Association Between Frequency and Severity of Dental Fluorosis and Molar Incisor Hypomineralization

INTRODUCTION

While we have seen a decrease in the dental caries experience, developmental defects of enamel (DDE) are increasingly evident in clinical practice.

Dental development is a slow, complex, multidimensional, and multilevel genetically-controlled process [1]. DDE are caused by interactions of local, systemic, genetic, and environmental factors, which can affect the functional ameloblast. Hence, the type of defect will depend on when the damage occurred during amelogenesis. If the damage occurs during the secretory phase, when the matrix is deposited, a hypoplasia type DDE can occur. In contrast, if the damage occurs during the maturation phase when the matrix is removed and the mineral content incorporated, a hypomineralization type DDE can occur, which can be diffuse or demarcated [2].

It is widely accepted that fluoride effectively controls dental caries [3]. However, it has been suggested that fluoride excess can stress the ameloblast affecting the synthesis of proteins responsible for the organic matrix removal [4], clinically resulting in a diffuse hypomineralization named dental fluorosis (DF). This type of DDE varies from thin, white, and fuzzy lines to a completely white surface. The most severe cases may display pigmentations and loss of structure [5]. DF affects symmetrically groups of teeth, especially the permanent maxillary premolars, second molars, and incisors [6]. Due to the increase in porosity, fluorotic enamel is less resistant to demineralization, and therefore more susceptible to dental caries [7].

Molar incisor hypomineralization (MIH) is a type of DDE of multifactorial origin with a possible genetic component [8] that asymmetrically affects the first permanent molars and permanent incisors [9]. Clinically, well-demarcated white, yellow, or brown opacities can be seen, which may be associated with post-eruptive fractures, atypical dental caries lesions, and atypical restorations. The MIH affected enamel presents significant structural and componential alterations, which may result in hypersensitivity and dental caries [10].

The studies that have evaluated the association between DF and MIH in fluoridated water regions have found lower MIH frequencies [11, 12, 13]. However, data from fluoridated salt regions are lacking. Also, the available studies have used the modified DDE index (mDDE), which according to Elfrink et al presents disadvantages since it does not consider classic MIH characteristics such as post-eruptive fractures, atypical dental caries lesions, atypical restorations, and dental extraction [14]. Thus, for the first time in a fluoridated salt region and using specific and validated indexes, we aimed to evaluate the association between the frequency and severity of DF and MIH.

MATERIALS AND METHOD

This study is described according to the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) statement.

RESULTS

The final sample consisted of 453 patients, of whom 228 (50.33 %) were females. The mean age was 14.49 ± 1.19 years. Figure 1 shows the frequency of DF’s and MIH’s involvement per tooth.

Figure 1. Frequency of Dental Fluorosis (DF) and Molar-Incisor Hypomineralization (MIH) involvement per tooth.

The percentage of patients with DF was 28.70% (n= 130); 23.07% presented at least one tooth with severe DF. The premolar was the most frequently affected tooth (18.15%), with the maxillary (20.84%) being more commonly affected than the mandibular (15.45%). The first permanent maxillary molars (4.09%) were the less affected teeth. The permanent maxillary incisors (7.88%) and canines (9.07%) were more frequently affected by DF than the mandibular (3.37% and 5.03%, respectively). The most commonly affected surface was the occlusal (13.03%), followed by the buccal (12.71%) and palatal/lingual (8.10%). Considering all the evaluated permanent teeth surfaces (n= 32441), the most frequently encountered score was TF2 (3.74%). No teeth/surfaces presented with TF ≥ 7.

The percentage of patients with MIH was 31.06% (n= 141); 38.3% presented at least one tooth (first permanent molar or permanent incisor) with severe MIH. 32.62% of the patients presented MIH in one tooth, 36.17% in two teeth, 18.44% in three teeth, 6.38% in four teeth, 3.55% in five teeth, and 2.84% in six teeth. The first permanent right maxillary molar was the most frequently affected tooth (7.73%), followed by the first permanent left maxillary molar (7.21%), the first permanent left mandibular molar (5.15%), and the first permanent right mandibular molar (4.19). Among the permanent incisors, the maxillary were the most frequently affected. The buccal was the most commonly affected surface (3.73 %), and the palatal was the least commonly affected (0.51%).

Table 1 shows the distribution of DF and MIH in first permanent molars and permanent incisors at surface level. TF2 was DF’s most frequent score, and white or cream demarcated opacities were the most frequent MIH defect.

