Chapter 3 Biomechanics of Articular Cartilage Copyright 2013

Introduction • Articular cartilage is found in synovial joints. • Articular cartilage functions: – Increase load distribution area – Allow movement while reducing friction and wear Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

Articular Cartilage Composition • Articular cartilage is multiphasic: – Matrix of collagen and proteoglycan (25%) – Free interstitial fluid (75%) – Ion phase Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

Water • Most abundant component of cartilage • Most concentrated near articular surface • Contains free mobile cations (e. g. Na+, K+, Ca 2+) • Allows movement of gas, nutrients, waste products • Its movement influences cartilage mechanical behavior. Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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. Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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. Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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. Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins

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. Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins