The root canal / dental crown relationship. -

Do teeth that have crowns always need root canal treatment? / If not, why do so many end up having it?

A) The relationship between dental crowns and root canal treatment.

Some people seem to think that if a tooth needs a crown that it also has to have root canal therapy. To the contrary, these procedures are entirely separate dental procedures and, most certainly, not every tooth that gets the former needs the latter.

Having said that, there is a relationship between the two, in the sense that (statistically speaking) a tooth that has had a crown placed on it is more likely to require root canal at some point in the future.

Dental research has attempted to quantify this association:

  • Bergenholtz (1991) found that on a long-term basis 9% of crowned teeth as opposed to 2% of uncrowned teeth required root canal treatment.
  • Felton (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 (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 the restoration's placement.
  • Cheung (2005) evaluated 122 teeth that received crowns that had no previous history of root canal treatment. At 10 years, 16% of the study teeth had experienced endodontic complications and at 15 years 18%.

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B) Why do some crowned teeth require root canal?

You might wonder why the above statistical relationships exist. For the most part, the answer lies in the fact that dental crowns are used to rebuild teeth that have experienced significant structural damage. And, as it happens, these same types of events can also damage a tooth's nerve tissue and therefore create a need for root canal treatment too.

An analogy.

It's a little bit like the similarity between automobile bodywork (getting a crown) and under-the-hood work (having root canal treatment). If you are in an accident (tooth breakage) you will need bodywork (the crown). If the accident has been especially severe and your radiator has been damaged, then you will need under-the-hood work also (root canal treatment), but it just depends on the specific nature of the accident.

So it's not that placing the crown has caused a need for root canal therapy, but instead that the events that have led to the need for the crown have actually caused a need for both.

Root canal treatment being performed before placing a crown.

Root canal treatment being performed.

1) The timing of the root canal treatment can be quite varied.

There can be a lot of variability as to when the need for the root canal therapy finally becomes apparent.

  • In many situations it will be obvious before any dental work is begun.
  • Less frequently, it may be during or immediately following the crowning process.
  • It's quite likely that the need may not become evident until a later point in time, possibly even many years later.
2) The timing of events may seem suspicious.

In some instances, the case might be made that the crowning process has precipitated the need for the root canal treatment. This might be but, in general, it's typically not the original causative factor. Here's an explanation.

Your tooth was probably worse off than realized.

Dental crowns aren't used to repair pristine teeth but instead those that have experienced some type of structural catastrophe (crack, fracture, large cavity, lost filling).

And due to having experienced a significant traumatic event, it can be that the tooth's pulp tissue, while still able to survive, lies in a compromised state. (As in it's no longer as resilient as it once was and therefore isn't as able to healthily rebound from trauma or insult.)

While in this state, it's possible that some type of stimulus (including the process of placing a crown) will trigger the final degenerative process of the tooth's nerve, and its subsequent need for root canal treatment.

C) How might placing a dental crown stress a tooth?

If you think about the procedure from your tooth's point of view, the crowning process is a big event.

A drill is used to grind away essentially all of a tooth's enamel layer, and a fairly substantial amount of the dentin encasing its nerve (pulp tissue) too. Related to this process:

  • The act of drilling creates friction and therefore heat that the dentist must then keep in check so the tooth doesn't become overheated. (That's why dental drills spritz a mist of water.) Anusavice (2007) suggests that the greatest risk of creating tooth trauma is heat generation during tooth preparation.
  • The drilling creates jarring vibrations that are transmitted throughout the tooth (physical trauma).
  • Because the dentin layer (the hard tooth structure that surrounds the tooth's pulp) is porous, the mechanical action of the dentist's drill can pump bacteria into this layer towards the nerve tissue, which can result in pulpal irritation.
  • Similarly, Anusavice (2007) suggests that the tooth's exposure to various chemicals used throughout the entire crowning process can result in trauma to its pulp tissue too.

The effects of each of the separate events mentioned above can be cumulative. And the thinner the layer of dentin that exists between the surface of the crown preparation and the tooth's nerve, the more likely that the level of trauma ultimately reached might be significant.

Isn't this reason enough not to have a crown made for a tooth?

No, if a crown is indicated, then that's the best plan. Yes, possibly the trauma of the procedure will, on its own, be the traumatic event that triggers the final degeneration of the tooth's pulp tissue.

But if so, it's most likely that the procedure was simply the event that finally pushed the health of the compromised tooth's nerve over the edge. The crowning process precipitated the timing of an outcome that would have ultimately occurred anyway.

D) Should your dentist have anticipated this problem?

You can't really expect your dentist to be that all-knowing. There are tests that they can perform so to evaluate the health of a tooth. And typically a dentist will perform them before beginning the crowning procedure.

But the results of these tests are often vague and difficult to interpret. If collectively they all point to the same conclusion, then fine. But when they don't, a dentist will probably not perform root canal treatment for fear that it's unneeded.

 

 
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