I. Introduction
Molar-incisor hypomineralization (MIH) is a clinical condition that was first defined in 2001. MIH is a defect with a systemic etiology that affects one or more first permanent molars or one-third of the occlusal surface or incisal edge of incisors, and is defined on the basis of the clinical appearance or presence of morphological defects[
1]. MIH clinically presents as soft and porous tooth enamel with clearly demarcated opacities. Furthermore, tooth brushing or masticatory forces could induce structural defects, which can occur posteruptive breakdown and can increase the sensitivity of teeth to external stimuli[
2,
3].
It is thought that MIH is not caused by a single specific condition. The medical complications including respiratory disorders and fever may increase the risk of MIH occurence[
4-
7]. Although the causative factor of MIH has not been clearly identified, genetic factors and correlation with hypomineralization of the second primary molar have been recently implicated[
8,
9].
The prevalence of MIH has been reported to range from 2.4% to 40.2%[
10-
12]. This variation of MIH prevalence may be attributed to differences in subject inclusion criteria, diagnostic categories for the disease, and methodologies.
Depending on the patient’s age, cooperation, eruption status and severity of structural defect, the treatment of MIH can range from preventive treatment to restoration and extraction[
6]. However, the treatment of MIH is characteristically associated with several clinical difficulties, including the hypersensitivity caused by MIH, rapid caries progression due to tooth structure breakdown and poor cooperation from young pediatric patients. Furthermore, achieving adequate anesthesia is a challenge, and tooth fracture repeatedly recurs even after restoration[
13].
The range and severity of MIH may influence the choice of restorative material and prognosis of treatment. Thus, the classification of MIH to mild, moderate, and severe stages of severity was suggested by Mathu-Muju and Wright[
14]. In this classification, previous studies suggested that for mild MIH, preventive treatment should be performed or composite resins (CRs) should be used to restore the defects. For moderate/severe MIH, restorations should be performed with glass ionomer cement (GIC) for partially erupted teeth and CRs for teeth with three or fewer affected surfaces. Restorations with a stainless steel crown (SSCr) or extraction should be performed for more severe defects[
15-
19]. It was showed that the treatment of MIH-affected first molars could involve various restorative materials depending on the severity and range of the defect, patient cooperation and eruption stages of the teeth. Therefore, restorative materials should be differentially chosen according to the extension and severity of the defects. However, only few studies have been reported and for this reason, there are limited prognostic information regarding degrees of MIH defect with restorative materials.
Therefore, this present study attempted to classify the range of defects of MIH-affected first molars and examines the prognoses of restorative treatment with different restorative materials in order to provide clinically useful information for treating MIH-affected first molar.
Ⅳ. Discussion
The present study classified the MIH-affected first permanent molars into different classes of severity and assessed the prognoses of restorations.
The selection and long-term stability of restorative material is a subject of controversy for MIH-affected first molars. Some previous studies have been reported that CR shows satisfactory treatment results[
17-
19], but showed limitation of not assesses the prognoses in accordance with the severity of defects.
The severity of MIH has been classified in a previous study[
20]. However, because the severity index was only divided into 2 classes, the effectiveness of assessing retreatment and the correlation between the severity of the defect and the choice of restorative material have been limited. In contrast, Mathu-Muju and Wright[
14] classified MIH into the mild, moderate, and severe classes depending on the severity of the defect, and recommended treatment modalities for each MIH class. However, the prognoses of the suggested treatment modalities were not assessed. Therefore, the present study assessed the prognoses of restorations in accordance with the severity of MIH-affected first molars into class I, II, or III, and investigated the survival rates of restorations as well as the retreatment frequency (
Table 2,
Fig. 1).
Results of the present study is consistent with those of the previous study which CR restorations had a significantly high survival rate and low risk of retreatment when used for MIH class I and II defects. However, CR restorations were associated with an increased risk of retreatment when used for MIH class III defects, but it was not statistically significant.
The 24 teeth were classified into MIH class I, for which the initial treatment included fluoride application, sealants, CR and GIC restoration as shown in
Table 4. Sealants were chosen most with class I defects, and 85.7% of the restorations survived during the study period. Furthermore, sealants showed the highest statistically significant survival rate with MIH class I defects, indicating that sealant resulted in a good prognosis for MIH class I defects. However, some MIH class I teeth required restorative treatment because of the breakdown of hypomineralized areas, and retreatment was performed with alternative material. Therefore, the teeth classified into the MIH class I required continuous follow-up, as it is necessary with other classes of defects.
