the cartilage biomechanics
Articular cartilage is a tissue composed of collagen fibers absorbed in a matrix of proteoglycans ; it is not innervated and it has no blood vessels. The only cellular elements are the chondrocytes, specialized to live in a environment continuosly stimulated.
Chondrocytes are long - lived and , in normal conditions ,stop to multiply at the end of growth , keeping their numbers unchanged for the rest of life. The articular cartilage has little intrinsic ability to repair and every defect of the tissue tends not to heal deteriorating over time in arthritis. Exercise can delay this process because chondrocytes are able to respond positively mechanical stimuli even if the response of cartilage cells will be different depending on the intensity of mechanical stimuli administered and on its constitution. Articular cartilage is mainly composed by water (68 - 85 %), collagen (10 - 20 %) and proteoglycans (5 -10%) that act together ; In particular the mechanical behavior depends on the amount of collagen and on the arrangment in the thickness of the cartilage itself. |
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Microscopically the cartilage can be divided in 4 depending on the collagen organization and on the proteoglycans density: the first area (superficial which is equivalent to 10 - 20 % of thickness ) has the higest content of collagen (85%) and fibers are parallel oriented to the surface; the fibrillar system assumes overall a multiple arches morphology and the collagen fibers tightly intersect each other to form a three - dimensional network in the interstices of which the liquid component is "retained".
When, because of compressive forces , the water contained in the fibrillar system is squeezed out and generates drag forces between the liquid component and the solid matrix that grow with compression increasing and make the exudation more difficult. So the cartilage becomes stiff when load increases.
The compaction degree of the proteoglycans affects fluids motion during compression and so on the permeability as a resistance of the flow through the cartilage matrix . In particular, when the cartilage is under compression there is a viscous friction due to the resistance that the interstitial fluid meets in its movement. Under increasing load, the fluid flow decreases and with it the permeability accompanying the compression that is the mechanical stress which acts on articular cartilages of the knee during deambulation that is the main motion action for moving.
When, because of compressive forces , the water contained in the fibrillar system is squeezed out and generates drag forces between the liquid component and the solid matrix that grow with compression increasing and make the exudation more difficult. So the cartilage becomes stiff when load increases.
The compaction degree of the proteoglycans affects fluids motion during compression and so on the permeability as a resistance of the flow through the cartilage matrix . In particular, when the cartilage is under compression there is a viscous friction due to the resistance that the interstitial fluid meets in its movement. Under increasing load, the fluid flow decreases and with it the permeability accompanying the compression that is the mechanical stress which acts on articular cartilages of the knee during deambulation that is the main motion action for moving.
The alternate compression
MK Barker and BB Seedhom (2000) , have analyzed the cartilage behavior in laboratory subjecting samples of it to alternate compression as occurs during deambulation. From their results it appears initially that, when the cartilage surface is solicited with alternate loads, the water exuded during the load phase is not completely reabsorbed during the recovery phase, it means that the cartilage does not completely compensate the deformation received. Continuing the number of phases load/recovery, instead, we have a reduction of liquid quantity loss : during compression the cartilage releases a water quantity equal to how much it absorb during unloading. |
So, we have an instantaneous deformation of the solid matrix during loading followed by an instantaneous and equal recovery of form during the unloading phase. During this behavior , defined "elastic", a minimal deformation is evident which is determined by the exudation and consequent absorption of small and equal volumes of liquids which depend on viscous friction.
So, the "visco - elasticity" determines a "damper" effects made according to the vertical loads alternation acting during deambulation.
So, the "visco - elasticity" determines a "damper" effects made according to the vertical loads alternation acting during deambulation.
The anisotropy Cartilage is a tissue with anisotropic properties that mainly resists to compression exercited by vertical forces as studied by Jurvelin JS, Buschmann MD , Hunziker EB in the work " Mechanical anisotropy of articular cartilage of the humans knee in compression" 2003. Recalling that anisotropy is the mechanical property for which a material has characteristics that depend on the direction along which forces act , with specific reference to the knee ,the Authors state that "anisotropy during compression (exercited in the orthogonal direction to the point of articular contact) may be essential for the cartilage function... this property must be considered development of advanced theoretical models for the cartilage biomechanics." |
It seems also relevant to report that MK Barker and BB Seedhom (2000), on the environment in which cartilages work, stated that : "the synovial fluid, especially under condition of static load , allows much better performances".
However, the performances of articular cartilages depend also from the type of synovial liquid there is in the articulation which , composed by blood plasma, hyaluronate and glycoproteins, acts as a lubricant and provides nourishment to the cartilages.
The adaptation to vertical loads
The more relevant concept to the development of a rational rehabilitational protocol , is based on MK Barker and BB Seedhom's sperimental concept (2000) for which "all the constituent components of matrix of cartilage fit as a function of compression deforming " in particulary to those forces that are exerted perpendicular to the surface plane.
In rehabilitational protocol , on the adaptability concet , the cartilage is associated to any biological function : in fact ,if cartilage tissue cannot regenerate ,the old one , remained intact if properly stimulated (respecting anisotropy concept ) and following principles that regulate the organic adaptation , can readjust to the new situation by increasing its efficiency and functionality.
However, the performances of articular cartilages depend also from the type of synovial liquid there is in the articulation which , composed by blood plasma, hyaluronate and glycoproteins, acts as a lubricant and provides nourishment to the cartilages.
The adaptation to vertical loads
The more relevant concept to the development of a rational rehabilitational protocol , is based on MK Barker and BB Seedhom's sperimental concept (2000) for which "all the constituent components of matrix of cartilage fit as a function of compression deforming " in particulary to those forces that are exerted perpendicular to the surface plane.
In rehabilitational protocol , on the adaptability concet , the cartilage is associated to any biological function : in fact ,if cartilage tissue cannot regenerate ,the old one , remained intact if properly stimulated (respecting anisotropy concept ) and following principles that regulate the organic adaptation , can readjust to the new situation by increasing its efficiency and functionality.