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Tooth decay and cavities



Dental caries disease, also called tooth decay, refers to demineralization or weakening of the teeth, and the end result of caries disease is a caries lesion.

An advanced caries lesion can progress to a point where the tooth surface forms a cavitation or a hole, which is the physical evidence of tooth breakdown.

Let's start by building a model of a tooth and its surrounding structures.

In the mouth, the bone beneath the bottom row of teeth is the mandible, and the bone above the top row of teeth is the maxilla. Both bones have an alveolus, or socket, for each tooth.

The socket is lined on the inside by a periodontal ligament.

Protecting the alveolus on the outside, is a layer of soft, supportive tissue called the gingiva, or gums, that sits on top of the bone and covers the root surface from the bone to the cementoenamel junction - where the cementum and enamel come together.

The tooth itself can be roughly divided into a few parts. The first part is the root, and it sits within the alveolus.

The root is covered by cementum, which is a bonelike substance that the periodontal ligament’s fibers attach to.

Next, there’s the neck, which is the transition between the root portion covered by bone and the crown.

The crown is the visible part of the tooth that protrudes from the gingiva, and it’s covered in enamel, which has such a high mineral content that it’s the hardest substance in the human body.

When the teeth are developing, enamel is made before the tooth erupts into the mouth by a group of cells called the ameloblasts that die once the tooth erupts - meaning that the teeth lose the ability to make more enamel forever.

Now, let’s fill the tooth in from the inside out. Blood vessels and nerves come from the jaw bones, and enter the center of the root through a narrow passage, called the apical foramen.

From there, they enter the soft center of the tooth, called the pulp, where they provide nutrition and sensation.

The outer wall of the pulp, contains odontoblasts which are cells that secrete a bonelike substance called dentin, which is filled with protein and minerals.

The odontoblasts have long processes, or arms, that lay within tiny tubules in the dentin - like an octopus sticking it’s arms through a block of cement.

These tubules are important for sensory nerves as well, which use them to travel through the dentin from the pulp to the dentin-enamel junction.

Finally, there’s saliva, which is secreted from several glands in the mouth.

Saliva contains bicarbonate ions, which helps to neutralize acid in the mouth, and glycoproteins like mucin which thickens the saliva.

Saliva also contains calcium and phosphate, which serve as replacement minerals, to help remineralization of the tooth surface.

So, when it comes to dental caries, there’s a tug-of-war between the pathogenic factors that promote demineralization of the tooth and protective factors that promote remineralization of the tooth.

Normally, there's a layer called the pellicle that covers the tooth, and bacteria in the mouth, like viridans group streptococci, settle on that pellicle layer.

These bacteria are called early colonizers, and they attach to the surfaces using bacterial surface proteins called adhesins.

Because they’re on the pellicle of the teeth, these bacteria come into contact with sucrose, a sugar found in various foods and drinks including table sugar.

Sucrose contains a glucose molecule bound to a fructose molecule.

So the bacteria use an enzyme called glucosyltransferase to cut the bond between them, use the fructose for energy, and add the glucose to a growing chain of glucose molecules called a glucan.

Glucans allow the bacteria to attach firmly to the surface of the tooth, and form dental plaque, which is a sticky collection of bacteria, proteins from saliva, and dead cells from the lining of the mouth.

Individual bacteria multiply and form many small microcolonies that coalesce, creating a layer of dental plaque - which is a type of biofilm.

Compared to a microcolony, the bacteria in a biofilm communicate with each other via chemical signaling and together they create a complex system where some bacteria work on tunneling between the microcolonies and to the surface in order to bring in a steady supply of food.

As an analogy, if bacteria were ants, then a microcolony would be a tiny group of ants, each doing it’s own thing, whereas a biofilm would be an ant farm with complex tunnels and rooms, and each ant carrying out a specialized task.

So typically, dental plaque can form on the surface of teeth, but if the pH of the local environment around the tooth surface remains above about 5.5, enamel will not demineralize.

One factor that tips the balance towards caries progression is having less bicarbonate-rich saliva, which can result from conditions like Sjogren syndrome, radiation treatment for cancer, gastric reflux, or as a side effect of a medication or recreational drugs.

A key factor that contributes to tooth demineralization is sugar.

Some bacteria ferment sucrose, and that produces lactic acid as a byproduct.