# The Pathology of the First Permanent Molar during the Mixed Dentition Stage--Review
## 3 Results and Discussion
### Eruption Pathology
#### 3.1.1 Ectopic Eruption
In his study on a sample of the Turkish population (7649 subjects), [PERSON], 2018 determined a prevalence of 2.65%, with a distribution of FPM ectopic eruption between the two dental arches of 57.5% for the maxilla and of 42.5% for the mandible [29]. A higher incidence of severe and very severe degrees of ectopic eruptions was registered in the maxilla as compared to the mandible [29]. In the study by [PERSON] et al., 2021, on a sample of the population in Saudi Arabia with a mean age of 6.90 \(\pm\) 0.985 years, 60 FPMs were found with an ectopic eruption in 45 of the 2014 investigated subjects [25]. The prevalence was higher in males (2.9%) than in females (1.6%), but with no statistically significant differences (\(p\) = 0.59). Forty-seven (78.4%) of the ectopic FPMs were in the maxilla and 13 (21.6%) in the mandible, with statistically significant differences between the two dental arches (\(p\) = 0.000). Similar rates between the right and left sides were determined [25]. In a sample of 772 Iranian children aged 5-8, [PERSON] et al., 2021, identified 79 subjects with an ectopic eruption of FPM, of which 61% were girls and 39% were boys [30]. The prevalence was 10.2%, the highest reported among all studies considered for this review. A mild ectopic eruption was found in 25.6% of the patients, a moderate one in 60.3%, and a severe one in 14.1%. The authors reported a higher frequency of ectopic eruption in the mandible (59.5%) as compared to the maxilla (19%) [30]. Of the subjects, 21.5% presented FPM ectopic eruptions in both the maxilla and the mandible [30]. In their 2021 study analyzing 11,403 Chinese patients aged 4-11, [PERSON] et al. found 409 children (3.6%) with 634 ectopically erupted FPMs, out of which 89.7% occurred in the maxilla. Two hundred sixty-five boys and 144 girls presented FPM ecology [31]. The male-to-female ratio was 1.79:1 in the group exhibiting self-corrected ectopic eruption and 1.53:1 in the group exhibiting the irreversible type. Correlating the pathology with the subjects' age, the highest prevalence was seen in six-seven-year-old children [31]. As can be seen from the listed papers, a higher prevalence of ectopically erupted maxillary FPMs, as compared to the mandibular ones, was determined by all studies investigating the anomaly in both dental arches. The severe ectopic eruption was also more prevalent in the maxilla than in the mandible. Since the eruption of the maxillary FPM is of interest, certain studies have analyzed the anomaly only regarding the upper dental arch. It is appreciated that the maxillary FPM is at risk for developing the anomaly due to its particular pattern of eruption, namely, not vertical, but in a forward and downward direction, through a rotary movement having the apices of the roots as a center [24]. Table 1 summarizes the main studies regarding the occurrence of FPM ectopic eruption. \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline \multicolumn{1}{c}{**Study**} & \multicolumn{1}{c}{**Year**} & \multicolumn{1}{c}{**Sample**} & \multicolumn{1}{c}{**Number of**} & \multicolumn{1}{c}{**Subjects with**} & \multicolumn{1}{c}{**Prevalence in Relation to Age Gender Distribution**} & \multicolumn{1}{c}{**Number of Entopic Molions**} & \multicolumn{1}{c}{**Prevalence in Relation to Small  
Predictive Factors for Irreversible Outcome  
Differentiation between the reversible and irreversible types is important for the therapeutic approach. The irreversible type can cause premature exfoliation of the primary second molar, which, in turn, leads to the mesial shift of the FPM and reduced space for the eruption of the second premolar [17; 21], deficiency in quadrant arch length and over-ruption of the opposing permanent molar [25]. In the reversible type, the primary second molar, even presenting an atypical resorption, remains mesial to the erupted FPM until normal exfoliation [19]. Therefore, several researchers were preoccupied with assessing the predicting factors for the irreversible outcome.