Гиподонтия: Недоразвитие зубов (исключая зубы мудрости)

Developmental absence of one or more teeth excluding the third molars

Hypodontia is defined as the developmental absence of one or more teeth excluding the third molars. It is one of the most common dental anomalies, and can have a negative impact on function, and also appearance. It rarely occurs in primary teeth (also known as deciduous, milk, first and baby teeth) and the most commonly affected are the adult second premolars and the upper lateral incisors. It usually occurs as part of a syndrome that involves other abnormalities and requires multidisciplinary treatment. The phenomenon can be subdivided into the following according to the number of teeth concerned:
Hypodontia: one to six missing teeth excluding the third molars
Oligodontia: six or more missing permanent teeth excluding the third molars
Anodontia: complete absence of teeth

Typically, all baby teeth will be present by the age of three. As for all adult teeth, they erupt between the ages 6 to 14, with the exception of the third molar, also known as the wisdom teeth which normally erupt between 17 and 25 years of age. If the tooth has yet to erupt by an appropriate age, panoramic x rays are taken.

Microdontia may be present in one or more of the other teeth. This means that the teeth appear smaller than normal, may be observed in both the primary and permanent dentition. This condition can be genetically linked and in severe cases, may present themselves in the form of ectodermal dysplasia, cleft lip or palate or Down Syndrome. A delay in tooth development may also serve as an indication, whereby the absence of an adult successor slows down the normal resorption of the roots of the baby teeth, which is the progressive loss of parts of the tooth. Misplaced (ectopic) positioning of the adult teeth may be discovered upon examination or a radiograph. One of the consequences may be an adult tooth intercepting with a baby tooth, causing premature loss or wrong positioning. This can be due to either the absence of neighboring teeth acting as a guide during eruption or the lack of space in the jaw for them to erupt into because of malocclusion.

Several studies have discovered that anteriorly missing teeth can accompany retrognathic maxilla, also known as an underbite, prognathic mandible, where the lower jaw protrudes out more than normal, and smaller posterior cranial base length. Occurrence of hypodontia can be associated with reduced anterior lower facial height and lip protrusion. This can be linked to lower maxillary to mandibular plane angles. A more acute mandibular angle and flatter chin may develop as a result. These characteristics become more prominent as the condition becomes progressively severe, particularly when more than one tooth is missing.

Several theories regarding the aetiology of hypodontia have been proposed in existing literature. There have been various theories mostly looking into genetic and environmental aspects and how they may both be involved. However, the exact cause remains unclear. The extent of individual influences of genetic and environmental factors is still widely debated. Theories regarding the mechanism through which hypodontia occurs can be categorised into evolutional or anatomical. Preliminary studies focused on an evolutionary approach which suggested shortening of the intermaxillary complex and thus shorter arches may contribute to a decrease in number of teeth. This was also suggested in 1945 by Dahlberg using Butler's Field Theory that focused on evolution and development of mammalian teeth into human dentition in an attempt to analyse different of agenesis. In each jaw, four morphological sites were identified (incisors, canines, premolars and molars). The tooth at the end of each region was less genetically stable and hence more prone to absence. In contrast, the tooth most mesial in each region seemed to be more genetically stable. A subsequent theory hypothesised the teeth at the end of each region were possibly "vestigial bodies" that became obsolete during the evolutionary process. At present, it has been theorised that evolutionary change is working to decrease the human dentition by the loss of an incisor, premolar and molar in each quadrant. According to Vastardis (2000), the size of jaws and number of teeth seem to decrease along with human evolution. Theories focusing on anatomical principle, hypothesised that specific areas of the dental lamina are especially prone to environmental effects during tooth maturation. In contrast, Kjaer et al. (1994) suggested regions where innervation developed were more sensitive than areas of fusion. Commonly affected regions were found to undergo innervation last, this might imply the developmental relationship between nerve and hard tissue. It is thought to be local nerve development that affects tooth agenesis rather than global development, as brainstem anomalies have not been seen to affect tooth development. Invasive environmental factors potentially affect tooth development and positioning leading to hypodontia and impaction. Examples include jaw fractures, surgical procedures and extraction of the preceding deciduous tooth. and ectodermal dysplasia. However, a definite etiological relationship has yet to be established. and candida. Exposure to PCBs (such as dioxin), allergies, and toxic epidermal necrolysis following a drug reaction may also be contributing factors. In a recent study assessing environmental risk factors for hypodontia, it was established that maternal smoking does play a causative role in hypodontia. Passive smoking and caffeine were also assessed but showed no statistical significance. 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.

