the rod directions are lower values, shown to have an elastic modulus between 70 and 77 GPa and a hardness value of 3.0-3.5 GPa. The anisotropy between the two directions can be as high as 30%. This is due partially to the structure of the material and the directionality of the rods in the c-direction. The structure of enamel is also composite in nature between the interrod and rods which also leads to anisotropy. Single crystallite hydroxyapatite, the mineral enamel is based on, is also anisotropic. Single crystallite hydroxyapatite has a higher hardness and young's modulus, which may be due to the defects present in enamel, such as substitutional ions as well as the presence of organic materials. The mechanical properties of enamel not only are anisotropic due to the structure of the rods and interrods. They are also varying across the length of enamel from the enamel at the surface of the tooth, the outer enamel, to the junction between the dentin and enamel, DEJ. The elastic modulus increases as the distance between the dentin-enamel junction (DEJ) increases within enamel. The fracture toughness is also anisotropic. The fracture toughness can vary by up to a factor of three due to the orientation of the rods. Also in enamel, Cracks do not easily penetrate the dentin, which may lead to the higher fracture toughness. Overall, enamel is a highly anisotropic material due to its microstructure, which leads to properties needed for the effective use of our teeth. See also Ameloblast Odontoblast Amorphous calcium phosphate Dental caries Ivory Tooth development References Bibliography ; (CD-ROM) Special Issue B. External links enamel Tissues (biology)