Editing Dentin (section)

=== Microstructure and crack propagation ===
During the dentinogenesis process, the odontoblast cells retreat from the DEJ to the outer lining of the pulp, leaving behind microtubules filled with cytoplasmic extensions and depositing intertubular dentin (ITD) in its place.<ref>{{Cite book |editor-last=Nanci |editor-first=Antonio |title=Ten Cate's oral histology: development, structure, and function. |date=2013 |publisher=Elsevier |isbn=978-0-323-07846-7 |edition=8th |location=St. Louis, MO |oclc=769803484}}</ref> ITD comprises the bulk of the dentin and, similarly to [[bone]], is a matrix composite of tablet-shaped [[hydroxyapatite]] [[nanoparticle]]s wrapped around collagen fibers. The mineralized [[collagen]] fibers are arranged in layers oriented perpendicular to the direction of the dentin microtubules<ref>{{Cite journal |last1=Kawasaki |first1=K |last2=Tanaka |first2=S |last3=Ishikawa |first3=T |date=1977 |title=On the incremental lines in human dentine as revealed by tetracycline labeling. |journal=Journal of Anatomy |volume=123 |issue=2 |pages=427–436 |pmid=858696 |pmc=1234542 }}</ref><ref name=":2">{{Cite journal |last1=Forien |first1=Jean-Baptiste |last2=Fleck |first2=Claudia |last3=Cloetens |first3=Peter |last4=Duda |first4=Georg |last5=Fratzl |first5=Peter |last6=Zolotoyabko |first6=Emil |last7=Zaslansky |first7=Paul |date=2015-06-10 |title=Compressive Residual Strains in Mineral Nanoparticles as a Possible Origin of Enhanced Crack Resistance in Human Tooth Dentin |url=https://pubs.acs.org/doi/10.1021/acs.nanolett.5b00143 |journal=Nano Letters |language=en |volume=15 |issue=6 |pages=3729–3734 |doi=10.1021/acs.nanolett.5b00143 |pmid=26009930 |bibcode=2015NanoL..15.3729F |issn=1530-6984|url-access=subscription }}</ref> which are lined with peritubular dentin (PTD), a 1-2&nbsp;μm thick layer of hydroxyapatite tablets with no preferred orientation and lacks<!-- What lacks? - if it's the "layer", improve the syntax! --> any supporting collagen fibers.<ref>{{Cite journal |last1=Gotliv |first1=Bat-Ami |last2=Veis |first2=Arthur |date=2007-09-01 |title=Peritubular Dentin, a Vertebrate Apatitic Mineralized Tissue without Collagen: Role of a Phospholipid-Proteolipid Complex |journal=Calcified Tissue International |language=en |volume=81 |issue=3 |pages=191–205 |doi=10.1007/s00223-007-9053-x |pmid=17674072 |s2cid=22634252 |issn=1432-0827}}</ref>

The hydroxyapatite tablets within the ITD were found to be compressed along the crystallographic c-axis due to tight interaction between the tablets and the collagen fiber. Tablets aligned parallel with the collagen fibers experience a significant increase in [[Stress (mechanics)|compressive stress]] of around 90 MPa and, for crack formation to occur, tensile stresses must first overcome this residual compressive stress. Since typical mastication stresses do not exceed 40 MPa,<ref>{{Cite journal |last=Anderson |first=D.J. |date=1956 |title=Measurement of Stress in Mastication. I |url=https://journals.sagepub.com/doi/10.1177/00220345560350050201 |journal=Journal of Dental Research |volume=35 |issue=5|pages=664–670 |doi=10.1177/00220345560350050201 |pmid=13367282 |s2cid=8312047 |url-access=subscription }}</ref> the ITD prevents cracks from forming during normal daily use and helps deflect cracks perpendicular to the dentin tubule and away from the pulp.<ref name=":2" /><ref>{{Cite journal |last1=Seknazi |first1=Eva |last2=Pokroy |first2=Boaz |date=October 2018 |title=Residual Strain and Stress in Biocrystals |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.201707263 |journal=Advanced Materials |language=en |volume=30 |issue=41 |pages=1707263 |doi=10.1002/adma.201707263|pmid=29766594 |arxiv=1902.08957 |bibcode=2018AdM....3007263S |s2cid=21719682 }}</ref>

Inelastic deformation of dentin primarily happens through microcracking. [[Fracture mechanics|Crack propagation]] within dentin travels preferentially along the interfaces of the ITD layers. Since the PTD, the hydroxyapatite tablets are not preferentially orientated; they are under less compressive residual stress, causing the microtubules to act as crack initiation sites. This manifests as cross-hatched shear microcracks forming at the microtubules in compression and as ring-shaped microcracks in tension. The tip of a larger crack creates a stress concentration that helps initiate microcracks around the microtubules ahead of it, consuming energy and resisting further damage. The imperfect linking of the microcrack to a larger crack also induces 'uncracked ligaments', which help arrest the larger crack.<ref>{{Cite journal |last1=Eltit |first1=Felipe |last2=Ebacher |first2=Vincent |last3=Wang |first3=Rizhi |date=2013-08-01 |title=Inelastic deformation and microcracking process in human dentin |url=https://www.sciencedirect.com/science/article/pii/S1047847713000889 |journal=Journal of Structural Biology |series=Special Issue in Recognition of Dr. Steve Weiner's Scientific Accomplishments |language=en |volume=183 |issue=2 |pages=141–148 |doi=10.1016/j.jsb.2013.04.002 |pmid=23583703 |hdl=2429/59407 |issn=1047-8477|hdl-access=free }}</ref> In comparison, enamel does not display the same fracture resistance, and fractures traveling across the DEJ are usually stopped within ~10 &nbsp;μm.<ref>{{Cite journal |last1=Imbeni |first1=V. |last2=Kruzic |first2=J. J. |last3=Marshall |first3=G. W. |last4=Marshall |first4=S. J. |last5=Ritchie |first5=R. O. |date=March 2005 |title=The dentin–enamel junction and the fracture of human teeth |url=https://www.nature.com/articles/nmat1323 |journal=Nature Materials |language=en |volume=4 |issue=3 |pages=229–232 |doi=10.1038/nmat1323 |pmid=15711554 |bibcode=2005NatMa...4..229I |s2cid=20947750 |issn=1476-4660|url-access=subscription }}</ref> The combination of the residual stress and the perpendicular orientation of the ITD mineralized collagen fibers significantly increases the [[fracture toughness]] and [[Fatigue (material)|fatigue]] endurance limit along the microtubule direction.<ref name=":2" />