# The Pathology of the First Permanent Molar during the Mixed Dentition Stage--Review
## 3 Results and Discussion
### Eruption Pathology
#### 3.1.1 Ectopic Eruption  
Ectopic eruption is one of the most frequent pathologies encountered during the mixed dentition period. Eruption is considered to be ectopic when a tooth erupts in an abnormal position or with an abnormal orientation [16]. The maxillary FPM is reported to be the tooth most affected by ectopy in the pediatric patient, followed by the maxillary canine [16; 17]. In the case of FPM, ectopic eruption is distinguished by a mesial shift from the normal path [18]. Therefore, the FPM is blocked from complete eruption by the primary second molar due to the close contact between the two teeth and becomes locked under the distal aspect of the deciduous molar [18; 19; 20]. When this happens, space loss has already occurred [21]. The etiology is multifactorial, with both genetic and local factors being involved. The high frequency in siblings [19] suggests a hereditary component. The increased prevalence of cleft children could be evidence of the contribution of genetic and local causes [22]. Ectopic eruption of the FPM is encountered in patients with a small maxilla, reduced arch length, a retruded position of the maxilla in relation to the cranial base [21], and when there is a lack of synchronization between the eruption moment of the FPM and tuberosity growth [23]. A steeper angle of eruption and a larger FPM width are also found [19]. Early eruption of the FPM could be another cause [24]. Diagnosis  
Ectopic eruption is diagnosed by radiographic examination, either following clinical suspicion or as an incidental finding detected during the evaluation for a different dental complaint. Clinical suspicion is raised when there is a delay of at least six months, or there is asymmetry in the eruption of one or more FPM as compared to other FPMs already erupted into the oral cavity [20; 25]. Another clinical situation that suggests the pathology is when the distal cusps are emerging into the oral cavity before the mesial cusps [17]. The first clinical sign that indicates the ectopic eruption of the FPM is a canting of the occlusal plane of the second deciduous molar [21]. On the radiograph, the ectopically erupted FPM appears in a superposed image and is impacted in the distobuccal root of the primary second molar [17]. Because the FPM has no deciduous predecessor, it is accommodated in the dental arch by the development of the jaw bones in the posterior regions as the tooth forms [24]. Under normal conditions, the FPM assumes its position behind the primary second molar without initiating resorption [24]. When the FPM ectopically erupts, resorption of the primary second molar occurs, this being an important radiological finding. [PERSON] et al., 2005, defined four grades of severity based on the resorption rate of the primary second molar [17]:  
* grade I (mild)--resorption limited to cement or with minimum dentin penetration
* grade II (moderate)--resorption of the dentin without pulp exposure
* grade III (severe)--resorption of the distal root resulting in pulp exposure
* grade IV (very severe)--resorption extended to the mesial root of the primary second molar. Another radiological finding is the magnitude of impaction (MOI), described by [PERSON] et al. as being the distance from the maximum mesial convexity of the FPM to a tangential line drawn to the distal contour of the primary second molar perpendicular to the occlusal plane [17]. In 1983, [PERSON] and [PERSON] described the two categories of FPM ectopic eruption according to its evolution: the reversible type, when the FPM breaks free, and the irreversible type, when the FPM remains in a locked position [19]. The authors showed that the type of ectopic eruption could be reliably established between the ages of 7 to 8 years because most of the FPMs had freed themselves in 7-year-old patients with reversible ectopic eruption [19]. Interestingly, in a 2017 study by [PERSON] et al., one-third of the forty-six self-corrected ectopic eruptions of the maxillary FPM occurred after the age of nine [26].