What Is An Underbite

Overview

With rapid advancements in Dentistry, it has never been easier to get a better smile. Orthodontic treatments like braces and even orthognathic surgery are options to treat malocclusions - misalignment of the teeth and/or jaws.¹ One of the most common malocclusions is an underbite, known by dentists as “mandibular prognathism”. An underbite is a misalignment of the jaws in which the mandible (the lower jaw) is projected forward beyond what’s considered normal. There are many factors that can contribute to this malocclusion, from genetics to environmental factors, and treatment varies greatly between patients.² Let’s understand more about underbites and their effect on patients.

Classification of malocclusion

Historically, Edward Angle was the first to classify malocclusion, and his work is still used today. Angle’s classification is based on the relative position of the upper and lower first molars of each side: a normal position of the jaws (known as class I) is characterized by the mesiobuccal cusp of the upper first molar aligning with the buccal groove of the lower first molar. A class II malocclusion (an overbite) means that the maxilla (the upper jaw) is pushed forward more than it should, and a class III (an underbite) means that the mandible is pushed forward more than what’s normal. In the case of an underbite, it means that either the maxilla did not develop enough, or that the mandible grew too much, or a combination of the two.

However, this isn’t the only classification of malocclusion, nor does it encompass all the different types of underbites known.

Later on, Tweed classified class III malocclusions into pseudo-class III and skeletal class III. The pseudo-class III is one in which bone development is normal and the relationship between the jaws is considered normal, but the teeth are misaligned and lead to a forward projection of the lower arch. The skeletal class III, on the other hand, is defined by an underdeveloped maxilla or an excessively large mandible. 

Moyers extrapolated this idea by proposing that the pseudo-class III exists as a “comfort position” enabled by a neuromuscular reflex: the patient is used to bringing the mandible forward because this position is more comfortable than the centric relation (the neutral position of the mandible) due to the number of contacts between teeth - the more contacts between teeth, the more “normal” and “comfortable” it appears to us when biting. A patient with a pseudo-class III has a higher number of contacts between teeth when bringing the mandible forward, and they instinctively do so to rest their mouth in a more comfortable position.³

Causes

Underbites can be caused by a variety of factors. As they are a heterogeneous group of malocclusion, presenting differently for each individual, causes also differ.

Genetics is one of the most influential aspects of a class III underbite. The Habsburg royal family is known popularly for their underbites, portrayed in many paintings, and the high prevalence of underbites (through mandibular overgrowth or maxillary deficiency) is an example of genetics and inbreeding playing a big role in malocclusion. Many other studies have been done in different populations and they have unanimously found that when a patient has an underbite, it is very likely that other family members have or had this malocclusion, with many pointing to an almost 50% chance of a child developing an underbite if one of the parents has an underbite as well.

Some of the genes associated with the development of class 111 malocclusion include

• IGF1 (insulin-like growth factor 1)
• GHR (growth hormone receptor)
• PLXNA2 

Epigenetic factors have also been considered. Although genes play a huge role in malocclusion, their expression is regulated by external factors and can impact the overall result, a phenomenon known as epigenetics. Changes in the dimensions of the cranium lead to different tension in the masticatory muscles: according to this theory, external factors can lead to variations in the strength of the masticatory muscles, which leads to the expression of epigenetic factors to regulate the growth and remodelling of the jaws.² 

Environmental factors also greatly contribute to class III malocclusion. Deleterious habits like thumb sucking, mouth breathing, abnormal tongue posture, and enlarged tonsils have been reported as possible causes or worsening factors for class III patients. These lead to a downward and backward growth pattern of the mandible, projecting the lower jaw forward.³

Signs and symptoms

In patients with class III malocclusion, the lower teeth cover the upper teeth when smiling, or in more severe cases, the patient’s side profile is altered into a more concave profile with a pronounced chin.⁴

But aesthetics aren’t the only thing affected by an underbite. If left untreated, it can have highly impactful consequences, such as

• Pain
• Speech impediments
• Airway restrictions (that can lead to breathing problems)
• Temporomandibular joint (TMJ) dysfunction
• Poor masticatory function
• Psychological problems: low self-esteem and self-consciousness can be side effects of the aesthetic problems brought on by malocclusion.⁵

