With recent improvements in the level of oral care, the prevalence of dental caries, a prominent oral disease, has been decreasing [1,2]. In contrast, the prevalence of enamel hypomineralization due to systemic factors has increased, which has heightened interest regarding this condition [3,4].

Crown formation in first permanent molars and incisors occurs at approximately the same time [5], during which enamel hypomineralization is frequently observed. Enamel hypomineralization in first permanent molars and anterior teeth was first reported in 1970 in Sweden [6]. In 2001, Weekheiim et al. [7] defined MIH as demarcated, qualitative enamel defects of systemic origin, affecting one or more permanent molars with or without involvement of incisor teeth. Since then, several studies worldwide have actively investigated the etiology, conditions, and prevalence of MIH, with most focusing on first permanent molars and incisors.

However, the clinical categorization of hypomineralization is broad, and clinically, all primary teeth, permanent canines, premolars, and second permanent molars can be affected by enamel hypomineralization [8-10]. Therefore, in a recent policy, the European Academy of Paediatric Dentistry (EAPD) recommended that whole dentition be included in future studies on enamel hypomineralization [11].

Several studies have investigated the causes of MIH [12]. A recent study reported that, while a significant correlation existed between MIH and polyvinyl chloride/dioxin discharge, MIH was only weakly correlated with nutritional status, birth conditions, childhood diseases, and fluoride intake [13]. In another study, water fluoridation and pregnancy at an older age were identified as risk factors for MIH [14,15]. High vitamin D concentration in blood has also been reported to be negatively correlated with MIH. While it appears that MIH is caused by various risk factors, no definite risk factors have been identified yet [16].

Teeth affected by MIH exhibit enamel defects and rapid progression of caries; therefore, in many cases, affected teeth require restorative treatment right after eruption. However, owing to the high organic content of enamel, the success rate of adhesion of restorations is low [17,18]. If the defect is deep and widespread, it can lead to severe loss of tooth material and early loss of permanent teeth because of caries progression. In addition, in adolescents with active social lives and a developing interest in appearance [19], slight defects can have a serious impact on self-esteem, depending on the location of the defect.

Despite the significant impact of MIH on adolescents, previous studies have only investigated MIH among elementary school students [20], and no study till date has evaluated MIH among a teenage population. The subjects of the present study were middle-school students with complete eruption of permanent dentition. The prevalence, defect patterns, and distribution of permanent tooth enamel hypomineralization, including MIH, and their effects on tooth condition and esthetic satisfaction were evaluated.

Enamel breakdown can occur on occlusal force impact, and enamel hypomineralization appears in a variety of colors, including yellow, tan, brown, and milky white [23]. The aprismatic enamel layer, thick prism sheath, and coarse crystals observed in hypomineralized enamel are responsible for the decreased hardness of hypomineralized teeth [24]. In particular, when bacteria penetrate porous enamel, the pulp enters into a state of chronic inflammation, which causes hypersensitivity and failure of local anesthesia [24,25]. Because of the resultant constant pain, young and adolescent patients affected by MIH often exhibit behavior management issues during dental treatment.

In addition, enamel hypomineralization typically occurs at locations other than those of dental caries, and when lesions affect the cusp, appropriate restoration becomes very difficult [26]. Consequently, maintaining restorations is challenging, and several claims are being raised that it is best to extract teeth with poor prognosis in a timely manner [27]. Traditionally, the ideal age for extraction of first permanent molars is 9 - 11 years [28]. However, recent studies are conflicted over the appropriate age for extraction; some studies have proposed that extraction should be performed at a younger age, whereas others have stated that early extraction could increase the risk of hypomineralization in second permanent molars [29].

Anterior teeth defects are treated with emphasis on esthetic improvement. However, because microabrasion, resin restoration, and whitening agents such as hydrogen peroxide usually cause mineral loss [30], casein phosphopeptide-amorphous calcium phosphate is now being used for active remineralization treatment [31].

In general, the ideal age for MIH epidemiological studies is 7 - 8 years, during which the first four permanent molars will have erupted and enamel loss due to dental caries or other diseases would not have started yet. In addition, at this age, children still possess a residual part of the second molar, which makes it possible to evaluate the correlation between MIH and second-molar hypomineralization. Elfrink et al. recently suggested that primary molar hypomineralization and MIH are correlated and that the former could be a predictor of MIH [32]. In the present study, although the entire permanent dentition was evaluated, hypomineralization of second permanent molars was the focus of evaluation. Therefore, subjects with complete dentition, between the ages of 14 and 16 years, were enrolled as study subjects.

