applied fracture mechanics

114Applied Fracture Mechanicsapatite crystallites. During enamel formation, the ameloblasts move from the dentin-enameljunction (DEJ) to the surface of the final enamel crown. When the enamel shell has reachedits final shape, the ameloblasts degenerate and die, explaining why mature enamel is a nonvitaltissue made up by ~85 vol-% hydroxyapatite and 15 vol-% proteins and water [2].Figure 5. Cross section of enamel rods shows the key-hole structutre (blue). The longitudinal orientationof the hydroxyapatite crystallites can be explained by considering how Tomes’ process controls thecrystallite orientation (Figure 4). Figure redrawn after [3].Figure 6. The keyhole shaped rods become more and more interwoven as the rods approaches the DEJ.Redrawn from [4]. The intervowen structure shown in the drawing is also characteristic for the cusptips, where that type of enamel is called “gnarled enamel”.