This study provides evidence regarding the clinical change of dental fluorosis and its severity after three years, using the TF index for DF. This is the first study to report dental fluorosis clinical change and its relationship with age, gender, and severity in a specific Colombian population with a high prevalence of DF.
The Thylstrup and Fejerskov Index is more applicable for measuring DF severity in areas with high or low fluoridated areas; it is considered a sensitive method because it allows carefully classify the affected tooth by correlating clinical and histological features. Despite being defined more than 40 years ago, it contributes to avoiding underestimating teeth affected by fluorosis; and is the most suitable for use in the clinical management of DF [21, 23].
The present study found that most DF scores remain stable with time, with a tendency to diminish at tooth level. Reduction in severity was noted in 29.6% and the increase in 24.0% of the teeth, while Do et al. [8] reports a reduction in 10.6% and an increase in only 1.0%. Furthermore, the cases of increased severity were observed in the scores 1, 2 (very mild), 3 (mild), and 5 (moderated), with association to TF 1 score (very mild) (p = 0.01); the canines-premolars-second molars as a group also showed association with increased DF severity (p = 0.005).
Regarding score changes, in general, the TF 1 increased until score 3, TF 2 had the greater tendency to remain the same, and scores 3–5 showed a greater tendency to reduce the score. The present study found severity TFI score, ranged from 1 to 6 after three years, differing to Do et al. [8], who reported a severity from 1 to 3, in children between 8 and 13 years old. Although in both studies, predominated a higher proportion of teeth with the same TFI score, after the following, our study presented more considerable change than the Do et al. [8].
After three years, some DF lesions became more evident. According to Thylstrup and Fejerskov [21], it's possible that a subsurface lesion was protected by an outer surface, previously diagnosed with a mild score. The increased severity of dental fluorosis lesions may be related to some intraoral conditions facilitating enamel mineralization [21]. So, in cases of slightly fluorosed enamel and even in moderate cases, macroscopic changes can be described as restricted to the coronal-incisal half of the crown at a specific time. In populations characterized by a low incidence of DF, teeth frequently exhibit opacities, preferentially located on the cusp tips [24], in contrast with the epidemiological scenery of the analyzed population of the present study.
In some cases, changes in the reduction of the DF score could be due to the influence of mechanical forces such as attrition and abrasion, which can cause wear on hypomineralized porous enamel surfaces as scores 1–3, causing lesions that disappear clinically and expose almost normal underlying enamel. As a result of the above, there might sometimes seem to be a gradual posteruptive regression of the severity of DF milder forms [1]. Even moderate and severe lesions can present fractures that can expose normal underlying enamel, decreasing the score of lesions previously classified as moderate or severe.
In this study, 5.2% (n = 59) of the teeth went from score 1 to 0, in agreement with Do et al. [8], that reported 6.6% (n = 97), but in opposite with Curtis et al. [25], that reported a higher percentage (47.0%-to-54.0%). It is noteworthy that some differences between these studies, as follow-up time and the Index applied. However, it has been reported that DF lesions do not disappear entirely; although clinically they may have a healthy appearance, this is due in most cases to changes in its refraction.
Regarding the tooth group, compared with maxillary incisor, the canines-premolars and second-permanent-molars teeth group presented a higher incidence of an increase in the severity of TFI score RR = 3.3; 95% CI 1.9–5.6 (p-value = 0.005). Probably function and mastication forces could be involved. In some cases, this tooth group was recorded with severe scores compared to the first-permanent-molars and permanent incisor as a group of teeth. To Fejerskov et al. [1], the increase in post-eruptive enamel loss depends on the degree of severity at the time of the eruption. In this study, an association was found between the change and the degree of severity of the lesions at the beginning. Canines, premolars, and second molars most frequently show dental fluorosis and more severe in comparison to a lower incidence and severity in the mandibular incisors and first molars [17, 23, 26]. This finding was also observed in the present study.
Some limitations of this study could be the sample size. Only 92 children examined in the first evaluation could be reassessed after three years, possibly due to accessibility or parents' occupation. Although there were linked to the same teeth after three years, there was not a systematic follow-up.
This study analyzed the DF clinical change, with variables as gender, age, tooth, and DF severity score. Nevertheless, some other variables could be omitted. The strengths of our study are the specific DF index used, and all permanent teeth present in the first exam were included, examiners were calibrated, and children had not received any previous treatment such as tooth whitening; whitening was a variable not controlled in other studies [8, 11, 25].
There is limited evidence to compare the longitudinal results of this study; only one study uses a specific index to DF as the one applied in this study, Do et al. [8], while other studies used non-specific indexes as the DDE Index, with controversial results. In 1988, de Liefde [10] found an increase in enamel opacities, while Wong et al. [11] found a decrease in diffuse opacities; other studies used a non-histological validated Index, and not sensitive enough as Curtis et al. [25] However, all of these studies, including ours, provide evidence of the change in DF, its trend, and the moment in which it occurs [2, 8, 10, 11, 25].
In this study, DF higher score after three years did not show statistical association with age, but it was found greater increase at 10 and 11 years of age, while at 12 years, this percentage was lower, showing a similar decline tendency as Curtis et al. [25] who reports a change in a follow-up longer time and to a greater age than this study, that could be evident in a more extensive sample size study. Therefore, factors such as age at the time of evaluation, time the tooth stays in the mouth, post-eruptive enamel maturation, diet, and oral hygiene habits, and its relationship with higher chewing forces in the lower arch, can influence the variation between studies in TFI scores [3, 23, 27, 28].
The post-eruptive environment can change the clinical appearance of dental fluorosis, which may be different according to age, severity and tooth type as was observed in this study; diagnosing DF and identifying the change in the lesion, knowing that a greater percentage of them remains the same or decreases and lower percentage increases, may be useful to define the most appropriate clinical management option for the fluorosed tooth.
Age and severity also influence the aesthetic perception of DF, and there is greater dissatisfaction when DF is severe compared with mild or very mild injuries; some studies have shown that there is no difference between individuals with DF and without DF in their aesthetic perception [23, 29, 30].
It is important higlited that several factors can influence the DF clinical change; in this study, the highest or severe TFI score was assigned to each tooth, which was compared after three years. In each tooth, the score could decrease, remain the same or increase and this alteration could occur in the different evaluated teeth in each child. This means that, in the same individual, some teeth could change to a lower, others remained the same and others increased their TFI score [21, 28]. Whether some teeth increase or decrease in the same individual depends of intrinsic and extrinsic factors, which in each individual can vary and affect clinical appearance and histology, in one or the other tooth following a different pattern between teeth and between individuals.
To better understand the longitudinal clinical change of DF, authors recommend carrying out new studies that allow comparison and approach to knowledge, with longer clinical follow-up. It's important to correlate the clinical diagnosis with the histological component, considering the enamel porosity degree in-depth and extension, using new technologies in clinical diagnosis. Moreover, to identify risk factors and use specific measurement systems for DF, such as the one used in this study based on histological validation. To establish damage prevention measures and inform the professional in clinical evolution.
DF has been one of the most prevalent conditions seen in pediatric dental practice, and it is proper of the clinical professional to guide the patient avoiding unnecessary and invasive treatments.