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Transcript
Collagen and Collagenous Tissues
• Structure of collagen
• Biomechanics of collagenous tissues
• Testing soft collagenous connective tissues
Collagen: Overview
• Collagen is the primary structural protein
in the body
• Collagen is the most prevalent protein
comprising ~30% of all proteins
• Collagen is highly conserved between
species (i.e. not undergone many
evolutionary changes)
• Molecules arranged in staggered pattern
• X-ray diffraction or electron
microscopy give rise to a banded
pattern
• Also relatively resistant to enzymatic
breakdown
Collagen fibrillar structure
Collagen Molecular Structure
• Triple Helix (Gly-X-Y)N
– X=proline
– Y=hydroxyproline
• Triple helix +
crosslinks:
Structure give rise to
a material that is very
stiff and stable
• Crosslinks (covalent
bonds) occur
between the ends
(insert diagrams) of
molecules
Band Spacing
D=670 Å
FIBRIL
Hole Zone (0.6D)
Overlap Zone (0.4D)
MICROFIBRIL
3000Å (4.4D)
COLLAGEN
MOLECULE
15Å Dia
104Å
TRIPLE
HELIX
PRIMARY
STRUCTURE
IN a-CHAIN
8.7Å
Glycine
Y
X
Glycine
Y
X
Collagen: Molecular Biology
• >20 different collagen types have been identified
• characterized by different  -chains each coded
by a different gene
• exons are often 54 bp long
– 3 bp in a codon
– 18 amino acids
– 6 sets of Gly-X-Y
Homotrimer
Type III=(a1(III))3
Heterotrimer
Type I=(a1(I))2a2(I)
Type XI=a1(XI)a2(XI)a3(XI)
Collagen Types
Classifications
Fibrillar
I
II
III
V
VI
Fibril Associated IX
XII
XIV
Network Forming IV
X
VIII
Filamentous
VI
Anchoring
VII
Examples
Tendon, Skin, Ligament
Cartilage
Skin Vessels, Tendon
Fetal Membranes - Assoc w/ Type I
Cartilage - Assoc w/ Type II
Cartilage, Cornea
Embryonic Tendon
Fetal Skin & Tendon
Basement Membrane
Hypertrophic Cartilage
Descemets Membrane
Vessels, Skin
Anchoring Filaments
Fibrillar Collagen (I (mostly), III) has greatest stiffness
Material Properties
Material
Collagen
Steel
Wood
Rubber
Bone
Elastin
Silk
Stiffness
1000 MPa
200 GPa
10GPa
1000-1400 kPa
18000 MPa
500-600 kPa
10000 MPa
UTS
100 MPa
1000 MPa
100MPa
125 MPa
100-500 MPa
But that's not enough information to predict
behavior in tissues...
Tissues are composites
Complex organization
Complex boundary conditions
Ligaments and Tendon
• connect bones together (Ligament)
• connect bones to muscle (Tendon)
• Transmit forces
• Aid in stabilizing joint motion
• Absorb impacts/stresses
• Prevent large displacements such as
dislocations
• Primarily uniaxial (1D) loading elements
Tendon Structure
Ligament and Tendon: Mechanical Properties
100
Tensile Strain
Tensile Stress
Stress (MPa)
75
50
ge
n
Ta
25
0
0
2
M
t
n
lu
u
od
4
Toe Region Linear Region
6
s
8
Yielding and
Microfailures
Strain (%)
10
Catastrophic
Failure
Ligaments
• Loading
– Fibers are parallel to
load axis
• Organization
– fascicular
organization
– Unloaded = crimped
– loaded = straight
• Composition
– Collagen 75-80%
– Elastin ‹5 %
– Proteoglycans 1-2%
http://drlowe.schipul.net
Lateral
Femur
Medial
Quadriceps
Tendon
Patella
Lateral
Collateral
Ligament Menisci
(LCL)
Medial
Collateral
Ligament
(MCL)
Posterior
Cruciate
Ligament
(PCL)
Fibula
Anterior
Cruciate
Ligament
(ACL)
Tibia
Patellar
Tendon
Knee Ligament Structure
Loaded
MCL
ACL
Unloaded
•
•
•
•
Similar changes occur in
collagenous tissues among
individuals and most species.
Progressive increases in
collagen which eventually
becomes more organized and
cross-linked until skeletal
maturity.
This results in increased
elastic stiffness and strength.
After skeletal maturity,
properties begin to deteriorate.
Stress, MPa
Age
Strain, %
Advancing Age
As a person becomes older, the maximal force their ACL can
tolerate decreases, this is has as much to do with changes in
geometry as it does changes in material properties
Immobilization
• Immobilization of the
knee causes
deterioration of the
MCL material
properties, but not the
ACL material
properties.
•
•
MCL is metabolically more active, so as it remodels the tissue it
lays down mechanically compromised material.
However, the ACL cannot produce new tissue, so it simply
atrophies.
Skin
• Collagen: 65 - 70 %
(more type III than ligament)
• Elastin: 5 - 10 %
• Proteoglycan: 1.5 - 2 %
• collagen crimp
decreases with age;
stiffness increases
• elastin crimp
increases with age;
decreasing recoil
• A mechanical
explanation for
wrinkles?
Young
Adult
Old
Skin: Mechanical Properties
• More compliant than
ligament or tendon;
needs to be for its
functions.
• orientation of coiled
fibers change with
load
• collagen is stiffer that
elastin but has greater
hysteresis (absorbs
more energy)
Ligament Tensile Testing
computer
Cross
correlation
Strain
computation
Connective Tissue Testing
Structural Properties
describe the behavior of the
actual tissue (e.g bone
ligament bone complex)
Mechanical properties
describe the behavior of the
tissue as a general material
Clamping considerations
Device to hold tissue and clamping must be stiffer and
stronger than the subject material. Otherwise the stiffness
of the device contributes to what you measure.
Ligaments—have their own "built in" clamps -- bones.
Usually drill holes in bone use steel rods.
• Do not want to clamp too far away or elongation
may include bone deformation.
• Do not want it too close because may damage
insertion (attachment) of ligament to bone. May
weaken bone.
Tendons only have 1 "natural" clamp
• Wherever you clamp, have to worry about
inhomogeneities and edge effects.
Measurement of strain
Deformation of biological tissues is nonhomogeneous, i.e. the
different regions can deform differently.
If we use the "clamp to clamp" strain the measurements would
be average over the whole region
any slippage in the clamping system would also affect the
measurement.
Some approaches to measuring regional strain in tissues
• Imaging
• Ultrasound
• Strain gauges--invasive
Strain Gauges
Sonomicrometry
(piezoelectric
crystals)
Mercury-in-rubber
www.sonometrics.com
F
Hg
F
V
Semiconductor
(resistive,
peizoresistive)
www.omego.com
Strain Tensors: 1D example
Cauchy
(infinitesimal)
L
 L
11    1    1 
 L0
 L0
Lagrangian
1 2
1  L2  L20 
E11    1  

