The chemistry of tooth decay- How and why cavities form. -
If you're curious about the science of tooth decay formation, this page outlines what goes on. It also gives suggestions based on this information covering steps you can take to prevent decay in your own mouth.
1) Cavities are caused by tooth "demineralization."
Decay formation is the result of what dentists refer to as "demineralization." This is a process that occurs when a tooth is exposed to an acidic environment of pH 5.5 and below. [Normal conditions for the mouth are neutral (around pH 7).]
The acids involved damage the tooth's hard tissues (enamel and dentin) by way of leaching mineral content (primarily calcium and phosphate ions) from them. Hence the term "de-mineralization."
2) Where do the attacking acids come from?
The acids that cause tooth demineralization tend to be produced by specific types of bacteria, specifically streptococci mutans and lactobacilli.
The primary home of these bacteria is within dental plaque:
- S. mutans plays an instrumental role in the initiation of cavity formation due to its ability to adhere to and subsequently colonize tooth surfaces.
- As a lesion progresses, conditions develop that are favorable for lactobacilli colonization too. And over time they play an increasingly damaging role in the decay process.
a) The acids that cause decay are bacterial waste products.
The bacteria that cause cavities are living organisms. And just like all living things they consume food and in return create waste products.
As it happens, the wastes that these types of bacteria create are very acidic (they have a pH of 4 and lower). The primary compound they produce, and the one primarily responsible for tooth demineralization, is lactic acid.
b) What kind of food is involved?
The primary food source for cariogenic (cavity-causing) bacteria is dietary sugars. This includes sucrose (table sugar), glucose, fructose, lactose and cooked starches.
The bacteria digest these sugars via a process called glycolysis, which is the primary means by which all bacteria create the energy they live on.
What's different about cariogenic bacteria is that they're able to take this process one step further, which results in the formation of the very potent compound lactic acid.
c) How quickly does the acid form?
The Stephan Curve.
Research data collected by Stephan (1944) [page references] included a graph that illustrated the pattern of acidic response that takes place in a person's mouth immediately after their rinsing with a sucrose (table sugar) solution.
- From the moment of exposure, oral conditions immediately become increasingly acidic. A maximum value is ultimately reached in approximately 5 to 20 minutes.
- After this point, a gradual recovery, taking between 30 to 60 minutes, begins that eventually returns the mouth to its original (pre-rinse) status.
Variables that affect the curve.
Each of the factors listed below will influence how intense the attack will be (how quickly and how much acid is produced), and therefore how long the pH of the mouth will remain below 5.5 (the level where demineralization will occur).
- The type and number of bacteria living in dental plaque. (Streptococcus mutans is the type of bacteria most associated with lactic acid formation and therefore cavity development.)
- The density of the plaque. Thicker plaque helps to protect bacteria and the acids they produce (see diagram below).
- The type of sugary meal consumed. Complex starches must be broken down into their component sugars before bacteria can digest them. In comparison, sucrose can be metabolized straight away.
3) Factors about dental plaque that affect cavity formation.
Everyone's mouth is inhabited by bacteria. In fact, a single human mouth can contain more microorganisms than there are people on planet Earth.
And while you can't sterilize your mouth, there are things you can do to minimize the impact of bacteria. You do this by preventing them from forming organized colonies.
These bacterial colonies are referred to as "dental plaque," and keeping it from building up and staying put is what brushing and flossing is all about.
a) Why plaque formation is so problematic.
Dental plaque not only provides a home for bacteria but it also acts as a medium that holds the acid they produce directly against a tooth's surface.
It's the acids that leach out underneath dental plaque that cause cavities.
Take a look at our diagram. When acid production is active (like after a sugary treat), the acid that's formed can seep out of the plaque in one of two directions.
- It can either accumulate underneath the plaque (directly against the tooth's surface).
- Or else seep out into the mouth.
b) Some of the acid gets neutralized.
Any of the acidic waste products that seep out into the mouth won't be able to cause decay. That's because they'll immediately get diluted, buffered, and/or washed away by saliva or any foods and beverages that are consumed.
c) It's the acid underneath the plaque that causes cavities.
The acid that's most instrumental in causing demineralization is that which seeps through the plaque and on down to the tooth's surface.
The thickness of dental plaque above tends to act as a protective covering that helps to shield the acid from dilution, buffering, or being washed away. Over time, saliva will finally penetrate through the dental plaque and begin to create its neutralizing effect. But this can take as long as two hours or more.
That means the acid that finds its way to the tooth's surface will remain relatively concentrated for an extended period. During this time, demineralization will take place.
