When does a crown make a better choice than a filling? -

Can a cavity be too big to fill? | Examples of large (risky) fillings. | How crowns strengthen teeth. | Applications where placing a filling makes the better choice.

Crowns vs. Fillings

You may find yourself in the situation where your tooth needs to be rebuilt. Your dentist says that either a dental crown or a filling might be placed. If so, the question then becomes how do you decide between the two?

When does choosing crown placement...

... make the better choice?


Factors to consider when choosing between a crown and filling.

a) Does the tooth require strengthening?

One of the biggest differences between a dental filling and a crown is the level of protection that the latter provides.

  • Crowns strengthen teeth. - One of the hallmarks of crown placement is the way it reinforces and strengthens a tooth. This is due to the way it cups over and encases it.

    The crown literally acts as a rigid splint that holds the tooth together. And that means that once one has been placed the tooth can withstand a greater level of chewing forces without risk of damage.

  • Dental fillings don't offer as much protection. - In comparison, placing a filling doesn't substantially increase the overall strength of its tooth. And as we explain below, placing one may place the tooth at greater risk for breaking.

    (Related page: The risks and outcomes of not placing a crown.)

Background -

Actually, you can't have a discussion about crowns vs. fillings, without first describing the size of the filling involved.

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

A small cavity repaired by a small dental filling.

For small repairs, placing a filling makes the right choice.

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

In this situation, even after the preparation (hole) for the filling has been drilled, the tooth remains relatively whole and intact. And because of this, it can be expected to still be able to withstand the chewing forces it's exposed to well.

Placing a filling (vs. a crown) is actually the superior choice.

In the case of relatively minor repairs, placing a crown instead of a filling can actually be detrimental to the tooth.

That's because the act of preparing (trimming) the tooth for its crown can stress it. Even to the point where irreversible harm may come to its nerve tissue, thus triggering the need for root canal treatment. We discuss the details of this complication here.

2) Large fillings frequently leave a tooth at risk.

A small cavity repaired by a small dental filling.

Large dental fillings can leave a tooth vulnerable to fracture.

In situations where a relatively larger amount of tooth structure has been lost, damaged or needs to be trimmed away when decay is removed, the picture may be different.

In these cases, the overall structural integrity of the tooth may be compromised, possibly significantly so and to the point where it is at risk of fracture.

An analogy.

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, 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 simple feat to crush each piece flat.
  • That's because the shell's structural integrity has been compromised. It's no longer an intact unit that's capable of withstanding forces well.

How filling preparation size affects tooth strength.

Reductions in tooth strength due to restoration size.

The numbers shown are reduction in tooth stiffness (a measure of tooth strength).

That's why teeth with big fillings can be fragile.

A tooth is somewhat the same, in the sense that once its structural integrity has been altered (because it has broken, decayed, or has been drilled away) it's simply no longer as sound.

Research evidence.

This effect was documented by a study by Reeh (1989). It evaluated the stiffness (a measure of strength and therefore potential for fracture) of premolars after cavity preparations (holes) for various sized fillings were drilled. [page references]

Just as you might expect, larger sized preparations resulted in greater loss of tooth stiffness (see graphic). [1 surface cavity preparation = -20%, 2 surface = -46%, 3 surface = -63%] This paper stated that this same general trend could be expected for molars too.

 

The question isn't "if" a filling can be placed.

Don't confuse the issue of "can a filling be placed" vs. "should one be." Even if your dentist feels they can anchor some type of restoration that will fill in your tooth's lost structure, quiz them about what the end result will be.

Any fragile or weakened tooth portions will still remain at risk for breakage. And you may find that placing the filling may really just be an interim solution. One whose service will abruptly end once the unreinforced tooth breaks down further.

A tooth whose filling played a role in it cracking.

A filling may create a wedging effect that ultimately cracks its tooth.

b) Fillings can create a wedging effect that leads to tooth fracture.

There's yet another concern that comes into play when making a 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.

A major difference between crowns and fillings.

In contrast to a dental crown that cups over and encases the tooth it's placed on, a filling is embedded within it.

And due to this configuration, it has the potential to act as a wedge when chewing pressure is applied to it. And if the biting forces are great enough, or small but perpetually reoccurring, at some point this wedging action may cause the tooth to crack or fracture. (See animation.)


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

Of course, these are the big questions and ones in which your dentist's judgment will play a critical role. As they make this determination, here are some of the issues they will consider.

Intercuspal distance.

Images of large dental fillings.

Fillings greater than 1/3 the distance between cusps tends to significantly weaken a tooth.

Crowns vs. fillings - Common guidelines.

As you would hope, the general rules of thumb that a dentist follows come from (or are at least confirmed by) research.

  • A study by Larson (1981) determined that fillings that take up just 1/3rd of the "intercuspal distance" of a tooth (see picture) reduces the tooth's resistance to fracture by more than one half.
  • Geurtsen (2003) stated that a tooth's risk for fracture increased substantially as its filling approached 50% of its intercuspal distance. This paper's recommendation was that fillings, composites (white) or amalgam (silver), should not exceed 1/3 to 1/4 this distance.
Your mileage may vary.

Not all dentists are as conservative in their recommendations as the guidelines above. And from a practical standpoint many teeth with "larger" fillings do just fine, even over the long term.

But at least from a standpoint of research findings, there is reason to expect that there is some level of risk involved with taking this approach.

These are all "big" dental fillings.

Images of large dental fillings.

The arrows indicate the most fragile portions of the tooth.

Examples of "large" fillings.

Take a look at the frames of our animation. Each one shows a dental filling that could be considered to be "big," and therefore the tooth a candidate for a dental crown.

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


Not all teeth with large fillings will be problematic.

Possibly by now you've already been to the mirror so you can judge the size of the dental fillings in your mouth. Did you see any "large" ones?

Now, ask yourself, how long have those big fillings been in place? What's your answer? Two years, five years, longer? If so, what's the deal? If teeth with large fillings are so weak, why haven't parts of yours already fractured off?

Even your dentist can't see the future.

Of course, the answer is that no dentist can know for certain which teeth will develop problems and which ones won't. No doubt, if your dentist could see into the future, they wouldn't be spending their time practicing dentistry.

Dentists do however, both from their dental training and clinical experience, have an idea of which teeth are at greater risk for breaking than others. And they have an obligation to report this information to you.

They may be considering the safest choice.

Clearly not every tooth with a big filling will crack or break. And without question many people get many years of service out of restorations such as these. Additionally, not every tooth that does fracture will be especially problematic to repair.

But if your dentist feels strongly about placing a crown, what they may be trying to relate to you is that they believe that by doing so they will create the most predictable, successful and least troublesome outcome for your tooth over the long run.

 

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