articular cartilage basic sciences. articular cartilage: location articular cartilage covers the...
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Articular Cartilage
Basic Sciences
Articular Cartilage: Location
Articular cartilage covers the joint surfaces:•Bottom of the femur•Top of the tibia•Back-side of the patella
Articular Cartilage: Primary Functions
• Transmits applied loads across mobile surfaces
• Lines the ends of bones • surfaces roll or slide during
motion– Hyaline cartilage is fluid-filled
wear-resistant surface– It reduces friction coefficient to
0.0025.
Articular Cartilage: Anatomy
1. Quadriceps femoris2. Patellar tendon3. Suprapatellar bursa4. Patella5. Joint cavity6. Infrapatellar fat pad7. Tibia8. Meniscus9. Articular cartilage10. Femur
Photo of knee joint cross section. The white tissue is the articular cartilage covering the distal femur (top) and proximal tibia (bottom) bone surfaces.
Articular Cartilage: Histology
By volume: 1% chondrocytes and 99% matrix•Chondrocytes produce collagen, proteoglycans, etc. as needed•Release enzymes to break down aging components.
Articular Cartilage: Chondrocytes Chondrocytes vary in size, morphology and arrangement
Changes with depth at tissue
Articular Cartilage: Chondrocytes
Articular Cartilage: Chondrocytes
Cells from surface to deeper levels:•Flatter Rounder•Scattered Organized into columns•Below tidemark, cells are encrusted with apatite salts (bone)
Articular Cartilage: Composition
Note that articular (hyaline) cartilage has the highest proportion of water and also the highest proteoglycan content
Articular Cartilage: Composition
Components are arranged in a way that is maximally adapted for biomechanical functions
•Chondrocytes (~ 1%)•Collagen (15%) (Type II in articular cartilage)•Proteoglycans (15%)•Water (70 %)
Collagen (15%)
•Majority is Type II collagen•Provides cartilage with its tensile strengthLook at Ligament & Tendon notes for structure of collagen fibers
Creates a framework that houses the other components of cartilage
Proteoglycans (15%) Each subunit consists of a combination of protein and sugar: Long protein chainSugars units attached densely in parallel
Proteoglycans (15%) Subunits are attached at right angles to a long filament Produce a macromolecules: the proteoglycan aggregate
Proteoglycans (15%)
Proteoglycans (15%) Each sugar has one or two negative charges, so collectively there is an enormous repulsive force within each subunit and between neighboring subunits
This causes the molecule to extend stiffly out in space
This property gives articular cartilage its resiliency to compression
The negative charges make the molecules extremely hydrophilic and cause water to be trapped within It is used during biomechanical or lubricant activity.
Water functions as the "shock absorber" in cartilage, lubricates and nourishes the cartilage.
Mechanical Behavior
Lateral Condyle
Patellar Groove
COMPRESSIVE AGGREGATE MODULUS (MPa)
0.70 0.53
POISSON’S RATIO 0.10 0.00PERMEABILITY COEFFICIENT (10-15m4/N•s)
1.18 2.17
Tensile Force
Toe region: collagen fibrils straighten out and un- “crimp” Linear region that parallels the tensile strength of collagen fibrils: collagen aligns with axis of tension Failure region
Tensile Force
A.) Random alignment of collagen fibrils
B.) Histogram of measurements made from micrograph show distribution of fibril alignment
Shear Force
Creep behavior
Creep of a visco-elastic material under constant loading
Stress rise during a ramp displacement compression of a visco-elastic material and stress relaxation under constant compression
Compressive force•Rate of creep is determined by the rate at which fluid may be forced out of the tissue
•This, in turn, is governed by the permeability and stiffness of the porous-permeable collagen-proteoglycan solid matrix
Permeability•Articular cartilage shows nonlinear strain dependence and pressure dependence
•The decrease of permeability with compression acts to retard rapid loss of interstitial fluid during high joint loadings
Permeability decreases in an exponential manner as function of both increasing applied compressive strains and increasing applied pressure
Permeability
Copious exudation of fluid at start but the rate of exudation decreases over time from points A to B to C
Compressive force
At equilibrium, fluid flow ceases and the load is borne entirely by the solid matrix (point C)
Compressive force
Creep behavior of collagen-proteoglycan matrix is analagous to an elastic spring
Compressive force
Creep behavior
Creep of a visco-elastic material under constant loading
Stress rise during a ramp displacement compression of a visco-elastic material and stress relaxation under constant compression
Stress Relaxation
The sample is compressed to point B and then maintained over time (points B to E)
Stress Relaxation
Increase stress to reach end of compressive phase
Stress Relaxation
Fluid redistribution allows for relaxation phase (points B to D) and matrix deformation
Stress Relaxation
Equilibrium is reached at point E