An underbite is a common misalignment where the lower teeth protrude, sitting forward of the upper teeth when the mouth is closed. This condition, which varies widely in severity, changes how the teeth meet, affecting both the facial profile and oral function. While mild cases are barely noticeable, severe examples can lead to difficulty chewing and speaking. An underbite results from genetic predispositions interacting with environmental factors that influence jaw growth.
Understanding the Mechanics of an Underbite
An underbite is clinically termed a Class III malocclusion, a structural discrepancy between the upper and lower jaws. The visual appearance occurs because the lower jaw (mandible) extends past the upper jaw (maxilla). This results in a reverse overlap where the bottom teeth sit in front of the top ones.
The underlying cause of this misalignment is typically skeletal, involving an imbalance in jaw size or position. This condition usually stems from an overgrowth of the lower jaw, underdevelopment of the upper jaw, or a combination of both. In rare instances, the misalignment is primarily “dental,” meaning the teeth are positioned incorrectly despite correctly sized jaws.
Genetic and Environmental Drivers of Development
Genetics are the primary determinant in the development of an underbite, indicating a strong hereditary component. If a parent or close family member has a Class III malocclusion, the likelihood of a child developing the condition increases. The inherited genes determine the foundational size and shape of the maxilla and mandible, setting the stage for the jaw relationship.
While the skeletal predisposition is inherited, environmental factors can contribute to the condition’s development and severity. Certain habits practiced during early childhood can apply pressure on the developing teeth and jaws. Prolonged pacifier use, habitual thumb sucking past the age of three, and extended bottle feeding can physically influence the direction of jaw growth.
Another contributing factor is tongue thrusting, where the tongue presses against the front teeth during swallowing, pushing them forward. Chronic mouth breathing, often linked to respiratory issues, alters the resting posture of the tongue. The tongue rests low in the mouth instead of against the palate, which can inhibit the natural forward growth of the upper jaw, exacerbating a mild skeletal tendency.
Age and Timing of Onset
The development of an underbite often begins during early childhood growth phases, when the facial bones are rapidly changing. The condition can be detected as early as age five, with the lower teeth sitting ahead of the upper teeth. This early period, often called Phase I Orthodontics, is when the jaw structure is highly adaptable and responsive to guided growth.
The most significant development and potential worsening typically occur during the adolescent growth spurt. This period, which happens earlier for girls than boys, is when a pre-existing skeletal pattern becomes more pronounced, as the lower jaw may continue to grow more than the upper jaw. While the skeletal structure is set after growth concludes, a mild underbite can still become noticeable later in life due to dental wear or minor tooth movement.
Treatment Pathways for Correction
Corrective measures depend on the patient’s age and the severity of the skeletal discrepancy. Early intervention, typically recommended between the ages of 7 and 10, takes advantage of the jaw’s ongoing growth. During this time, growth modification techniques are effective in guiding the development of the jaws into a better relationship.
Orthodontic appliances like a reverse-pull headgear (face mask) are used to gently pull the underdeveloped upper jaw forward. Palatal expanders may also be used to widen the upper arch, creating more space and aiding correction. A chin cap is another appliance that can redirect the growth of the lower jaw, preventing it from jutting forward excessively.
For adolescents and adults whose jaws are finished growing, treatment for mild to moderate cases often involves fixed appliances like braces or clear aligners. These appliances, sometimes combined with elastics, move the teeth into better alignment to compensate for the jaw discrepancy. In severe cases where the skeletal misalignment is significant and growth is complete, orthognathic surgery is necessary to physically reposition the upper and lower jaws.