Fig 7 The above molar was extracted due to periodontal sequelae. Upon trans-illumination, multiple crack extensions are observed, yet they are contained only within the enamel layer. Since the process is gradual, the cracks will be continuously replenished of protein rich oral fluids, thereby utilizing a self- healing mechanism.
MICROSTRUCTURE: ENAMEL VS. DENTIN
The microstructure of enamel is dominated by hydroxyapatite crystal-rich enamel rods, cemented together by an organic matrix protein polymer. Enamel being brittle yet stiff, undergoes only minimal deformation while transferring loads to the underlying dentin. The key to the unusual properties of enamel lies in its unique three dimensional structural arrangement, which consists of very long rods of carbonated apatite arranged in directional bundles. These bundles are progressively interwoven at higher hierarchical levels. The rise in crack growth resistance is largely attributed to a combination of mechanisms that included crack bridging, crack bifurcation and crack curving, which are induced by prism decussation of the inner enamel. 24 The microstructure of coronal dentin appears to be that of a mineralized collagen fiber bio-composite, the intertubular dentin being the matrix and the dentin tubule lumens with their associated cuffs of peritubular dentin forming the cylindrical fiber reinforcement. Dentin posses both elastic and plastic material properties that vary significantly from region to region and along different orientations within very small distances. Uncracked-ligament bridging presents as
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