Anomalies of Tooth Structure
Delayed Eruption

Eruption is the physiologic movement of the teeth from the place of development within the jawbone to its functional position inside the oral cavity.⁷ Typically, the teeth erupt into the mouth when the first part of the tooth crown peaks through the gingival tissue.⁷ This usually happens once the tooth root is approximately ⅔ of its maximum length.⁷ Tooth eruption occurs over a wide age range and can differ by both gender and race.⁷ Eruption is considered to be delayed if the tooth has not surfaced 12 months after the normal eruption time or when the tooth root is ¾ complete.⁷ There are a number of reasons why teeth might be delayed in their eruption. The problem can be localized in which the eruption path is obstructed (Figure 12) or more widespread when a systemic disorder is implicated.⁷ Teeth continue to erupt after emergence to offset masticatory wear and jaw growth.⁷

Ankylosis
Ankylosis is the cessation of eruption after tooth emergence.⁷ This is caused by fusion of the tooth dentin or cementum with the alveolar bone.⁷ The pathogenesis of this process is not known.⁷ While ankylosis can occur at any age, it is most common in children 8 to 9 years of age.⁷ The teeth that are most frequently involved include the primary mandibular first molar, followed respectively by the primary mandibular second molar (Figure 13), the primary maxillary first molar, and the primary maxillary second molar.⁷ The involved ankylosed tooth usually has a submerged occlusal plane compared to the adjacent teeth and may, upon percussion, produce a sharp, solid sound.⁷ The periodontal ligament space may be absent radiographically.⁷ Permanent teeth rarely become ankylosed.⁷

Impacted Teeth
The term used to describe teeth that fail to erupt is impacted.⁸ Primary teeth impactions are uncommon but when they do happen, the primary second molar is usually involved.⁷ In the permanent dentition, the most commonly impacted teeth in order by frequency are the mandibular third molars, the maxillary third molars, and the maxillary canine teeth.⁷ See Figures 14‑17 for examples of impacted teeth. Impacted teeth are classified according to their position angulation relative to the rest of the erupted dentition such as: mesioangular, distoangular, horizontal, vertical, or inverted.⁷ They can be completely encased in the bone or partially erupted.