with strengthening a tooth and restoring it to its original shape. Root canal (endodontic) treatment, on the other hand, only involves treating the nerve space inside the tooth. There is, however, a relationship between the two in the sense that (statistically speaking) a tooth that has had a dental crown placed is more likely to require root canal treatment at some point in the future.

Since there are so many variables involved with this issue, it is a difficult relationship to quantify. Some researchers, however, have attempted to do so:

Bergenholtz et al. (1991) - found that on a long-term basis 9% of crowned teeth as opposed to 2% of uncrowned teeth required root canal treatment.
Felton et al. (1989) - Found that 13.3% of crowned teeth required root canal treatment over the long-term as opposed to only .5% of unrestored teeth.
Whitworth et al. (2002), who reviewed the above studies as well as a number of others, suggested that a realistic estimate might be that 4 to 8% of crowned teeth will require root canal treatment within the ten years that follow their placement.

Why will some crowned teeth subsequently require root canal treatment?

You might wonder why the above-mentioned statistical relationship between dental crown placement and the need for (not previously diagnosed) root canal treatment exists. The answer lies in a status of the tooth's nerve tissue that could be referred to as "stressed dental pulp."

Dental crowns aren't used to repair pristine, intact teeth but instead are placed on those teeth that have experienced at minimum at least one, if not a lifetime, of cumulative insult. The effect of this trauma can be that the tooth's nerve tissue has survived but in a compromised state. It is no longer as resilient as it once was in the sense that it is no longer able to repair itself and healthily rebound from insult. Because of this, there can be at some point a stimulus (like placing a dental crown) that finally triggers the final degeneration of the nerve tissue and the subsequent need for root canal treatment.

What type of events can compromise the reparative nature of a tooth's pulp tissue?

Any type of event that causes trauma to a tooth can stress it. It could be obvious trauma such as that sustained as a result of a blow to the face. Or it could be other forms that are less obvious such as that less intense but more frequent trauma created by people who clench and grind their teeth. Other types of stresses are outright breakage or fracture of a tooth, the irritation caused by the presence of a large amount of tooth decay or the trauma associated with removing decay and placing a restoration (quite possibly repeatedly over the course of the person's lifetime). Each of these types of traumatic events can stress the nerve tissue inside a tooth, thus placing it in a state where its reparative and recovery abilities are compromised.