Introduction

Tooth agenesis is defined as the genetic absence of baby (primary) or permanent (secondary) teeth, resulting from complications during the initial stages of dental development. These early disturbances in tooth formation cause certain teeth to develop in babies or adults.

While the degree of tooth agenesis varies, the most commonly affected permanent teeth are:¹

• Maxillary lateral incisors (small teeth on either side of the two front teeth)
• Mandibular second premolars (teeth before the molars)
• Third molars (wisdom teeth)

Genetic and environmental factors equally promote the development of this medical condition, with numerous genes being involved. Tooth agenesis can be evident on its own or be linked to several syndromes.

Figure 1. Teeth types in adults. MRCEM Revision Notes. Adult and children's teeth [Internet]. Available from: Bing Images. Licensed under CC BY-SA 3.0

Tooth agenesis can affect appearance, emotional well-being, and functionality (e.g., chewing), demanding a comprehensive, multidisciplinary approach for effective management.^1,2,3

Depending on the total number of absent teeth, it can be categorised into three groups:

• Hypodontia: failed development of one to five teeth
• Oligodontia: failed development of six or more teeth
• Anodontia: all teeth fail to develop

Causes for tooth agenesis

The origin of tooth agenesis is complex, involving a combination of genetic, environmental, and evolutionary factors.¹ 

Genetic factors 

Alterations in tooth development genes, such as MSX1, PAX9, and AXIN2, or inherited conditions like ectodermal dysplasia, can lead to tooth agenesis.⁴

Environmental factors

Maternal infections during pregnancy (e.g., rubella virus), drug use during pregnancy (e.g., thalidomide, anticonvulsant medications), exposure to ionising radiation or chemotherapy agents, and nutritional deficiencies can affect tooth development.^1,5

Additionally, the following can impact tooth formation:⁵

• Trauma or injury to a tooth germ (which later develops into a tooth) during early developmental stages
• Endocrine disorders like hypothyroidism (low thyroid hormones) and growth hormone deficiency
• Inflammation or infection in the region of the developing tooth may also impact its formation

Evolutionary factors

Evolutionary trends, particularly the progression of human evolution, have also played a vital role in the reduction of jaw and tooth size and number owing to a reduction in functional needs.⁶

Conditions with tooth agenesis

Tooth agenesis can be linked to underlying medical reasons such as cleft lip and palate, syndromes like Ehlers-Danlos syndrome and Down syndrome.⁵

Signs and symptoms

The prime symptoms of tooth agenesis include:

• Missing teeth: the visible absence of one or more teeth from birth, which may be detected in early infancy
• Delayed eruption: teeth adjacent to the missing ones may erupt later than usual or in abnormal positions
• Malocclusion: the asymmetrical bite patterns and difficulties in chewing due to absent teeth
• Speech difficulties: lost anterior (front) teeth might lead to pronunciation issues.
• Facial asymmetry: the absence of multiple teeth that affects facial structure and symmetry
• Compromised alveolar bone growth: reduced stimulation from tooth roots might affect the underdeveloped jawbone structure
• Retained primary teeth: primary (baby) teeth may be retained longer than usual due to the absence of permanent successors (adult teeth)
• Spacing issues: the gaps or spaces between teeth, particularly in areas where teeth are absent

Types of tooth agenesis

Based on the total number of absent teeth, tooth agenesis can be divided into three groups: hypodontia, oligodontia, and anodontia.

Hypodontia

Hypodontia is a condition in which one to five teeth fail to develop. This condition both impacts baby and permanent teeth, but frequently impacts upper lateral incisors and adult second premolars (see Fig. 1).¹

The characteristics of hypodontia are:^7,8,9

• Frequently affected teeth: the second premolars, upper lateral incisors, and lower central incisors are usually absent
• Delayed eruption: the nearby teeth can erupt later than usual or in a changed position
• Malocclusion: hypodontia may also cause spacing, occlusal disharmony (incorrect teeth position), and midline shifts (the centreline of upper and lower front teeth don’t align)
• Altered alveolar bone growth: hypodontia affects the alveolar bone (which supports the teeth) development, leading to local atrophy 

Oligodontia

Oligodontia is a condition in which six or more teeth, excluding third molars, are absent.¹ This condition is a more advanced case of hypodontia, potentially affecting both primary and permanent teeth.

