Anomalies in Tooth Development: Size, Shape, and Eruption

Neural Crest Cells and the Stages of Tooth Development

• Developmental Origins: Tooth development begins with the migration of cranial neural crest cells into the brachial arches, resulting in the formation of two dental arches.

• Dental Lamina and Induction: The dental lamina forms first, followed by the processes of induction and proliferation.

• Anomalies by Stage:

  • Induction and Proliferation: Disruption at these stages results in conditions including hypodontia, oligodontia, anodontia, supernumerary teeth, and double teeth.

  • Morphodifferentiation (Bell Stage): Disruptions during the physiologic process of morphodifferentiation lead to macrodontia, microdontia, dense invaginatus, dense evaginatus, and talon cusps.

  • Histodifferentiation: Anomalies in this stage produce regional odontodysplasia.

Microdontia: Characteristics, Etiology, and Prevalence

• Definition: A microdontic tooth possesses a crown that is smaller than normal for its tooth type.

• Commonly Affected Teeth: This condition most frequently affects the maxillary lateral incisors ($12$ and $22$) and maxillary third molars ($18$ and $28$). Notably, these are also the teeth most commonly missing in cases of hypodontia.

• Extent and Terminology:

  • Localized: Affects a single tooth or a small group of teeth.

  • Generalized: Affects all teeth in the dentition. This is often seen in cases of severe hypodontia.

  • Alternative Terminology: The microdontic maxillary lateral incisor is frequently referred to as a "peg-shaped lateral incisor."

• Morphology:

  • The mesiodistal width is narrower than normal.

  • The crown typically narrows to a point, lacking a distinct incisal surface.

• Differential Diagnosis (Relative Microdontia):

  • Broad or spacious jaws can create multiple spacings between teeth, making teeth of normal size appear small.

  • This is often secondary to heredity, such as inheriting large jaws from one parent and normal-sized teeth from another.

• Clinical Presentation and Management:

  • A disruption during the bell stage can result in two equally small premolars. If the crowns are too malformed to restore, extraction may be the optimal choice.

  • Varying degrees of disruption can result in unequal sizes within the same tooth type (e.g., tooth $12$ being mildly affected while tooth $22$ is more severe).

  • Treatment: Teeth can be restored to normal width using resin-based composite or all-ceramic crowns.

• Prevalence Data (Permanent Dentition):

  • Maxillary lateral incisors: Approximately $2.5\%$.

  • General occurrence: Extremely rare in primary teeth ($0.1\%$ to $0.5\%$\text{ range depending on study}).

Macrodontia (Megadontia)

• Definition: The tooth crown is larger than normal for that specific tooth type. It is also referred to as megadontia or megadont teeth.

• Prevalence:

  • Permanent dentition: approximately $1.1\%$.

  • Primary dentition: Unknown, but considered very rare.

  • Most commonly affected single tooth: Central incisor.

• Clinical Observations:

  • Macrodontia can involve multiple teeth and may present alongside other anomalies, such as supernumerary teeth.

  • Case Example: A patient presented with enlarged crowns of teeth $16, 26, 36, 46, 47$ featuring extra cusps and grooves. Radiographs revealed extra roots on $36$ and $46$, alongside congenitally missing (proliferation stage anomaly) teeth $15, 25, 35, 34, 45$.

• Management Strategies:

  • Enamel Stripping: Stripping enamel from the mesial and distal sides to reduce width. Risk: Excessive stripping can expose the pulp, leading to pulpitis or necrosis.

  • Contralateral Build-up: Adding material to the adjacent or contralateral tooth to restore aesthetic balance.

  • Extraction and Orthodontics: Removing the macrodontic tooth and using braces to close the space. This allows for the correction of crowding or rotations simultaneously.

  • Prosthodontics: Extraction of the macrodontic tooth followed by replacement with a bridge of normal dimensions.

  • Acceptance: No treatment, focusing on general aesthetics instead.

Double Teeth: Fusion, Germination, and Concrescence

• Fusion:

  • Definition: The union of enamel and possibly dentin between two or more teeth.

  • Morphology: Usually presents as two crowns joined by a groove, often with two separate roots and separate canals. It results in a reduced tooth count in the arch.

  • Etiology: Believed to be caused by the close proximity of developing tooth buds.

  • Clinical Concern: Fusion in primary teeth is highly correlated with anomalies (missing or impacted) in the underlying permanent successors.

• Germination (Gemination):

  • Definition: The budding of a second tooth from a single tooth germ.

  • Morphology: Typically presents as a large crown with a single root and a single large pulp canal.

• Concrescence:

  • Definition: The union of two or more teeth by cementum only.

  • Diagnosis: Usually an incidental finding on radiographs; it cannot be determined clinically. It is important to identify before extraction to avoid unintentionally removing both teeth.

  • Etiology: Likely due to space restriction causing roots to fuse as cementum develops.

• Prevalence of "Double Teeth":

  • Primary Dentition: Range of $0.5\%$ to $4.5\%$ (one study cited $1.6\%$).

  • Permanent Dentition: Approximately $0.1\%$.

• Management:

  • The labial and palatal grooves of double teeth are prone to dental caries; sealants are recommended.

  • Surgical separation is not possible as cutting through the union results in periodontal defects and pulp devitalization.

