Anterior open bite: there is no vertical overlap of the incisors when the buccal segment teeth are in occlusion.
Aetiology of anterior open bite
In common with other types of malocclusion, both inherited and environmental factors are implicated in the aetiology of anterior open bite. These factor include skeletal pattern, soft tissues, habits, and localized failure of development.
Individual with a tendency to vertical rather than horizontal facial growth exhibit increased vertical skeletal proportions. Where the lower face height is increased there will be an increased inter-occlusal distance between the maxilla and mandible. Although the labial segment teeth appear to be able to compensate for this to a limited extent by further eruption, where the inter-occlusal distance exceeds this compensatory ability an anterior open bite will result. If the vertical, downwards and backwards pattern of growth continues, the anterior open bite will become more marked.
In this group of patients the anterior open bite is usually symmetrical and in the more severe cases may extend distally around the arch so that only the posterior molars are in contact when the patient is in maximal interdigitation. The vertical development of the labial segments results in typically extended alveolar processes when viewed on a lateral cephalometric radiograph.
Soft tissue pattern
In order to be able to swallow it is necessary to create an anterior oral seal. In younger children the lips are often incompetent and a proportion will achieve an anterior seal by positioning their tongue forward between the anterior teeth during swallowing. Individuals with increased vertical skeletal proportions have an increased likelihood of incompetent lips and may continue to achieve an anterior oral seal in this manner even when the soft tissues have matured. This type of swallowing pattern is also seen in patients with an anterior open bite due to a digit-sucking habit. In these situations the behaviour of the tongue is adaptive. An endogenous or primary tongue thrust is rare, but it is difficult to distinguish it from an adaptive tongue thrust as the occlusal features are similar. However, it has been suggested that an endogenous tongue thrust is associated with stigmatism (lisping), and in some cases both the upper and lower incisors are proclined by the action of the tongue.
The effects of a habit depend upon its duration and intensity. If a persistent digit-sucking habit continues into the mixed and permanent dentitions, this can result in an anterior open bite due to restriction of development of the incisors by the finger or thumb. Char-acteristically, the anterior open bite produced is asymmetrical (unless the patient sucks two fingers) and it is often associated with a posterior crossbite. Constriction of the upper arch is believed to be caused by cheek pressure and a low tongue position.
After a sucking habit stops the open bite tends to resolve, although this may take several months. During this period the tongue may come forward during swallowing to achieve an anterior seal. In a small proportion of cases where the habit has continued until growth is complete the open bite may persist.
Localized failure of development