# High Resolution STEM Images of the Human Tooth Enamel Crystals
## 1 Introduction
### Dental Caries and Defects at the Nanoscale  
Dental caries and periodontal diseases are the most common bacterial infections in humans [12]. As observed in Figure 2, at the nanoscale, during carious processes or other acidic attack, human tooth enamel crystals were destroyed systematically: first a central lesion elongated along the [11-20] direction appeared on the (0001) basal planes of the crystals; secondly, this lesion developed anisotropically along the [0001] direction across the crystals; and third, the crystals were break open [7]. No full explanation for the anisotropic crystal dissolution has yet been put forward. Neither the structural reason for the anisotropic dissolution nor the implications for the mineralization/demineralization process have yet been fully understood.  
Nevertheless, several common-sense observations have been made (Figure 2): (1) there was a structural feature on the center of the crystal basal faces different from the adjoining surface; (2) after dissolution, the lesion was hexagonal in shape and did not split the crystal along the [11-20] direction; (3) the dissolution process propagated anisotropically along the [0001] direction, which indicated that the structural feature was 3D.  
Several approaches are followed to conduct the ongoing research on that subject such as the analysis of etch pits at the HAP surface [13,14] and surface energy calculations [15]. Defects in tooth enamel crystals aligned along the [11-20] direction have been analyzed by HRTEM. [PERSON] et al. [6] identified some types of disorders that may act as dissolution sites. Recent work by [PERSON] et al. [16] and by [PERSON] et al. [3] have shown a high concentration of Na\({}^{+}\) and Mg\({}^{2+}\) ions at the center of enamel crystals. These findings have been specified by [PERSON] et al. [2], who have shown the existence of two nanometric layers enriched with Mg\({}^{2+}\) flanking a core rich in Na\({}^{+}\), F-, and CO\({}_{3}\)\({}^{2-}\) ions. This sandwich core being  
Figure 5: (**a**) Three-dimensional (3D) representation of the hexagonal HAP unit cell indicating the position of the CDL plane (the [10-10] plane). (**b**) View along the [0001] axis. The rectangle indicates the position of the CDL. (**c**) View along the [1-011] axis. Ca atoms are in blue, P atoms are in green, O atoms are in red, and H atoms are in yellow.  
surrounded by a shell with lower concentrations of substitutional defects. The residual stress created by the chemical gradients may favor the anisotropic cautious dissolution.  
All these works are still in progress. It must be noted that no explanation has yet been given on the elongated shape of the lesion along the [11-20] direction and for the fact that it does not split open the crystals as would a grain boundary. Another approach to understanding the structural mechanisms of the dissolution anisotropy is the analysis of a contrast phenomenon observed at the very dissolution sites such as the CDL.  
In this paper, we present the analysis of the aberration-corrected high-resolution HAADF-STEM images of the CDL of human tooth enamel crystals along the [0001] (0\({}^{\circ}\)/a), [10-11] (54\({}^{\circ}\)/a), and [1-210] (90\({}^{\circ}\)/a) directions, which will allow obtaining more detailed information on the structural characteristics of the CDL at the nanometric level.