The majority of MIH class II defects were treated by using CR, GIC, and SSCr. The CR had a significantly lower risk of retreatment and a higher 24-month survival rates. Furthermore, the average periods until first retreatment were 19.2 and 11.8 months for CR and GIC, respectively. Overall results showed that CR restoration had the best prognosis for MIH class II.
In MIH class III defects, the retreatment ratio for CR restorations increased to 63.2%, and the 24-month survival rate decreased to 54.1%. In contrast, the retreatment ratio and 24-month survival rate for GIC restorations did not differ significantly from those of the MIH class II. These results imply that the range of the defect is a greater impact factor on the prognosis of the restoration than does the choice of restorative material for MIH class III defects. Although restorations with SSCr were performed on a relatively fewer number of teeth, it showed the best prognosis, with a 24-month survival rate of 75% in MIH class III. Furthermore, 5 teeth were retreated with SSCr, and these required no additional retreatments. This finding supported the recommendations to use SSCr for defects that affect 3 or more tooth surfaces[
15,
16].
The frequency of retreatment was assessed with each material (
Table 5). The proportion of cases requiring retreatment was 47.5% for CR, 63% for GIC, and 16.7% for SSCr. In addition, the frequency of retreatment was greatest in restorations with GIC, with an average of 1.20 retreatments in each tooth. The median survival time was 34 months for CR and 18 months for GIC, showing that CR were retained longer than GIC. CR had a significantly high the median survival time and 83.3% of SSCr survived for 24-months. These results supported the findings of previous studies that consistently suggested better prognosis with CR and SSCr.
Interestingly, root canal treatment was performed for postrestoration pulpal complications in 6 teeth with class III defects (
Table 4). However, no correlation was found between retreatment frequency and pulpal involvement (data not shown). Thus, it could be speculated that the proximity of the defect to the pulp has a greater impact than retreatment frequency, but additional studies would be needed to substantiate this.
A distinct finding of this study was the numerous choices of GIC for MIH-affected first permanent molars, influenced by the personal preferences of dental clinicians. The preference of GIC could be affected by several factors, including partial eruption, low patient cooperation, uncertain adhesiveness of CR, short treatment time, and tooth sensitivity, treatment cost and etc. Notwithstanding the high choice of GIC, CR showed a greater survival rate in all classes of defects. These findings once again showed the similarity to the validity of Mathu-Muju and Wright’s[
14] recommendation of the use of CR for the treatment of MIH-affected first permanent molars.
In conclusion,
Fig. 2 illustrates a treatment protocol that takes into account the range of defects, symptoms and patient cooperation during the treatment of the first permanent molars affected by MIH. First, the MIH-affected first permanent are identified. For firm hypomineralized enamel without enamel defects, sealants are applied, while for soft hypomineralized enamel, restoration after the removal of the hypomineralized area is recommended. CR restoration is performed for defects limited to less than 2 tooth surfaces; this restoration is also recommended if the defects are observed on more than 2 tooth surfaces, provided the eruption of the first permanent molar is complete, and the removal of the entire hypomineralized enamel area is possible. However, if the removal of the entire hypomineralized enamel is not possible because of extensive structural defects, SSCr is recommended as the initial restoration. If defects are present on more than 2 tooth surfaces and the tooth is partially erupted, GIC restoration, which is later replaced with SSCr or CR restoration, may be considered depending on the pediatric patient’s ability to cooperate. For cooperative pediatric patients, SSCr or CR is recommended for the initial restoration. Higher treatment success rates may be achieved by following this protocol.
Although many previous studies have recommended SSCr for restoring class III defects, only limited data on the long-term prognoses of SSCr restorations are available, thus the need for further long-term studies essential on this subject. Furthermore, limitations of this study includes that the range of defects classified as MIH class III is broad in a measure. Though future studies, further clinical implications of various restorative materials could be revealed by assessing the prognosis of restorations in accordance with subdivided the MIH class III.