Chapter 3 Biomechanics of Articular Cartilage

Introduction Articular cartilage is found in synovial joints. Articular cartilage functions: Increase load distribution area Allow movement while reducing friction and wear

Articular Cartilage Composition Articular cartilage is multiphasic: Matrix of collagen and proteoglycan (25%) Free interstitial fluid (75%) Ion phase

Collagen Gives a layered character to cartilage Presents 3 zones: Superficial tangential zone (10% to 20% thickness) Middle zone (40% to 60% thickness) Deep zone (30% thickness) Has a high tensile stiffness and strength

Proteoglycan Large protein-polysaccharide molecule Not homogenously distributed in collagen: Highest concentration in middle zone Lowest concentrations in superficial and deep zones Adds stability and rigidity to ECM

Water Most abundant component of cartilage Most concentrated near articular surface Contains free mobile cations (e.g. Na+, K+, Ca2+) Allows movement of gas, nutrients, waste products Its movement influences cartilage mechanical behavior.

Interaction among cartilage components PG interacts with collagen: It forms a porous matrix. This matrix is swollen with water. When load is applied to cartilage: Fluid runs outside the matrix. It protects against excessive stress and strain.

Biomechanical Behavior of Articular Cartilage Cartilage treated as a biphasic material with: Interstitial fluid phase Porous-permeable solid phase Cartilage is a highly stressed material To understand cartilage response to stress: Intrinsic mechanical properties must be determined. Compression, tension, and shear are considered.

Biomechanical Behavior of Articular Cartilage (continued) Intrinsic material properties and resistance to flow of solid matrix define interstitial fluid pressurization. Interstitial fluid pressurization influences: Load-bearing capacity Lubrication capacity 9

Lubrication of Articular Cartilage Lubrication processes limit wear of cartilage Fluid-film lubrication: Uses film of lubricant causing a bearing surface Load is supported by pressure developed in fluid-film Boundary lubrication: Surfaces protected by adsorbed layer of boundary lubricant Prevents surface to surface contact

Lubrication of Articular Cartilage (cont’d) Mixed lubrication Combination of fluid-film and boundary lubrications: Temporal coexistence of both at distinct locations Boosted lubrication: shift of fluid-film to boundary 11

Role of interstitial fluid pressurization in joint lubrication Fluid-film lubrication contribution is transient because of rapid dissipation of fluid-film thickness by joint loads. When interstitial pressurization is high, friction coefficient is low. As creep equilibrium is reached, friction coefficient is high. Effective friction coefficient decreases: With increasing rolling and sliding joint velocities With increasing joint load

Wear of articular cartilage Is unwanted removal of material from solid surfaces by mechanical action. Can be: Interfacial wear: Bearing surfaces come into direct contact, with no lubricant film separating them. Fatigue wear: Accumulation of microscopic damage within the bearing material under repetitive stressing Wear due to synovial joint impact loading

Wear of articular cartilage (continued) Once collagen-PG matrix is disrupted it can induce: Further disruption of collagen-PG matrix due to repetitive matrix stressing Increased “washing out” of PGs due to violent fluid movement and thus impairment of articular cartilage’s interstitial fluid load support capacity Gross alteration of normal load carriage mechanism in cartilage, thus increasing frictional shear loading on the articular surface 14

Hypotheses on the Biomechanics of Cartilage Degeneration Cartilage failure progression relates to: Magnitude of imposed stresses Total number sustained stress peaks Changes in intrinsic molecular and microscopic structure of collagen-PG matrix Changes in intrinsic mechanical property of tissue This is associated with decreased cartilage stiffness and increased cartilage permeability.

Functional Tissue Engineering of Articular Cartilage Cartilage has poor healing capacity. Tissue engineering is: Incorporating an appropriate cell. This cell will grow fabricated tissues. These tissues will be used for repair and replacement of damaged or diseased tissues and organs. 16