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F - CamTools
Joints, Osteoarthritis and Biomaterials
•  Natural materials:
–  Joints and articular cartilage, anatomy and
function
–  Cartilage and joint failure, osteoarthritis
•  Artificial biomaterials:
–  Total joint replacement
•  Tissue engineering:
–  Neo-cartilage and bone tissue engineering
challenges and outlook
Lecturer: Michelle Oyen, [email protected]
Biomechanics Topics
•  Natural tissues:
–  Baseline behavior of structural tissues (bone, connective tissue)
and materials (wood)
–  Healthy vs. Pathological: can see increasing (calcification) or
decreasing (degradation) modulus with disease
•  Implant materials:
–  Baseline (pre-implantation) analysis of metal, ceramic, and
polymeric materials used in implantation
–  Wear testing and fatigue testing pre-implantation
–  Testing and analysis of retrieved implants
•  Artificial tissues:
–  Matching properties for implantation and restoration
Joint Definitions
•  Joint
–  A joint is the location at which two or more bones make
contact. They are constructed to allow movement and
provide mechanical support
•  Arthrology
–  the science concerned with the anatomy, function,
dysfunction and treatment of joints.
•  Arthritis
–  (from Greek arthro-, joint + -itis, inflammation)
–  a group of conditions where there is damage caused to
the joints of the body. Arthritis is the leading cause of
disability in people over the age of 55.
Classification of Joints
•  Synarthrodial
–  Provides little or no motion
•  Amphiarthrodial
–  Provides minimal motion
–  Cartilaginous joints
•  Diarthrodial
–  a form of articulation that permits maximal motion
–  Synovial joints
• 
• 
• 
• 
• 
• 
Ball and Socket - such as the shoulder or the hip and femur.
Hinge - such as the elbow.
Pivot - such as the radius and ulna.
Condyloid - such as the wrist between radius and carpals, or knee
Saddle - such as the joint between carpal thumbs and metacarpals.
Gliding - such as between the carpals.
Classification of Joints
•  Synarthrodial (A,B) Provides little or no motion
•  Amphiarthrodial (C,D) Provides minimal motion
•  Diarthrodial (E,F,G,H,I,J) Permits maximal motion
Diarthrodial Joints
•  Diarthrodial
–  a form of
articulation
that permits
maximal
motion, as
the knee
joint
Idealized Synovial Joint
Tissues = Cells + ECM
•  In biology, extracellular matrix (ECM) is any
material part of a tissue that is not part of any cell.
•  Extracellular matrix dominance is the defining
feature of connective tissue.
•  Most connective tissues are:
–  Involved in structure and support.
–  Characterized largely by the traits of non-living tissue.
–  Examples: bone, cartilage, ligaments and tendons
epithelial, muscle, and nervous tissue
connective tissue
Collagen
http://www.youtube.com/watch?v=NK2VKpVyk2s&feature=related
Collagen Structures
m
Collagenous
Tissue
mm
Collagen
Fibrils
µm
Collagen
Molecules
nm
Atoms
Image © M. Buehler and M. Oyen 2007
Collagen Types
Fibrils and Self-Assembly
Graham et al. 2000 (Kadler group)
Collagen
Julian Voss-Andreae's sculpture Unraveling Collagen (2005), stainless steel.
Cell-ECM Interactions
Cells make the ECM and then live in it. Cells have an active role
in maintaining the ECM: they produce new material, resorb
existing material through enzymes and acids, and thus remodel.
