mechanical properties of biological tissues specific: cartilage in class

what is mechanical static equilibrium?

when net forces within a system is zero

state where no physical changes occur

AKA

• object is at rest

• object is in unaccelerated motion

what articular cartilage is in static equilibrium what are cartilage’s internal forces equal to?

external forces (loading)

what structures contribute to cartilage’s internal forces?

• collagen

• proteoglycans (GAGS)

what do the proteoglycans (+water), and the collagen network create?

these forces create a swollen, hydrated tissue that resists compression

what crates a swelling pressure?

the dense concentration of negatively charged GAGs repel each other and attract water

How does the collagen network respond to external forces in cartilage?

The collagen network helps contain proteoglycans and water within the cartilage. When external forces are applied, the pressure generated by the proteoglycans and water increases, which creates tensile stress on the collagen fibers, known as hoop stress.

is it preferable for articular cartilage to maintain mechanical equilibrium at all times?

no! it should undergo cycles of loading and unloading

articular cartilage requires fluid flow to bring nutrients in from the synovial fluid

certain levels of stress are needed to ensure cartilage maintenance

what are the two parts of the biphasic fluid?

• fluid phase: water is primarily responsible for resisting external forces

• solid phase: physical structures of collagen and proteoglycans primarily responsible

how does the fluid phase resist compression when external forces are applied?

• internal pressure developed with loading

  • GAGS are pushed closer together, thus increasing the repulsion force

• frictional drag created by low permeability of collagen network

  • fluid tries to flow out of the cartilage, into the synovial fluid

  • primary factor in viscoelastic behavior of cartilage

is viscoelasticity rate dependent?

yes! tissue deformation is inversely related to velocity

what is rapid loading (ex: jumping)?

• elastic response

  • deform and recover instantaneously

  • little time for fluid to flow out of the tissue

what is slow loading? (ex: prolonged standing)

• viscoelastic response

  • deformation will continue to increase over time

what phase may provide up to 95% resistance to external forces?

fluid phase! it helps protect the solid matrix

what phase resists compression when external forces push water out of the cartilage?

solid phase!

internal pressure developed by the physical structure of the collagen fibers and proteoglycans

what are the effects of prolonged loading on cartilage?

study with continuous comrpession with 150% body weight= cartilage with increased deformation

this is creep!

increased deformation of the patellar cartilage when it was subjected to a constant load over time

does patellar volume recover with the removal of external loading?

yes! patellar volume recovers

what impairments in the collagen and proteoglycans would reduce cartilages ability to resist compressive forces?

• disruption of collagen fibers

  • allows water AND proteoglycans (thus GAGs) to move more freely out of the cartilage tissue and into joint

• reduction in proteoglycans

  • reduced repulsive force and reduced ability to attract water

if the meniscus is removed from the knee what happens to the stresses experienced by the articular cartilage?

the stress increases and it becomes more concentrated

what does the acetabular labrum do?

it deepens the acetabular cup and adds stability to the joint

which meniscus has a better potential for healing?

the lateral meniscus because it has more vascularization

what is needed to maintain cartilage structure and mechanical properties?

joint loading and motion are required to maintain cartilage structure and mechanical properties

what does injury begin with?

some type of cartilage breakdown (fissuring)

what are the consequences of cartilage injury?

collagen network is disrupted which will increase permeability

what are the consequences of increased permeability?

• increased permeability will result in loss of proteoglycans (decreased density) and increased and faster fluid flow OUT of the articular cartilage

what are the consequences of decreased proteoglycan density and increased fluid flow?

decreased proteoglycan density and increased fluid flow results in decreased swelling pressure this reducing the fluid phase resistance to external load and increasing stress on the solid matrix

what are some of the mechanisms underlying the development of osteoarthritis?

• total load on joint

  • high frequency or magnitude

    • knees: football players, miners

    • hips: farmers, elite athletes (NFL)

• distribution of the load over the contact area

  • joint incongruity resulting in a small area of contact

    • fracture involving the joint

    • menisectomy

    • acetabular dysplasia