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Cartilage Mechanics

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costal, nasal, tracheo-bronchial, and articular cartilage; cartilage model (pre-skeletal) ... intervertebral disc, articular disc (knee, wrist, jaw), bony ... – PowerPoint PPT presentation

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Title: Cartilage Mechanics


1
Cartilage Mechanics
  • CARTILAGE
  • Types, Locations, Mechanical Functions
  • FUNCTIONAL COMPONENTS OF CARTILAGE
  • DISEASES INVOLVING CARTILAGE
  • FRICTION, LUBRICATION AND WEAR
  • TENSILE PROPERTIES
  • SHEAR PROPERTIES

2
Cartilage Types Locations
  • hyaline cartilage (hylos glassy)
  • costal, nasal, tracheo-bronchial, and articular
    cartilage cartilage model (pre-skeletal)
  • white fibrocartilage
  • intervertebral disc, articular disc (knee, wrist,
    jaw), bony grooves lodging tendons
  • yellow elastic fibrocartilage
  • external ears, larynx, epiglottis, arytenoids

3
Prenatal development of long bone
A, the mesenchymal model, 5 weeks B, the
cartilage model, 6 weeks C and D, the beginning
of ossification, 7 to 12 weeks
4
Growth of Long Bone
Later stages of prenatal development of a long
bone. A, 6 months. B, at birth
Growth of a long bone (the femur) during
childhood. In freely movable joints the ends of
the bones are capped with articular cartilage.
The epiphyseal cartilage plate provides for the
longitudinal growth of the bone.
5
Mechanical Functions of Articular Cartilage
  • Joint motion with minimal friction or wear, lower
    extremities undergo 1 million loading
    cycles/year
  • distribute and transmit stress to bone

6
Functional Components of Cartilage
Two major phases - solid and fluid (biphasic)
7
Components of Cartilage Cells
  • ( chondrocytes)
  • occupy only 1-10 of the volume of cartilage
    (young cartilage has a higher density and volume
    fraction of cells than adult cartilage)
  • ? in mechanical measurement and analysis,
    contribution of chondrocytes usually neglected.
  • Functions
  • synthesize the extracellular matrix (ECM)
  • synthesize enzymes that degrade ECM
  • respond to changing functional biomechanical
    demands

8
Functional Components of Cartilage
Electron micrographs of chondrocytes originating
from the different zones of mature articular
cartilage. A Tangential zone chondrocyte B
Transitional zone chondrocyte C, D Upper (C)
and lower (D) radial zone chondrocytes.
Light micrograph of mature rabbit articular
cartilage. S, tangential zone T, transitional
zone UR, upper radial zone LR, lower radial
zone C, calcified cartilage B, subchondral bone
plate.
9
Extracellular Matrix Solid Phase
  • Collagen
  • 60 of dry weight of cartilage
  • mostly type II collagen
  • (a chain ? triple helical molecule ? fibril ?
    fiber)
  • zonal collagen architecture (superficial
    tangential, middle, deep)
  • functions
  • tensile stiffness and strength
  • mesh that immobilizes proteoglycan aggregates

Collagen fibril structure
10
Cartilage Collagen Network
Layered structure of collagen network showing
three distinct regions (A) corresponding SEM
collagen fibrillar arrangement (B).
11
Components of Cartilage Proteoglycan
  • 30 of the dry weight of cartilage
  • terminology
  • proteoglycan aggregate proteoglycan monomers,
    aggregated onto a hyaluronate backbone, in an
    interaction stabilized by link proteins
  • proteoglycan monomer - protein core to which are
    covalently attached 20-100 sugar polymers
    (glycosaminoglycans)
  • glycosaminoglycans are highly charged (acidic)
    molecules due to -SO3- -COO- that are
    negatively ionized at physiologic pH
  • charge density of articular cartilage is high
    (negative 0.05-0.2 mol/L tissue fluid)

12
Proteoglycans (cont'd)
  • electrostatic repulsion of neighboring GAG
    polymers makes proteoglycans tend to swell and
    resist compression
  • electrostatic attraction and concentration of
    positive counter-ions (Na, H), and repulsion
    and dilution of negative co-ions (Cl-) e.g., for
    a bathing medium a concentration of NaCl of 0.11
    M and a charge density of -0.2 mol/L, the
    extracellular matrix concentration Na 0.26 M
    and Cl- 0.05 M
  • functions
  • compressive stiffness
  • modulates ionic milieu of extracellular fluid

