Developmental Disturbances of the Teeth, Anomalies of Number

Etiology

Anodontia refers to the complete absence of all teeth due to agenesis. Hypodontia involves the agenesis of fewer than 6 teeth, while oligodontia describes the absence of 6 or more teeth. Hyperdontia involves the development of supernumerary teeth, which appear in addition to the normal complement of teeth.

Despite sharing a common etiology, anodontia, hypodontia, and oligodontia present with varying phenotypes based on the severity of the underlying genetic or developmental cause. Research has identified 15 genes linked to the failure of tooth development in nonsyndromic cases.[1] When tooth agenesis occurs in association with a broader systemic condition, the term "syndromic hypodontia" applies, as demonstrated in disorders, eg, ectodermal dysplasia.[2][3] Additionally, orofacial clefting may lead to tooth displacement or developmental failure involving 1 or more teeth.[4]

The cause of hyperdontia remains unclear, with evidence pointing to multifactorial influences. In cases where multiple unerupted supernumerary teeth are present, clinicians should consider syndromic conditions, eg, cleidocranial dysplasia, a rare congenital skeletal disorder inherited in an autosomal dominant pattern.[5] Other syndromes frequently associated with multiple supernumerary teeth include cleft lip and/or palate, trichorhinophalangeal syndrome, and Gardner syndrome.[6]

Epidemiology

Hypodontia and oligodontia often fall under the broader classification of “hypodontia,” with specific distinctions made based on the number of missing teeth. Hypodontia represents the most common dentofacial malformation in humans.[7]  A clear correlation exists between hypodontia in the deciduous (primary) dentition and the corresponding permanent dentition; patients missing a primary tooth frequently lack its permanent successor.[8]

In the deciduous dentition, hypodontia appears in <1% of individuals of White racial background, whereas Japanese populations demonstrate a significantly higher prevalence.[9] In contrast, the permanent dentition displays a broader prevalence range, with incidence reported between 1.6% and 36.5%, depending on the studied population.[7] Gender-related trends show no predilection for hypodontia in the primary dentition; however, a slight female predominance appears in permanent dentition cases.[7]

Hyperdontia, or the presence of supernumerary teeth, constitutes another common dentofacial anomaly. Research conducted by Brooks et al reported supernumerary teeth in 0.8% of primary dentitions and 2.1% of permanent dentitions.[10] A slight male predilection has been observed, with higher incidence rates in Hispanic populations. The anterior maxillary midline region represents the most frequent site for these teeth, commonly referred to as “mesiodens.”[11] Multiple supernumerary teeth rarely occur in patients without an associated syndrome or underlying systemic condition.[6]

Pathophysiology

Dental agenesis may result from both intrinsic and extrinsic factors. In non-syndromic cases, 7 distinct genes—AXIN2, EDA, LRP6, MSX1, PAX9, WNT10A, and WNT10B—have demonstrated associations with this condition. Mutations in these genes were detected in 91.9% of molecular analyses conducted on affected patients, with PAX9 and WNT10A mutations accounting for the majority of identified cases.[1] The WNT/β-catenin signaling pathway plays a central role in numerous stages of embryonic development and holds particular importance during tooth formation.[12]

Mutations in WNT10B disrupt this signaling process, leading to reduced odontoblastic differentiation and impaired angiogenesis of dental pulp cells, ultimately resulting in deficient tooth structure or agenesis.[13] The number of missing teeth closely reflects the severity and prevalence of these genetic mutations. Recent findings have also underscored the role of MSX2, a transcription factor essential for ameloblastic differentiation and enamel formation. Alterations in MSX2 interfere with normal enamel production, contributing to the development of dental anomalies.[14]

Hyperdontia, or the presence of supernumerary teeth, remains poorly understood, with several competing developmental theories.[15] One hypothesis suggests that during histodifferentiation, a developing tooth bud may split into 2 separate but unequal segments, giving rise to an additional tooth.[16] A more widely accepted theory attributes hyperdontia to hyperactivity of the dental lamina, the tissue responsible for forming teeth.[17] This excessive cellular activity leads to the overproduction of enamel and the formation of one or more supernumerary teeth.

Histopathology

In regions where 1 or more teeth are absent, microscopic examination typically reveals bone levels that range from normal to slightly reduced. These changes reflect the lack of mechanical stimulation and natural development that occurs when teeth are missing.

