Structural, mechanical and chemical evaluation of molar-incisor hypomineralization-affected enamel: A systematic review

Highlights

• Structural, mechanical and chemical properties of MIH-enamel were assessed.

• Mineral quantity, quality, hardness and elasticity were reported to be reduced.

• The association between appearance and severity are not clear.

• Heterogeneity in current research findings was found.

• Standard protocols and outcome measures in future studies are needed.

Abstract

Objectives

To systematically assess and contrast reported differences in microstructure, mineral density, mechanical and chemical properties between molar-incisor-hypomineralization-affected (MIH) enamel and unaffected enamel.

Methods

Studies on extracted human teeth, clinically diagnosed with MIH, reporting on the microstructure, mechanical properties or the chemical composition and comparing them to unaffected enamel were reviewed. Electronic databases (PubMed, Embase and Google Scholar) were screened; hand searches and cross-referencing were also performed.

Results

Twenty-two studies were included. Fifteen studies on a total of 201 teeth investigated the structural properties, including ten (141 teeth) on microstructure and seven (60 teeth) on mineral density; six (29 teeth) investigated the mechanical properties and eleven (87 teeth) investigated the chemical properties of MIH-affected enamel and compared them to unaffected enamel. Studies unambiguously found a reduction in mineral quantity and quality (reduced Ca and P content), reduction of hardness and modulus of elasticity (also in the clinically sound-appearing enamel bordering the MIH-lesion), an increase in porosity, carbon/carbonate concentrations and protein content compared to unaffected enamel.

Findings

were ambiguous with regard to the extent of the lesion through the enamel to the enamel-dentin junction, the Ca/P ratio and the association between clinical appearance and defect severity.

Conclusions

There is an understanding of the changes related to MIH-affected enamel. The association of these changes with the clinical appearance and resulting implications for clinical management are unclear.

Clinical significance

MIH-affected enamel is greatly different from unaffected enamel. This has implications for management strategies. The possibility of correlating the clinical appearance of MIH-affected enamel with the severity of enamel changes and deducing clinical concepts (risk stratification etc.) is limited.

Introduction

Dental enamel is the hardest and most mineralized structure in the human body. Amelogenesis commences with the bell stage of tooth development. Ameloblasts are located in the inner enamel epithelium and undergo a sequence of maturation stages, which eventually enable them to produce a protein-rich enamel matrix. Once secreted by the ameloblast, matrix granules aggregate as nanospheres, which form a complex 3-dimensional framework for the arrangement of the enamel crystals. At the same time, a series of enzymes control the resorption of the enamel matrix, so that human dental enamel eventually consists of 95 wt.% mineral in the form of hydroxyapatite (Carlstrom, Glas, & Angmar, 1963; Gustafson and Gustafson, 1968, Robinson et al., 1979; Robinson et al., 1987, Suckling and Thurley, 1984).

When one of the relevant processes, i.e. matrix production, matrix secretion, matrix arrangement, crystal formation or matrix resorption is altered or disturbed, a compromised enamel structure may result. Macroscopic quantitative defects, which are mainly caused by a disturbance to amelogenesis during the matrix secretion phase, are defined as enamel hypoplasia (Clarkson, 1989; Mahoney, Ismail, Kilpatrick, & Swain, 2004; Commission on Oral Health, Research & Epidemiology. Report of an FDI Working Group, 1992; Suckling and Thurley, 1984; Suckling, 1989, Suckling, 1998), however, qualitative defects, caused by disturbances in either the calcification or maturation phase are defined as enamel hypomineralization (Suckling, Elliott, & Thurley, 1983; Suckling and Thurley, 1984, Suga, 1983, Suga, 1989).

The term Molar Incisor Hypomineralization (MIH) was first defined in 2001 by Weerheijm and colleagues as demarcated, qualitative developmental defects of enamel affecting a minimum of one permanent molar with or without involvement of the incisors (Weerheijm, Jälevik, & Alaluusua, 2001). Similar defects also occur in second primary molars – hypomineralized second primary molar (HSPM) (Elfrink, Schuller, Weerheijm, & Veerkamp, 2008; Elfrink et al., 2012, Elfrink et al., 2013; Mittal & Sharma, 2015). MIH defects have different grades of severity (mild to severe); moreover, the clinical appearance varies from creamy/white through yellow to brown color with or without post-eruptive breakdown (PEB). A relatively high MIH prevalence of 3 to 22% and of 2 to 40% has been reported in Europe and in the world, respectively, although indices used have varied (Garcia-Margarit, Catala-Pizarro, Montiel-Company, & Almerich-Silla, 2014; Jälevik, 2010, Kukleva et al., 2008).

Despite intensive efforts to understand the etiology of MIH, it has not been fully elucidated. A multifactorial pathogenesis with a possible genetic component has been hypothesized (Alaluusua, 2010, Crombie et al., 2009; Elhennawy & Schwendicke, 2016; Silva, Scurrah, Craig, Manton, & Kilpatrick, 2016). The management of MIH is considered challenging for the patients, the caregivers and the dentist. Different treatment modalities are available, ranging from prevention of PEB and dental caries, management of hypersensitivity and pain, through restorative treatment or extraction with or without subsequent orthodontic alignment of adjacent teeth (Elhennawy and Schwendicke, 2016, Lygidakis, 2010, Weerheijm, 2003).

To understand the pathogenesis of MIH, but also to derive appropriate management strategies, knowledge of the alterations of MIH-affected compared with unaffected enamel is needed. A number of authors have reported on structural, mechanical and chemical properties of MIH-affected enamel, however, these studies have, at first glance, reported ambiguous findings and applied a vast range of different analytical methods. By appraising the available evidence systematically and comparing study methods and findings, common and conflicting aspects might be highlighted, and possible reasons for the heterogeneity between studies might be better understood. This, eventually, should contribute to (a) a better understanding of the changes in MIH, with the discussed relevance for both basic and applied dental sciences, and (b) lead to recommendations of what studies and how future investigations in the field should be conducted, i.e. identify knowledge gaps or derive a set of methods which seem valid and reproducible for application here. The aim was to systematically collect, evaluate and contrast available studies on the structural, physical and chemical alterations of MIH-affected compared with unaffected enamel.