Table 2 shows the presence of MIH, according to the presence of DF at surface level. The frequency of MIH frequency at surface level was higher in the absence of DF (PR= 0.03; 95% CI: 0.01–0.08) and presence of dental caries (PR= 3.80; 95% CI: 2,39–6.03). Age, sex, and dental caries experience did not influence the association between MIH’s and DF’s presence (aPR= 0.03; 95% CI: 0.01–0.08). However, we observed heterogeneity in the association’s strength between DF and MIH, being higher in the absence of dental caries (aPR= 0.022; 95% CI: 0.01–0.09). No statistically significant difference was observed in the presence of dental caries (aPR= 0.41; 95% CI: 0.06–2.98).

Table 3 shows the association between MIH and dental fluorosis according to severity at surface level. After adjusting for age, sex, and dental caries experience, the severity of MIH was lower among those with mild DF than those without DF (aOR= 0.02; 95% CI: 0.01–0.07). Regarding severe DF, no significant difference was observed in the severity of MIH (P= 0.174).

DISCUSSION

This study showed that the frequency and severity of MIH are lower in the presence of DF.

One of the strengths of this study is that the observation bias was controlled, as two different researchers evaluated DF and MIH independently. This is one of the advantages of using photographs in research since conducting such evaluations clinically can be challenging and time-consuming. The use of photographs in research also prevents patients from being subject to multiple clinical examinations performed by various examiners.

This is one of the first studies to utilize valid criteria to independently classify DF (TF index) and MIH (MIH index), a crucial methodological feature that strengthens our study’s internal validity. According to Thylstrup and Fejerskov, the TF index has high precision and sensitivity and accurately characterizes DF even in populations with different severities (Thylstrup and Fejerskov 1987). It also allows relating the clinical characteristics with the histologic characteristics, a particular and crucial feature for clinicians, researchers, and epidemiologists [5]. On the other hand, the MIH index includes validated criteria that classify the clinical status and extent of this type of DDE [16, 18]. Although it also allows differentiation of MIH from other enamel defects, the severity can only be determined for MIH. Other studies have used the mDDE index to evaluate DF and MIH [11, 12, 13]; however, this index does neither allow evaluating the defects’ severity nor consider common MIH characteristics such as post-eruptive fractures, atypical restorations, atypical dental caries lesions, and atypical dental extractions [14].

As the examination was based on photographic records, this study’s limitation is that the history of fluoride exposure could not be identified. However, to control this information bias, we established that all participants should have born and resided in the same city (as obtained from the medical record) as one of the inclusion criteria. The use of photographs could represent a methodological limitation due to the risk of under- or overestimating the frequency and severity of enamel defects. As for MIH and dental caries, the use of standardized photographs for the diagnosis of DF is a reliable and reproducible method compared to clinical diagnosis [20, 21].

DF’s pathologic mechanism has been widely discussed in the scientific literature. Chronic fluoride intake during amelogenesis affects the dental enamel’s structure and composition [22]. In this study, the permanent premolars and second permanent premolars were the most frequently affected teeth by DF (Figure 1). The frequency and severity of DF tend to be higher in permanent teeth with delayed eruption because fluoride exposure increases with age [23]. According to Bårdsen’s meta-analysis, fluoride exposure duration seems to be more critical than specific risk periods during amelogenesis [24]. In contrast, the first permanent molar was the least affected tooth by DF. This is interesting since the first permanent molar is the most frequently affected tooth in MIH.

Similar to other studies conducted in fluoridated water regions [11, 12, 13, 25, 26, 27, 28, 29], we found that the frequency and severity of MIH was lower in the presence of DF (Table 2). Balmer et al (2012) [11] suggest that fluoride could be a MIH protective factor, while Koch (2003) [25] affirms that MIH cannot be related to fluoride intake. Based on our findings, we hypothesize that DF and MIH “compete” with each other; thus, studies are needed that evaluate the temporality of these defects. We also believe that the difference in fluoride exposure between regions/populations could explain the variability in the prevalence and severity of MIH.

Contrary to what one might believe, enamel affected by DF is more susceptible to dental caries’ demineralization [7]. It should also be considered that the fluorotic enamel presents higher porosity and, thus, a greater mineral area to be dissolved by acids, which facilitates its penetration into the dental enamel [7]. Likewise, the enamel affected by MIH presents significant alterations in its structure and composition, such as increased porosity, a greater amount of carbon and carbonate, and a decreased mineral content [10]. These characteristics have been associated with a higher dental caries experience [10, 30]. Hence, it is crucial for the clinician to know the source of exposure to fluoride of individual and collective use, determine the patient’s individual risk, and implement effective measures for the control of dental caries in patients with DF or MIH.

CONCLUSION

Our findings showed that in a fluoridated salt region, the MIH frequency and severity tends to be lower in the presence of DF.