The third molars, commonly known as wisdom teeth, are the final set of teeth to develop, generally appearing between the ages of 17 and 25. They frequently fail to emerge correctly, often becoming impacted when they do not fully break through the gum line. This condition affects a large percentage of the population. The reasons for non-eruption fall into two broad categories: a physical obstruction blocking the tooth’s path or a biological lack of the tooth developing at all.
Mechanical Reasons for Non-Eruption
The most frequent reason a wisdom tooth fails to erupt is a lack of sufficient space within the jawbone, leading to impaction. As human jaws have evolved to become smaller, the limited area at the back of the mouth often cannot accommodate these late-arriving third molars. This lack of room traps the tooth beneath the gums or bone, preventing its normal alignment and emergence.
The angle at which the developing tooth attempts to grow is a primary determinant of impaction. A wisdom tooth may grow horizontally, lying completely on its side and pushing into the roots of the adjacent second molar. Alternatively, it might be angled toward the front of the mouth (mesial impaction) or toward the back (distal impaction), preventing a clean vertical path through the gum tissue.
In some instances, the physical barrier is not just a neighboring tooth but also the surrounding bone structure. The tooth may be fully enclosed by dense bone, referred to as a hard tissue impaction, which completely halts its upward movement. Even if the tooth manages to push through the bone, it can still be blocked by the soft gum tissue, resulting in a soft tissue impaction.
The adjacent second molar acts as a major physical obstacle, especially if it is fully rooted. Pressure from a misaligned wisdom tooth attempting to erupt pushes against the second molar, making it impossible for the third molar to find a clear pathway. This crowding effect results from a mismatch between the size of the teeth and the available space in the jawbone.
When Wisdom Teeth Never Form
A completely different reason for the absence of wisdom teeth is a congenital condition known as dental agenesis, meaning the tooth never forms. This absence results from genetic factors that affect tooth development during the embryonic stage. Researchers estimate that up to 35% of individuals may be born without one or more of their wisdom teeth.
This phenomenon is strongly connected to human evolution and a shift in dietary habits over time. Ancestral humans required large, powerful jaws and a full set of molars to grind tough, uncooked foods. As diets became softer with the advent of cooking, the evolutionary pressure to maintain a large jaw size lessened, leading to a gradual reduction in jaw size.
Genetic mutations that prevent the formation of third molars were not selected against because the teeth were no longer necessary for survival, allowing these traits to persist and become common. Specific genes, such as PAX9, MSX1, and AXIN2, have been identified as influential factors governing tooth number and development. Variations in these genes can disrupt the normal pattern, resulting in the congenital absence of wisdom teeth.
This genetic predisposition is distinct from impaction because the tooth germ, the initial tissue from which the tooth develops, simply fails to initiate. The absence of the tooth is determined before birth, unlike a tooth that forms but later becomes physically trapped. The likelihood of having this agenesis can also be linked to family lineage and is sometimes observed alongside the absence of other permanent teeth.
What Happens When Teeth Stay Buried
When a wisdom tooth forms but remains trapped beneath the gum line, it can lead to complications. A common issue with partially erupted teeth is pericoronitis, an infection and inflammation of the gum tissue surrounding the crown. This occurs because the small flap of gum creates a pocket that traps food particles and bacteria, making effective cleaning impossible.
The pressure and positioning of the impacted tooth can cause direct harm to the adjacent second molar. As the third molar pushes against the root, it can lead to decay or the reabsorption of the second molar’s root structure. This damage compromises the stability and health of an otherwise functional tooth.
In some instances, the sac of tissue that surrounds the developing wisdom tooth within the jawbone can fill with fluid, forming a cyst. These cysts can grow over time, potentially damaging the surrounding jawbone, nerves, and adjacent teeth. Though rare, a benign (noncancerous) tumor can also develop from this tissue.