Editing Cartilage (section)

=== Tendon-bone interface ===
In addition to its role in load-bearing joints, cartilage serves a crucial function as a gradient material between softer tissues and bone. Mechanical gradients are crucial for your body’s function, and for complex artificial structures including joint implants. Interfaces with mismatched material properties lead to areas of high [[stress concentration]] which, over the millions of loading cycles experienced by human joins over a lifetime, would eventually lead to failure. For example, the elastic modulus of human bone is roughly 20 GPa while the softer regions of cartilage can be about 0.5 to 0.9 MPa. <ref>{{cite journal |last1=Handorf |first1=Andrew |title=Tissue Stiffness Dictates Development, Homeostasis, and Disease Progression |journal=Organogensis |date=27 April 2015 |volume=11 |issue=1 |pages=1–15 |doi=10.1080/15476278.2015.1019687 |pmid=25915734 |pmc=4594591 }}</ref><ref>{{cite book |last1=Mansour |first1=Joseph |title=Biomechanics of Cartilage |publisher=MDPI |pages=66–79 |url=https://web.mit.edu/cortiz/www/3.052/3.052CourseReader/27_BiomechanicsofCartilage.pdf}}</ref> When there is a smooth gradient of materials properties, however, stresses are distributed evenly across the interface, which puts less wear on each individual part. 

The body solves this problem with stiffer, higher modulus layers near bone, with high concentrations of mineral deposits such as hydroxyapatite. Collagen fibers (which provide mechanical stiffness in cartilage) in this region are anchored directly to bones, reducing the possible deformation. Moving closer to soft tissue into the region known as the tidemark, the density of [[chondrocytes]] increases and collagen fibers are rearranged to optimize for stress dissipation and low friction. The outermost layer near the articular surface is known as the superficial zone, which primarily serves as a lubrication region. Here cartilage is characterized by a dense extracellular matrix and is rich in proteoglycans (which dispel and reabsorb water to soften impacts) and thin collagen oriented parallel to the joint surface which have excellent shear resistant properties. <ref>{{cite journal |last1=Chen |first1=Li |title=Preparation and Characterization of Biomimetic Functional Scaffold with Gradient Structure for Osteochondral Defect Repair |journal=Bioengineering |date=6 February 2023 |volume=10 |issue=2 |page=213 |doi=10.3390/bioengineering10020213 |doi-access=free |pmid=36829707 |pmc=9952804 }}</ref> 

Osteoarthritis and natural aging both have negative effects on cartilage as a whole as well as the proper function of the materials gradient within. The earliest changes are often in the superficial zone, the softest and most lubricating part of the tissue. Degradation of this layer can put additional stresses on deeper layers which are not designed to support the same deformations. Another common effect of aging is increased crosslinking of collagen fibers. This leads to stiffer cartilage as a whole, which again can lead to early failure as stiffer tissue is more susceptible to fatigue based failure. Aging in calcified regions also generally leads to a larger number of mineral deposits, which has a similarly undesired stiffening effect. <ref>{{cite journal |last1=Lotz |first1=Martin |title=Effects of aging on articular cartilage homeostasis |journal=Bone |date=28 March 2012 |volume=51 |issue=2 |pages=241–248 |doi=10.1016/j.bone.2012.03.023 |pmid=22487298 |pmc=3372644 }}</ref> Osteoarthritis has more extreme effects and can entirely wear down cartilage, causing direct bone-to-bone contact.<ref>{{cite web |title=Osteoarthritis |url=https://www.mayoclinic.org/diseases-conditions/osteoarthritis/symptoms-causes/syc-20351925 |website=Mayo Clinic |access-date=13 May 2024}}</ref>