How does a dental crown strengthen a tooth? / When is a crown a better choice than a filling?

The strengthening capability of a dental crown is due to the fact that it cups over and encases the tooth on which it is placed.

Dental crowns cup over and strengthen teeth.

A crown literally acts as a splint that holds a tooth together. Once a crown has been placed, the tooth can withstand a greater level of chewing forces without a risk of fracture.

In comparison, dental fillings don't substantially increase the overall strength of a tooth. And, in fact, in some instances they can actually place a tooth at greater risk for breakage. Here's an explanation.

1) With small fillings, there's usually no concern.

A small cavity repaired by a small dental filling.

The ideal situation for placing a filling is when it's used to repair a relatively minor amount of tooth damage, such as a small cavity.

In these instances, even after the preparation for the filling (the filling's "hole") has been created, the tooth remains relatively intact. And because of this, the tooth can be expected to withstand chewing forces well.

2) Large fillings may leave a tooth at risk.

A big cavity repaired by a big dental filling.

In situations where a relatively larger amount of tooth structure has been lost or damaged, the picture may be different. With these cases, the overall structural integrity of the tooth may have been compromised, possibly significantly so.

It's somewhat like this ...

The structural integrity issue we're discussing here is a little like what you find with an egg shell.

If you take a raw egg and you want to break it open, well it really takes a pretty firm rap. That's because an egg shell, as an intact unit, is a surprisingly strong object.

Now, in comparison, say you've broken the egg open and the two halves of the empty shell are lying on their sides. In this condition, it's a fairly simple feat to crush each piece flat. The shell's structural integrity has been compromised. It's no longer an intact unit that's capable of withstanding forces well.

This is why teeth with fillings can be fragile.

Teeth are somewhat the same, in the sense that once a large portion of a tooth is lost (because it has broken, decayed, or has been drilled away) it is simply no longer as sound. The parts that remain can be fragile (thin, weakened, unsupported) and therefore at greater risk for fracture.

It's true that, yes, a filling can be placed for teeth in this condition. That's not really the issue here. The problem is that if a filling is placed, the strength of the tooth is not greatly enhanced.

A cracked tooth.

A filling doesn't reinforce a tooth (like a crown does). And, in fact, all a filling does is occupy the space of the lost tooth structure. Any fragile or weakened tooth portions that remain are still at risk for breakage.

Some fillings may create a wedging effect that causes tooth fracture.

There's yet another concern that comes into play when making to decision about whether to place a dental crown or a comparatively large filling. It has to do with the potential (harmful) affect that the filling may have.

In contrast to a crown that cups over, a filling is placed within a tooth (it relies on the tooth's remaining structure to anchor it). And due to this configuration, the filling has the potential to act as a wedge when chewing pressure is applied to it.

If the biting forces are great enough, or continually reoccurring, at some point this wedging action may cause the tooth to crack or fracture.

3) What constitutes a big dental filling? When should a crown be placed?

Well, of course, this is the big question. And it's more or less what you are relying on your dentist to advise you about. It's your dentist's obligation and responsibility to provide you with opinions, information, and options concerning how best to restore your teeth.

Here are some common guidelines.

At minimum, any filling that is greater than about 1/3rd of the width of its tooth should probably be considered to be a filling large enough that the tooth's overall strength has been compromised and it now makes a reasonable candidate for a dental crown.

Pictures of large dental fillings.

Some dentists are even more conservative in their opinion. They feel that if a filling is greater than about 1/3rd of the distance between a tooth's cusp tips, it should have a dental crown placed.

Here are some examples of "large" fillings.

Take a look at the pictures to the right. Each of the dental fillings shown could be considered to be "large," and therefore the tooth a candidate for a dental crown.

In each illustration, an arrow(s) points to that portion(s) of the tooth which would be expected to be most prone to cracking or breaking off.

4) Not all teeth with large fillings will be problematic.

Possibly by now you have already been to the mirror so you can judge the size of the dental fillings in your teeth. Did you see any "large" ones? Now, ask yourself, how long those big fillings have been in place? What was your answer? Two years, five years, longer? If so, what's the deal? If teeth with big fillings are so weak, why haven't parts of these teeth already fractured off?

Of course, the answer is that no dentist can know for certain which teeth will develop problems and which teeth won't. No doubt, if your dentist could see into the future, they wouldn't be spending their time practicing dentistry. Dentists do have, however, both from their dental training and clinical experience, an idea of which teeth are more at risk for breaking than others. And they have an obligation to report this information to you.

Clearly not every tooth with a "large" filling will crack or break. Certainly many people get many years of service out of these types of restorations. Additionally, not every tooth that does fracture will be especially problematic to repair. What your dentist is trying to relate to you, however, is that they believe a crown will produce the most predictable, successful outcome for your tooth over the long run.

The relationship between dental crowns and root canal treatment.

Some people seem to think that if a tooth needs a dental crown that it also has to have root canal treatment. To the contrary, these procedures are entirely different and separate types of dental work and, most certainly, not every tooth which has a crown placed on it needs root canal treatment. [ Crowned tooth complications: Why some teeth that have had dental crowns placed will need root canal treatment. ]

The relationship between dental crowns and root canal treatment is similar in nature to that between automobile bodywork (dental crown treatment) and under-the-hood work (root canal treatment). If you are in an accident (tooth breakage) you will need bodywork (the dental 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 depends on the specific nature of the accident.

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