Hypodontia

# Overview
In dentistry, hypodontia is the condition at which the patient has missing teeth as a result of their failure to develop.  Hypodontia describes a situation where the patient is missing up to 6 teeth, excluding the 3rd molars. Missing third molars occur in 9-30% of population.  In primary dentition the maxilla is more affected, with the condition usually involving the maxillary lateral incisor.

# Classification
The condition of missing over 6 teeth, excluding 3rd molars or wisdom teeth, is called oligodontia.  The condition for missing all teeth, either primary and/or permanent), is called anodontia.  A similar condition is hyperdontia, in which there are more than the usual number of teeth.
Many other terms to describe a reduction in number of teeth appear in the literature: oligodontia, anodontia, aplasia of teeth, congenitally missing teeth, absence of teeth, agenesis of teeth and lack of teeth.

# Pathophysiology
The cause of isolated missing teeth remains unclear, but the condition is believed to be associated with genetic or environmental factors during dental development.  Missing teeth have been reported in association with increased maternal age, low birth weight, multiple births and rubella virus infection during embryonic life.
There is a possible correlation between tooth agenesis and innervation. A relationship was also postulated between abnormalities of the brainstem and the presence of agenesis.
Hypodontia is often familial, and can also be associated with genetic disorders such as ectodermal dysplasia or Down syndrome.  Hypodontia can also been seen in people with cleft lip and palate.
Among the possible causes are mentioned genetic, hormonal, environmental and infectious.
Etiology due to hormonal defects: idiopathic hypoparathyroidism and pseudohypoparathyroidism. Exists the possibility that this defect depends on a moniliasis (candidiasis, candida endocrinopathy syndrome).
Environmental causes involving exposure to PCBs (ex.dioxin), radiation, anticancer chemotherapeutic agents, allergy  and toxic epidermal necrolysis after drug.
Infectious causes of hypodontia: rubella, candida.
The Journal of the American Dental Association published  preliminary data suggesting a statistical association between hypodontia of the permanent teeth and epithelial ovarian cancer (EOC). The study shows that women with EOC are 8.1 times more likely to have hypodontia than are women without EOC. The suggestion therefore is that hypodontia can serve as a "marker" for potential risk of EOC in women.
Also the increased frequency of hypodontia in twins and low birth weight in twins with hypodontia suggests that environmental factors during perinatal are responsible hypodontia.

## Genetics
Genetic causes also involve the genes MSX1 and PAX9. But MSX1 and MSX2 are excluded as candidate genes for hypodontia.
Genetic associations for selective tooth agenesis ("STHAG") include:
==Epidemiology and Demographics
In caucasians, the most common missing teeth are the wisdom teeth (25-35%), the upper lateral incisors (2%) the lower second premolars (3%), or the upper second premolar, with a 4:1 female to male ratio. The prevalence of missing primary teeth is found at 0.1-0.9%, with a 1:1 male to female ratio. Excluding the third molars, missing permanent dentition accounts for 3.5-6.5%. Similar trends of missing teeth can be seen in approximately 3-10% of orthodontic patients. 
30-50% of people with missing primary teeth will have missing permanent teeth, as well.