Research paper
Nano-scale structure and mechanical properties of the human dentine–enamel junction

https://doi.org/10.1016/j.jmbbm.2010.09.003Get rights and content

Abstract

Despite being an interface between two mechanically mismatched phases of the soft dentine and hard enamel, the dentine–enamel junction (DEJ) in a human tooth is in general capable of withstanding a long working life of repeated dynamic loading. The current poor understanding of the structure and properties of the DEJ has presented a major obstacle to designing better therapeutic protocols for complications concerning the DEJ. In this investigation, it was discovered that the DEJ is a thin, but gradual interface with characteristics transiting from those of dentine to those of enamel. The collagen fibres in dentine enter into the enamel side of the DEJ and terminate in a region in which the hydroxyapatite crystals begin to show enamel characteristics. Using focused ion beam machining, micro-beams were fabricated from regions within 50 μm of the DEJ and were subjected to bend tests. In spite of the similarity in the flexural strength of the DEJ and enamel, fractographs revealed cracks in the DEJ that propagated along structures with dentine characteristics. To the best of our knowledge, this is the first report on the testing of the mechanical properties of the DEJ.

Introduction

The crown of the human tooth is composed of two calcified tissues, namely, enamel and dentine. Enamel, on the outer surface, is the hardest tissue in the human body, comprising ∼95 vol% of apatite crystals and ∼5 vol% of water and organic materials arranged in ∼5 μm keyhole-shaped structures known as prisms. It protects the underlying dentine and acts as the cutting and grinding surface during mastication. Dentine is made up of ∼50 vol% apatite crystals, 20 vol% water, and 30 vol% organic matrix. It is distributed throughout the crown and root and so forms the bulk of tooth and has the function of absorbing and distributing stresses within the tooth. The structural and compositional dissimilarities between these two mineralised tissues induce marked differences in their mechanical behaviour—enamel is much stiffer (Young’s modulus E80GPa) and harder (hardness H4GPa) than dentine (E20GPa, H1GPa) (Angker and Swain, 2006). Sharp interfaces between such dissimilar materials are usually subjected to concentrated stresses which often cause delamination. Yet, surprisingly, a tight and durable junction known as the dentine–enamel junction (DEJ) exists between the two calcified tissues in the crown of the tooth and persists throughout the millions of cycles of mastication forces during the working life of a tooth, with only rare cases of mechanical failures. The life-long success of the bond between enamel and dentine stands in stark contrast to the adhesion of dental restorative materials applied to the oral mineralized tissues of the tooth which dislodge relatively easily. These restorations typically fail either by gross structural failure which results in dislodgement of the applied materials, or by micro-leakage at the restoration-tissue interface, leading to bacterial invasion and subsequent secondary caries or pulpal death (White et al., 2005). An accurate understanding of the structure–function relationship governing the DEJ would have significant clinical relevance and may permit the creation of improved interfaces between restorations and the odontogenic mineralized tissues.

The DEJ has been described as a complex interface with at least three levels of microstructure: the 25–100 μm scallops with their convexities directed towards the dentine and concavities towards the enamel; the 2–5 μm micro-scallops housed within each scallop; and a finer nano-level structure within each micro-scallop (Marshall Jr. et al., 2001, Marshall et al., 2003). Although it has been proposed that the crack resistance of the DEJ may arise from gradual, rather than abrupt, changes in the structure and mechanical properties (White et al., 2000), optical microscopy and scanning electron microscopy (SEM) examinations have indicated a clear demarcation between enamel and dentine, such that the DEJ appears as a narrow, distinct interface (Hayashi, 1992, Lin et al., 1993). Despite the DEJ being renowned for its crack resistance, only a limited number of studies have investigated the fracture properties (Dong and Ruse, 2003, Fong et al., 2000, Habelitz et al., 2001, Imbeni et al., 2005, Marshall Jr. et al., 2001, Stock et al., 2008, White et al., 2000, Zaslansky et al., 2006, Zhang et al., 2007). None of the investigators reported the occurrence of a fracture along the DEJ due to their high fracture toughness, and this cast doubts on the accuracy of the measured values in representing the related fracture properties of the DEJ. The remarkable mechanical success of the DEJ has been attributed to structural characteristics such as the large surface area due to the scalloping structure, the collagen fibre reinforcement, and the intimate contact between the adjacent enamel and dentine crystals (Imbeni et al., 2005, Marshall et al., 2003). However, the contribution of these factors to the mechanical integrity of the DEJ has not been systematically studied, and interestingly, most of the known features of the DEJ were actually identified by microscopy and their roles in fracture resistance were inferred from the observed microstructure. Furthermore, as three different levels of microstructure are present in the DEJ, techniques such as traditional tensile testing would only be able to produce values which represent the collective effects from the three different levels. In this study, a micro-cantilever technique will be used to investigate the submicron-scale structures of the DEJ, so as to gain an insight into the structure–function relationship at this level of hierarchy. The microstructure and fracture properties of the DEJ will be compared to those of the enamel and dentine immediately next to the biological junction, so as to identify the possible toughening mechanisms at the nano-level isolated from the influence of the scallops and micro-scallops.

Section snippets

Materials and methods

Four extracted non-carious human permanent molars were used in the present experiments. Prior to processing, the teeth were stored in Hank’s balanced salt solution (HBSS) at 4 °C so as to prevent demineralization. The growth of micro-organisms in the medium was prevented by the addition of sodium azide. The teeth were sectioned, using an EXAKT 300CP diamond saw, into 1 mm thick slices which were then mounted into matching holes drilled in brass blocks to facilitate subsequent polishing (Tang

Functional width of the DEJ

The elastic modulus and hardness data from the nano-indentation tests conducted across the DEJ are shown in Fig. 2. Enamel was found to have an elastic modulus of ∼95 ± 15 GPa and a hardness of ∼7 ± 2 GPa, whereas dentine had an elastic modulus of ∼19 ± 2 GPa and hardness of ∼1 ± 0.1 GPa. A sharp change in mechanical properties was observed across the DEJ as revealed under the present experiment with a spatial resolution of ∼10 μm.

Microstructure of the DEJ

Fig. 3 shows a low-magnification SEM micrograph of the region

The width of the DEJ

Although there have been reports on mechanical testing across the DEJ using a variety of techniques such as micro-indentation, nano-indentation and AFM indentation (Fong et al., 2000, Marshall Jr. et al., 2001, White et al., 2005), the width of the region within which the transition in mechanical properties takes place remains controversial. An earlier micro-indentation investigation revealed a 100 μm wide region adjacent to the DEJ within which the mechanical properties gradually changed (

Conclusions

Based on the apatite crystal size and presence of collagen as seen in the TEM, a 0.5–1 μm thick layer with distinct difference in morphology from bulk enamel and dentine was identified as the DEJ. The flexural strength of this layer was slightly lower than the adjacent enamel, but much higher than dentine. Upon rehydration, the layer demonstrated a crack arresting capability similar to dentine, without significant increase in its flexural strength. At the nano-level distant from the influence

Acknowledgements

This investigation was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, PR China (Project No. HKU7156/08E), as well as a grant from University Grants Council (SEG HKU06). AHWN is grateful to support from the Croucher Foundation in the form of a Senior Research Fellowship.

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