What is tooth decay?
In short, tooth decay is a location on a tooth where so much of the tooth's mineral content as been dissolved away that a defect (a hole or a "cavity") has formed.
Now, let's back up a few steps and start a discussion about tooth decay so this blurb of a description makes more sense.
Tooth anatomy as it relates to tooth decay.
First off, you need to think in terms of a tooth being a hard calcified object. Yes, teeth do have nerves in their centers and this tissue is soft, but the surface of a tooth (where tooth decay begins) is formed from types of tissues that are very high in mineral content. These tissues are called enamel and dentin. Our mock up of a dental x-ray shown to the right illustrates where the dentin and enamel portions of a tooth are located.
Enamel
The vast majority of the surface portion of a tooth that is visible is covered by enamel. You've probably heard that tooth enamel is the hardest tissue found in the human body. This is true. Enamel is more than 95% mineral in composition. Most of this mineral content is a compound called hydroxyapatite which, as you probably already know, is rich in calcium. A tooth's calcified tissues are enamel and dentin.
Dentin
You may be surprised to learn that teeth are not solid enamel. Only the portion of a tooth that nature intended to lie at and above the gum line is covered by enamel. The bulk of a tooth, both its root and inner aspects, is composed of another calcified tissue called dentin. Dentin also contains the mineral hydroxyapatite, but to a lesser degree than enamel. Only about two thirds of the content of dentin is mineral so, relatively speaking, dentin is "softer" than enamel.
Synonymous Terms for Tooth Decay
There are two terms that are frequently used to refer to tooth decay. The most common of these is the word "cavity," which no doubt simply refers to the hole that often forms as a result of the tooth decay process. Another term that can be used interchangeably with "tooth decay" is the word "caries." This is the term you will most frequently find used in dental literature. The word "caries" is derived from the Latin word for "rot," which seems to be a reasonably accurate description of the tooth decay process.
Has tooth decay always been a problem for mankind?
No doubt throughout the entire history of humankind there have always been at least some individuals who have severely suffered from the effects of tooth decay. Cavities first became pandemic (an epidemic spread over a wide geographic region) however with the establishment of sugar plantations in the 1700's in the "New World." Subsequently, tooth decay affected yet greater numbers of people with the widespread cultivation of the sugar beet in Europe in the 1800's.
How does tooth decay form?
Earlier we defined tooth decay is a location on a tooth where enough of its mineral content has been lost that a hole has formed. The process by which this occurs is called demineralization and it takes place because of the presence of acids on a tooth's surface.
Where do the acids that cause tooth decay come from?
The acids that cause tooth demineralization (cavity formation) are produced by specific types of bacteria (mutans streptococci and lactobacilli) that live in dental plaque.
These bacteria are living organisms just like we are. When we consume foods and beverages, we create waste products. Bacteria do the same thing. The bacteria that cause tooth decay utilize sugars (glucose, sucrose, fructose, lactose, or cooked starches) as their food source. The waste products created from digesting these sugars are the acids (especially lactic acid) that cause the demineralization of tooth enamel and dentin.
Since the bacteria that live in our mouth eat when we do, as we ingest foods which contain sugars (such as soda, candy, milk, and even fruits and vegetables) these bacteria get a meal too. And within minutes they start producing the acids that cause tooth decay.
Dental plaque provides a home for the oral bacteria that cause tooth decay.
Everyone's mouth is inhabited by bacteria, in fact a single human mouth can contain more microorganisms than there are people on planet Earth. While you can't sterilize your mouth, you can minimize your potential for having tooth decay. You do this by not allowing the bacteria that are present to form organized colonies. These bacterial colonies are referred to as "dental plaque."
Tooth decay forms underneath dental plaque. Tooth decay is caused by acids produced by bacteria. Dental plaque not only provides a home for oral bacteria but it also acts as a medium that holds the acid they produce directly against a tooth's surface. Take a look at our diagram to the left. When acid production is active (when we consume sugar) some of the acid that is formed will seep from the dental plaque's surface and into our mouth. This isn't the acid that will cause tooth decay. Any acid finding its way into our mouth will get diluted, buffered, and/or washed away by saliva and the foods and beverages we eat and drink.
The acid that is instrumental in demineralizing a tooth's surface, and therefore causing tooth decay, is the acid that seeps in the opposite direction, on through the plaque and down to the tooth's surface. This acid will lie underneath the dental plaque and directly against the tooth. And because the dental plaque covering over this acid will act as a protective layer there is less effective dilution, buffering, and washing action produced by saliva. This in turn means that the acid will tend to remain in very concentrated form (a pH of 4 and lower) for quite some time after it has been created, and therefore tooth demineralization will take place for an extended period of time also.
With time saliva will penetrate through the dental plaque and begin to have its neutralizing effect on the acids that have been created in response to the sugary meal. But this can take as long as two or more hours.
The amount of tooth demineralization that takes place after an exposure to sugar is in part related to the age of the dental plaque. Characteristics such as the thickness, chemical nature, and types of bacteria living in dental plaque correlate with its age. Plaque that is only a few hours old will not be able to produce as much tooth demineralization as plaque which is several days old (when each is provided with an identical exposure to sugar).
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