Collagen
Proteoglycans
http://www.steve.gb.com/science/extracellular_matrix.html
Mechanotransduction
ECM macromolecule networks are linked to intracellular
networks and mechanical transduction occurs network to
network. Cells thus respond directly to mechanical forces.
http://www.steve.gb.com/science/extracellular_matrix.html
Proteins and sugars
Connective Tissue
•  Cells, proteins, sugars
Tissues in Diarthrodial Joints
Bone
Cartilage
Cartilage
Bone
Meniscus
(Fibrocartilage)
Bone Composition
Bone is a hydrated organic-inorganic composite with approximately 50%
mineral by volume
Mineral
Organic
90% collagen Ca5OH(PO4)3
Water
• Water
– 15-20% of total volume
• Organic (“osteoid”)
– 30% of total volume
– Fibrillar type I collagen
• Hydroxyapatite mineral
– 45% of total volume
– 75% of total weight
Bone Hierarchy
Osteoblasts make bone
Osteoclasts dissolve it
Osteocytes maintain it
http://www.bartleby.com/107/
A bit about Bone
Bone Biomechanics
Bone Biomechanics
Demineralized Bone Biomechanics
Tissue Elastic Properties
Approximate
elastic modulus, E
100 GPa
10 GPa
Bone
1 GPa
100 MPa
Calcified or Calcifying Cartilage
10 MPa
Articular Cartilage, Tension
1 MPa
0.1 MPa
Articular Cartilage, Compression
Cartilage Composition
Cartilage consists of
chondrocyte cells (<1%)
in an Extracellular Matrix
(ECM) made of
•  Water
–  65-80% of total weight
•  Collagen Type II, fibrillar
–  75% of dry weight
•  Aggregating Proteoglycans (PGs)
–  25% of dry weight
Interpenetrating collagen &
PG networks
Length Scales in Diarthrodial Joints
Cartilage Proteoglycans
•  1 proteoglycan = 1 core protein + many GAG
(glycosaminoglycan) chains
–
–
–
–
Core protein
GAG
(KS, CS)
–
–
–
–
H 2O
•  GAG chains on proteoglycans are negatively
charged and attract water
Cartilage Proteoglycan Aggregates
•  Aggregating proteoglycans attach to a
hyaluronic acid (HA) backbone to make
enormous macromolecule aggregates
Core protein
HA
GAG
(KS, CS)
Cartilage Proteoglycan Aggregates
Cartilage Collagen (type II)
“Arcade” structure for collagen in diarthrodial joints
Fibers parallel to surface at surface
Fibers perpendicular to surface
at bony insertion
SEM Image courtesy of Jack
Lewis, U of MN
The PG aggregates in healthy cartilage are compressed to 20% of
their free volume, and are then contained within a collagen
network that is thus pre-strained
Cartilage Inhomogeneity
Synovial Fluid
Normal synovial fluid contains 3-4 mg/ml
hyaluronan (hyaluronic acid), a polymer of
disaccharides composed of D-glucuronic
acid and D-N-acetylglucosamine.
Hyaluronan is synthesized by the synovial
membrane and secreted into the joint
cavity to lubricate the surfaces between
synovium and cartilage
Joints and Friction
Joints and Friction
Synovial fluid is responsible
for only part of the lowfriction effect!
Tissue Biomechanics
•  Concerned with baseline structure (composition) - properties relations
•  Continuum mechanics approaches based on classical elasticity
incorporating the “special features” of the mechanical behavior of
tissues
–  Nonlinear elasticity
–  Anisotropy
–  Time-dependent mechanical responses
Reminders from Basic Mechanics
σ = Eε
From strength of materials
F
δ
=E
A
L
F
F
F = kδ
EA
k=
L
Reminders from Basic Mechanics
Springs in series
F = F1 = F2
δ = δ1 + δ2
Springs in parallel
δ = δ1 = δ 2
F = F1 + F2
Reminders from Basic Mechanics
Springs in series
F = F1 = F2
δ = δ1 + δ2
Springs in parallel
δ = δ1 = δ2
F = F1 + F2
F = kδ = k1δ1 + k2δ2
kδ = k1δ + k 2δ
k = k1 + k 2
Reminders from Basic Mechanics
Springs in series
F = F1 = F2
δ = δ1 + δ2
1 1 1
= +
k k1 k 2
Springs in parallel
δ = δ1 = δ2
F = F1 + F2
F = kδ = k1δ1 + k2δ2
kδ = k1δ + k 2δ
k = k1 + k 2
Elastic Recruitment