2
2  L20 
Eulerian
1
1  1  L2  L20 
e11  1  2   

2    2  L2 


Stress-free state
How can we identify the best stress-free 'reference'
state for the stress and strain calculations?
• The soft tissues buckle under compression
• Long toe region makes it difficult to identify
transition from compressive to tensile forces
Solution:
•
Use a small tare load to repeatably identify the
initial state
Anelastic Properties
Hysteresis
• Loading & unloading curves
are different
• Area between curves
represents energy absorbed
by material
Preconditioning
• Apparent material properties
are history dependent
• Becomes repeatable with
multiple cycles (in ligaments
and tendons tested in vitro,
this occurs between 4-7
cycles)
Viscoelastic properties
Stress-Relaxation
• stress decreases with time
but reaches an equilibrium
for a step increase in strain
Creep
• Strain gradually increases
with time but reaches an
equilibrium for a step load
Strain-Rate Effects
• increased strain rate results in increased stiffness due to
viscous forces
• These effects are small in ligaments and tendons for the
normal range of strain rates, but can be important in relation
to prevention of injury
6 DOF Knee
Testing Rig
Collagenous Tissues: Key Points
• Collagen is a ubiquitous structural protein with many types all
having a triple helix structure that is cross-linked in a
staggered array.
• Some of the most common collagen types are fibrillar and the
collagen can be organized in 1-D, 2-D or 3-D in different
tissues to confer different material properties.
• The 1-D hierarchical arrangement of stiff collagen fibers in
ligaments and tendons gives these tissues high tensile
stiffness
• The 2-D arrangement of collagen fibers in tissues such as
skin is often quite wavy or disordered to permit higher strains
• Crimping, coiling and waviness of collagen matrix gives the
tissue nonlinear properties in tension.
• Collagen structure in tissues changes with disease & ageing.
• Different tissue types require different testing configurations.