The most active research on prevalence of MIH is conducted in Europe, where prevalence levels ranging from 3.6% to 37% have been reported [10]. In Asian countries, prevalence of MIH has been reported to be 2.8% in Hong Kong [33], 25.5% in China [34], 20% in Thailand [35], and 6.0% in South Korea [20]. This variation in prevalence might be attributable to environmental or genetic factors or even differences in research methodology and criteria among various studies [10]. In the present study, the prevalence of MIH was 13.8%, which increased to 23.2% upon expanding the study sample to include subjects in the stage prior to permanent dentition. This value is slightly higher compared to that reported in a nationwide survey in 2010, probably because the previous study included children in Pusan and larger cities as test subjects, while the present study only included subjects from a provincial area in Yangsan. Because the residents of this province have lower socio-economic status and limited access to health services in comparison with residents of bigger cities, the risks of development and progression of oral diseases among the former are known to be high. Epidemiological studies on MIH have also reported that children living in provincial areas exhibit higher prevalence of MIH than those living in cities [36].

In the present study, the average number of teeth with lesions in the EH and MH groups were 3.88 and 5.18, respectively, with no significant differences according to sex. These values were higher compared to those reported in 6 to 12-year-old children in Brazil at 3.3 [37], 12-year-old children in Hong Kong at 2.6 [33], and 8-year-old children in Spain at 3.5 [38].

In the EH group, mandibular first permanent molars were the most commonly affected teeth, followed by maxillary central incisors, maxillary first permanent molars, and mandibular second permanent molars, in that order. These results indicating high prevalence of MIH in first permanent molars and maxillary central incisors are consistent with those of previous studies [36]. It is noteworthy that the proportion of affected second permanent molars, which are generally excluded from MIH diagnosis, was relatively high. It is, therefore, necessary to study hypomineralization in second permanent molars, which experience a relatively high proportion of occlusal forces. In the MIH group, where subjects exhibited defects in first permanent molars, the results of correlation analysis demonstrated that presence of defects in first permanent molars indicated a high possibility of presence of defects in the remaining permanent teeth, with the correlation being especially strong among central incisors and second permanent molars. Given the strong correlation between MIH in first and second permanent molars, extraction of MIH-affected first permanent molars-which is one of the treatment choices-should be planned carefully.

In the present study, the pattern of MIH defects varied according to tooth type. Demarcated opacity was the most commonly observed defect at 99.7% in anterior teeth, probably because the positional characteristics of anterior teeth renders them unaffected by bite forces and also decreases their chances of developing crown fractures from external factors. First permanent molars exhibited similar proportions of demarcated opacity at 49.6% and atypical restoration at 46.1%, which indicated a history of hemorrhoid damage. The highest prevalence of demarcated opacity was observed in second permanent molars at 85.8%. Since the present subjects had just experienced eruption of second permanent molars, we believe that they might not have yet suffered enamel loss due to occlusal forces or dental caries. In the absence of appropriate regulated treatment for demarcated opacity, there is a very high chance that the condition could progress into first permanent molars and atypical restorations.

Subjects with enamel hypomineralization in anterior teeth frequently experienced esthetic dissatisfaction. In this study, among the six anterior teeth, high prevalence of enamel hypomineralization was observed in in the middle and 1/3 of the incisal edge of maxillary central incisors. Hypomineralization in these teeth becomes evident during smiling, talking, and mastication, which might adversely affect esthetic perception. The present results of the questionnaire survey for subjective esthetic satisfaction revealed lower esthetic satisfaction and greater need for esthetic treatment in presence of defects in anterior teeth, which also reflected the effect of MIH on esthetic satisfaction among patients. Adolescence is a period of transition from childhood to adulthood, which involves physical and psychological changes. Therefore, it is necessary to consider the possibility of loss of self-esteem due to physical defects among adolescents.

The present study aimed to investigate the prevalence and distribution of enamel hypomineralization among adolescents. The results indicated a 13.8% prevalence of MIH, which increased to 23.2% upon inclusion of permanent teeth. Enamel hypomineralization in first permanent molars was significantly correlated with that in central incisors and second permanent molars. In anterior teeth, the middle and 1/3 of the incisal edge of maxillary central incisors were the most affected by hypomineralization. Patients with defects in anterior teeth exhibited low esthetic satisfaction and high demand for esthetic treatment.