Wisdom teeth (third molars) become impacted because they don't have enough room to come in (erupt) or develop normally.
Wisdom teeth usually emerge sometime between the ages of 17 and 25. Some people have wisdom teeth that emerge without any problems and line up with the other teeth behind the second molars. In many cases, however, the mouth is too crowded for third molars to develop normally. These crowded third molars become trapped (impacted).
An impacted wisdom tooth may partially emerge so that some of the crown is visible (partially impacted), or it may never break through the gums (fully impacted). Whether partially or fully impacted, the tooth may:
Grow at an angle toward the next tooth (second molar)
Grow at an angle toward the back of the mouth
Grow at a right angle to the other teeth, as if the wisdom tooth is "lying down" within the jawbone
Grow straight up or down like other teeth but stay trapped within the jawbone