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