"Wisdom teeth" are a type of molar. Molars are the chewing teeth found furthest in the back of the mouth. Most humans have first, second, and third molars.
A person's third molars are their wisdom teeth. These teeth come in behind the 2nd molars (if there is room for them and they are aligned properly), usually during a person's late teens or early twenties. Typically a person will have four wisdom teeth: upper left, upper right, lower left, and lower right.
In dental terminology an "impacted" tooth refers to a tooth that has failed to emerge fully into its expected position. This failure to erupt properly might occur because there is not enough room in the person's jaw to accommodate the tooth, or because the angulation of the tooth is improper.
Dentists use specific terms to describe the positioning of impacted wisdom teeth. The most common type of impacted wisdom tooth is one that is impacted mesially. The term "mesial" simply means that the wisdom tooth is angled forward, toward the front of the mouth.
The other types of impactions, in order of frequency of occurrence, are the vertical, horizontal, and distal types.
In addition to mesially, vertically, horizontally, and distally impacted, wisdom teeth can also be classified as soft tissue or bony impactions. The term "bony" impaction indicates that the wisdom tooth is still fully encased in the jaw's bone. A "soft tissue" impaction is one where the upper portion of a wisdom tooth (the tooth's crown) has penetrated through the bone, but has not yet erupted fully through the gums.
The reason why some wisdom teeth are impacted is not an easy question to answer. A primary cause of wisdom tooth impaction is simply that there is inadequate jawbone space behind the person's second molar. Why this lack of space exists is not fully understood, however there does seem to be a correlation between large tooth size, tooth crowding, and the presence of impacted wisdom teeth. It has been theorized that the coarse nature of stone age man's diet had the effect of producing extensive tooth wear, not only on the chewing surface of the teeth but also on the sides of the teeth where neighboring teeth rest against each other. The net effect of this wear would be a reduction in the collective "length" of the teeth as a set, thus creating enough jawbone space to accommodate the wisdom teeth by the time they erupted. In comparison the diet of modern man does not usually cause a significant amount of this type of tooth wear. It has also been argued that the coarse nature of stone age man's diet, as compared to modern man's relatively soft diet, probably required more activity of the "chewing" muscles. This activity could have stimulated greater jawbone growth, thus providing more space for wisdom teeth. The harsh and threatening world of the caveman no doubt often lead to the occurrence of broken teeth and even tooth loss. Once a tooth (or a portion of it) is missing the teeth behind it have a tendency to move forward. This shifting would make more jawbone space available for the wisdom teeth. In comparison, with the advent of modern dentistry there are relatively few reasons why a tooth should be lost or remain in a state of disrepair.
The precise reasons why an individual's wisdom teeth might need to be extracted should be explained to them by their dentist, after an examination which almost certainly will need to include dental x-rays. While each person's situation will be unique, some of the more common topics dentists discuss when they describe to a person why their wisdom teeth should be removed are listed below. The rationale associated with the first group of reasons given here is based on the fact that wisdom teeth, either partially or fully erupted, are often very difficult to clean. (The term "erupted" used here means that the wisdom tooth has, at least to some degree, penetrated through the gums.) Any location in the mouth where plaque is allowed to accumulate and persist, such as around a partially erupted wisdom tooth, a person will run the risk of developing dental problems such as tooth decay ("cavities"), periodontal disease ("gum disease"), or recurring infections. Typically it is the partially erupted impacted wisdom tooth that is more unpredictable or troublesome for a person than an impacted wisdom tooth that has never penetrated through the gum tissue.