An underbite, clinically known as a Class III malocclusion, is a bite misalignment where the lower teeth and jaw sit forward, overlapping the upper teeth when the mouth is closed. This reverses the normal bite relationship where the upper front teeth slightly overlap the lower teeth. The condition, which ranges from mild to severe, is often noticeable by a protruding lower jaw and a concave facial profile. The causes of an underbite are complex, stemming from interactions between inherited bone structure, early developmental factors, and the final alignment of the teeth.
Inherited Skeletal Factors
The most significant factors determining the likelihood of developing an underbite are related to the inherited dimensions of the jaw bones. This skeletal misalignment is often referred to as mandibular prognathism, describing a lower jaw (mandible) that is disproportionately large or positioned too far forward relative to the upper jaw. Conversely, the condition can also arise from maxillary hypoplasia, where the upper jaw (maxilla) is underdeveloped or set too far back. In many cases, an underbite results from a combination of both an overgrown mandible and an undersized maxilla.
The influence of heredity is strong, with individuals significantly more likely to have an underbite if family members, such as a parent or sibling, also have the condition. This familial pattern suggests a genetic component determines the shape and size of the jawbones. However, the inheritance is not typically controlled by a single gene but is instead considered polygenic, meaning multiple genes contribute to the final jaw structure.
The specific genetic mechanisms are complex, often involving a polygenic threshold model where a certain number of contributing genes must be present for the trait to be expressed. Researchers have identified variations in specific chromosomal locations that are associated with the Class III phenotype. These genes often relate to growth factors and cartilage development, influencing jaw size discrepancies.
The severity of the skeletal discrepancy can vary widely, which is often explained by the concept of variable expressivity—where the same genetic predisposition can manifest as a mild or a pronounced underbite. Because jaw growth continues throughout childhood and adolescence, the full extent of the inherited skeletal pattern may not become apparent until the individual reaches full maturity. This underlying skeletal disharmony often represents the most challenging aspect of underbite correction, requiring advanced orthodontic or surgical intervention.
Environmental and Developmental Influences
While genetics set the template for jaw growth, various external factors during a child’s development can disrupt this process and contribute to or worsen an underbite. Prolonged oral habits in early childhood, particularly non-nutritive sucking past the age of three, can negatively influence the developing bone structure. Excessive or intense thumb or pacifier sucking can apply forces that inhibit the forward growth of the upper jaw, potentially leading to a relative maxillary deficiency.
Atypical tongue posture or a tongue thrust swallow pattern is another contributing developmental factor. In a proper swallow, the tongue rests against the roof of the mouth. However, a tongue thrust involves the tongue pressing forward against the front teeth during swallowing. This constant forward pressure, occurring thousands of times a day, can push the lower front teeth outward and the upper front teeth inward, which can exacerbate an existing minor skeletal tendency toward an underbite.
Premature Tooth Loss
The premature loss of primary (baby) teeth can alter the paths of the erupting permanent teeth. This misalignment encourages the lower jaw to shift forward into an underbite position.
Jaw Trauma
Trauma to the jaw during the growth period can cause asymmetrical or restricted growth of either the maxilla or the mandible. This directly contributes to a skeletal imbalance.
Contributing Dental Alignment Issues
Beyond the skeletal structure, the specific alignment of the teeth themselves can create or compound the appearance of an underbite. The position and angle of the front teeth, independent of the underlying jaw size, play a significant role in the final bite relationship. This is often categorized as a dental Class III malocclusion, as opposed to a skeletal one.
In some cases, the upper front teeth may be tipped inward, while the lower front teeth are tipped outward. This particular dental arrangement can mask a minor skeletal issue, or conversely, create a reverse bite relationship even when the jaw bones are relatively well-positioned. This condition is sometimes called a “pseudo-underbite,” where the teeth are compensating for a minor skeletal discrepancy or are simply misaligned within a normal jaw structure.
Crowded or misplaced teeth can also force the lower jaw into a forward position when the individual bites down to achieve maximum contact. This habitual positioning of the jaw, driven by the need for the teeth to meet, can cause the lower jaw to functionally protrude. Correcting these purely dental alignment issues often involves orthodontic treatment focused solely on moving the teeth into their proper position relative to the opposing arch.