  • Primary double teeth must be monitored until exfoliation, and the permanent successors must be tracked for eruption or developmental anomalies.

Dense Invaginatus (Dense in Dente)

• Definition: An invagination of an accessory enamel organ into the dental papilla, creating a tooth-like structure within the pulp chamber.

• Morphology: A pear-shaped infolding of enamel and dentin. The enamel lining is more radiodense than the surrounding tooth material.

• High-Risk Nature: The invagination is thin and often communicates with the oral cavity. Caries can quickly penetrate the pulp, causing necrosis of both the "dense" and the main pulp chamber.

• Prevalent Teeth: Most common in maxillary lateral incisors, followed by central incisors, premolars, canines, and molars. It can occur bilaterally.

• Diagnosis: Requires specialized imaging like radiographs or Cone Beam Computed Tomography (CBCT). Look for deep pits in unusual locations.

• Treatment:

  • Preventive: Seal the pit with fissure sealant immediately upon eruption to prevent bacterial ingress.

  • Curative: If necrosis occurs, root canal treatment is required. In cases with periapical infection, the root may stop developing, leaving a thin-walled canal with an open apex.

Accessory Cusps: Talon Cusp and Cusp of Carabelli

• Talon Cusp:

  • Description: A horn-like projection from the cingulum of a maxillary or mandibular incisor. It contains enamel, dentin, and sometimes a pulp horn.

  • Development: Caused by abnormal proliferation of the internal enamel epithelium and dental papilla into the stellate reticulum during the bell stage.

  • Prevalence: $1\%$ to $2\%$ in the permanent dentition.

  • Management: If it interferes with occlusion, subtle, gradual reduction of the cusp height can be performed to avoid pulp exposure. Pit and fissure sealants should be placed in the developmental grooves.

• Cusp of Carabelli:

  • Description: A large accessory cusp found on the palatal aspect of permanent first molars (e.g., tooth $16$).

  • Clinical Note: Can complicate the placement of dental dam clamps; however, once the clamp is seated, the cusp shape may help retain it.

Dense Evaginatus (Leon's Premolar)

• Definition: An enamel projection protruding from the occlusal surface of premolars.

• Demographics: Predominantly seen in Asian ethnic groups (Chinese, Thai, Japanese, Indian, Filipino).

• Morphology: More common in mandibular premolars than maxillary. Often contains dentin and pulp.

• Clinical Risks:

  • The projection is prone to fracture, which can suddenly expose the pulp.

  • Fractures often lead to pulp necrosis and periapical infection shortly after eruption.

• Management:

  • Smooth rough surfaces if the projection wears down.

  • Apply resin-based composite for reinforcement or to cover exposed dentin.

  • If necrotic, root canal therapy is necessary, which is challenging in young children/mixed dentition due to behavior and immature root development.

Disruptions in Tooth Eruption: Infra-occlusion and Ankylosis

• Ankylosis: The fusion of cementum directly to the alveolar bone, characterized by the absence of a periodontal ligament (PDL). This acts as a mechanical obstruction to eruption.

• Infra-occlusion: A condition where a tooth ceases to erupt and remains below the occlusal plane of adjacent teeth. No orthodontic force can move an ankylose tooth.

• Management for Primary Molars:

  • Monitoring: Most infra-occluded primary molars will shed within $6$\text{ months} of their normal exfoliation time.

  • Successor Present: If the primary tooth does not show root resorption after $6$\text{ months}, consult an orthodontist regarding extraction to prevent tipping of adjacent teeth (e.g., tooth $36$ tipping over $75$).

  • Successor Absent: If there is no permanent successor, the primary tooth will not exfoliate spontaneously. Options include extraction for orthodontic space closure or keeping the tooth long-term to preserve bone for a future dental implant.

Dilaceration: Traumatic Etiology and Impact on Eruption

• Definition: A sharp bend or twist in the tooth root or crown.

• Etiology: Typically caused by trauma to the primary predecessor. The trauma pushes the primary root against the developing permanent tooth germ.

• Timing: Permanent central incisor crowns complete formation between age $3$ and $5$. Injury during this window leads to the developed portion being angled relative to the developing portion.

• Clinical Presentation: A permanent maxillary incisor failing to erupt by age $7$ to $8$. A delay of more than $6$\text{ months} compared to the contralateral tooth warrants investigation.

• Treatment: Management involves creating space (orthodontic opening coils), surgical exposure, and orthodontic guidance to pull the tooth into the arch.

Ectopic Eruption

• Definition: A tooth erupting in an abnormal position, often physically obstructed by an adjacent tooth.

• Common Site: The maxillary first permanent molar ($16, 26$) getting stuck behind the distal bulge of the second primary molar ($55, 65$).

• Prevalence: $2\%$ to $6\%$ in the maxilla; significantly rarer in the mandible.

• Management:

  • Early diagnosis via bitewing or OPG radiographs.

  • In most cases, the permanent molar will eventually free itself (self-correct).

  • If it remains stuck, it may cause pain, infection, or premature loss of the primary molar. Orthodontic distalization of the permanent molar is required to regain space for the second premolar.

  • Always check the contralateral side, as ectopic eruption is frequently bilateral.