Definition and scoring system of molar incisor hypomineralization: A review

Objectives: A definition and classification system for molar incisor hypomineralization (MIH) was first suggested by the European Academy of Pediatric Dentistry (EAPD) in 2003. Since then, there has been growing interest in these mineralization defects due to esthetic and cariogenic susceptibility concerns. The aim of this review was to evaluate studies on MIH in an attempt to provide evidence for consistency and standardization of MIH definition and severity scoring reporting by clinicians and researchers. Methods: Included were studies (research papers and reviews) focusing on MIH affected populations and containing a definition and scoring system. Only English language articles published in the period 2001 to 2015 were considered. Study selection and data extraction were performed by two independent investigators. Results: More than half of the publications used the EAPD definition and fewer than half of the prevalence reports have either used the EAPD criteria or the developmental defects of enamel (DDE) index as their classification system. Others have either incorporated modifications or created their own scoring criteria. The lack of standardized definition and heterogeneity of scoring systems used might explain the considerable variability in prevalence figures. Conclusion: In order to better define the diagnostic criteria and the treatment needs for those patients, a globally accepted standardized protocol for scoring system is required. Correspondence to: Katherine Kula, Diplomate, American Board of Orthodontics, Chair and Jarabak Endowed Professor, Department of Orthodontics and Oral Facial Genetics, Indiana University School of Dentistry, 1121 W. Michigan St., Indianapolis, IN 46202, USA, Tel: 317-278-9915, E-mail: kkula@iu.edu


Introduction
Molar incisor hypomineralization (MIH) is the term used to describe a special pattern of enamel defects. The terminology was first used by Weerheijm et al. [1] describing developmental defects affecting first permanent molars and permanent incisors, ranging from distinct, isolated, white, yellowish, or brown-colored demarcated opacities to severe post eruptive structural breakdown. Shortly after MIH terminology was proposed, the European Academy of Paediatric Dentistry (EAPD) announced criteria specifically aiming at diagnosing and recording MIH [2,3].
Information available on the exact etiology of MIH is limited. A combination of factors that may affect the ameloblasts resulting in abnormal enamel formation has been suggested in most studies. Systemic or environmental insults during the maturation stages of enamel development as well as the possibility of genetic predisposition of ameloblasts to environmental stressors have been reported [4][5][6]. Some authors suggest that MIH may be an autosomal recessive condition or an unrecognized form of localized amelogenesis imperfecta. Others have indicated that there is a possible association between MIH and variants in genes related to enamel formation such as AMBN, TUFT1, and TFIP11 [7,8]. A genome-wide association study identified the SCUBE1 gene on chromosome 22 as a potential genetic locus for MIH [9].
There is wide disparity in the reported prevalence figures of MIH. The majority of reports are from European populations, with prevalence rates ranging from 5.9% to 38% [10,11]. Other Non-European prevalence data also demonstrate extreme variations with a prevalence rate as low as 2.8 % in Hong Kong [12] and a rate as high as 40.2% in Brazilian children [13]. The extensive variation in the described rates is suggested to be either due to actual differences in MIH prevalence between different populations, variations in age cohorts, masking of MIH by other conditions, or most importantly, due to the different clinical examination protocols used to define this condition and the non-uniform methods of classification and scoring indices [14][15][16][17]. The objective of the current review was to systematically evaluate the studies on MIH to determine the consistency of the use of the current definition and scoring systems.

Methods
A broad search of MEDLINE, EMBASE, ResearchGate, and Google Scholar databases was conducted for the years 2001 to 2015, using the index terms 'molar-incisor-hypomineralization', 'MIH', 'idiopathic enamel defects in permanent teeth', 'hypomineralized permanent molars' and 'hypomineralized permanent incisors'. The search was limited to English language articles. All abstracts were read and the full text of available relevant articles was then reviewed and data were extracted by two authors independently. The reference list including a contemporary definition and scoring system or to estimate the prevalence in a certain population excluding articles focusing on generalized or hereditary enamel defects and other age groups.

Results
Criteria of the 52 articles included in the current review (12 reviews and 40 clinical prevalence studies) are summarized in Tables 1 and 2. A total of 25 reports used either Weerheijm et al. [1] or Weerheijm of key articles was also examined to include any citations that were not located in the search engines.
Of the 175 initial citations, 94 were selected based on abstracts and the remaining articles dealing with generalized enamel defects or inherited dental anomalies were excluded. After reviewing each publication, 52 articles (40 research paper and 12 reviews) relevant to the formulated research question were chosen for inclusion.
Articles were included if the aim was either to describe MIH

Study
Year Study type Location Classification Definition   [3] characterized the hypomineralization as a demarcated opacity, not diffuse. Some authors, however, modified the definition of hypomineralization to include other clinical criteria such as "color changes or ill structuring" or changing the word "systemic origin" into developmental, qualitative, morphological, or asymmetrical [10, 13,[18][19][20]. Some authors also included other teeth such as the permanent canines, permanent second molars, or primary teeth in the definition [19,21].
While some of the prevalence reports indicated that they used the EAPD criteria or the modified developmental defects of enamel (DDE) index as their classification system, other authors have either added modifications or created their own judging criteria. For example, Fteita et al. [22], Laisi et al. [23], and da Cost-Silva et al. [24] classified their cases into mild, moderate, or severe according to the degree of tooth structure loss. Lygidakis et al. [25] and Kevrekidou et al. [20] suggested that it's better to classify MIH cases into two groups only (mild and severe) combining the previously designed two categories of moderate and severe into one group. Some investigators [13,26] based their data only on a classification of 0 and 1, where 0 indicates sound teeth or a normal dentition while 1 denotes MIH affected case without reporting grading or severity.

