applied fracture mechanics

Fracture of Dental Materials 113Figure 4. After the first layer of structureless enamel has formed on the mantel dentin, the ameloblastdifferentiate its end closest to the precipitated enamel into the so called Tomes’ process. This unit can bedescribed as a concave formation from which hydroxyapatite crystallites precipitate. The c-axis of thesecrystallites are perpendicular to the surface of Tomes’s process, explaining the the well organized precipitationof the hydroxyapatite crystallites in each enamel rod.The secretion from the peripheral site of the Tomes’ process results in the formation of whatis referred to as the enamel matrix wall. These walls enclose pits into which the Tomes’processes fit. These sites are then filled with matrix proteins acting as nucleating agents forthe hydroxylapatite crystallites. The crystallites that precipitate in these two matrices (thematrix wall and the central pit) have different orientation. It is important to emphasize thatthe final wall and pit enamel have the same composition. The only difference is the orientationof the crystallites in these two enamel types.A cross-section of the enamel rods reveals that the individual rods have a key-hole shapedstructure (Figure 5).As the ameloblasts move toward their final destiny, they produce enamel rods that aresomewhat wavy and interwoven (Figure 6). Independent on these waves, the enamel rodsform angles that are roughly perpendicular to the outer as well as inner surfaces of theenamel shell (Figure 7). The hard enamel can be described as a hard shield protecting theunderlying dental tissue of the visible part of the tooth.Enamel consists mainly of hydroxyapatite crystallites, which are oriented in very wellorganized larger bundles of crystallites. These larger bundles are referred to as enamel rods.Each enamel rod is made by enamel forming cells, the so called ameloblasts. The diametersof the rods range from 4-8 μm. During enamel formation, the ameloblasts secret differentproteins (amelogenins and enamelins), which act as nucleating agents for the hydroxyl