S1 File -

Aim
To evaluate the association between the frequency and severity of dental fluorosis and Molar Incisor Hypomineralization (MIH) in 8-12-year-old schoolchildren living in an area with a high concentration of fluoride in the drinking water.
Methods
The present cross‑sectional study was conducted on Mexican children (n = 573) selected from one community presenting a drinking water fluoride concentration of 1.39 ppm/F. The prevalence of dental fluorosis was ascertained using the Thylstrup and Fejerskov Index (TFI). The presence and severity of MIH was evaluated using the European Academy of Pediatric Dentistry (EAPD) criteria. A multinomial regression model was used to estimate the odds ratio (OR) and the 95% confidence intervals (CI), using the severity of MIH as the result.
Results
The prevalence of MIH was 37.7% and, by severity, was 16.1% mild, 14.3% moderate, and 7.3% severe. The prevalence of dental fluorosis in permanent dentition was 70.9% (TFI ≥1) and, by severity, was 29.2% (TFI = 0), 45.6% (TFI 1–3) and 25.3% (TFI ≥4), while 54.5% of subjects were found to have poor oral hygiene. Schoolchildren with fluorosis (TFI ≥4) were 49% less likely [OR = 0.51; p = 0.025] to present mild MIH than children with fluorosis (TFI <4). Similarly, children with fluorosis (TFI ≥4) were 53% [OR = 0.47; p = 0.019] and 62% [OR = 0.38; p = 0.036] less likely to present moderate and severe MIH than children with fluorosis (TFI <4).
Conclusion
An inverse relationship between the presence of fluorosis and MIH was found. The results obtained by the present study may contribute to both the early identification of disorders affecting the enamel and the creation and implementation of long-term oral health prevention, promotion, and intervention programs in the affected population.