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Hypodontia is defined as the developmental absence of one or more teeth excluding the Wisdom tooth. It is one of the most common dental anomalies, and can have a negative impact on function, and also appearance. It rarely occurs in Deciduous teeth (also known as deciduous, milk, first and baby teeth) and the most commonly affected are the adult Premolar and the upper Incisor. 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:
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.
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.
Cephalometric tracing is commonly used to study a patient's dentofacial proportions in the craniofacial complex. This can aid in predicting growth changes, allowing dentists, especially orthodontists, to develop a suitable treatment plan. Coupled with that, findings consistent among individuals include:
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. Svinhufvud et al. (1988) suggested that teeth that were more prone to absence developed in areas of initial fusion of the jaw. For instance, maxillary lateral incisors originate where the lateral maxillae and medial nasal bone processes fuse. 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.
Presently, the role of polygenic and environmental factors on hypodontia is recognised in most theories.
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. Treatment such as irradiation has been shown to have severe effects on developing teeth. In a smaller capacity, chemotherapy was also found to have a similar effect. Thalidomide (N-phthaloylglutamine) was also discovered to have a causative effect on mothers who took the drug during pregnancy, resulting in congenitally missing teeth in their children. A link was found between systemic diseases, endocrine disruption (i.e. idiopathic hypoparathyroidism and pseudohypoparathyroidism) and ectodermal dysplasia. However, a definite etiological relationship has yet to be established. Examples of infections include rubella 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.
Genetic associations for selective tooth agenesis ("STHAG") include:
| STHAG1 | MSX1 | 4p16 | |
| STHAG2 | ? | 16q12 | |
| STHAG3 | PAX9 | 14q12 | |
| STHAG4 | WNT10A | 2q35 | |
| STHAG5 | ? | 10q11 | |
| STHAG6 | LTBP3 | 11q12 | |
| STHAGX1 | EDA | Xq13.1 |
Failure of tooth formation due to disturbances during the early stages of development could be the cause of congenital missing teeth; this is also known as tooth agenesis. A variety of studies show that missing teeth are commonly associated with genetic and environmental factors. Some literature also shows that a combination of both factors may contribute to the occurrence of hypodontia.
Most craniofacial characteristics are influenced by both genetic and environmental factors through complex interactions. The variable expressivity of traits can be either completely genetically determined, environmentally determined, or both. That genetics plays an important role in hypodontia is shown in many different cases. There are hundreds of genes expressed and involved in regulating tooth morphogenesis. Although a single gene defect may contribute to hypodontia, more studies propose that hypodontia is the result of one or more points of closely linked genetic mutations, or polygenic defects.
The pattern of congenitally missing teeth seen in monozygotic twins is different, suggesting an underlying epigenetics factor, which may be due to the simultaneous occurrence of two anomalies. This multifactorial aetiology involves environmental factors which trigger the genetic anomalies, resulting in the occurrence of dental agenesis. Common environmental factors include infection, trauma and drugs which predispose to the condition. In hereditary cases, evidence of dental germ developing after surrounding tissues have closed the space required for development may be a large contributing factor, as well as such genetic disorders as Down syndrome, ectodermal dysplasia, cleidocranial dysplasia, and cleft lip and cleft palate.
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.
PAX9 and TGFA are involved in regulating between MSX1 and PAX9, causing hypodontia of the molars.
Hypodontia can be found in isolated cases too. The familiar or sporadic type of isolations are more frequently reported than the syndromic type. Isolated cases of autosomal dominant, 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.
Meta-analyses and theoretical reviews have demonstrated that attractive children are seen by others as more intelligent and exhibit more positive social behaviour and traits, other than receiving much more positive treatment than their less attractive counterparts. 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.
It has been found that individuals with hypodontia experience more difficulty during mastication or functioning movements due to smaller occlusal table available. A recent cross-sectional study showed that hypodontia patients have more difficulties in chewing, especially if the deciduous teeth associated with the missing permanent teeth had been exfoliated. Despite currently limited evidence to support this statement, it is plausible that hypodontia may pose functional limitations, which eventually affect that individual's general well-being and quality of life.
Hypodontia can indeed pose limitations on the chewing ability of a patient. 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.
Before determining a treatment plan, the following should be determined:
Traditionally, the management of hypodontia has involved replacing missing teeth. By replacing the missing teeth, it can prevent neighbouring teeth tilting or drifting and also prevents the overeruption of opposing teeth which could then impact on occlusion and temporo-mandibular joint dysfunction and impact the patient's susceptibility to gum disease, tooth wear and tooth fractures. 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. If tooth movement was to occur, another form of management may be required depending on the severity and nature.
The following below are the methods used to manage hypodontia:
The primary molars present also functions as a space maintainer, prevent alveolar bone resorption and delays future Prosthodontics space replacement by acting as a semi permanent solution going into adulthood Previous studies also shown a good prognosis of retained primary molars 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.
Despite this, the retention of primary teeth, particularly molars, are more susceptible to occlusal wear, over-eruption of opposing teeth and the loss of inter-occlusal space.
When moving the canine into the space of the lateral incisor, the dimensions of the canine, root position and gingival position differ from a lateral incisor and therefore preparation of the canine is necessary in order for it to mimic the incisor. This may involve:
The use of veneers can also be used instead of composite however, these are more expensive and more time-consuming.
There have been several studies which showed the advantages of orthodontic space closure without Prosthodontics space replacements. The main advantage mentioned is the early completion of the treatment during early adolescence and the long lasting result of the treatment outcome. In individuals with a high smile line, the mesial re-positioning of canine maintains the normal soft tissue architecture is important in maintaining the aesthetic appearance. This option also negates the risks and costs that comes with prosthodontic treatment and the impression that there is no missing tooth.
Some factors need to be considered when making a decision whether to undergo space closure. These include facial profile, size and dimension of canine, the shade of colour of the teeth and the gingival contour and height. Group function occlusion is usually present as a result of the mesial movement of the canine. In order to maintain the stability of the closed space, direct-bonded lingual retainers are usually required.
Space opening and prosthodontic treatment is indicated where there is a Class I molar relationships in the absence of malocclusion Class III molar relationships presenting with a concave facial profile. However, the alteration in appearance during orthodontic treatment (e.g. creating diastema for placement of prostheses) before the filling up the space, although temporary, can negatively impact the oral health-related quality of life in adolescents.
In the permanent dentition third molars are most commonly absent, and one study found prevalence rates of between 20–22%. When third molars are ignored the prevalence rate for each tooth varies from study to study. In Caucasian studies mandibular second premolars and maxillary lateral incisors are most often absent. Several UK studies have found the lower second premolar to be most commonly absent. Studies from Asian populations report that the mandibular incisor is most commonly absent.
A higher prevalence of hypodontia in females has been reported. The most extensive studies have been in Caucasian populations and suggest a prevalence of 4–6%.
One study looked at 33 previous studies with a sample size of 127,000, and concluded that the prevalence of hypodontia in the permanent dentition varied between continents, racial groups and genders. In the white European population they suggested a prevalence of 4.6% in males and 6.3% in females. In an African-American sample they found this to be 3.2% in males and 4.6% in females. The same study found that in the permanent dentition the most likely teeth to be missing and the frequency of these missing teeth was:
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