Ribbon-like hypomineralization in human dental enamel

Human dental enamel is a uniquely hard and tough biological realization of the inherently brittle apatite crystal. In sound enamel both structure and chemistry are organically fine-tuned, while developmental disorders - such as hypomineralization - often lead to severe enamel alterations resulting in weaker mechanical performance and shorter lifetime as in case of molar-incisor hypomineralization (MIH) which has a global prevalence of 13.5 %. However, causes of the development of hypomineralization disorder remain unclear up to date. In the present paper, a new form of dental hypomineralization is reported for the first time. Structural and chemical alterations of dental enamel exhibiting a special ribbon-like hypomineralization (RLH) are compared to the characteristics of sound and MIH-affected teeth. Microporosity, mineral density and characteristic 3D macroscopic shape of the RLH affected volumes were captured by micro-CT analysis. On the submicron-scale, nanoporosity, unusual morphologies and reduced size of apatite nanocrystals have been revealed. These RLH specific features are coupled with a particular chemical fingerprint in Raman spectroscopy, which allows its clear separation from MIH affected and sound enamel. Based on these nanostructural features we conclude different failure mechanisms in the biochemical control during crystallization of the RLH and MIH enamel. STATEMENT OF SIGNIFICANCE: Molar-incisor hypomineralization (MIH) is a developmental enamel disorder with up to 14 % prevalence, characterized by reduced mineral content, hypersensitivity, and post-eruptive enamel breakdown. The causes of the development of this disorder remain unclear up to date. In this study, we examine molars with hypomineralization in a ribbon-like macroscopic morphology. These molars show increased porosity, organic content, and reduced mineral content, while carbonate levels remain unchanged in both affected and unaffected areas, unlike MIH-affected enamel. Ribbon-like hypomineralized (RLH) enamel also features unusual apatite nanocrystal morphologies, including isometric crystals in the prism boundary zone and bean-shaped crystals with a soft central zone. Nanoscale analysis reveals RLH as a developmental anomaly distinct from MIH, offering novel insights into the biochemical controls of hypomineralization.