Variations of Organic and Inorganic Components across Human Dentin-enamel Junction Revealed by SEM-EDS

The dentin-enamel junction (DEJ) is a natural junction that unites two dissimilar mineralized tissues in the human tooth: enamel and dentin, which differ in their structures, compositions, and biomechanical properties. These two distinct and functionally different dental components work together for decades without delamination or catastrophic failures, largely due to the unique interface structure of the DEJ. Tooth enamel is the hardest tissue in the human body and is very brittle, which consists of 95–96% inorganic components, 1–2% organic components and 3% water by weight. Dentin, on the other hand, is softer and more flexible, containing 70% inorganic components, 20% organic components, and 10% water by weight [1]. It has been suggested that the DEJ is not a simple inert interface between two mineralized structures. Rather, it is a complex dynamic interface that plays an important role in maintaining the structural integrity of teeth [2]. The mechanical and functional properties of the DEJ play an essential role in the performance, function, and longevity of the whole tooth. Its excellent biomechanical properties have also drawn interest as a biomimetic model for connecting different materials [3]. Microstructurally, transmission electron microscopy (TEM) revealed that mineral crystals in dentin and enamel are closely interconnected with each other at the DEJ [2]. SEM revealed that organic collagen fibers from dentin penetrate perpendicularly into the enamel at the DEJ and provide reinforcement to the junction [4]. Several collagens have been identified in the DEJ of human teeth, including Types I, VII, and IV collagens [5, 6]. Mechanically, the DEJ is a smooth transition of mechanical properties such as hardness and elastic modulus between dentin and enamel [7, 8]. Functionally, the DEJ has been shown to arrest crack propagations and prevent catastrophic tooth failure [9]. The remarkable success of the DEJ has been attributed to its structural features, such as large interface area due to the scalloping and micro-scalloping structures, mechanical property gradient, collagen fiber reinforcement, and the intimate contact between adjacent enamel and dentine crystals [10]. In the current study, the morphology and elemental distribution of the DEJ interface of human third molars were investigated qualitatively and quantitatively using scanning electron microscopy with energy-dispersive x-ray spectroscopy (SEM-EDS). The scallop and micro-scallop structures of the DEJ interface were observed in the backscattered electron images (Fig. 1). For the first time, the width of DEJ interface was estimated from the variation of the elemental distribution across the DEJ. The SEM-EDS linescans revealed abrupt changes in the distributions of six elements (C, N, Ca, P, Na, and Mg) across the DEJ interface. From the enamel to the dentin side of the DEJ, the inorganic mineral elements Ca, P, Na, decreased while Mg increased by 21–25%, and the two organic elements C and N increased by approximately 150% and 270%, respectively. Based on the width of the slope of the nitrogen elemental distribution curves, the width of DEJ was estimated to be approximately 2 µm (Fig. 2). The knowledge of the variations of organic and inorganic components and morphological structure of the DEJ not only helps us better understand its unique properties, but also provides valuable information for biomimetic engineering of superior dental restorations and prosthetics. An in-depth understanding of the DEJ may shed light on the treatment of some dental diseases, such as radiation-induced enamel delamination, and inspire the design of highly functional interphase layer for robust and durable connections between different materials. (A) Picture of a cross-sectioned human tooth specimen; (B) & (C) Enamel and dentin structures near the DEJ observed using SEM gaseous secondary electron imaging at 120× and 1000× magnifications; (D) & (E) Scallop and micro-scallop structures of the DEJ interface observed using SEM backscattered electron imaging at 1000× and 8000× magnifications. (A) Comparisons of atomic percentages of elements C, N, O, Ca, P, Na and Mg in the dentin and enamel sides of the DEJ. (B) Representative EDS linescans across the DEJ in backscattered electron images. (C) Representative measurements of the width of DEJ based on the linescan curves of nitrogen [11].