12.2. Variation in Dental Development

Tooth Number

Most vertebrates have dozens of teeth and some variation is usually found around the most common number of teeth within each species. Most humans, for example, have 20 teeth in the primary dentition and 32 in the permanent dentition. One or more third molars are absent in 20–23% of the population, followed in prevalence by missing second premolars and lateral incisors. Without considering the third molars, about 3.5–8.0% of humans lack some other tooth.

The condition of naturally having fewer teeth than the most common number in a species is called hypodontia. The condition mostly results from developmental abnormalities with genetic origin. Selective Tooth Agenesis ("STHAG") has been associated with the genes MSX1, PAX9, LTBP3 and several others associated with the differentiation of the ectoderm. When identical twins lack a tooth, it tends not to be the same one. This indicates that environmental factors, in addition to genetic ones, are involved in the patterning of hypodontia. Common environmental factors include infection, trauma and some drugs.

If many (more than five) teeth are missing the condition is called partial anodontia or oligodontia. Anodontia is the complete absence of primary or permanent teeth. It generally has a genetic basis and involves congenital obstruction of the development of the dental lamina. Partial or complete anodontia frequently accompany major genetic syndromes (ectodermal dysplasia, Down syndrome, Riegar syndrome), particularly those that affect ectodermal development.

Hyperdontia is the development of extra (supernumerary) teeth. They can appear in any area of the dental arch and they are most common in the permanent maxillary arch. It occurs in 1–3% of Caucasians but it is more frequent in Asians. It is more frequent in men than in women. A single extra tooth is most common (86% of cases) and it is most commonly found among the maxillary incisors. Many supernumerary teeth never erupt and not cause any complication. They may, however, delay eruption of nearby teeth or cause other problems which might require their extraction. Dental X-rays are often used to diagnose hyperdontia.

Supernumerary teeth can be classified by shape and by position. The shapes include the following:

• Supplemental (where the tooth has a normal shape for the teeth in that class); 

• Tuberculate (also called barrel shaped);

• Conical (also called peg shaped);

• Compound odontoma (multiple small tooth-like forms);

• Complex odontoma (a disorganized mass of dental tissue);

When classified by position, a supernumerary tooth cay be a:

• Mesiodens. The most common form. A conical tooth that occurs between the maxillary central incisors.

• Paramolar. Extra tooth found buccal or lingual to the molars. Most commonly small, rudimentary and located in the maxillary arch.

• Distomolar. Fourth and fifth molars that form distal to the third molars.

• Accessory tooth. Located anywhere else in the mouth.

Figure 2. Plaster model showing mesiodens and accessory tooth immediately palatal to the right central incisor. Paramolar tooth buccal to the maxillary third molar. More details (left, right).

Like hypodontia, hyperdontia tends to be hereditary with a strong genetic basis modified by environmental factors. It can be found in association with a variety of genetic syndromes that affect ectodermal differentiation.

Timing, Shape and Composition

Most babies are born without teeth. Natal and neonatal teeth involve teeth erupting before birth or sooner than usual. It occurs in about one in 2,500 births. Natal teeth are more frequent, approximately three times more common than neonatal teeth. The most common location is the mandibular region of the central incisors. Natal teeth and neonatal teeth can be caused by genetic syndromes.

Tooth development can be delayed by deficient nutrition or endocrine disorders, anemia, renal failure, heavy metal intoxication or tobacco smoke. Enamel hypoplasia is specific to the development of the enamel’s organic matrix and it is clinically visible as enamel defects. It may be caused by nutritional factors, trauma or diseases (celiac disease, chicken pox, congenital syphilis).

Dilaceration is an abnormal bend found on a tooth, and is nearly always associated with trauma that moves the developing tooth bud. As a tooth is forming, a force can move the tooth from its original position, leaving the rest of the tooth to form at an abnormal angle. Cysts or tumors adjacent to a tooth bud are forces known to cause dilaceration, as are primary (baby) teeth pushed by trauma into the gingiva where it moves the tooth bud of the permanent tooth.

Impacted teeth

An impacted tooth fails to erupt into the dental arch within the expected developmental window. It may be retained throughout the individual's lifetime unless extracted or exposed surgically. Teeth may become impacted because of adjacent teeth, dense overlying bone, excessive soft tissue or a genetic abnormality. Most often, the cause of impaction is lack of space in the dental arch. The wisdom teeth (third molars) are most frequently impacted and among them, mandibular molars are more frequently impacted than maxillary ones.

Erupted teeth that are adjacent to impacted teeth are predisposed to periodontal disease. Since the distal surface of the last teeth are difficult to reach, an impacted third molar frequently results in gingival inflammation around the second molar that can result in periodontitis and compromise the tooth. Impacted third molars frequently contract dental caries when a minor communication is established with the oral cavity.

Pericoronitis is an infection of the soft tissue that covers the crown of an impacted tooth and is usually caused by the normal oral microbiota. For most people there exists a balance between the host defenses and the oral micriobiota but if the host defenses are compromised like during minor illness such as a flu or a cold, pericoronitis may result. Pericoronitis can present as a mild infection or severe infection.

Impacted teeth are classified with base on position, orientation and degree of eruption for dicussion of a treatment plan. The most common treatment is extraction.

Summary

Developmental variation in tooth number is not uncommon. One or a few missing teeth is common, with the third molars being the most commonly missing teeth. Extranumerary teeth are less common and normally involve a single tooth between the central incisors or around the maxillary molars. An entire set of teeth can be missing in rare conditions that impair ectodermal differentiation. Teeth that fail to erupt may be harmless, may contract caries or may compromise erupted teeth. In the later cases, management most likely includes their extraction.

Key terms

Tooth number, hypodontia, hyperdontia, anodontia, oligodontia, down syndrome, Riegar syndromes, ectodermal dysplasia, supplemental tooth, tuberculate tooth-like, conical tooth, compound odontoma, complex odontoma, mesiodens, paramolar tooth, distomolar tooth, accessory tooth, enamel hypoplasia, impacted tooth, tooth impaction, periodontal disease, pericoronitis.

Figure credits

Figure 1 by Lisbet K Lind, Christina Stecksén-Blicks, Kristina Lejon, Marcus Schmitt-Egenolf. EDAR mutation in autosomal dominant hypohidrotic ectodermal dysplasia in two Swedish families. BMC Med Genet. 7, 80. 2006. PMID 17125505., CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=2747431

Figure 2a by User:Nizil Shah - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=28144494

Figure 2b by Сергей Васильков - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=47167424

Figure 3 by Coronation Dental Specialty Group - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31045471