MSX1 (muscle segment homeobox 1) is involved in condensation of ectomesenchyme in the tooth germ. Among the members of homeobox genes, MSX1 and MSX2 are crucial in mediating direct epithelial mesenchymal interactions during tooth development by expressing in regions of condensing ectomesenchyme in the tooth germ. MSX1 mutations have been identified as a contributing factor in missing second premolars, third molars, and a small percentage of first molars. MSX1 is less likely to cause anterior agenesis. Heterozygous mutations in PAX9 (paired box gene 9) could arrest tooth morphogenesis as it plays a role of transcription the gene expressed in tooth mesenchyme at the bud stage during tooth development. A study showed that single nucleotide polymorphisms in PAX9 were highly associated with missing upper lateral incisors. Its polymorphic variant may be associated with hypodontia such as missing lower incisors or in a more severe form of agenesis like oligontia (lack of six or more permanent teeth).

EDA provides instructions for making a protein called ectodysplasin A. It encodes transmembrane protein that is part of TNF (tumour necrosis factor) family of ligands. EDA gene defects cause ectodermal dysplasia, which is also known as X linked hypohidrotic ectodermal dysplasia. Common dental features of ectodermal dysplasia are multiple missing teeth and microdontia. PAX9 and TGFA are involved in regulating between MSX1 and PAX9, causing hypodontia of the molars. autosomal recessive, or X linked inheritance patterns may have an impact on the isolation conditions in expressing variation of both penetration and expressivity of traits. Mutations in MSX, PAX9 and TGFA genes are known to cause congenitally missing teeth in some racial groups.

In the 1960s and 1970s, several studies were conducted sponsored by the U. S. Atomic Energy Commission, with the aim of finding a link between genetics and hypodontia.

There are numerous studies and research reports on the prevalence, aetiology, and treatment of hypodontia and the dentoskeletal effect of hypodontia. A few studies have investigated Oral Health related Quality of Life (OHRQoL) in individuals with hypodontia Therefore, a divergence from perceived ideal dentofacial aesthetic, particularly in children, might adversely affect self esteem and self confidence besides attracting mockery from peers. It is therefore reasonable to theorize that deviations from "normal" or "ideal" dentofacial aesthetic could be destructive to an individual's psychosocial and emotional well being, which brings upon some psychosocial distress in that individual as a result of their condition. Further deepening of the bite can also be seen on individuals with missing posterior teeth. Apart from that, hypodontia may lead to non working interferences, poor gingival contours and over eruptions of the opposing teeth. The condition can be associated with split in the upper lips – a condition known as oral cleft. Hypodontia can have impacts on speech, aesthetics and function of muscles in the mouth. As a result, hypodontia can have negative impacts on the quality of life, although the condition can be well managed and treated by dentists and orthodontists. To manage the condition, the patient will need to have long term orthodontic treatment.

Patient with hypodontia requires careful treatment plan due to complex case in order to ensure the best treatment outcomes. Such treatment plans require multi disciplinary approach, which usually come at a financial cost to both patient and possibly their family. Due to this reason, a team consists of different dental specialties is involved in the patient care.

Hypodontia is a condition that can present in various ways with differing severities. This results in a wide range of treatment methods available. Early diagnosis of hypodontia is critical for treatment success. The treatment of hypodontia involves specialists in departments such as oral and maxillofacial surgery, operative dentistry, pediatric dentistry, orthodontics and prosthodontics. The size and number of remaining teeth in both arches However, studies have suggested that a stable occlusion can be achieved even with a shortened arch of 10 occluding pairs of teeth. The findings support the concept that a healthy and stable occlusion can exist despite missing teeth as long as an acceptable number of teeth are in occlusion. However, this management technique may not be suitable for those with gum disease, parafunctioning activity (tooth grinding or clenching) or malocclusion. It should also be noted that spaces within the dental arch should be monitored, especially in younger patients, as teeth are more likely to drift, tilt or over erupt. To do this, study models and clinical photographs could be taken in order to record baseline records. Appearance may not be a problem in some cases, for example, when spacing present behind the canines may not be particularly visible, depending on the individual. Therefore, with a presence of healthy primary teeth in the absence of a permanent successor, retaining the primary teeth can be a feasible management of hypodontia. The primary molars present also functions as a space maintainer, prevent alveolar bone resorption and delays future prosthodontic space replacement by acting as a semi permanent solution going into adulthood However, leaving the primary teeth in place may run the risk of tooth infraocclusion where the occlusal surface is below that of adjacent teeth.