Hypodontia

Hypodontia is defined as the developmental absence of one or more teeth (excluding the third molars) which can affect both the primary and permanent dentition.[1] It is the most common developmental dental anomaly and can be challenging to manage clinically.[2] The term hypodontia is used to describe the whole range of the disorder from one missing tooth to the complete absence of all teeth, anodontia.

See also: List of dental abnormalities associated with cutaneous conditions

Hypodontia
Clinical photograph of a hypodontia case of missing upper lateral incisors
SpecialtyDentistry

Terminology in use varies. One text [3] has proposed the following descriptive terms:

  • Hypodontia: A developmental or congenital condition characterised by fewer than normal teeth.
  • Severe hypodontia: A developmental or congenital condition characterised by absence of 6 or more teeth.
  • Oligodontia: A developmental or congenital condition characterised by fewer than normal teeth in the presence of systemic manifestations.
  • Anodontia: a developmental or congenital condition characterised by absence of all teeth.

The descriptive terms agenesis or multiple dental agenesis are often used in the US.[3]

Individuals with hypodontia often exhibit other dental anomalies such as microdontia,[4][5] impaction of permanent canines [6] and transposition.[7]

Supernumerary teeth are teeth present in addition to the normal complement. This can occur in both dentitions.

Signs and symptoms

Most people with hypodontia are only missing one or two teeth, but can vary from one missing tooth to a full missing dentition.

The teeth most likely to be missing are permanent 2nd premolars and upper lateral incisors.[8]

Deciduous dentition

In the deciduous dentition the teeth most likely to be missing are upper laterals and lower central incisors, these account for about 50-90% of affected deciduous teeth.

It is most common for the missing teeth to be unilateral and agenesis only affecting one or two teeth.[8]

Permanent dentition

In the permanent dentition, the teeth most likely to be missing are the lower 2nd premolars and the upper lateral incisors. This pattern seems to vary more in individuals that have hypodontia as part of a syndrome.[8]

Dental features

Most of the time tooth agenesis is not associated with a syndrome but it is a common feature in cleft-lip or cleft-lip and palate patients, particularly individuals with more severe clefts.[9] In these cases, agenesis often presents in the upper lateral incisor.

Examples of more conditions that may present with hypodontia as one of their features include Down Syndrome[10] and ectodermal dysplasia.[11]

It is known that hypodontia can contribute to abnormal occlusion, especially in its more severe cases, and also has linked to developmental delays, microdontia, taurodontia, enamel hypoplasia and ectopic positioning of permanent teeth due to the inability of neighbouring teeth to guide eruption.[12] It has been reported that 50% of patients with a canine/first premolar transposition also experience hypodontia, there are suggestions that this may be due to a genetic link.

Skeletal features

The skeletal features and growth patterns associated with hypodontia are unknown although there is some evidence to suggest that the cranio-facial features of individuals with missing teeth are different from those with a full natural dentition. Patients with hypodontia commonly present with a smaller lower anterior face height and lip protrusion which are linked with lower mandibular plane angles. Smaller lengths of maxilla and mandible and a Class III skeletal base are also associated with hypodontia. The tendency of the patient to have these skeletal effects becomes more significant as the severity of agenesis increases, especially when more than one type of tooth is missing.[13] Patients may appear over-closed due to the short face height and large freeway space associated with hypodontia.

Children

Studies suggest that children with mild hypodontia do have significant associated skeletal and dental features. It was found that it was associated with; a high-angle cranio-facial pattern, a class III malocclusion (in maxillary hypodontia) and a significant reduction in posterior facial height (with posterior and mandibular hypodontia).[14]

Associated anomalies
Illustration of cleft lip and palate

[15]

Cause

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.[8] However, the exact cause remains unclear. The extent of individual influences of genetic and environmental factors is still widely debated.[2]

These theories can be categories into evolutional or anatomical.[16]

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.[17] 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.[18] 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.[19]

Theories focusing on anatomical principle, hypothesised that specific areas of the dental lamina are especially prone to environmental effects during tooth maturation.[16] 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.[20] In contrast, Kjaer et al. (1994) suggested regions where innervation developed were more sensitive than areas of fusion.[21] 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.[21]

Presently, the role of polygenic and environmental factors on hypodontia is recognised in most theories.

Environmental factors

Environmental factors can be classified into 2 main groups, invasive and non-invasive.  These factors act additively or independently, ultimately affecting positioning and physical development of the tooth.[22]

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.[22] 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).[23] and ectodermal dysplasia. However, a definite etiological relationship has yet to be established.[22] Examples of infections include rubella[24] and candida.[25] Exposure to PCBs (ex.dioxin),[26][27][28] allergy [29] and toxic epidermal necrolysis after drug.[30] 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.[31]

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.[32]

Genetics

Genetic causes also involve the genes MSX1 and PAX9.[33][34]

Genetic associations for selective tooth agenesis ("STHAG") include:

