Biomechanics of Cartilage

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About Cartilage
Mechanical Properties of the Articular Cartilage
Lubrication Mechanism
Failure of the Cartilage

1. Mow VC & Hung CT (2001). Biomechanics of articular cartilage. In Nordin M & Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System, 3^rd ed. Philadelphia, PA, USA: Lippincott Williams & Wilkins. pp.60-100
2. Chaffin DB, Andersson GBJ,  Martin BJ (1999). Occupational Biomechaincs, 3rd ed.  New York, John Wiley & Sons.

About Cartilage

Types of the Cartilage

• hyaline cartilage (articular cartilage)
• fibrocartilage

Characteristics of articular cartilage

• 1-5 mm hyaline cartilage： dense connective tissue

• translucent： no blood vessels, lymphatic channel, or nerve innervation
  How does the cartilage obtain nutrition and remove metabolites?

• components： low cellular density
  • condrocyte： < 10%
  • extracellular matrix
    • collagen fibers
    • ground substance： proteoglycans
    • water： 65-80%： interstitial fluid movement is important in mechanical property and joint lubrication

Functions of articular cartilage

• spread load over a wide area
• allow movement of two articulating bones with minimal friction and wear
• deformed under loading, exuding synovial fluid

Collagen fibers in articular cartilage

• biological unit： tropocollagen
• mechanical properties: tensile stiffness and strength
• distribution of collagen in articular cartilage
  • superficial tangential zone： parallel to the articular surface
  • middle zone： randomly distributed
  • deep zone： perpendicular to cartilage-calcified cartilage interface (tidemark)

Proteoglycans in articular cartilage

basic unit： glycosaminoglycans (GAGs) mutually repelled between neighboring GAGs proteoglycan hyaluronic acid link protein GAG chains： 200-400 nm in length
protein core chondroitin sulfate chains (CS)：  decrease with aging keratan sulfate chains (KS)： increase with development and aging CS/KS ratio：  10：1 at birth and 2：1 in adult

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Mechanical Properties of the Articular Cartilage

Biphasic creep response

• exudation of fluid： up to 50% of the fluid can be squeezed out
• creep phenomenon of the collagen fiber

Biphasic load relaxion phenomeon

• stress increased as fluid exudation
• stress decreased as fluid redistribution

Non-linear permeability

Rate dependency of the material behavior

• rapid loading： like elastic material
• slow loading： like viscoelastic

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Lubrication Mechanism

Boundary lubrication

• the chemical adsorption of a monolayer of lubricant molecules onto the articular surfaces
• depends on the chemical property of lubricants

Fluid film lubrication

• a much thicker film of lubricant causing a relatively large separation of the two bearing surface
• Elastohydrodynamic fluid films of both the sliding and the squeeze type probably play an important role in lubricating the joint
• With high load and low speeds of relative motion, the fluid film will decrease in thickness as the fluid is squeezed our from between the surfaces.
• Under very high loading conditions, the fluid film may be eliminated, allowing surface-to-surface contact

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Failure of the Cartilage

• mechanical loading and unloading prevent cartilage degeneration

• limited ability to remodel itself if articular cartilage is damaged

• types of failure
  interfacial wear： wear resulting from the direct interaction of bearing surfaces

  adhesion or abrasion wear only takes place in an impaired or degenerated joint

  traumatic arthritis
  fatigue wear： wear resulting from bearing deformation under repetitive loads

  failure of collagen-PG matrix + loss of PG

  e.g. chondromalacia patella
  damage from a high impact

• loads leading to wear
  • acute injury： active loading or impact loading
  • chronic injury： interfacial or fatigue loads

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