Mechanical properties of cartilage

Question 1

what are the 4 main mechanical properties of cartilage

Answer 1

heterogenous
biphasic
viscoelastic
anisotropic

Question 2

why is cartilage labelled heterogenous

Answer 2

has a depth dependent modulus
- exponentially increases as a function of depth (from 0.2-2 MPa)

Question 3

what does biphasic mean

Answer 3

cartilage has two phases and is a fluid saturated porous material
- fluid phase = interstitial fluid carrying load
- solid phase = collagen, PGs and cells carry load
- phase taking most of the load at a given time can change

Question 4

why is cartilage labelled viscoelastic

Answer 4

has a time dependent response
exhibits both viscous and elastic characteristics when deformed

Question 5

what does viscoelasticity depend on

Answer 5

internal friction of molecules as they slide by one another
- high friction = water can’t get out
- low friction = water escapes
speed of mvmt
- high speed = water doesn’t leave
- low speed = water escapes

Question 6

what 5 behaviours do viscoelastic materials demonstrate

Answer 6

creep
stress relaxation
load dependent effective stiffness/modulus
hysteresis
energy loss during impact

Question 7

what is creep

Answer 7

constant compressive stress applied (monitor strain)
strain rapidly increases and then slowly increases (non-linear)
when stress is removed - strain does not immediately go back to 0

Question 8

what is stress relaxation

Answer 8

constant strain applied (monitor stress)
strees first rapidly increases and slowly relaxes during strain hold
occurs due to fluid redistribution

Question 9

how is the effective stiffness/modulus and load related

Answer 9

both depend on the load applied (equal the slope of their graph)
modulus is higher when the load is higher

Question 10

what is hysteresis

Answer 10

the curve when load is applied and when the load is released look different
- due to energy loss and reorganisation during load

Question 11

hwo does cartilage react during loading

Answer 11

fluid escapes cartilage
ECM starts to compact
load is borne mostly by fluid phase
(dynamic modulus)

Question 12

how does cartilage react during equilibrium

Answer 12

fluid flow stops when internal pressure = external pressure
load is borne mostly by solid phase
(equilibrium modulus)

Question 13

what does the speed of fluid exudation depend on

Answer 13

permeabililty of the cartilage
^depends on:
- amount of water, PGs, and collagen

Question 14

is cartilage more or less permeable the faster you load

Answer 14

less permeable the faster you load

Question 15

why is cartilage less permeable at higher loads

Answer 15

increased compression = decreased diamater of pores and increased intrinsic friction
BECOMES STIFFER

Question 16

is the loading of cartilage even through each zone

Answer 16

no - it varies from zone to zone

Question 17

which zone resists load the best

Answer 17

middle/transitional zone
- least on the surface

Question 18

what is cartilage’s response during tensile loading

Answer 18

toe region = reorientation of collagen fibres in the direction of tension
stretched = fibres start to elongate and carry load

Question 19

what are the tensile properties of cartilage

Answer 19

highly anisotropic due to different orientation of collagen fibres
stronger when loading is parallel to collagen compared to perpendicular
tensile strength of cartilage is far less than tensile strength for tendons and ligaments