The third molars, commonly known as wisdom teeth, are the final set of teeth to develop and erupt in the human mouth, typically appearing between the ages of 17 and 21. For many people, these teeth fail to emerge correctly because the jaw structure cannot physically accommodate them. This mismatch between tooth size and jaw space leads to impaction, a condition where the tooth becomes trapped in the jawbone or gum tissue.
The Evolutionary Shift in Jaw Size
The jaws of early humans were significantly larger and more robust than those of people today, an anatomical necessity driven by their diet. Ancestral humans consumed tough, uncooked foods like raw meats, nuts, and fibrous plants, requiring extensive and forceful chewing. This rigorous use stimulated the growth of larger, stronger jawbones, providing ample room for all 32 teeth, including the third molars.
A major turning point occurred with the advent of cooking and the transition to agriculture, which introduced softer, processed foods into the human diet. This shift dramatically reduced the functional demand for powerful chewing, lessening the forces that stimulate jawbone growth during development. Over generations, the human jawbone and overall facial structure began to shrink.
The size reduction in the jaw has progressed faster than the evolutionary reduction or loss of the third molars themselves. While the need for an extra set of large grinding teeth diminished, the genetic programming for them largely remained, creating an evolutionary mismatch. Modern jaws are simply smaller versions of their ancestors’ jaws, but they still contain the same number of teeth, resulting in a common lack of space for the final molars to emerge correctly.
How Lack of Space Causes Impaction
Impaction is the direct physical consequence of this evolutionary reduction in jaw size, where the wisdom tooth is physically blocked from fully entering the dental arch. Because the third molars are the last teeth to start developing, they often find that the space has already been claimed by the first and second molars by the time they are ready to erupt. The jaw structure is largely set by the late teenage years, leaving no room for the final teeth to push through.
When the tooth is trapped, it often grows at an abnormal angle, resulting in different types of impaction, which are classified by the tooth’s orientation. The most common is mesial impaction, where the tooth tilts forward toward the second molar. In a horizontal impaction, the tooth is completely parallel to the jawbone, lying on its side and pushing directly into the root of the adjacent tooth.
Other forms include distal impaction, where the tooth is angled backward toward the back of the mouth, and vertical impaction, where the tooth is in a near-normal upright position but remains trapped below the gum line or bone. Regardless of the angle, the physical obstruction prevents the tooth from serving its function and instead forces it against the neighboring second molar or keeps it permanently embedded in the bone.
Health Consequences of Poor Eruption
When a wisdom tooth is impacted or only partially erupted, it can lead to several specific pathologies within the mouth. Partially emerged teeth create a small flap of gum tissue over the tooth crown, which is difficult to clean, allowing food debris and bacteria to become trapped. This condition frequently leads to pericoronitis, a painful, inflammatory infection of the gum tissue surrounding the partially emerged tooth.
The hard-to-reach position also makes the third molar highly susceptible to dental decay, or caries, which can rapidly develop and potentially spread to the adjacent second molar. Furthermore, the sac of tissue that surrounds the developing wisdom tooth within the jawbone can fill with fluid if the tooth remains impacted, forming a dental cyst. If left untreated, these fluid-filled sacs can expand, causing significant structural damage to the jawbone, the roots of neighboring teeth, and even surrounding nerves.
In rare instances, these cysts can even develop into a benign tumor. The pressure from an angled, impacted wisdom tooth can also cause damage to the root of the second molar, leading to potential bone loss and infection in the area. While the effect of wisdom teeth on overall crowding of the front teeth is often debated, the direct damage to the second molar remains a primary concern.