Anatomy of a tooth
Teeth aren't actually that simple! There's a lot of different things that make up a tooth and they all have different roles to play. It is really a great work of engineering.
From the most outer surface, the most famous part of a tooth is the enamel. This is the white outer surface of your teeth and does not exist anywhere else in your body except for your teeth. It is harder than bone and it needs to be! Did you know your jaws can bite with 120 pounds/60kg of force? (That is around 600 newtons if you want me to be strict about my terminology) That's heavier than I am!
Tooth enamel is 96% mineral, which makes it so strong. The main mineral of enamel is calcium hydroxyapatite. As with most minerals, they can be dissolved by acids, which is the basis of tooth decay. How fluoride protects our teeth from decay is by chemically remineralising the mineral and forming calcium fluorapatite. Calcium fluorapatite is more acid resistant (dissolves at a lower pH) than the original calcium hydroxyapatite, which is the basis of fluoride preventing decay.
The second layer of the tooth is called dentine. Dentine is significantly different to enamel and makes up the bulk of a tooth. It is a pale yellow colour and contributes to the colour of a tooth significantly, because enamel is relatively translucent. Much of teeth whitening is actually working in this dentine layer. It is still harder than bone, but softer than enamel.
The mineral content of dentine is 70% calcium hydroxyapatite and contains 20% organic material. The rest is water. This decreased mineral content gives it more compressive, tensile and flexural strength than enamel, which means it is less brittle.
Dentine has microscopic holes in its structure, called, dentine tubules. The holes actually travel the entire thickness of the dentine! It is because of these tubules that forms the basis of tooth sensitivity to hot and cold. When the dentine is exposed to temperature change, it causes fluid to move inside the tubules and stimulate sensors inside the nerve. This is what the pain is from. Sensitive toothpaste can either work by sealing these tubules or desensitising the sensors. That's why if one sensitive toothpaste does not work for you, you should try another brand.
On the root surface, there is no enamel covering the dentine. It is different layer called cementum. This layer is what attaches your tooth to the bone and is very thin, around 15 microns near the enamel. Teeth are actually not fused to your jaw bones. Instead, they are attached by millions of fibres that run from the bone to the cementum. It is because of this design that teeth can be moved orthodontically and teeth can be removed just by pulling them out.
The nerve of the tooth is what is called the pulp. This is where all the action is when a root canal is performed. Cells, blood vessels and nerves make up most of the pulp.
Dentine is continually being produced throughout life, by the pulp and can be produced rapidly under irritation. This can make the physical space of the pulp become very small and very difficult to perform a root canal.
As you can see in the photo of a real tooth, the pulp is well protected by the dentine and enamel. When people say that the dentist "hit the nerve", is it actually triggering the sensors inside the pulp. The only times when the nerve will be "hit" is when there a is a really deep filling or a root canal is being performed.
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