Articular Cartilage

Structure

Hyaline Cartilage

Ends of long bones (1-5 mm thick)

Avascular

Aneural

Function Synovial Joints

Distribute loads

Allow for movement

CompositionCellular – chondrocytes (10% of volume)

CompositionExtracellular Matrix Organic – collagen (type II) (10-30% of

weight) & proteoglycans (3-10% of weight)

Water (most abundant component), inorganic salts, glycoproteins, lipids (60 - 87%)

Composition

Collagen fibers offer little resistance to compressive forces

Highly organized stiffness and tensile strength

Composition

Isotropic – material properties of substance are same regardless of loading

Hyaline cartilage is anisotropic: Collagen arrangement Cross link density Collagen/PG interaction

Composition

Composition

Fluid Component Permits diffusion of gases,

nutrients, wastes SYNOVIAL FLUID

Important to the structural organization of collagen load bearing /mechanical behavior (80% surface / 65% deep)

Collagen-PG Interaction Plays direct role in organization of

extracellular matrix

Important to mechanical properties resists compression

AC under Compression

AC under Compression

AC under Compression

AC under Compression constant load rapid initial

deformation slow (time dependent) deformation equilibrium

20 to exudation of interstitial fluid

AC under tension

AC under tension

AC under tension Toe region – alignment of collagen

fibers

Linear region – stretching of collagen fibers

AC under tension

Osteoarthritis Collagen cross link alteration fibrillation OA deterioration of tensile properties

of collagen-PG solid matrix

Loosening of collagen network increased swelling

Synovial Fluid Lubrication

Reduce Friction

Nutrition

Synovial Fluid Plasma-like

High in hyaluronate lubrication to reduce friction

Lubricin – has an affinity for AC - cartilage lubrication

Synovial Fluid Hyaluronate (HA) – responsible for

viscosity of synovial fluid

Resistance to shear forces

Lubrication of Articular Cartilage

Boundary Lubrication Fluid-Film Lubrication

Hydrodynamic (non-// surfaces) Squeeze-film ( surfaces)

Mixed Lubrication Boundary - Fluid-film Boosted

Type of LubricationBoundary – single layer of lubricant molecules on

each bearing surface (lubricin has affinity for AC)

Type of Lubrication

Fluid Film thin fluid film provides greater surface separation rigid bearings (stainless steel)

Fluid Film Lubrication Hydrodynamic – a wedge of fluid is

formed when non-parallel surfaces slide over each other

Fluid-Film LubricationSqueeze film – pressure is created in the

fluid film by bearing surfaces that are

Mixed Lubrication

Mixed Lubrication

Boosted ultrafiltration of synovial fluid thru

collagen-PG matrix H2O & electrolytesarticular

cartilage (squeeze-film) concentrated gel of HA protein

complex coats surfaces (boundary)

Type of Lubrication

Boundary high loads low relative speeds long durationFluid-film low/oscillated magnitude high relative speeds

Wear of ACInterfacial Wear Fatigue Wear High Impact

Loading Wear

Bearing surfaces in direct

contact - no film separating

Microscopic damage 20

repetitive loading

High load w/ insufficient

time for fluid redistribution

Strain rate - microtrauma exceed reparative process

Disruption of collagen-PG matrix, PG “wash-out”, alteration of load reducing mechanism

DISRUPTION OF CARTILAGE MICROSTRUCTURE

Adhesion

Abrasion Stress

Strain

results from results fromresults from

Cartilage Degeneration

Magnitude & distribution of imposed stress

# of sustained stress peaks

Degenerative changes to matrix

Changes in tissue’s mechanical properties

stress conc.Swelling & weak tissue destruction by normal forces

Insult to molecular structure of collagen-PG matrix

in load frequency & magnitude

Loosening of collagen network, abnormal PG expansion, tissue swelling, cartilage stiffness, permeability Abnormal: Stresses & strains

Mechanoelectrochemical stimuli

ECM remodeling by chondrocytes

OA

Wear of AC Interfacial

- adhesive- abrasive

Fatigue- disruption of collagen-PG solid matrix due to repetitive stress

Cartilage Degeneration Magnitude of imposed stresses Total number of sustained stress peaks Changes in collagen-PG matrix Changes in mechanical properties of

tissue

Cartilage Degeneration Loosening of collagen network PG expansion Tissue swelling Decrease in stiffness and increase in

permeability Altered cartilage function