13
Functional Components of Cartilage
Extracellular Fluid Phase (water, ions see above)
A. Schematic depiction of an aggregating
proteoglycan monomer composed of keratin sulfate
and chondroitin sulfate chains bound covalently
to a protein core molecule.
B. A representation of a proteoglycan aggregate
which is composed of monomers noncovalently
attached to hyaluronic acid with stabilizing link
proteins.
14
Schematic Summary Joint Loading
A. Chondrocytes synthesize collagens,
proteoglycans, hyaluronate, and link protein.
These components assemble into a stable
extracellular matrix subsequent catabolic
processes cause the release of certain components
from the tissue. Alternatively, some biosynthetic
products may never become incorporated into the
matrix. These metabolic processes may be
influenced by mechanical, chemical, or electrical
phenomena.
15
Schematic of Joint Loading
B. Physical phenomena chondrocyte deformation,
hydrostatic pressurization, fluid flow, electric
fields (streaming potentials and currents),
matrix consolidation, and physico-chemical
alterations (altered ion concentrations and
osmotic pressure).
16
Compression of Cartilage
17
Diseases of Cartilage Osteoarthritis
  • degenerative joint disease, a wear and tear
    phenomenon
  • gross changes
  • erosion of the joint surface
  • reactive bone formation
  • microscopic changes
  • fissures in the articular surface
  • cell division
  • increased incidence with aging, obesity, joint
    injury
  • extremely common (80 of Americans gt age 65 have
    radiological evidence)
  • treatment
  • analgesics
  • ? total joint replacement
  • potential cause (see figure)
  • excessive joint loading
  • matrix degradation process unknown

18
Diseases Involving Cartilage
19
Diseases Involving Cartilage
20
Diseases Involving Cartilage
Hypothesis of the etiopathogenesis of
osteoarthrosis due to excessive joint loading.
(I) Normal articular cartilage. Dots represent
proteoglycans, curved lines represent collagens,
and oval structures represent chondrocytes. (ii)
Proteoglycan loss exposes the superficial
collagen fibrils condition is still reversible.
(iii) Injury of collagen fibrils in the cartilage
surface additional loss of proteoglycans point
of no return.
21
Diseases of Cartilage Rheumatoid Arthritis
  • autoimmune disease
  • gross changes
  • synovial thickening due to inflammation
  • pannus formation
  • erosive destruction of cartilage and bone
  • microscopic changes
  • synovial cell proliferation
  • cartilage destruction most severe under pannus
  • familial (inherited)
  • much less common than osteoarthritis
  • treatment of flares
  • immunosuppressive drugs
  • -gt total joint replacement
  • potential cause (see figure)
  • initiating/triggering factor unknown
  • inflammatory process and enzymes
  • stimulation of chondrocyte-mediated matrix
    degradation