Hyperdontia, when examined histologically, reveals supernumerary teeth with a structure resembling that of a normal tooth. These teeth display clearly defined layers of enamel, dentin, and pulpal tissue. Most supernumerary teeth—particularly mesiodens—appear smaller than standard teeth, yet they maintain the same internal architecture. The enamel, dentin, and pulp layers remain present, though reduced in overall size or proportion.[18]

History and Physical

Developmental dental anomalies related to tooth number, including anodontia, hypodontia, oligodontia, and hyperdontia, have varying clinical presentations. These anomalies can significantly impact oral health and function. These dental anomalies have the following manifestations:

• Anodontia refers to the complete absence of tooth development, resulting in the total failure of the dentition to form.
• Hypodontia involves the absence of 1 to 6 teeth and often serves as a general term to describe the lack of any number of teeth, regardless of quantity.
• Oligodontia denotes a more severe form of dental agenesis, characterized specifically by the absence of 6 or more teeth.
• Hyperdontia describes the presence of 1 or more additional teeth beyond the normal complement, leading to supernumerary tooth formation.

Evaluation

Hypodontia, regardless of severity, often becomes apparent early in life. The condition may first be recognized during the eruption of the deciduous dentition when a tooth fails to appear. In many cases, however, diagnosis occurs later as the permanent dentition begins to emerge, and 1 or more teeth do not erupt as expected. Clinical examination provides an initial assessment, while panoramic radiographs serve as essential diagnostic tools for confirming the presence or absence of teeth.[19]

Multiple supernumerary teeth rarely appear in patients without an associated syndrome or developmental disorder. In contrast, a single supernumerary tooth represents a relatively common dental anomaly and typically does not require further diagnostic workup. Supernumerary teeth most frequently occur in the anterior maxilla, mandibular premolar region, or around the third molars.[20]

Treatment / Management

The extent of treatment required for hypodontia depends on its severity, ranging from relatively simple interventions to complex, multiphase care. Early diagnosis plays a critical role in optimizing outcomes by allowing for timely treatment planning and implementation during early developmental stages. Identifying developmental disturbances early helps minimize future complications and often reduces the complexity of treatment later in life.[21] (B2)

Only licensed dental professionals should manage treatment for missing teeth. The least invasive option involves the use of removable prostheses, which typically avoid the need for tooth structure modification or surgical intervention. For a single missing tooth, treatment options may include observation, fixed prosthodontics, or dental implants. When multiple teeth are absent, treatment becomes more complex and may involve advanced surgical and prosthetic techniques.

Management of hyperdontia often involves extraction when the supernumerary tooth contributes to periodontal issues, functional interference, or restorative challenges. If sufficient space exists in the dental arch and no complications arise, observation without intervention remains a valid option. In cases where supernumerary teeth remain impacted and encased within soft tissue or bone, ongoing radiographic monitoring becomes essential to detect potential pathology. When crowding occurs due to the presence of additional teeth, maintaining excellent oral hygiene and attending regular follow-up appointments is crucial for supporting long-term oral health and stability.

Differential Diagnosis

Hypodontia

Any hypodontia should be considered for the following:

• Ectodermal dysplasia if additional symptoms or structural involvement are noted
• Early exfoliation of teeth
• Otodental dysplasia

Hyperdontia

For patients with hyperdontia, the following differential diagnoses should be considered:

• Genetic syndromes, eg, cleidocranial dysplasia, Gardener syndrome, or trichorhinophalangeal syndrome
• Variation of normal anatomy

Prognosis

Anodontia, hypodontia, and oligodontia generally have a good prognosis for treatment outcomes, especially with early detection and diagnosis. Depending on the extent and number of missing teeth, various prosthodontic and restorative options are available. With severe hypodontia being associated with syndromes, management of the syndrome itself generally precedes and occasionally precludes management of the dentition.

Isolated tooth agenesis has been associated with a long-term systemic risk. A population-level study identified correlations between childhood tooth agenesis and an elevated incidence of colorectal, associated with the AXIN2 gene, and ovarian cancers in adulthood.[22] Early dental anomalies may prompt the referring clinician to evaluate the systemic health of their patients further.

Hyperdontia and supernumerary teeth typically have a good prognosis, as they are treated or extracted if they pose a threat to erupting teeth, esthetics, or masticatory function.

Complications

Depending on the extent of anodontia, hypodontia, or oligodontia, different potential complications can arise. In patients missing most or all of their dentition, speech and mastication are generally affected, significantly impacting their quality of life. Patients missing 2 to 4 teeth may not be as severely impacted, as more restorative or prosthodontic treatment options are available.

Supernumerary teeth, if malpositioned, can prevent the normal eruption of developing teeth, which would require removal, a potentially invasive procedure. The multiple supernumerary teeth are generally indicative of an underlying syndromic or developmental condition, which the appropriate clinician should evaluate.