The parts of a tooth, explained
A plain-language tour of a tooth, from the hard enamel outside to the living pulp at its center, and why each layer matters when something goes wrong.
When a dentist says a cavity “has reached the dentin,” or that a tooth “needs a root canal,” it helps to know what those words point to. A tooth isn’t a solid white pebble. It’s built in layers, and each layer has a job. Once you know the layers, most of what happens at the dentist starts to make sense.
This lesson walks through a tooth from the outside in. It uses a molar, one of the wide back teeth, as the example. Figure 1 shows one cut down the middle.
- Enamel
- Dentin
- Pulp
- Gum
- Root canals
- Bone
Four kinds of teeth
A full set of adult teeth has 32, counting the four wisdom teeth. Children have 20 baby teeth. Adult teeth come in four shapes, each suited to its job:
- Incisors, the eight flat teeth at the front, cut food.
- Canines, the four pointed teeth at the corners, grip and tear it.
- Premolars, just behind the canines, crush it.
- Molars, the widest teeth at the back, grind it.
Whatever its shape, every tooth has the same basic parts.
The crown and the root
The crown is the part of the tooth you can see above the gums. That’s why a cap that covers a tooth is also called a crown. The root is the part below the gumline, set into the jawbone, and it’s usually longer than the crown.
Front teeth usually have one root. Molars usually have two or three, spread apart like the legs of a stool. That spread helps them stand up to the force of chewing.
Enamel: the hard outer shell
Enamel covers the crown. It’s the hardest material in your body, harder even than bone. It protects the softer layers inside from the force of chewing and from heat and cold.
Enamel has one weakness: acid. The bacteria in plaque, the sticky film that builds up on teeth, make acid when they feed on sugar. Over time, that acid pulls minerals out of the enamel and leaves a weak, chalky spot. If it keeps going, the spot breaks down into a hole, which is a cavity.
Enamel has no living cells, so it can’t repair a cavity on its own. An early weak spot, though, can sometimes harden again with fluoride and good brushing before it becomes a hole. That’s one reason checkups matter: they can catch these spots while they can still be reversed.
Dentin: the layer underneath
Under the enamel is dentin, which makes up most of the tooth. Dentin is softer than enamel and slightly yellow. It’s full of tiny tubes that run toward the center of the tooth.
Those tubes explain a lot. If enamel wears thin, or the gums pull back from the root, the tubes are left open. Then hot, cold, or sweet things can set off a quick, sharp sting, which dentists call sensitivity.
Dentin’s softness matters, too. Decay spreads faster once it gets through the enamel, so a cavity that looks small on the surface can be larger underneath.
The pulp: the living center
At the center of the tooth is the pulp, the soft tissue that holds the tooth’s nerves and blood vessels. The pulp fills a space inside the crown called the pulp chamber. From there, it runs down through each root in a thin channel called a root canal.
The pulp helps a tooth grow and form. Its nerves react to heat, cold, and damage, mostly by sending pain. When decay or a crack lets bacteria reach the pulp, it can become inflamed or infected. The pulp is sealed inside hard walls, so swelling has nowhere to go, which is one reason an infected tooth can ache so badly.
A fully grown tooth can keep working without its pulp, because the tissues around the root still hold it in place. That’s why root canal treatment works: it removes the damaged pulp and seals the canals, and the tooth stays. What actually happens during a root canal explains the steps.
What holds a tooth in place
A tooth isn’t glued into the bone. A thin layer called cementum covers the root. Tiny fibers, called the periodontal ligament, run from the cementum to the bone and hold the tooth in its socket.
These fibers work a bit like a hammock. They let the tooth move very slightly when you bite, which cushions the force of chewing. They also sense pressure, which is how you can tell when you’ve bitten down on something hard.
Gums and bone: the foundation
The gums wrap around each tooth like a snug collar. Between the gum and the tooth is a shallow groove. At a checkup, your dentist may measure how deep this groove is, because a deeper space, called a pocket, can be a sign of gum disease.
Under the gums is the bone of the jaw, which holds the roots. Healthy gums are firm and fit snugly, and they don’t bleed when you brush. When plaque stays along the gumline, the gums can turn red and swollen and bleed easily. This is gingivitis, the early stage of gum disease, and it can usually be reversed.
If it goes on, the infection can spread below the gums and start to break down the bone. This later stage is called periodontitis. Bone lost this way doesn’t grow back on its own, which is why gum disease is a common reason adults lose teeth.
Those numbers at your checkup
During a gum check, you may hear a string of numbers, like “three, two, three.” Each one is the depth of the groove around a tooth, in millimeters. One to three is usually healthy. Four or more can be a sign of a pocket, and your dentist will explain what it means for you.
How problems move from the outside in
Put the layers together, and you can see how a small problem grows. Decay starts in the enamel, where a small filling, or sometimes just fluoride, can stop it. Once it reaches the dentin, it spreads faster and needs a larger filling, or a crown if a lot of the tooth is lost. If it reaches the pulp, the tooth usually needs a root canal or has to come out.
Each stage needs more treatment than the one before. That’s the real reason for regular checkups and X-rays: they find problems while they’re small and the fix is simpler.
Seeing it in your own mouth
Every mouth is a little different. Your exam and X-rays show these same layers in your own teeth: how much enamel is left, whether decay has reached the dentin, and how much bone surrounds each root. At Dental Academy, we’ll show you what we see and explain it in these terms.
To read about the care each layer needs, see exams and cleanings, tooth-colored fillings, root canal therapy, and gum disease treatment. This lesson explains teeth in general. An exam is how you find out what’s right for your own teeth.
Questions about your own teeth?
An exam is how you find out what’s right for you. Tell us a little about what’s going on, and we’ll call you to find a time.