What Is Tooth Agenesis?

Overview

People are born with all of the teeth they will have throughout their entire lives: both the deciduous dentition (“baby teeth”) and permanent dentition exist in a newborn as tooth buds, which will develop individually in their due time.¹ As a baby grows, their first set of teeth erupts little by little until the deciduous dentition is complete, and then comes the permanent teeth once children reach the age of six¹. However, in some cases, one or more teeth, whether deciduous or permanent, can be missing. This condition is known as tooth agenesis and can present as mild (with one or two teeth missing) or as severe as the complete lack of teeth.² In this article, we will cover some of the factors causing the dental anomaly of tooth agenesis. 

Types of tooth agenesis

Tooth agenesis can be divided into different groups based on their cause and severity. Nonsyndromic Tooth Agenesis (NSTA) or Syndromic Tooth Agenesis (STA) are two forms of agenesis. STA is related to conditions such as Down syndrome, ectodermal dysplasia and Apert syndrome¹. There can even be a complete absence of teeth in severe cases.² 

The NSTA form is more common and affects certain groups of teeth more than others: second premolars (bicuspids) of the mandible, lateral incisors of the maxilla, and second premolars of the maxilla are the most common teeth to be missing in an adult.² It is generally milder, affecting only one side of the face (unilateral); the exception to this rule is the upper lateral incisors, as a patient is more commonly missing both (bilateral) instead of one.²

When it comes to severity, tooth agenesis can be classified into three categories:²

• Hypodontia: Agenesis of five or fewer teeth²
• Oligodontia: Agenesis of six or more teeth²
• Anodontia: Complete agenesis of teeth; this form is rare and almost always associated with syndromes²

One important thing to note is that although third molars (wisdom teeth) can be missing, and this phenomenon is known as agenesis, they are not included in the count that separates hypodontia, oligodontia and anodontia². This is because natural selection has been slowly working at eliminating wisdom teeth, as our diets have significantly changed to include softer foods that do not require extra help to chew, and their presence or absence varies between ethnic groups greatly. While aboriginal populations in Tasmania nearly always have all of their wisdom teeth, indigenous Mexicans lack them entirely in almost all cases³. Nowadays, people can have one, two, or even all of their wisdom teeth missing, and this phenomenon doesn’t cause significant problems as when other teeth are missing; statistics show that one in every five people lacks one wisdom tooth⁵.

Causes

Generally, tooth agenesis can be attributed to genetic or environmental factors.¹ Nonsyndromic forms can be sporadic (a random mutation in a patient’s DNA causes one or more teeth to be missing) or familial (in this case, more family members have a history of one or more missing teeth, and the patient inherited this trait).¹

Genetic factors can be attributed to about 80% of cases of tooth agenesis.² Although not always syndromic, sometimes genes known to cause a certain syndrome can be involved: in this case, tooth agenesis can be a milder presentation of the syndrome, or the patient has a different mutation than what could cause this syndrome, though it still causes tooth agenesis.²

Common genes known to cause agenesis are:²

• Syndrome associated genes:
  • AXIN2: related to the oligodontia-colorectal cancer syndrome, known to cause hypodontia²
  • EDA and EDAR: related to hypohidrotic ectodermal dysplasia, known to cause oligodontia²
  • FGFR1: related to Kallmann syndrome, known to cause hypodontia²
  • MSX1: related to Witkop syndrome, Wolf-Hirschhorn syndrome, and Pierre Robin syndrome, known to cause hypodontia or oligodontia²
• Non-syndrome-associated genes:
  • GREM2: known to cause hypodontia, as well as other dental anomalies like taurodontism and microdontism²
  • PAX9: known to cause hypodontia or oligodontia, as well as microdontism²
  • WNT10B: known to cause oligodontia²

Syndromic presentations of tooth agenesis can be related to many syndromes.¹ Currently, there are more than 90 syndromes known to cause either moderate or severe agenesis.⁴ Some of the most common ones are:¹

• Ectodermal dysplasia: can cause hypodontia, oligodontia, or anodontia, as well as conical teeth¹
• Orofacial digital syndrome: can cause hypodontia or oligodontia, besides supernumerary teeth and dental malocclusion¹
• Orofacial clefting (cleft lip and/or cleft palate): can cause hypodontia or oligodontia¹
• Pierre Robin syndrome: related to hypodontia, as well as microdontia and supernumerary teeth¹
• Van der Woude syndrome: can cause hypodontia and cleft lip and/or palate¹
• Apert syndrome and Soto syndrome: related to hypodontia and enamel hypoplasia¹
• Ellis-van Creveld syndrome: causes hypodontia, as well as conical teeth and taurodontism¹
• Wolf-Hirschhorn syndrome: related to hypodontia, taurodontism, and microdontia¹
• Down syndrome, Moebius syndrome and Witkop tooth and nail syndrome can cause hypodontia¹

Environmental factors can be attributed to about 20% of cases of NSTA². Common risk factors are chemotherapy and radiotherapy at a young age (when the formation of the tooth bud is still ongoing), exposure to the rubella virus during pregnancy, and use of medications like thalidomide during the formative years of the tooth bud². Orofacial trauma can also cause agenesis in the affected area.¹

Signs and symptoms

Unusual spacing in the dentition is usually a sign of tooth agenesis.¹ Tooth agenesis rarely occurs in the deciduous dentition, but when it does, it usually points to the lack of a permanent successor as well.²