Discussion
MIH can affect both esthetics and cariogenic susceptibility and is considered to be a global dental problem. It is described by the EAPD as hypomineralization of systemic origin of 1-4 first permanent molars (FPM) frequently associated with affected incisors [3]. Since then, the defining criteria for the condition seem to be the systemic origin and the demarcated opacities in FPM with or without associated incisors. Clinically, MIH presents as opacities of different colors in the affected teeth that will sometimes undergo post-eruptive breakdown due to the porous and weak enamel surface. Accordingly, the condition can represent a serious and challenging clinical management problem. Children with MIH require higher levels of treatment needs and demonstrate considerable management problems [27][28][29]. Although most of the published research is epidemiological data, there is still insufficient evidence about the significance of consequences of MIH and how it may impact the dental healthcare delivery system in general.
Based on these review findings, there seems to be many reasons why the current MIH terminology and criteria are considered questionable and should be addressed:

Teeth involved
In some reports, demarcated opacities were not limited to the permanent first molar and incisors. Similar lesions have been observed on second primary molars, permanent canines, second permanent molars and premolars [19,21]. Consequently, some authors [30] described the terminology MIH as "misleading" and claimed that this greatly contributes to an under-estimation of the actual defect. They suggested that mineralization defects on all teeth should be recorded in order to reflect the extent of involvement and if sufficient evidence exist that MIH is not predominantly restricted to molars and incisors, revising the name should be considered.
While most of the studies consider children as having MIH when one or more FPM meet the diagnostic criteria, others [21] indicated that in the absence of FPMs, MIH diagnosis should be based on the anterior teeth. Balmer et al. [31] introduced other terms such as Incisor Hypomineralization (IH) and Molar Hypomineralization (MH). They indicated that the definition of all three conditions (MIH, IH and MH) are mutually exclusive and that demarcated opacities named as IH, MH, and deciduous molar incisor hypomineralization (DMH) should all be considered parts of a spectrum that includes MIH given that these defects share characteristics and risk factors which overlap with MIH and are expected to have the same etiology. The original inclusion of only molars and sometimes incisors was based on the mixed dentition status of the children examined. These teeth form and erupt earlier than the other permanent teeth and could show either early environmental effects or staged genetic effects on tooth formation. We agree with Weerheijm et al. [3] that if examinations of older children could show that more teeth are involved in hypomineralization than the molars and incisors, then a name change would be appropriate.

Lesion size
The minimum size of lesions included in the diagnosis of MIH is also inconsistent between studies. While some studies included only defects larger than 2 mm [16,[32][33][34], others have included all hypomineralized lesions regardless of their size [30,35]. The exclusion of smaller defects might have thus contributed to lower prevalence rates in some reports.

Severity indices or scoring systems
The majority of published literature basically report prevalence data in certain population. Prevalence figures varied considerably. To a great extent, the lack of standardization and heterogeneity of the scoring or characterization systems used in each study probably contributed to the variability. Although a few studies used the current EAPD characterizing system (Table 3), some authors have either used some modifications or created their own scoring system which makes it obvious that there is a certain degree of confusion and lack of consensus [3,[22][23][24][25][26]. The usefulness of the early EAPD criteria should also be reevaluated due to the fact that it only categorizes the enamel conditions but does not quantify them. Ghanim et al. [17], following the 2014 EAPD Congress in Poland, developed a complex scoring system to quantify the severity of MIH based on the number of involved teeth as well as the type and extent of the enamel defect.
The severity of MIH may differ significantly between individuals and within different teeth of the same individual. The DDE index was implemented to classify the affected teeth according to the type and extent of the defect into 3 categories: demarcated opacities, diffuse opacities, or hypoplasia. Modifications were later proposed to make classification simpler while more specific and meaningful (Table 4).
According to current EAPD criteria, the most extensively used index, diagnosis of MIH includes recording either absence or presence of demarcated opacities, post-eruptive enamel breakdown, atypical restorations, extractions due to MIH and the failure of eruption of a molar or incisor. The use of different scoring systems and severity indices results in confusing and contradictory results. If a case was only scored according to the most severe defect, this may not reflect meaningful results as it may represent involvement of a few or even one tooth whereas the same score may be given to a case with multiple minor defects in another study using summation of scores of all affected teeth. This inconsistency does not allow accurate representation of the patients' overall oral condition, actual restoration needs, and financial requirements.
Children with MIH have higher levels of treatment needs and satisfactory restoration of affected teeth might be very challenging. A study reported that by the age of nine years, children affected with enamel hypomineralization required dental treatment about ten times more frequently compared to unaffected children and that each hypomineralized tooth was treated on average twice [36]. Actual management needs were underestimated when considering only decayed or restored teeth as most of these children would also require continued pediatric restorative, orthodontic and preventive care [37,38]. This highlights the importance of directing future research to high-quality studies that help to improve our understanding of the exact causes of MIH, allow development of standardized diagnostic criteria and universal severity index, and update the dental profession on the overall impact on the oral health care delivery system.

Conclusions
Although MIH seems to be a significant dental public health concern, there is insufficient evidence regarding how accurate the currently used definition and judgement criteria reflect the clinical challenges, treatment needs, and cost associated with the condition. Studies continue to be published using alternative definitions and scoring systems and there is lack of agreement on important criteria. Such inconsistency makes it difficult to compare studies and develop evidence based guidelines. A globally accepted definition and scoring system is necessary.   Table 4. Modified DDE index for use in general purpose epidemiological studies [64].