Editing Tooth enamel (section)

== Mechanical properties ==
Due to the unique structure of enamel, the mechanical properties are very interesting. Enamel is the hardest material in the body and is one of the most durable load-bearing biological materials. It has been shown to have a [[fracture toughness]] three times greater than that of geological [[hydroxyapatite]], the main mineral in enamel.<ref>{{cite journal |last1=White |first1=S.N. |title=Biological organization of hydroxyapatite crystallites into a fibrous continuum toughens and controls anisotropy in human enamel |journal=J. Dent. Res. |date=2001 |volume=80 |issue=1 |pages=321–326 |doi=10.1177/00220345010800010501 |pmid=11269723 |s2cid=25482660 }}</ref>

Consisting of rod and interrod regions within the microstructure of enamel, the mechanical properties of enamel vary depending on the location within the microstructure.<ref name="Mechanical properties of human dent">{{cite journal |last1=Habelitz |first1=S |title=Mechanical properties of human dental enamel on the nanometre scale |journal=Archives of Oral Biology |date=2001 |volume=46 |issue=2 |pages=173–183 |doi=10.1016/S0003-9969(00)00089-3 |pmid=11163325 }}</ref> The rod and interrod structure causes [[anisotropy]] in enamel as both components have different mechanical properties. The interrod enamel has around 53% and 74% decreased [[hardness]] and [[elastic modulus]] when compared to the rod structures. This leads to a composite like hierarchical structure of enamel.<ref>{{cite journal |last1=Ge |first1=J |title=Property variations in the prism and the organic sheath within enamel by nanoindentation |journal=Biomaterials |date=2005 |volume=26 |issue=16 |pages=3333–3339 |doi=10.1016/j.biomaterials.2004.07.059 |pmid=15603829 }}</ref> The hardness and stiffness parallel to the rod axis results in high hardness and modulus, shown to have modulus values of 85–90 GPa and hardness values of 3.4-3.9 GPa.<ref name="Mechanical properties of human dent" /> The hardness and modulus in the direction perpendicular to 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.<ref name="Mechanical properties of human dent" /> The anisotropy between the two directions can be as high as 30%.<ref name="Mechanical properties of human dent" /> This is due partially to the structure of the material and the directionality of the rods in the c-direction.<ref name="Mechanical properties of human dent" /> The structure of enamel is also [[Composite material|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.<ref name="Mechanical properties of human dent" />

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.<ref>{{cite journal |last1=Park |first1=S |title=Mechanical properties of human enamel as a function of age and location in the tooth |journal=J Mater Sci: Mater Med |date=2008 |volume=19 |issue=6 |pages=2317–2324 |doi=10.1007/s10856-007-3340-y |pmid=18157510 |s2cid=25159868 }}</ref> 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.<ref>{{cite journal |last1=Xu |first1=H |title=Indentation Damage and Mechanical Properties of Human Enamel and Dentin |journal=Journal of Dental Research |date=1998 |volume=77 |issue=3 |pages=472–480 |doi=10.1177/00220345980770030601 |pmid=9496920 |s2cid=21928580 }}</ref> Overall, enamel is a highly anisotropic material due to its microstructure, which leads to properties needed for the effective use of our teeth.