Prolonged retention of a deciduous tooth can also be a sign of tooth agenesis: without the permanent tooth, there are no stimuli to exfoliate the deciduous one.¹

Patients with hypodontia can present with other findings caused by the agenesis: lower mandibular plane angle, smaller lower anterior face height, lip protrusion, smaller maxillary and mandibular length, and class III skeletal profile.⁵ A pattern of short face height and large spaces between teeth (diastema) are signs of hypodontia.⁵

Diagnosis

If there is suspicion of tooth agenesis, X-rays can help confirm the hypothesis.² In children, a panoramic X-ray after the age of six, when all permanent tooth buds (with the exception of third molars) are present, can confirm tooth agenesis.² In adults, it should be advised to confirm agenesis through the exclusion of other causes: periodontal disease (gum disease) and trauma can lead to tooth loss or extraction of the tooth; severe cavities (tooth decay) and dental abscesses can also motivate extractions.²

A thorough medical history should be collected in case there's suspicion of syndromes (through the severity of the agenesis or through the presence of other symptoms), and genetic testing can be done to confirm or deny the hypothesis.² A family history of agenesis is also important to assess the risk of agenesis in children.²

Other dental anomalies can also be present and should be observed during the intraoral exam: microdontism, taurodontism and malocclusion are some of the most common findings².

Management and treatment

Treatment depends on each individual case, based on which teeth were affected and the severity of tooth loss. When there is agenesis of the permanent tooth, but its deciduous counterpart is present, maintaining the deciduous tooth for as long as possible is the best option². Good oral hygiene, a healthy diet with fewer carbohydrates, and fluoride supplementation (either through varnishes, sealants, mouthwashes or other forms) can help prevent tooth decay and keep the deciduous tooth as healthy as possible.² If, for whatever reason, there is premature loss of the deciduous tooth lacking its successor, maintaining the dental arch's length is necessary and can be done through orthodontic devices.²

Once the patient has grown, other options are available: prosthodontics or dental implants can give back the patient both function and aesthetics, being a permanent solution to agenesis.²

In severe cases of agenesis like oligodontia or anodontia, prosthodontics can be made as early as six years old and periodically remade to accommodate the patient's facial growth until adulthood². Although prosthodontics is a tool to primarily recuperate functions like speech and mastication, aesthetics can be greatly improved and consequently improve self-esteem by helping patients to smile confidently².

Aside from maintaining the dental arch's length, orthodontics can help treat malocclusion in agenesis patients: space redistribution, correction of misaligned teeth and correction of bite problems caused by the agenesis itself or by syndromes that can alter craniofacial development.² In severe cases, when there are skeletal discrepancies in adulthood, orthognathic surgery can be performed to correct this problem.

Syndromic patients often have other dental anomalies aside from agenesis: microdontia, taurodontism, short roots, impacted teeth, delayed formation of teeth, enamel hypoplasia and transposition of canines and premolars are some of the most common findings.¹ These conditions might require treatment (like orthodontic treatment for transposition of teeth) or might just require observation (like taurodontism).

Epidemiology

Tooth agenesis has a prevalence that varies between 3% and 10% in its mildest form (hypodontia).² Oligodontia is more rare, varying between 0.1% and 0.5%.² Excluding wisdom teeth, rates drop slightly, varying between 1.6% to 6.9%.⁵ Still, tooth agenesis is the most prevalent craniofacial malformation in humans.⁵

The teeth most commonly affected are the third molars (wisdom teeth), with a 23% incidence rate.¹ Wisdom teeth have been slowly disappearing, and 1 in every 5 people lack at least one-third of the molar.⁵ This is followed by second premolars and upper lateral incisors, but different ethnicities can have different patterns of missing teeth². 60% of cases are unilateral.¹

Incidence varies between ethnic groups: Asians are the most affected, followed by North Europeans and African Americans.² There is also a slight preference for individuals assigned female at birth (AFAB) when compared to assigned males at birth (AMAB) in a 3:2 ratio.²

Summary

Tooth agenesis is the most common craniofacial malformation and is characterised by the absence of one or more teeth: mild forms like hypodontia (five or fewer teeth absent) are usually nonsyndromic, while moderate forms like oligodontia (six or more teeth missing) and anodontia (complete absence of teeth) are present in patients with genetic syndromes like ectodermal dysplasia, Down syndrome and cleft lip and/or palate. Causes of agenesis include genetic mutations, as well as environmental factors like early childhood chemotherapy, radiotherapy, use of medications like thalidomide, orofacial trauma, and maternal exposure to the rubella virus. Signs and symptoms include unusual spacing of the teeth, prolonged retention of deciduous teeth, and malocclusions such as class III skeletal defects (severe overbites and underbites); other dental anomalies can appear in a patient with agenesis, like microdontia and taurodontism. 

Diagnosis is made through clinical examination of the oral cavity and possible other syndromic features, a panoramic X-ray in children of ages 6 and older, and exclusion of other tooth loss causes in adults; genetic testing can be done if there is suspicion of syndromes. Treatment consists of oral rehabilitation: maintenance of the deciduous tooth for as long as possible through good oral hygiene, healthy diet and fluoride supplementation, orthodontic devices to maintain or create space in the dental arch and to correct malocclusion, and prosthodontics and dental implants to regain function. Most cases of agenesis are mild and nonsyndromic, affecting mostly Asians, North Europeans and African Americans, with a slight preference for AFAB individuals.