# The Pathology of the First Permanent Molar during the Mixed Dentition Stage--Review
## 3 Results and Discussion
### Eruption Pathology
#### 3.1.1 Ectopic Eruption
Prevalence and Characteristics in Different Populations  
FPM ectopic eruption was investigated by many researchers in different population samples. [PERSON] and [PERSON], 1998, in a group of 3612 Thai pediatric population 6-9 years old, determined a prevalence of 0.75% for FPM ectopic eruption (27 children) [27]. Ectopic eruption in the maxilla was present in 0.39% of the studied cases (14 subjects) and in the mandible in 0.36% (13 subjects). The majority of the ectopic eruptions in the maxilla were severe, with subsequent root resorption of the primary second molar, while those in the mandible were minimal [27]. Concerning the correlations with the type of malocclusion, the ectopic eruption of the maxillary FPMs was mostly found in patients with Class I, but also in Class II and III malocclusions, while the ectopic eruption of the mandibular FPMs was predominantly found in Class I and not at all in subjects with Class III malocclusion [27]. [PERSON] et al., 2005, carried out an extensive retrospective study concerning ectopically erupted maxillary FPM on a group of 509 consecutive Spanish patients in the early mixed dentition stage, aged six to nine, who were reviewed every six months, from January 1998 up to December 2002 [17]. The criterion for including the subjects in the study was that the mandibular FPM had to have emerged [17]. Twenty-two patients, representing 4.3% of the total, and having an average age of seven years and six months, showed ectopic eruption of the maxillary FPM. Bilateral location was found in 63.6% of the cases and unilateral in 36.4% of the cases, but no statistically significant differences were determined between one side vs. two sides located ectopic eruptions (\(p>0.05\)). The unilateral locations were predominant on the right side (75%); statistically significant differences were found between right/left localizations (\(p<0.05\)). The difference between the average impaction on the right (2.91 mm) versus the left side (1.6 mm) was statistically significant (\(p<0.05\)). In addition, 69.4% of the total 36 ectopic molars self-corrected, and the rest remained impacted [17]. In a sample of British children (aged 7.0-11.4 years, mean 8.5 years \(\pm\) 1.2) with radiographic evidence of ectopic eruption of one or more FPMs, [PERSON] et al., 2007, recorded 49 affected molars [20]. The prevalence was higher in the maxilla (n = 45; 91.8%) than in the mandible (n = 4; 8.2%), with an almost equal distribution between the left/right sides [20]. A similar number of reversible and irreversible ectopic eruptions was registered (24 and 25, respectively) [20]. In another 2017 study investigating 1052 Italian individuals in the early mixed dentition stage, the prevalence of maxillary FPM ectopic eruption was 2.5% (26 subjects), and the ratio between unilateral and bilateral locations was 1:5 (in 6 and in 20 subjects, respectively) [18]. The distribution of patients according to gender was 14 boys and 12 girls. No differentiations were carried out between the reversible and irreversible types due to the early mean age of the sample [18]. In their study conducted on ectopic eruption over a group of 505 Spanish children aged between 5 and 8 years, [PERSON] et al., 2014, identified the pathology in 34 patients (6.7% of the cases) [28]. The distribution according to gender was 18 males (52.9%) and 16 females (47.1%), with no statistically significant differences (\(p>0.05\)). The highest prevalence of FPM ectopic eruption was among the eight-year-old children (41.2%), followed by the seven-year-old children (32.4%), six-year-old group (20.6%), and five-year-old group (5.9%). This study pointed out a higher incidence of bilateral occurrence (67.6%) as compared to unilateral occurrence (32.4%) [28]. The reversible type of ectopic eruption was diagnosed in 58.8% of the cases, and the irreversible type in 41.2% of the cases [28].