Articular Cartilage

Structure Hyaline Cartilage Ends of long bones (1-5 mm thick) Avascular Aneural

Function Synovial Joints Distribute loads Allow for movement

Composition Cellular – chondrocytes (10% of volume)

Composition Extracellular 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 Lubrication Boundary – 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 Lubrication Squeeze 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 & electrolytesarticular cartilage (squeeze-film) concentrated gel of HA protein complex coats surfaces (boundary)

Type of Lubrication Boundary high loads low relative speeds long duration Fluid-film low/oscillated magnitude high relative speeds

Wear of AC High Impact Loading Wear Interfacial Wear Fatigue Wear results from results from results from 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  Stress  Strain Adhesion Abrasion Disruption of collagen-PG matrix, PG “wash-out”, alteration of load reducing mechanism DISRUPTION OF CARTILAGE MICROSTRUCTURE

Cartilage Degeneration Magnitude & distribution of imposed stress # of sustained stress peaks Degenerative changes to matrix Changes in tissue’s mechanical properties Swelling & weak tissue destruction by normal forces  stress conc.  in load frequency & magnitude Insult to molecular structure of collagen-PG matrix 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