Diagnosis

Diagnosis is made by a dentist through clinical evaluation and complementary exams (like X-rays). A few clinical parameters to look for in class III patients are:

• Age: younger patients have a more favourable prognosis when compared to older patients; the earlier you treat an underbite, the better.
• Dental malocclusion: as stated before, class III can be either skeletal or dentoalveolar; the relationship between first molars (Angle classification), between incisors, and the existence of crossbites can determine the severity of the malocclusion.
• Skeletal elements: the angle of the mandible plane, type of facial growth (brachiocephalic/horizontal growth or dolichocephalic/vertical growth), and Wits appraisal (a method that evaluates the angle between the maxilla and the mandible using the occlusal surface of the teeth as a guide) are used to determine skeletal involvement.

Cephalometric analysis is used to identify whether the malocclusion is skeletal or not. This type of X-ray captures the side profile of the patient and is used to analyze the relationship between the upper and lower jaw⁵. This can identify the source of the malocclusion: if it is skeletal, is it due to maxillary deficiency, or due to mandibular overgrowth or both.⁴

Management and treatment

Treatment of underbites can begin as early as during mixed dentition. In fact, early treatment can greatly benefit the patient as it expands the treatment options. A few combinations of treatment are possible:

• Orthodontic treatment only: braces or other orthodontic devices like palatal expanders are used to treat dentoalveolar cases of class III, like pseudo-class III and dental crossbites. In these cases, there’s no skeletal involvement.
• Orthopaedic and orthodontic treatment combined: when there is skeletal involvement but the patient is still young enough for orthopaedic treatment, this combination is ideal.⁵ Cases like retrognathic maxillary class III (maxillary deficiency) or prognathic mandibular class III (mandible overgrowth) can be treated through a combination of orthopaedic devices like a reverse pull mask and braces or other orthodontic devices like rapid palatal expander.
• Orthognathic surgery: this can be an option if treatment is initiated late (during permanent dentition) or if there are other skeletal problems. The skeletal problems include bimaxillary retrusion, bimaxillary protrusion, or a combination of maxillary retrognathism and mandibular prognathism and also craniofacial deformities caused by syndromes like Crouzon’s syndrome, Beckwith-Weidemann syndrome, Apert’s syndrome, and Antley-Bixler syndrome. In this case, surgery is performed to align the jaws after orthodontic treatment.
• Microimplant therapy: as an alternative to orthognathic surgery, temporary anchorage devices (TADs) can be surgically inserted into the jaws of the patient, and they can be combined with distalizing mechanisms to set the mandible back or bring the maxilla forward.
• Camouflage extractions: another alternative to orthognathic surgery, camouflage of class III in permanent dentition, can be performed through teeth extraction and orthodontic treatment.⁴

Epidemiology

The frequency of class III malocclusion varies between different ethnicities. In Caucasians, it is about 4%, but it reaches 14% in Asians, being more common in people of Asian ancestry.⁴

Prevention

Though genetic and epigenetic factors cannot be controlled, deleterious habits play a significant role in malocclusions. Controlling deleterious habits like thumb sucking, and mouth breathing can help in avoiding causes that can lead to an underbite.³ . Enlarged tonsils can be removed surgically and should be evaluated by a medical professional.

Summary

Underbite is a type of malocclusion, that causes the lower teeth and the mandible to be set forward beyond normal parameters. It can be caused by genetic mutations that are inherited, abnormal epigenetic expression of genes, or through external factors like deleterious habits such as thumb sucking and mouth breathing. Common signs and symptoms include lower teeth that cover the upper teeth and a concave facial profile. Underbite is linked to undesirable consequences like problems with speech and breathing, pain, temporomandibular joint disorders, and psychological problems like low self-esteem due to aesthetic concerns. Diagnosis is made through clinical evaluation and an X-ray known as cephalometric analysis. Treatment should begin as early as mixed dentition and can be through orthodontic devices, a combination of orthodontic and orthopaedic devices,  orthognathic surgery,  microimplant therapy, or through camouflage with extractions and orthodontic alignment. Underbites are more common in Asians and those of Asian ancestry. Prevention can be done through elimination of deleterious habits like thumb sucking and mouth breathing, though genetic and epigenetic factors cannot be controlled.