An underbite is a common misalignment of the jaw where the lower teeth protrude beyond the upper front teeth when the mouth is closed. Medically known as a Class III malocclusion, this condition creates a distinct profile where the lower jaw appears more prominent. While severity varies, this misalignment can cause difficulties with chewing, speaking, and lead to jaw pain over time.
Defining the Condition
Dental and maxillofacial professionals often use the term mandibular prognathism to refer to the protruding lower jaw. Underbites are broadly separated into two categories: skeletal and dental. A skeletal underbite is the more severe form, resulting from a fundamental misalignment in the jawbones themselves. This occurs when the lower jaw (mandible) is oversized or positioned too far forward, or when the upper jaw (maxilla) is underdeveloped and set back. Conversely, a dental underbite occurs when the jawbones are correctly sized, but the position of the teeth causes the lower teeth to overlap the upper teeth.
Genetic and Hereditary Factors
The most common cause of a significant underbite is genetics, which heavily influences the size and position of the jawbones. The inheritance pattern is often described as polygenic, meaning multiple genes interact to determine the final jaw structure. If a parent has a noticeable underbite, the child has an increased likelihood of developing a similar misalignment.
Inherited Skeletal Discrepancy
The severe skeletal discrepancy, known as mandibular prognathism, is primarily an inherited trait where the mandible grows excessively or the maxilla’s growth is restricted. Researchers have identified multiple genetic factors and specific gene loci associated with this development. For example, genes like MYO1H, PLXNA2, and MATN1 have been implicated, suggesting the underlying issue relates to the formation and growth pathways of bone and connective tissues. The inheritance of jaw size is sometimes linked to an autosomal dominant pattern with incomplete penetrance, explaining why the condition can skip generations or present with varying degrees of severity.
Developmental and Habitual Influences
Childhood Habits
Beyond genetics, certain prolonged habits during childhood can influence jaw development and contribute to an underbite. These acquired causes primarily affect the dentition, potentially leading to a dental underbite or exacerbating a pre-existing skeletal tendency. Habits like extended pacifier use or persistent thumb sucking past the age of three exert constant pressure, pushing the upper teeth backward and the lower teeth forward.
Posture and Breathing
Another significant influence is chronic mouth breathing, often linked to enlarged tonsils or adenoids, which alters the resting posture of the tongue and jaw. When the mouth is habitually open, the upper jaw may not develop its full width, allowing the lower jaw to sit in a more forward position. Tongue thrusting is a specific habit where the tongue pushes against the back of the front teeth during swallowing, which can gradually push the lower incisor teeth forward.
Associated Medical Conditions
In a smaller number of cases, an underbite may manifest as a secondary symptom of a broader medical condition. Certain craniofacial syndromes are characterized by abnormal bone development that directly affects the relationship between the upper and lower jaws. Conditions like Apert syndrome or Crouzon syndrome involve premature fusion of skull bones, which restricts the forward growth of the maxilla. Other developmental issues, such as a cleft lip or cleft palate, are frequently associated with an underbite because the resulting scar tissue inhibits the forward development of the upper jaw bone. Less commonly, hormonal imbalances can also contribute to jaw overgrowth; for instance, excessive production of growth hormone, as seen in acromegaly, can cause a dramatic enlargement of the mandible.