Type OMIM Gene Locus
STHAG1 106600 MSX1 4p16
STHAG2 602639 ? 16q12
STHAG3 604625 PAX9 14q12
STHAG4 150400 WNT10A 2q35
STHAG5 610926 ? 10q11
STHAG6 613097 LTBP3 11q12
STHAGX1 313500 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 shows that missing teeth are commonly associated with genetic and environmental factors. Some literature also show that a combination of both factors may contribute to the occurrence of missing teeth.[35]

Genetics has always played a crucial role in dental aplasia. The pattern of congenitally missing teeth seen in monozygotic twins is different, suggesting an underlying epigenetic factor, which may be due to the simultaneous occurrence of two anomalies.[36] This multifactorial aetiology involves environmental factor which triggers the genetic anomalies resulting 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 huge contributing factor as well such as genetic disorders like down syndrome,[10] ectodermal dysplasia,[11] cleidocranial dysplasia,[37] orofacial clefting, especially cleft lip and palate.[38]

Most the craniofacial characteristics are influenced by the both genetic and environmental factors through complex interactions. The variable expressivity of traits can be either completely genetic determined, environmental determined or both. The multiplicity of hypodontia theories suggest that missing teeth are strongly influenced by genetic factor. Although single gene defect may contribute to the cause of hypodontia, but more studies propose that hypodontia is mainly the result of one or more point of genetic mutations which are closely linked, which is known as polygenic defect.[39][40][41]

Despite the aetiology of missing teeth is remained consensus, but genetic plays an important role for hypodontia is shown in many different cases.[42] There are over hundreds genes are expressed and involved in regulating tooth morphogenesis.[43][44]

MSX1

MSX1 (muscle segment homeobox 1) 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.[45] MSX1 mutations have been identified as one of the contributing factors of missing second premolars, third molars, with a small percentage of first molars. MSX1 is less likely to cause anterior agenesis.[45][46][47]

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.[48][49] A study showed that single nucleotide polymorphisms in PAX9 were highly associated with missing upper lateral incisors.[44]

AXIN2

AXIN2 (AXIS inhibition protein 2) gene is negative regulator of the Wnt signalling pathway, which is important in regulating cell fate, proliferation, differentiation and apoptosis.[50] It's 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).[51][52]

EDA

The EDA provides instructions for making a protein called ectodysplasin A.[53] 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.[54] Common dental features of ectodermal dysplasia are multiple missing teeth and microdontia.[55]

PAX9 and TGFA are involved in regulating between MSX1 and PAX9 causing hypodontia of the molars.[36]

Hypodontia can be found in isolated cases too. The familiar or sporadic type of isolations are more frequently reported than syndromic type. Isolated cases autosomal dominant,[56][57] autosomal recessive [58][59] or X-linked [60] inheritance patterns may have an impact on the isolation conditions in expressing variation of both penetration and expressivity of traits.[61] Mutations in MSX, PAX9 and TGFA genes are known to cause congenitally missing teeth in some racial groups.

Research

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.[62][63]

Impacts

There is numerous studies or research report on the prevalence, aetiology,[64] treatment of hypodontia and dentoskeletal effect of hypodontia.[65] A few studies have investigated Oral Health-related Quality of Life (OHRQoL) in individuals with hypodontia[64] and provided some evidences that hypodotia may have an impact on quality of life.

Psychosocial

Cosmetic dentistry and facial aesthetic are notable for today's popular culture.[65] Interpersonal relationships and perceived qualities, for instances intelligence, friendliness, social class, and popularity from infancy to adulthood, can be affected by dentofacial appearances.[65] Some studies showed that the extent of complaints made by patients was associated with the severity of the condition and the number of missing permanent teeth.[65]

Meta-analyses and theoretical reviews 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.[66] Therefore, a divergence from perceived ideal dentofacial aesthetic, particularly in children, might adversely affect self-esteem and self-confidence besides attracting mockery from peers.[67]

It is therefore reasonable to theorize that deviations from “normal” or “ideal” dentofacial aesthetic could be destructive to an individual's psychosocial and emotional[68] well-being, which brings upon some psychosocial distress in that individual as a result of his or her condition.[65]

Functional

Individuals with hypodontia tend to have deeper bites and spaces.[69] 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.[69]

It has been found that individual 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.[65] 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.[69]

Financial

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.[70] Due to this reason, a team consists of different dental specialties is involved in the patient care.[71][72]

Management

Hypodontia is a condition that can present in various ways with differing severities. This results in a wide range of treatment methods available.[73] Those affected should be allowed to consider and select the most suitable option for themselves.[74] Early diagnosis of hypodontia is critical for treatment success.[75] The treatment of hypodontia involves specialists in departments such as oral and maxillofacial surgery, operative dentistry, pediatric dentistry, orthodontics and prosthodontics.[36]

Before determining a treatment plan, the following should be determined:

  • Evaluate the number of teeth missing[36][76]
  • The size and number of remaining teeth in both arches[36]
  • Malocclusion[36][76]
  • Facial profile[36][76]
  • Bone volume[36]
  • Age: Definitive treatment for hypodontia only commences once all permanent teeth erupt, or upon the completion of orthodontic treatment[75][76]
  • Primary teeth condition[76]
  • Patient's motivation towards treatment[76]

