Wisdom Teeth

- Which teeth are they? | What does the term "impacted" mean? | Impaction classifications.

Which teeth
are wisdom

Link to 3rd molar slideshow.

Types of
wisdom tooth

Link to types of impactions animation.

1) What are they?

"Wisdom teeth" are a type of molar. Molars are the large chewing teeth found furthest in the back of the mouth.

Most people have 1st, 2nd and 3rd molars. A person's third molars are their wisdom teeth. (See our graphic below.)

a) When do they come in?

For most people, the eruption process takes place during their late teens or early twenties (usually ages 18 to 24 years), although eruption outside of this age range is not uncommon.

An x-ray showing a person's upper and lower teeth on their right side.

Use the slideshow button to see examples of which teeth are a person' wisdom teeth.

If there is not enough room for the teeth, or they are not aligned properly, they may never fully erupt. (See "Impacted Teeth" below.)

b) How many wisdom teeth does a person have?

People usually have four: upper left, upper right, lower left, and lower right.

If they don't, it's due to their genetic makeup. It's been estimated that about 25% of people are lacking one or more. (Faculty, 1997)

An x-ray showing different classifications of tooth impactions.

Examples of different classifications of impacted teeth.

2) What are "impacted" wisdom teeth?

In dental terminology, an "impacted" tooth refers to one that has failed to fully erupt (emerge into its expected position).

[Our slideshow and animations below give examples of impactions.]

This failure to erupt properly might occur because:

  • There is not enough room in the person's jaw to accommodate the tooth.
  • The tooth's eruption path is obstructed by other teeth.
  • Because the angulation of the tooth is improper.

3) Types of impactions (classifications).

Dentists use a number of terms, in combination, to describe the positioning of impacted teeth. They are mesial, distal, horizontal, vertical, soft-tissue and bony.

a) Mesial, vertical, horizontal and distal.

These terms are used to refer to the general angulation (positioning) of the tooth.

Graphic showing mesial, distal, vertical and horizontal impaction types.

Mesial, distal, vertical and horizontal wisdom tooth impactions.

  • The term "Mesial" (also mesio-angular) means that the tooth is angled forward, toward the front of the mouth. This is the most common type of wisdom tooth impaction.

The other types of impactions, in order of frequency of occurrence, are the vertical, horizontal, and distal types.

  • Vertical impactions have a relatively normal orientation.
  • Horizontal (also traverse) impactions have an alignment where the tooth is lying on its side.
  • A Distal (also disto-angular) impaction has an angulation that is generally directed towards the rear of the mouth.
Takeaways from this section.

When making an assessment of the level of difficulty that's expected for an extraction, the dentist will pay particular attention to its angulation.

In general, here's how each type ranks (from easier to harder): Vertical < Mesio-angular < Horizontal < Disto-angular.

Graphic illustrating soft tissue and bony impaction types.

Full-bony, partial-bony and soft-tissue impactions.

b) Soft-tissue and bony wisdom tooth impactions.

In combination with the classifications above, wisdom teeth are also categorized as soft tissue or bony impactions.

  • 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 fully erupted through the gum tissue.
  • The term "bony" or "hard tissue" impaction indicates that the tooth still lies primarily within the jawbone. A full-bony impaction is entirely encased by bone tissue, whereas a partial-bony one has erupted through it somewhat.

Takeaways from this section.

The cost to remove an impacted wisdom tooth is typically based on its soft-tissue/bony classification.

And as you might expect, within this group soft-tissue impactions are typically the easiest to perform, followed by partial and then full-bony.

c) How likely is it that your wisdom teeth will be impacted?

Studies suggest that the incidence of having at least one that's impacted runs on the order of 65 to 72%. (Faculty, 1997)

Lower wisdom teeth are more likely to be impacted than upper ones.


Graphic illustrating how a lack of jawbone space can lead to problems.

Inadequate jawbone space leads to impaction.

4) What causes wisdom tooth impaction?

The reason why some wisdom teeth are impacted is not an easy question to answer. A primary cause simply seems to be a condition of inadequate jawbone space behind a person's second molar.

Why this lack of space exists is not fully understood. There does, however, seem to be a correlation between large tooth size and/or the presence of generalized tooth crowding and having impacted wisdom teeth.

The human diet has changed.

The dietary changes adopted by modern man have been theorized as playing a role in the incidence of 3rd molar impaction.

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The coarse nature of stone-age man's diet had the effect of producing extensive tooth wear (not just on the chewing surface of the teeth but also in between, where neighboring teeth touch against each other).

When this type of wear takes place, it tends to reduce the "length" of the teeth (as a set), thus creating additional jawbone space to accommodate the wisdom teeth by the time they erupt. In comparison, the diet of modern man does not usually cause a significant amount of this type of 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 chewing muscle activity. This activity could have stimulated greater jawbone growth, thus providing more space for wisdom teeth.

Additionally, the harsh world of the caveman no doubt often led to the occurrence of broken teeth and tooth loss. Once a tooth (or a portion of it) is missing the teeth behind it have a tendency to shift forward. This type of movement would make more jawbone space available for wisdom teeth. In comparison, with the advent of modern dentistry there are relatively few reasons why a tooth should remain unrepaired or be lost.



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