The characteristics of oligodontia are:¹⁰

• Multiple missing teeth: affects both primary and permanent dentition (teeth), causing major gaps in dental arches
• Delayed or failed eruption: the remaining teeth may erupt later than usual or fail to develop entirely
• Severe malocclusion: owing to the large gaps, patients tend to suffer from open bite, deep bite, or crossbite
• Compromised masticatory function: reduced number of teeth affects chewing efficiency and leads to nutritional deficiencies
• Alveolar bone hypoplasia: absence of dental roots causes abnormal development of the jawbones and affects prosthetic treatment

Individuals with oligodontia, especially the young, have psychological concerns, including self-esteem issues owing to the aesthetics of the condition, i.e., absent teeth. 

Anodontia

Anodontia is a condition that arises from an inherited genetic mutation causing a failure to develop complete primary or permanent teeth.

The characteristics of anodontia:

• Complete absence of teeth: both primary and permanent teeth fail to develop
• Severe alveolar bone deficiency: inadequate dental roots cause underdevelopment of the jawbones, which in turn causes extensive bone resorption (bone breakdown and its mineral release)
• Facial development issues: individuals with anodontia usually have underdeveloped mandibles (lower jaw bone) and maxillae (upper jaw bone), resulting in a concave profile
• Difficulty in mastication and speech: a lack of teeth greatly disrupts chewing efficiency and proper speech articulation

Usually, anodontia is associated with genetic syndromes such as ectodermal dysplasia, which also affects nails, hair, and sweat glands.¹¹

Occlusion

Occlusion is defined as the alignment and contact between upper and lower teeth when the jaws are closed. Proper occlusion enhances efficient chewing, clear speech, and facial aesthetics.

Effects of tooth agenesis on occlusion

Tooth agenesis affects teeth alignment and how the upper and lower teeth contact, causing improper bites, early tooth wear, and jaw pain.⁶

Misalignment

Absent teeth cause remnant teeth to shift out of their natural positions, leading to improper bites and misaligned teeth known as malocclusion. The following outcome of misalignment affects occlusion:

• Midline shift: lack of tooth structure on one side leads to facial asymmetry and shift in dental midline towards the side of the absent teeth
• Crowding: due to insufficient space, teeth may become crowded or overlap
• Spacing: gaps or spaces form between teeth where the missing teeth should be

Bite problems

The absence of teeth affects the way the upper and lower teeth come together. This can lead to various bite issues such as:

• Overbite: the upper front teeth overlap pointedly over the lower front teeth
• Underbite: lower front teeth protrude beyond the upper front teeth
• Crossbite: upper teeth sit inside the lower teeth when the jaws are closed

Overeruption

Overexposure of a tooth, known as overeruption, occurs when the tooth on the opposite side (opposing tooth) that normally limits its vertical exposure from the gums is absent due to tooth agenesis. Thus, overeruption disrupts normal occlusion.

Altered jaw function

Tooth agenesis can affect jaw development and function, causing:

• Impaired chewing: due to absent teeth, there is difficulty in chewing food
• Temporomandibular joint (TMJ) disorders: asymmetrical teeth eruption leads to TMJ disorders or pain in the joints of the jaw
• Jaw pain: discomfort or pain occurs due to strain on the jaw muscles and joints

Improper occlusion (malocclusion) from teeth misalignment, overeruption, or crowding caused by tooth agenesis can alter jaw functions, including inefficient chewing due to reduced bite force, which can affect overall digestion and nutrition. 

Implications for jaw development

As teeth play a crucial role in controlling jaw advancement, tooth agenesis impacts the development and morphology of the jawbones.

