Dear Dr. Hall,
I started off looking for information on teeth too white and then found this ask a dentist link so I thought I’d ask a different question that has been on my mind.
I have an eye tooth that had a root canal done on it perhaps 40 years ago. It has been suggested to me to have it capped to prevent it from breaking. What do you think? Thanks!
– Holly from Cleveland
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Thanks for your question. It’s the type of question I enjoy because it’s one where political correctness gives you one answer but science gives you a different answer.
A root canal treatment makes a tooth more brittle and more susceptible to breaking. And the easy answer is to just crown every tooth with a root canal. The insurance will pay for it, after all, and dentists like doing crowns much more than fillings because they’re much more profitable.
But there is a fundamentally different type of stress on a front tooth from the stress on a back tooth. Molars have four cusps, and the stress on a molar tends to push those cusps apart. So almost all fractures of molars are splits of one type or another. The same is largely true of premolars that have two cusps. Molars and premolars also are designed for grinding, where front teeth are designed for biting and tearing.
The potentially damaging stress on a front tooth, however, is almost entirely a shearing stress. The canine tooth (eye tooth), in particular, is subject to strong horizontal stresses. In fact, it is supposed to absorb most of the horizontal stresses to keep them from affecting the back teeth—a function called canine protection. This is why it has the longest root. When you grind your teeth side to side, it is usually the canine tooth that, by its slope, forces your back teeth apart and helps relieve the sideways stresses on those back teeth. The other front teeth take the stress of what is called anterior guidance—when you slide your lower jaw forward you do so against the inside surfaces of the upper front teeth. While this isn’t as great as the stresses on a canine tooth, they are significant stresses and are again horizontal.
Protecting a Root Canal Tooth from Fracture
So how do you protect a root canal tooth from fracture? For a back tooth, the answer is clear. A crown or onlay covering the chewing surface will absorb the vertical stresses and prevent splitting of the tooth.
But for a front tooth, the traditional, politically correct answer of crown every tooth with a root canal will actually weaken the tooth against its most seriously stresses.The weakest part of the tooth is at the neck of the tooth. A porcelain crown preparation requires reducing the tooth about one millimeter all around. If the diameter of the tooth is 7 mm, that will be reduced to 5 mm, a reduction of almost 30%. Complicating the problem, the center of the tooth is where the pulp of the tooth used to be and is now empty space. What’s left of the tooth after the crown preparation could be as little as 50% of its original fracture resistance or less.
Read the story of Amber from Oregon who wrote to me several years ago, whose dentist told her she needed to have crowns on her three front teeth with root canals. Two weeks after putting the crowns on, one front tooth broke off at the gumline.
Well, some dentists may say, we can prevent that horizontal fracture by putting a titanium post down through the middle of the tooth. But, while a stiff post will help prevent fracture at the neck of the tooth, research has shown that it will transfer stress to the tip of the post, deep in the tooth, and will increase the likelihood of a vertical root fracture.
No, a crown on a front tooth with a root canal isn’t needed to protect the tooth against fracture, unless there is a lot of tooth structure already missing due to decay or pre-existing fracture. A crown may be needed, however, for aesthetic reasons, because a tooth with a root canal treatment can turn dark.
Here is my recommendation for treating a front tooth with a root canal treatment, assuming there is a great deal of healthy tooth structure remaining. First, I would clean out the inside part of the crown of the tooth—the visible part of the tooth—from all root canal filling materials and cement. These filling materials are the major source of darkening of a tooth after root canal treatment. Then I would cement in a white fiberglass post to help strengthen the tooth. The fiberglass post, since it flexes, will add resistance against horizontal fracture without transferring stress to deep in the root of the tooth. Then I would fill in all of the rest of the missing tooth structure with a strong composite material. From my experience, cleaning out all of the root canal filling material will delay the darkening of the tooth several years. When the tooth begins to turn dark, I would veneer the tooth with porcelain.
I hope this is helpful.
– Dr. Hall
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