The growth and development of maxillary bones and teeth is subject to general (heredity, functions of endocrine glands, metabolism) and local factors. The role of heredity in inheriting facial features is well-known: the shape of the nose, the size of the jawbones, the size of teeth. In orthodontics, some abnormalities are acknowledged as having a genetic cause: diastema, deep occlusion (bite), exaggerated mandible (mandibular prognathism), the absence of certain teeth (frequently, the lateral upper incisor is missing), supernumerary teeth. The growth of jawbones, the alignment of teeth and the occlusion (bite) ratios are strongly influenced by local factors, by the balance of the perioral muscles and bad habits. The repercussions on the growth of the dental arcades vary according to the age of the child, their duration and frequency.

Mouth breathing (through the mouth, instead of the nose), deglutition with lingual interposition, sucking or interposition of the lower lip (lip biting), speech disorders, create imbalances of the muscles, and various abnormalities can occur. The teeth and the bones move in the direction of the higher muscle pressure. For instance, children breathing through their mouths keep their mouth open, the lips no longer press on the front teeth, which move forward. Demineralization and cavities appear on the teeth which are no longer protected by saliva, jawbones become narrower, the front facial size increases, causing a typical facial aspect: adenoid facies.

In the atypical deglutition, the tongue interposes between the two dental arcades and a gap appears between the teeth: open occlusion.

Sucking or interposing of the lower lip often appears after deconditioning other sucking related habits. It may determine a widening of the groove between the lip and the chin, causing an old person look, dental protrusion, open occlusion.

Bad thumb or pacifier suckling habits:

For children under 2-3 years of age, suckling their thumb or pacifier is normal. However, the persistence of these habits in older children is dangerous, as it stimulates certain muscle groups, in the detriment of others, and through the direct pressure of the thumb on the teeth, it changes their direction and position. Open occlusions appear, where the upper incisors do not touch the lower incisors, the posterior positioning of the mandible, the narrowing of the arcades, the pushing forward of the upper incisors and backward of the lower incisors. In very rare cases, suckling habits do not change the position of the arcades. If such habits are broken at an early age, minor dental changes can be corrected spontaneously.

Milk teeth cavities and their extraction before time are the causes of numerous cases of crowding of permanent teeth. The role of milk teeth is also to maintain the gap required for the alignment of permanent teeth. If milk teeth are lost and space maintainers are not applied, the teeth behind the arcade migrate forward, occupying the space necessary for the permanent replacement teeth.