4) "Young" dental plaque is less harmful than "old."
The amount of tooth damage that occurs after an exposure to sugar is, in part, related to the age of the dental plaque. (Age = How long a particular glob of plaque has been sitting on a particular location on a tooth.) Factors such as thickness, chemical nature, and the types of bacteria living in it all correlate with its age.
With other factors being equal, plaque that's just formed over the last few hours doesn't have the potential to create as much demineralization as that which is several days old. That's why it's important to brush both thoroughly and frequently.
There's more to this story ...
You might be surprised to learn that the process of tooth demineralization is only half of the story about how cavities form.
It does explain how and why tooth decay starts and advances. But the whole process of cavity development is more complicated than just that. We explain on our page "How long does it take for a cavity to form?"
Tooth anatomy as it relates to cavity formation.
Every aspect of the outer surface of a tooth is composed of either enamel, dentin or cementum. They are each calcified tissues (they have a high mineral content) and are the ones in which decay forms.
But beyond that they are very different. Here's information and some interesting facts about them.
a) Tooth Enamel
The primary calcified (hard) tissues of a tooth.
Dentin makes up the bulk of a tooth. Enamel just covers the portion above the gum line.
When you look at your teeth, most of what you see is those portions covered by a layer of tooth enamel (this would include everything you see that's white, and frequently even 100% of what you see at all). (See diagram.)
You've probably heard that tooth enamel is the hardest tissue found in the human body and that is true. It's composed of around 95% minerals.
Most of this mineral content consists of calcium and phosphate ions that have combined to form crystals of hydroxyapatite (hydroxyapatite crystals are the basic building-block element of tooth enamel).
Collectively the crystals are organized into minute enamel "rods" (a tooth will have 5 to 12 million of them). These long, strong rods make up the structure of the enamel itself.
Factors affecting decay formation.
Enamel has some unique characteristics that influence how it is affected by the decay process.
- As glossy and smooth as the surface of dental enamel seems, at a microscopic level it contains pores (minute spaces that exist between its rods of hydroxyapatite).
This porosity can create differences in the density and hardness of enamel in different areas of a tooth, thus creating some locations that are more susceptible to damage by the demineralization process.
- Unlike most body tissues, after the formation of tooth enamel has been completed it's incapable of regenerating or healing itself. That means that once an outright cavity has developed (like the case where a hole has formed) the damage is permanent.
You might be surprised to learn that teeth are not solid enamel. It only makes up the surface layer of a tooth's "crown" (that portion that lies at and above the gum line).
The bulk of a tooth, both its root and interior aspects, is composed of a light yellow, hard tissue called dentin (see diagram above).
It too contains a high percentage of minerals (in the form of hydroxyapatite), but not as high as enamel. Only about 70% of its composition is mineral, so relatively speaking, dentin is the "softer," more cavity-prone, type of calcified tooth tissue.
Factors affecting decay formation.
- Dentin is damaged by a less acidic environment. Enamel undergoes demineralization at a pH 5.5 or lower. For dentin this value is just 6.2 (or lower). (Koenigs 2013)
- A part of the comparative "softness" of dentin vs. enamel has to do with the fact that its hydroxyapatite crystals are substantially smaller (10 to 30 times, in all dimensions).
- Unlike tooth enamel, dentin is a living tissue. And as such it can undergo changes in response to the decay process.
The loss of tooth structure caused by cavity formation can't be replaced. But changes within the dentin itself or the formation of additional amounts of it within the tooth can help to create protection from advancing decay.
Cementum is an ultra-thin mineralized layer that covers the surface of a tooth's root.
It's affected by the decay process, just like the other calcified tissues. But it's so thin and destroyed so readily that we've confined our discussion to just dentin and enamel.
Is a tooth's nerve involved with decay formation?
It's only the tissues mentioned above that are directly involved with the process of tooth decay formation (a condition caused by tooth demineralization).
Yes, a tooth's nerve (an interior, non-calcified tissue) can be damaged during the process. But that's a secondary issue (a side effect) and fortunately doesn't always occur.
Terminology - Dental caries.
The terms dental caries, tooth decay and cavity all refer to the same thing and can be used interchangeably.
It's pretty easy to see why the term "cavity," which means a hole, has come into usage. The same can be said for the term "decay," which makes reference to the destructive (decaying) aspect of this disease. Why the word "caries" is used may not be so obvious.
"Caries" is derived from the Latin word for "rot," which is also a suitable description of the decay process. "Dental caries" is the term you will find most frequently used in scientific literature.
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