Editing Cartilage (section)

=== Mechanical properties ===
The mechanical properties of articular cartilage in load-bearing joints such as the [[knee]] and [[hip]] have been studied extensively at macro, micro, and nano-scales. These mechanical properties include the response of cartilage in frictional, compressive, shear and tensile loading. Cartilage is resilient and displays [[Viscoelasticity|viscoelastic]] properties.<ref name="pmid5111002">{{cite journal | vauthors = Hayes WC, Mockros LF | title = Viscoelastic properties of human articular cartilage | journal = Journal of Applied Physiology | volume = 31 | issue = 4 | pages = 562–8 | date = October 1971 | pmid = 5111002 | doi = 10.1152/jappl.1971.31.4.562 | url = http://jap.physiology.org/content/31/4/562.full.pdf }}</ref>

Since cartilage has interstitial fluid that is free-moving, it makes the material difficult to test. One of the tests commonly used to overcome this obstacle is a confined compression test, which can be used in either a 'creep' or 'relaxation' mode.<ref name="mansour2013">{{cite book | last=Mansour | first=J. M. | date=2013 | title=Biomechanics of Cartilage | pages=69–83}}</ref><ref name="patel2019">{{cite journal |last1=Patel |first1=J. M. |last2=Wise |first2=B. C. |last3=Bonnevie |first3=E. D. |last4=Mauck |first4=R. L. |date=2019 |title=A Systematic Review and Guide to Mechanical Testing for Articular Cartilage Tissue Engineering |url=https://doi.org/10.1089/ten.tec.2019.0116 |journal=Tissue Eng Part C Methods |volume=25 |issue=10 |pages=593–608|doi=10.1089/ten.tec.2019.0116 |pmid=31288616 |pmc=6791482 }}</ref> In creep mode, the tissue displacement is measured as a function of time under a constant load, and in relaxation mode, the force is measured as a function of time under constant displacement. During this mode, the deformation of the tissue has two main regions. In the first region, the displacement is rapid due to the initial flow of fluid out of the cartilage, and in the second region, the displacement slows down to an eventual constant equilibrium value. Under the commonly used loading conditions, the equilibrium displacement can take hours to reach.

In both the creep mode and the relaxation mode of a confined compression test, a disc of cartilage is placed in an impervious, fluid-filled container and covered with a porous plate that restricts the flow of interstitial fluid to the vertical direction. This test can be used to measure the aggregate modulus of cartilage, which is typically in the range of 0.5 to 0.9 MPa for articular cartilage,<ref name="mansour2013"></ref><ref name="patel2019"></ref><ref name="korhonen2002">{{cite journal |last1=Korhonen |first1=R. K. |last2=Laasanen |first2=M. S. |last3=Töyräs |first3=J. |last4=Rieppo |first4=J. |last5=Hirvonen |first5=J. |last6=Helminen |first6=H. J. |last7=Jurvelin |first7=J. S. |date=2002 |title=Comparison of the Equilibrium Response of Articular Cartilage in Unconfined Compression, Confined Compression and Indentation |url=https://doi.org/10.1016/S0021-9290(02)00052-0 |journal=Journal of Biomechanics |volume=35 |issue=7 |pages=903–909|doi=10.1016/S0021-9290(02)00052-0 |pmid=12052392 }}</ref> and the Young’s Modulus, which is typically 0.45 to 0.80 MPa.<ref name="mansour2013"></ref><ref name="korhonen2002"></ref> The aggregate modulus is “a measure of the stiffness of the tissue at equilibrium when all fluid flow has ceased”,<ref name="mansour2013"></ref> and Young’s modulus is a measure of how much a material strains (changes length) under a given stress.

The confined compression test can also be used to measure permeability, which is defined as the resistance to fluid flow through a material. Higher permeability allows for fluid to flow out of a material’s matrix more rapidly, while lower permeability leads to an initial rapid fluid flow and a slow decrease to equilibrium. Typically, the permeability of articular cartilage is in the range of 10^-15 to 10^-16 m^4/Ns.<ref name="mansour2013"></ref><ref name="patel2019"></ref> However, permeability is sensitive to loading conditions and testing location. For example, permeability varies throughout articular cartilage and tends to be highest near the joint surface and lowest near the bone (or “deep zone”). Permeability also decreases under increased loading of the tissue.

Indentation testing is an additional type of test commonly used to characterize cartilage.<ref name="mansour2013"></ref><ref name="kabir2021">{{cite journal |last1=Kabir |first1=W. |last2=Di Bella |first2=C. |last3=Choong |first3=P. F. M. |last4=O’Connell |first4=C. D. |date=2021 |title=Assessment of Native Human Articular Cartilage: A Biomechanical Protocol |url=https://doi.org/10.1177/1947603520973240 |journal=Cartilage |volume=13 |issue=2 Suppl |pages=427S–437S|doi=10.1177/1947603520973240 |pmid=33218275 |pmc=8804788 }}</ref> Indentation testing involves using an indentor (usually <0.8 mm) to measure the displacement of the tissue under constant load. Similar to confined compression testing, it may take hours to reach equilibrium displacement. This method of testing can be used to measure the aggregate modulus, Poisson's ratio, and permeability of the tissue. Initially, there was a misconception that due to its predominantly water-based composition, cartilage had a Poisson's ratio of 0.5 and should be modeled as an incompressible material.<ref name="mansour2013"></ref> However, subsequent research has disproven this belief. The Poisson’s ratio of articular cartilage has been measured to be around 0.4 or lower in humans <ref name="mansour2013"></ref><ref name="kabir2021"></ref> and ranges from 0.46–0.5 in bovine subjects.<ref name="jin2004">{{cite journal |last1=Jin |first1=H. |last2=Lewis |first2=J. L. |date=2004 |title=Determination of Poisson's Ratio of Articular Cartilage by Indentation Using Different-Sized Indenters |url=https://doi.org/10.1115/1.1688772 |journal=Journal of Biomechanical Engineering |volume=126 |issue=2 |pages=138–145|doi=10.1115/1.1688772 |pmid=15179843 }}</ref> 

The mechanical properties of articular cartilage are largely anisotropic, test-dependent, and can be age-dependent.<ref name="mansour2013"></ref> These properties also depend on collagen-proteoglycan interactions and therefore can increase/decrease depending on the total content of water, collagen, glycoproteins, etc. For example, increased glucosaminoglycan content leads to an increase in compressive stiffness, and increased water content leads to a lower aggregate modulus.