How does a dental crown strengthen a tooth? When are crowns a better choice than dental fillings?

The strengthening capability of dental crowns is related to the fact that they cup over and encase the tooth on which they are placed. This means that a crown can act as a splint that binds a tooth together. This is a very important feature of dental crowns and one that makes them a very valuable type of restoration for a dentist to have available to them.

In contrast, dental fillings, especially large ones, can have a weakening effect on the teeth in which they are placed. Dental fillings rely on a tooth's remaining structure to hold and support them. Fillings, in general, don't strengthen a tooth and can't protect a tooth from the forces generated by biting and chewing.

If only a small amount of tooth decay is present then it's usually no problem for a dentist to make a repair for a patient by way of placing a dental filling. In these cases there will still be a substantial amount of healthy tooth structure remaining after the decay has been removed and the filling has been put in place.

On the other hand, repairing a large cavity often requires the removal of so much tooth structure that the overall strength of the tooth is reduced, in some cases greatly. This is because the tooth is no longer as intact as it once was, its structural integrity has been compromised.

Of course it's not exactly the same thing but... the structural integrity aspect of a tooth is a little like an egg. 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, in that specific shape, is really a surprisingly strong object.

Now, in comparison, say you have broken the egg open and the two halves of the empty shell are lying on their sides. It's fairly simple to crush each piece of the shell now. Its structural integrity has been compromised. The shell is no longer in the shape it was meant to be so to be able to withstand forces. Teeth are somewhat the same in the sense that once a large portion of the tooth is missing (because it has broken, decayed, or has been drilled away) it is simply no longer as sound.

What constitutes a big dental filling?

Well of course this is the $64,000 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.

Really, any filling which is greater than about a third of the width of its tooth overall could be considered to be a filling large enough that the strength of the tooth has been compromised and the tooth is now a good candidate for the placement of a dental crown. Some dentists are even more conservative in their opinion and feel that if a filling is greater than about one third of the distance between a tooth's cusp tips, the tooth should have a dental crown placed.

Take a look at the animation to the right. Each of the dental fillings shown could be considered to be "large." 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.

What size are the dental fillings in your teeth?

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? So what's the deal? If teeth with big fillings are so weakened, 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 break. Many people get many years of service out of these "large" restorations. Additionally, not every tooth that does crack or break will be especially problematic to repair. What your dentist is trying to relate to you however when they suggest that a dental crown should be placed instead of a filling is that they believe a crown will produce the most predictable successful outcome for your tooth, in the long run.

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The relationship between dental crowns and root canal treatment.

Some people seem to equate a tooth's need for a dental crown with the need for root canal treatment for that same tooth also. While both of these dental treatments may be required they are entirely separate procedures and, most certainly, not every tooth which has a dental crown placed on it needs 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.