Now lets take a look at the next hard tissue of the tooth - the Dentin. Dentin makes up almost all of the tooth and there is a lot more to it than meets the eye. In fact there is a lot more to it than you have been shown in almost every diagram you have ever seen of a tooth.
At right is the diagram you saw before. Using this image you can see that the enamel is merely a "cap" that fits over the dentin. This hard cover is important because, unlike enamel, dentin is not as mineralized or as densely packed. It wears quicker than enamel, severely erodes chemically in the presence of acids, and it decays quicker than enamel.
Oh, there is also one more thing - dentin, unlike enamel, is really, really sensitive! It will react to hot, to cold, to metal, to sweets, or if the dentist touches it with any of their instruments.
Some of you may remember moments when you were at the dentist who began working on the tooth and then - bam - you felt an intense, stabbing pain. Perhaps the dentist said to you something terribly reassuring like "we must have hit the nerve."
You about to find out how incredibly inaccurate that statement actually is, and why it is important to investigate a little further into the complex, but easy to understand material that we call dentin.
Why is Dentin Sensitive?
Now lets take another look at our diagram to the right. Here I have drawn in a few changes. Ignore the blue circle for a moment and concentrate on the lines which seem to radiate outward from the pulp chamber through the dentin. These lines are the source of all that tooth sensitivity and pain.
So, they must be the nerves right?
No. The truth be told, there are no nerves within any of the hard tissues of the tooth. There is actually no living tissue within the dentin or the enamel. There is living tissue in the center of the tooth, the area drawn in red called "the pulp." We will get into that a bit more in the section describing the soft tissue of the tooth.
These lines drawn in the dentin are actually tubes - water filled pipes that radiate outward from the central pulp chamber within the tooth to the outer perimeter of the dentin itself.
I cannot stress this anatomical point enough. In any given tooth there are probably hundreds if not thousands of these tubules coursing through the dentin, intimately connected to the pulp tissue throughout the life of the tooth. And each of these tubules is connected to a nerve ending within the pulp. So, no there are no nerves in dentin, but that doesn't mean they don't have feeling. They do, a lot of feeling. For if you disturb the water in that tubule in any way, you are going to feel it.
Where do Dentin Tubules come from?
The dentin and the tubules within the dentin are formed by pulpal cells which are called odontoblasts. These cells begin to lay down the mineralized matrix of the dentinal tubule in a direction down and inwardly toward the center of the tooth. Ultimately, the pulpal cells get trapped within the very chamber they have formed, in the center of the tooth.
Look at the diagram to the right which shows the cross section of the dentin within the blue circle in the diagram above. The outer perimeter of the pulp chamber is lined with the layer of odontoblasts. Each odontoblast is actually stuck to the dentin it created by a little "foot' that sticks into the tubule itself. Lastly, as I said before, each dentin tubule is filled with water which comes from the pulp as well.
Unlike enamel, this arrangement allows the dentin to regenerate itself over the life of the tooth. In fact look at the radiographs below.
The one on the left was taken on a lower right first molar of my daughter when she was 9 years old. The one in the middle was taken of the exact same tooth this past year, 13 years later. You can see how the dentin is getting thicker, and in effect, the pulp seems to be getting smaller. The image on the extreme right is of a 90 year old patient where the pulp is barely visible.
By the way, this thickening of the dentin, and shrinking of the pulp chamber is one of the reasons that teeth get darker as we age. In the previous section on enamel, we discussed how bleaching changes the "reflective properties" of the tooth. This effectively compensates for this darkening of the dentin, but it is only temporary - because the darkness of the tooth comes from within.
What purpose do dentinal tubules have?
There is nothing in the body that forms for no reason. And dentinal tubules are no exception. So, why do we have thousands of these pipes, filled with water, connected to nerves in each tooth?
Well it can't be just to tell us we have a problem in our tooth. I don't think cavemen really needed to know they had to visit their cave-dentists after all.
It turns out that each one of these tubules serves as a pressure sensor, telling the brain when the tooth has come into contact with any object, thus allowing the brain to direct the muscles of chewing in a proper coordinated fashion.
Let me say that again - if not for the dentinal tubules the brain would not be able to tell if it your teeth have come into contact with an object you were eating, or even perhaps your own opposing tooth. As it turns out, in order to protect your teeth the brain is programmed to open immediately upon contacting something. Very important when you consider that the force on a back molar is somewhere in the neighborhood of 1000 pounds per square inch! If not for this reflex, we could easily break our teeth when we chew, something that actually could happen following a dental visit, while you are numb.
How does this dentin tubule anatomy affect diseases of the tooth?
We will talk about tooth decay, fractures of teeth, and other diseases that damage teeth later. The most important take home message here is that once a tooth begins to suffer from tooth decay, the minerals in both the enamel and dentin begin to dissolve away. Once this begins it will not stop until the diseased portion is removed and the tooth is restored.
Remember that last statement - tooth decay only gets worse over time.
When it is just in the enamel, you don't feel it. Enamel has no nerves. Once it breaks through into the dentin it begins to stimulate the tubules and you begin to feel symptoms like sensitivity to cold, metal or sweets. And once it breaks through into the dentin it will follow the tubules toward the pulp chamber and eventually break through into that sterile chamber - destroying the pulp tissue and causing what could be a fatal infection of the jaw if left untreated.
So take this to the bank - all cavities end up as pulp exposures over time.
The difference in treatment between an early cavity in the enamel, a moderately shallow cavity into the dentin, and a pulp exposure could mean saving or losing the tooth. It could also be the difference between treatment that may cost a few hundred dollars or several thousand.
It is all a matter of time and tubules.
The images below show various stages of decay in the same patient. The tooth on the left has an old silver amalgam filling but clinically looks fine. However when you look at the radiograph you see the dark shadows that have broken through the enamel and are starting to follow the tubular highway toward the pulp. The tooth on the right is far more severely decayed, with the pulp chamber nearly exposed. Clinically there is a broken down hole in the tooth with the soft, decayed dentin visible to the naked eye.
What happens when you get a pulp exposure will be addressed in the section about pulp therapy appropriately entitled "Endodontics" which means "inside the tooth."
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