# The Pathology of the First Permanent Molar during the Mixed Dentition Stage--Review
## 3 Results and Discussion
### Pathology of the Dental Hard Tissues
#### 3.2.1 Molar Incisor Hypomineralization
Decay-resistance is a distinguishing feature [52]. The Burden of MIH  
Due to MIH's rapid increase in incidence and the large geographic area covered, numerous researchers have focused on this pathology, even considered an \"emerging disease\" [72]. The prevalence, the patterns of distribution in relation to gender and age groups, and the MIH clinical picture were the preoccupations of many investigators. [PERSON] et al., 2012, in a group of 505 Spanish pediatric population aged 6-14 years old, determined a prevalence of 17.85% for MIH (90 children) [73]. An equal distribution was found regarding gender. The authors showed that 57.7% of all MIH-type lesions were located in the maxillary FPMs and 42.4% in the mandibular FPMs. Of the subjects, 16% presented more than one affected FPM. Of the cases, 58.88% had both molars and incisors affected, 8.88% of the cases had four affected FPMs, 11.11% had three affected FPMs, 11.11% had two affected FPMs, and 10% had one affected molar. Most of the lesions encountered were mild forms (50%), 28.89% were moderate, and 21.11% were severe. The most frequently affected was the maxillary right FPM [73]. In the study by [PERSON] et al., 2014, 164 (20.2%) of the 810 Iranian children, 9- to 11 years old, had MIH lesions, and over half of them (53.7%) presented developmental defects on all four FPMs [74]. Furthermore, the authors showed that the subjects with all molars affected were over three times more likely to have four to seven incisors affected than those presenting MIH lesions on one FPM [74]. Regarding the severity of MIH lesions, the most common type was the mild form, with creamy/white lesions in 17.1% and yellow/brown defects in 41.6%, followed by moderate lesions (31.3%) and severe defects (10.0%) [74]. In another study on 1511 children aged between 8-11 years in Istanbul, MIH lesions registered a prevalence of 14.2% [75]. Out of 215 patients diagnosed with MIH, 102 were boys (47.4%) and 113 girls (52.6%) [75]. Regarding the distribution of MIH defects in relation to age, a higher percentage was found in 11-year-old children (n = 144; 18.2%) than in 8-year-old children (n = 71; 9.9%), showing statistically significant differences [75]. On a sample of 705 Spanish children aged 6-14 years and 11 months, MIH defects were found in 56 of them, representing a prevalence of 7.94% [76]. Regarding gender distribution, MIH lesions were more frequently found in females (9.41%) than in male subjects (6.39%), representing a ratio of 1:1.54 between males and females. The maxillary teeth were more commonly affected (n = 137) than the mandibular ones (n = 78) from a total of 215 teeth (both FPMs and incisors) with MIH lesions [76]. In a sample of 686 Mexican children, [PERSON] et al. reported a prevalence of 35.4% for MIH lesions (in 243 subjects) [77]. In what concerns the severity of the MIH lesions found, the moderate category was the most frequent (n = 163; 67.1%), followed by mild lesions (n = 45; 18.5%) and severe lesions (n = 35; 14.4%) [77]. [PERSON] et al., 2019, investigating a group of 375 children aged 7-12 years in Milwaukee, Wisconsin, determined an MIH prevalence of 9.6% (n = 36), with a higher distribution upon the maxillary FPMs (n = 26) in comparison to the mandibular FPMs (n = 18) [78]. The authors reported a higher distribution in females (n = 25, representing 11.1% of the 226 female participants) than in males (n = 11, representing 7.7% of the 142 male participants). Mild lesions were found in 69.4% (n = 25) and severe forms in only 30.6% (n = 11) of the total MIH cases investigated. A higher frequency of severe defects was registered in the mandibular FPMs (n = 13) than in the maxillary FPMs (n = 5). Another aspect highlighted by the study was that over half of the subjects diagnosed with MIH (n = 19; 52.8%) had one FPM affected, whereas all four FPMs with MIH lesions were found only in 8.3% of cases (n = 3) [78]. [PERSON] et al. carried out a cross-sectional study concerning MIH prevalence on 2000 Indian patients aged 8-16 years [56].