Reduced jaw growth

The absence of teeth can lead to immature jawbones. Teeth play a significant role in strengthening jawbone development through chewing. Without these forces, the jawbone might not grow to its full capacity and resulting in a smaller jaw size.¹²

Altered facial structure

Lost teeth can influence the facial structure. The need for teeth can lead to an indented appearance within the cheeks and lips.

Compensatory growth

The jaw sometimes grows compensatorily and abnormally in the missing teeth. This tends to result in asymmetry, creating other issues.

Diagnosis

Diagnosing tooth agenesis involves a specific technique of assessment. Here are the key steps:

Clinical examination

• Visual inspection: Examination of the oral cavity to determine poorly placed teeth and evaluate this situation
• Occlusion evaluation: The upper and lower teeth configuration and interaction are evaluated for any form of abnormal occlusion

Radiographic assessment

•  X-rays: X-rays of the development of the jawbone and surrounding structures are advised to confirm the absence of tooth buds
• Panoramic radiographs: Runs to examine the whole oral cavity, aiding in detecting the absent tooth and associated anomalies

Genetic testing

• Family history: A comprehensive family history is taken to assess any genetic predisposition to tooth agenesis.
• Genetic analysis: Suggestion of detecting the specific gene mutations associated with this condition⁶

Treatment

A comprehensive approach is suggested for the absence of teeth that leads to occlusion and jaw development. Here are the key treatment strategies:¹

Orthodontic treatment

• Braces or clear aligners can be used for the realignment of teeth and making room for prosthetic replacements
• Space maintainers retain the space available for future implants or other restorative treatments required

Prosthetic solutions

• Dental implants: surgically installed titanium posts into the jawbone, which act as artificial tooth roots to provide a stable foundation for crowns
• Bridges are fixed devices that connect the gap generated by the absence of teeth
• Dentures: a detachable device that is an artificial replacement for absent teeth and the surrounding (associated alveolar and palatal tissues) tissues

Restorative dentistry

• Crowns: used to cover and protect natural teeth or dental implants
• Veneers: the designed thin shells made to cover the front surface of the tooth, improving the functionality and appearance

Surgical interventions

• Bone grafting: increases the volume and density of the jawbone for assembling dental implants
• Jaw surgery: It may be essential in critical situations to correct and improve occlusion and jaw abnormalities

FAQs

What are the most commonly affected teeth in tooth agenesis?

The most often affected teeth are the maxillary lateral incisors, mandibular second premolars, and third molars.

What are the main causes of tooth agenesis?

Evolutionary factors, environmental factors, such as nutritional deficiencies, ionising radiation exposure, and genetic factors – mutations in associated genes – are the causes of tooth agenesis.

What are the implications of tooth agenesis for jaw development?

The absence of a tooth causes reduced stimulation from the roots and chewing pressures, which leads to underdeveloped, altered face anatomy, jawbones, and decreased jaw development.

What are the implications of tooth agenesis for occlusion? 

Improper occlusion (malocclusion) from teeth misalignment, overeruption, or crowding caused by tooth agenesis can alter jaw function, including inefficient chewing due to reduced bite force, which can affect overall digestion and nutrition.

When should I see a doctor?

If you suspect signs, such as difficulty in chewing, speaking, or misalignment, it's apparent that you should consult with a dentist or orthodontist. Thereafter, the dentist or orthodontist can run a complete evaluation and recommend suitable treatment options.

Summary

Tooth agenesis, a genetic condition that causes the absence of teeth, results from disturbances in early dental development. It includes hypodontia (one to five missing teeth), oligodontia (six or more missing teeth), and anodontia (complete absence of teeth). The appearance is influenced by environmental, evolutionary and genetic factors. The dental occlusion is impacted by altered jaw function, biting issues, misalignment, and cosmetic difficulties. It also causes underdeveloped, altered face anatomy, leading to compensatory jaw growth and decreased jaw development. Diagnosis is a precise assessment, such as genetic examination, radiographic assessment, and clinical examination. To account for correcting tooth loss and its impact on occlusion and jaw development, there are numerous treatment options, including orthodontic therapy, prosthetic solutions, restorative dentistry, and surgical procedures.