The following below are the methods used to manage hypodontia:

Accept spacing

This is a method suitable to individuals if the space from a missing tooth is not deemed to be an aesthetic concern.[77] Spacing present behind the canines may not be particularly visible, depending on the individual.[77]

Management of retained primary teeth

When there is a case of hypodontia of the permanent premolar teeth, the primary molar teeth would often remain in situ beyond the time they are meant to exfoliate.[78] 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[76] Previous studies also shown a good prognosis of retained primary molars going into adulthood.[79][80] However, leaving the primary teeth in place may run the risk of tooth infraocclusion i.e. submerging or progressive root resorption.[36][76]

Orthodontic space closure

Orthodontic space closure[73][81] can be an option for hypodontia management in the case of missing maxillary lateral incisors through the reshaping, and mesial re-positioning of the adjacent canine.[81] This management is indicated in hypodontia cases of Class I molar relationship with severe crowding in the mandibular anterior region where the extraction of lower premolar leads to a predictable outcome, and Class II molar relationship in the absence of crowding and protrusion of the mandibular anterior dentition.[81][82][83]

There have been several studies which showed the advantages of orthodontic space closure without prosthodontic space replacements.[82][84] 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.[81][82][84]

Some considerations are to be taken into account 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.[81][82][83] 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.[81]

Orthodontic space opening prior to prosthodontic treatment

The need for orthodontic space opening prior to prosthodontic management depends on the amount of edentulous space available in relation to adjacent teeth, occlusion and aesthetic concerns.[81] To determine the amount of space needed, three methods in the literature can be used which are the golden proportion, the Bolton Analysis and comparing the edentulous space with the contralateral tooth size if present.[85][86]

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.[81] 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.[87]

Removable partial dentures

Removable partial dentures are known to be an effective interim method for maintaining functional and aesthetic demands in a growing patient, where definitive fixed restorations are not suitable yet.[77] Removable dentures act as a space maintainer and also prevent the migration of adjacent or opposing teeth, thereby preserving the face height.[77] They are also easy to adjust or add on to in the event of further tooth eruption.[77] However, it may be difficult for young individuals to adhere with wearing removable dentures, due to their bulk.[77] Some patients also find the idea of dentures functionally and socially unacceptable, making them unwilling to comply.[77] Removable prosthetic devices are also known to cause damage to the remaining teeth if worn over a long period of time.[77]

Dental implant

Conventional crown and bridgework

Fixed restorative options are generally preferred over removable ones.[77]

  • Resin-bonded bridges: Due to its minimal preparation required, this method of replacing teeth is more suited to young adults.[77] It is a definitive restoration with the ability to fill up one or two tooth spaces.[77] Research has reflected a survival rate of 80% over a period of 6 years or longer,[88] and that cantilever resin-bonded bridges are at least as good as conventional fixed-fixed bridges. Restoring teeth with this method can only be done after orthodontic treatment and will need an element of retention to ensure that tooth contacts are not misplaced.[77]  
  • Conventional bridges: Teeth tend to be prepared for conventional bridgework if there are large restorations present, thus being more commonly done on adult patients with hypodontia.[77] Treatment with conventional bridgework[87][75] will require significant reduction of tooth structure, which will put the tooth at risk of biological damage i.e. loss of vitality.[75] This risk is especially high in young patients with large pulp chambers.[75]

Tooth autotransplantation

Autotransplantation[74] involves the removal of a tooth from one socket and relocating to another socket in the same individual. If done successfully, it is able to ensure stable alveolar bone volume as there is continuous stimulation of the periodontal ligament.[36]

Implant supported tooth replacement

Placing dental implants has proven to be a predictable and reliable method of treating hypodontia, along with bringing excellent aesthetic results.[75] Implant placement should be delayed until jaw growth in an individual is complete.[36] One limitation of implant placement would be the need for a sufficient amount of bone volume, which if not met, may affect the positioning of the implant.[75] However, bone grafting can be carried out to overcome this.[75]

Epidemiology

Hypodontia is less common in the primary dentition [2] with reported prevalence rates ranging from 0.5% in Icelandic population [89] to 2.4% in Japanese population.[90] In the primary dentition the teeth reported as most likely to be missing are the lateral incisors, both maxillary and mandibular.[3][90] If a deciduous tooth is missing this will increase the risk of an absent successor.[91]

In the permanent dentition third molars are most commonly absent and one study [91] found prevalence rates of between 20 and 22%. When third molars are ignored the prevalence rate for each tooth varies from study to study.[2] In Caucasian studies mandibular second premolars and maxillary lateral incisors are most often absent.[2] Several UK studies have found the lower second premolar to be most commonly absent.[92][93] Studies from Asian populations report that the mandibular incisor is most commonly absent.[94]

A higher prevalence of hypodontia in females has been reported, 3:2.[93] The most extensive studies have been in Caucasian populations and suggest a prevalence of 4-6%.[92][93]

One study [95] looked at 33 previous studies with a sample size of 127000 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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