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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.

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Presentation on theme: "Articular Cartilage Basic Sciences. Articular Cartilage: Location Articular cartilage covers the joint surfaces: Bottom of the femur Top of the tibia."— Presentation transcript:

1 Articular Cartilage Basic Sciences

2 Articular Cartilage: Location Articular cartilage covers the joint surfaces: Bottom of the femur Top of the tibia Back-side of the patella

3 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

4 Articular Cartilage: Anatomy 1. Quadriceps femoris 2. Patellar tendon 3. Suprapatellar bursa 4. Patella 5. Joint cavity 6. Infrapatellar fat pad 7. Tibia 8. Meniscus 9. Articular cartilage 10. 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.

5 Articular Cartilage: Histology By volume: 1% chondrocytes and 99% matrix Chondrocytes produce collagen, proteoglycans, etc. as needed Release enzymes to break down aging components.

6 Articular Cartilage: Chondrocytes  Chondrocytes vary in size, morphology and arrangement  Changes with depth at tissue

7 Articular Cartilage: Chondrocytes

8 Cells from surface to deeper levels: Flatter  Rounder Scattered  Organized into columns Below tidemark, cells are encrusted with apatite salts (bone)

9 Articular Cartilage: Composition Note that articular (hyaline) cartilage has the highest proportion of water and also the highest proteoglycan content

10 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 %)

11 Collagen (15%) Majority is Type II collagen Provides cartilage with its tensile strength Look at Ligament & Tendon notes for structure of collagen fibers Creates a framework that houses the other components of cartilage

12 Proteoglycans (15%)  Each subunit consists of a combination of protein and sugar: Long protein chain  Sugars units attached densely in parallel

13 Proteoglycans (15%)  Subunits are attached at right angles to a long filament  Produce a macromolecules: the proteoglycan aggregate

14 Proteoglycans (15%)

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.

16 Mechanical Behavior Lateral Condyle Patellar Groove COMPRESSIVE AGGREGATE MODULUS (MPa) POISSON’S RATIO PERMEABILITY COEFFICIENT (10-15m4/Ns)

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

18 Tensile Force A.) Random alignment of collagen fibrils B.) Histogram of measurements made from micrograph show distribution of fibril alignment

19 Shear Force

20 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

21 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

22 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

23 Permeability decreases in an exponential manner as function of both increasing applied compressive strains and increasing applied pressure Permeability

24 Copious exudation of fluid at start but the rate of exudation decreases over time from points A to B to C Compressive force

25 At equilibrium, fluid flow ceases and the load is borne entirely by the solid matrix (point C) Compressive force

26 Creep behavior of collagen-proteoglycan matrix is analagous to an elastic spring Compressive force

27 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

28 Stress Relaxation The sample is compressed to point B and then maintained over time (points B to E)

29 Stress Relaxation Increase stress to reach end of compressive phase

30 Stress Relaxation Fluid redistribution allows for relaxation phase (points B to D) and matrix deformation

31 Stress Relaxation Equilibrium is reached at point E


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