22
Diseases Involving Cartilage
Progressive stages in joint pathology. 1. Acute
inflammation of synovial membrane (synovitis) and
beginning proliferative changes. 2. Progression
of inflammation with pannus formation beginning
destruction of cartilage and mild
osteoporosis. 3. Subsidence of inflammation
fibrous ankylosis. 4. Bony ankylosis advanced
osteoporosis.
23
Diseases Involving Cartilage
24
Diseases Involving Cartilage
The chondrocyte has a primary role in joint
cartilage matrix turnover.
25
Friction, Lubrication and Wear
Synovial Fluid
Synovial transport pathways (not drawn to scale)
26
Coefficient of Friction
27
Friction, Lubrication and Wear
A joint friction experiment using an isolated
animal synovial joint.
Schematic of typical test configuration
28
Coefficient of Friction
29
Friction, Lubrication and Wear
A. A simple pendulum device with the human hip
joint as its fulcrum is used to measure the
coefficient of friction between femoral head
(shown) and the acetabulum (not shown) by the
decay of the amplitude of the pendulum motion.
30
Friction, Lubrication and Wear
B. A typical set of curves for the coefficient of
friction versus the number of cycles from one hip
joint specimen under suddenly loaded conditions -
unlubricated with synovial fluid - showing a
longer period of swing and a lower coefficient of
friction with increasing load.
C. Differences of the coefficient of friction
between unlubricated and synovial fluid
lubricated hip joints with varying applied loads.
At a load gt600 N, no differences in coefficients
of friction were observed.
31
Explant Studies
Specimens cut from the bovine humeral head and
glenoid surface provide conforming surfaces for
studies on interfacial friction.
Variation of the coefficient of friction under
various lubrication and loading conditions.
32
Tensile Properties
Dependence on Collagen Alignment Fibrillation
Typical test configuration
Typical stress-strain curve for articular
cartilage in a uniaxial experiment
33
Tensile Properties
Dependence on collagen fibrillation
34
Youngs Modulus Cartilage cf. Tendon and Skin
35
Shear Properties
Dependence on Collagen Content
Typical test configuration
Shear test of a circular specimen
A schematic of cartilage in pure shear. The
tension of collagen provided shear stiffness.
36
Shear Properties
Dependence on collagen content
A direct correlation between the collagen content
(by net weight) and magnitude of dynamic modulus
G for bovine articular cartilage. The
compressive clamping strain is 20 and frequency
f 1 Hz.
37
Confined Compression Swelling
Equilibrium confined compression modulus
(Above) Schematic of measurement of the bulk
longitudinal modulus of a mechanically linear
tissue equilibrated at NaCl concentration co.
38
Confined Compression Swelling
Role of fixed charge density
39
Bulk Longitudinal Modulus
Bulk longitudinal modulus HA(c) for articular
cartilage (b) as a function of NaCl concentration.
40
Confined Compression Stress-relaxation
Schematic representation of fluid exudation and
redistribution within cartilage during a
rate-controlled, compression stress-relaxation
experiment (lower left figure). The horizontal
bars in the upper figures indicate the
distribution of strain in the tissue. The lower
graph (right) shows the stress response during
compression phase (O, A, B) and the relaxation
phase (B, C, D, E).
41
Confined Compression Creep
Schematic representation of fluid exudation and
redistribution within a poroelastic material such
as cartilage during a step application of stress
and the resulting creep deformation.
42
Confined Compression Swelling
43
Confined Compression Swelling
ASSUMPTIONS
(1) the cells do not contribute to the physical
properties and can be ignored (2) cartilage
consists of homogeneous (spatially uniform) solid
and fluid phases (3) the solid matrix behaves as
a linearly elastic and isotropic (idential in all
directions) material (4) there is frictional drag
that is proportional to the fluid velocity
(relative to the solid) and characterized by the
hydraulic (open circuit) permeability (5) the
fluid and current flow are coupled to the
hydrostatic pressure and electrical potential
gradients through electrokinetic coupling
coefficients that obey Onsager reciprocity (6)
the stresses and strains are infinitesimal
44
Poroelastic Modeling of Cartilage
45
Poroelastic Modeling of Cartilage
46
Darcys law hydraulic permeability
The fluid-solid interaction is described by the
phenomenological relations of non-equilibrium
thermodynamics where the electrokinetic
coupling coefficients are equal (k12 k21) by
Onsager reciprocity and k22 is the electrical
conductance. The open circuit, Darcy hydraulic
permeability is
47
Poroelastic Modeling of Cartilage
48
Solutions of the Poroelastic Equation of Motion
Sinusoidal steady-state In the sinusoidal steady
state, each of the variables, such as the
displacement u(z,t), can be written in the
form The equation of motion becomes
49
Solutions of the Equation of Motion
50
Solutions of the Equation of Motion
51
Solutions of the Equation of Motion
52
Solutions of the Equation of Motion
53
Solutions of the Equation of Motion
A curve-fit of the theoretical solution for the
surface displacement of a cartilage specimen in
confined compression from the linear KLM theory
and the actual experimental data.
54
Physical Properties of Cartilage in Osteoarthritis
DECREASED PROTEOGLYCN CONTENT INCREASED WATER
CONTENT gt explanation? DECREASED EQUILIBRI
UM CONFINED COMPRESSION MODULUS INCREASED HYDRAU
LIC PERMEABILITY DECREASED STREAMING POTENTIAL
55
Cartilage Summary of Key Points
  • Hyaline cartilage occurs at articular joints,
    fibrocartilage is found at other joints and
    locations
  • Long bone begins as cartilage
  • Articular cartilage bears frictional and
    compressive loads at joints
  • Cartilage is highly hydrated with few cells and
    no vessels fibrillar collagen solid phase
    enmeshes charged proteoglycans and synovial fluid
  • Solid matrix elasticity, fluid pressure and
    viscosity, electrical charge density and osmotic
    chemical balance all contribute to viscoelastic
    cartilage mechanical properties
  • Osteoarthritis and rheumatoid arthritis lead to
    joint degeneration
  • Cartilage coefficient of friction is very low
    (0.01)
  • Cartilage is often tested in compression
  • Poroelasticity theory models cartilage as a
    composite of elastic solid and viscous fluid
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