Figure 1.

The hierarchical structure of bovine enamel. (a) Enamel on the smallest size scale consists of hydroxyapatite (HAP) nano-fibres, which are glued together by proteins (first hierarchical level). These nano-fibres are bundled together to form micro-fibres (a), building the so-called enamel rods. Hence, the first hierarchical level can also be called the ‘intra-rod structure’. The enamel rods are arranged straight and parallel in the enamel layer close to the tooth surface (b,d; second level of hierarchy; multiple rods) and form periodically interwoven decussation patterns in the enamel layer close to the inner dentin core of the tooth (c,d; third hierarchical level; decussated rods). Groups of rods of same orientation within the decussation structure form the so-called Hunter–Schreger bands. (e) Dental enamel is mineralized up to 80–90 vol% and covers the tooth crown.